ALBERT

Description: Five non-eddy-resolving oceanic general circulation models driven by atmospheric fluxes derived from the NCEP reanalysis are used to investigate the link between the Gulf Stream (GS) variability, the atmospheric circulation, and the Atlantic meridional overturning circulation (AMOC). Despite the limited model resolution, the temperature at the 200-m depth along the mean GS axis behaves similarly in most models to that observed, and it is also well correlated with the North Atlantic Oscillation (NAO), indicating that a northward (southward) GS shift lags a positive (negative) NAO phase by 0–2 yr. The northward shift is accompanied by an increase in the GS transport, and conversely the southward shift with a decrease in the GS transport. Two dominant time scales appear in the response of the GS transport to the NAO forcing: a fast time scale (less than 1 month) for the barotropic component, and a slower one (about 2 yr) for the baroclinic component. In addition, the two components are weakly coupled. The GS response seems broadly consistent with a linear adjustment to the changes in the wind stress curl, and evidence for baroclinic Rossby wave propagation is found in the southern part of the subtropical gyre. However, the GS shifts are also affected by basin-scale changes in the oceanic conditions, and they are well correlated in most models with the changes in the AMOC. A larger AMOC is found when the GS is stronger and displaced northward, and a higher correlation is found when the observed changes of the GS position are used in the comparison. The relation between the GS and the AMOC could be explained by the inherent coupling between the thermohaline and the wind-driven circulation, or by the NAO variability driving them on similar time scales in the models.

Description: This research was supported by the PREDICATE project of the European Community, and for M. Bentsen by the Research Council of Norway through RegClim, NOClim, and the Programme of Supercomputing.

Description: Published

Description: 2119–2135

Description: 3.7. Dinamica del clima e dell'oceano

Description: JCR Journal

Description: reserved

Description: In this paper results from the application of an ocean data assimilation (ODA) system, combining a multivariate reduced-order optimal interpolator (OI) scheme with a global ocean general circulation model (OGCM), are described. The present ODA system, designed to assimilate in situ temperature and salinity observations, has been used to produce ocean reanalyses for the 1962–2001 period. The impact of assimilating observed hydrographic data on the ocean mean state and temporal variability is evaluated. A special focus of this work is on the ODA system skill in reproducing a realistic ocean salinity state. Results from a hierarchy of different salinity reanalyses, using varying combinations of assimilated data and background error covariance structures, are described. The impact of the space and time resolution of the background error covariance parameterization on salinity is addressed.

Description: This work has been funded by the ENACT Project (Contract EVK2-CT2001-00117) for A. Bellucci and P. Di Pietro, and partially by the ENSEMBLES Project (Contract GOCE-CT-2003-505539) for A. Bellucci.

Description: Published

Description: 3785-3807

Description: 3.7. Dinamica del clima e dell'oceano

Description: JCR Journal

Description: reserved

Description: The effect of horizontal resolution on tropical variability is investigated within the modified SINTEX model, SINTEX-F, developed jointly at INGV, IPSL and at the Frontier Research System. The horizontal resolutions T30 and T106 are investigated in terms of the coupling characteristics, frequency and variability of the tropical ocean-atmosphere interactions. It appears that the T106 resolution is generally beneficial even if it does not eliminate all the major systematic errors of the coupled model. There is an excessive shift west of the cold tongue and ENSO variability, and high resolution has also a somewhat negative impact to the variability in the East Indian Ocean. A dominant two-year peak for the NINO3 variabilty in the T30 model is moderated in the T106 as it shifts to longer time scale. At high resolution new processes come into play, as the coupling of tropical instability waves, the resolution of coastal flows at the Pacific Mexican coasts and improved coastal forcing along the coast of South America. The delayed oscillator seems the main mechanism that generates the interannual variability in both models, but the models realize it in different ways. In the T30 model it is confined close to the equator, involving relatively fast equatorial and near-equatorial modes, in the high resolution, it involves a wider latitudinal region and slower waves. It is speculated that the extent of the region that is involved in the interannual variability may be linked to the time scale of the variability itself.

Description: This research was partially supported by the Italy–USA Cooperation Program of the Italian Ministry of Environment and by the EU projects ENSEMBLES and DYNAMITE.

Description: Published

Description: 730-750

Description: 3.7. Dinamica del clima e dell'oceano

Description: JCR Journal

Description: reserved

Description: The accuracy of all types of Vaisala radiosondes and two types of Snow White chilled-mirror hygrosondes was assessed in an intensive in situ comparison with reference hygrometers. Fourteen nighttime reference comparisons were performed to determine a working reference for the radiosonde comparisons. These showed that the night version of the Snow White agreed best with the references [i.e., the NOAA frost-point hygrometer (FPH) and University of Colorado cryogenic frost-point hygrometer (CFH)], but that the daytime version had severe problems with contamination in the humid upper troposphere. Since the RS92 performance was superior to the other radiosondes and to the day version of the Snow White, it was selected to be the working reference. According to the reference comparison, the RS92 has no bias in the mid- and lower troposphere, with deviations 〈±5% in relative humidity (RH). In the upper troposphere, the RS92 has a 5% RH wet bias, which is partly due to the RS92 time lag error and the termination of the heating cycle. It was shown that the time lag effects relating to Vaisala radiosondes can be corrected. Because these were nighttime comparisons, they can be considered to be free from solar radiation effects. Neither the radiosondes nor the Snow White succeeded in reproducing reference class hygrometer profiles in the stratosphere. According to the 29 radiosonde intercomparisons, the RS92 and the modified RS90 (FN) had the best mutual agreement and no bias. The disagreement is largest (〈±10% RH) at low temperatures (T ≪ −30°C), where the FN underestimated (overestimated) in high (low) ambient RH. In comparison with the RS92, the RS90 had a semilinearly increasing wet bias with decreasing temperature, where the bias was 10% RH at −60°C. The RS80-A suffers from a large temperature-dependent dry bias in high RH conditions, being over 30% RH at −60°C and 5% RH near 0°C. The RS80-A dry bias can be almost totally removed with the correction algorithm by Leiterer et al., which was chosen as the best available. The other approach tested tends to overcorrect in high RH conditions when T 〈 −50°C. For T 〉 −30°C it is ineffective and does not correct the RS80-A dry bias in high ambient RH.

Description: Advances in computer power, new forecasting challenges, and new diagnostic techniques have brought about changes in the way atmospheric development and vertical motion are diagnosed in an operational setting. Many of these changes, such as improved model skill, model resolution, and ensemble forecasting, have arguably been detrimental to the ability of forecasters to understand and respond to the evolving atmosphere. The use of nondivergent wind in place of geostrophic wind would be a step in the right direction, but the advantages of potential vorticity suggest that its widespread adoption as a diagnostic tool on the west side of the Atlantic is overdue. Ertel potential vorticity (PV), when scaled to be compatible with pseudopotential vorticity, is generally similar to pseudopotential vorticity, so forecasters accustomed to quasigeostrophic reasoning through the height tendency equation can transfer some of their intuition into the Ertel-PV framework. Indeed, many of the differences between pseudopotential vorticity and Ertel potential vorticity are consequences of the choice of definition of quasigeostrophic PV and are not fundamental to the quasigeostrophic system. Thus, at its core, PV thinking is consistent with commonly used quasigeostrophic diagnostic techniques.

Description: Synoptic and mesoscale meteorology underwent a revolution in the 1940s and 1950s with the widespread deployment of novel weather observations, such as the radiosonde network and the advent of weather radar. These observations provoked a rapid increase in our understanding of the structure and dynamics of the atmosphere by pioneering analysts such as Fred Sanders. The authors argue that we may be approaching an analogous revolution in our ability to study the structure and dynamics of atmospheric phenomena with the advent of probabilistic objective analyses. These probabilistic analyses provide not only best estimates of the state of the atmosphere (e.g., the expected value) and the uncertainty about this state (e.g., the variance), but also the relationships between all locations and all variables at that instant in time. Up until now, these relationships have been determined by sampling in time by, for example, case studies, composites, and time-series analysis. Here the authors propose a new approach, ensemble synoptic analysis, which exploits the information contained in probabilistic samples of analyses at one or more instants in time. One source of probabilistic analyses is ensemble-based state-estimation methods, such as ensemble-based Kalman filters. Analyses from such a filter may be used to study atmospheric phenomena and the relationships between fields and locations at one or more instants in time. After a brief overview of a research-based ensemble Kalman filter, illustrative examples of ensemble synoptic analysis are given for an extratropical cyclone, including relationships between the cyclone minimum sea level pressure and other synoptic features, statistically determined operators for potential-vorticity inversion, and ensemble-based sensitivity analysis.

Description: The pioneering large-scale studies of cyclone frequency, location, and intensity conducted by Fred Sanders prompt similar questions about lesser-studied anticyclone development. The results of a climatology of closed anticyclones (CAs) at 200, 500, and 850 hPa, with an emphasis on the subtropics and midlatitudes, is presented to assess the seasonally varying distribution and hemispheric differences of these features. To construct the CA climatology, a counting program was applied to twice-daily 2.5° NCEP–NCAR reanalysis 200-, 500-, and 850-hPa geopotential height fields for the period 1950–2003. Stationary CAs, defined as those CAs that were located at a particular location for consecutive time periods, were counted only once. The climatology results show that 200-hPa CAs occur preferentially during summer over subtropical continental regions, while 500-hPa CAs occur preferentially over subtropical oceans in all seasons and over subtropical continents in summer. Conversely, 850-hPa CAs occur preferentially over oceanic regions beneath upper-level midocean troughs, and are most prominent in the Northern Hemisphere, and over midlatitude continents in winter. Three case studies of objectively identified CAs that produced heal waves over the United States, Europe, and Australia in 1995, 2003, and 2004, respectively, are presented to supplement the climatological results. The case studies, examining the subset of CAs than can produce heat waves, illustrate how climatologically hot continental tropical air masses produced over arid and semiarid regions of the subtropics and lower midlatitudes can become abnormally hot in conjunction with dynamically driven upper-level ridge amplification. Subsequently, these abnormally hot air masses are advected downstream away from their source regions in conjunction with transient disturbances embedded in anomalously strong westerly jets.

Description: Oklahoma Mesonetwork data are used to illustrate important atmospheric features that are not well shown by the usual synoptic data. For example, some shifts of wind from south to north that are shown as cold fronts on synoptic charts are not cold fronts by any plausible definition. As previously discussed by Fred Sanders and others, such errors in analysis can be reduced by knowledge of the wide variety of weather phenomena that actually exists, and by more attention to temperatures at the earth's surface as revealed by conventional synoptic data. Mesoscale data for four cases reinforce previous discussions of the ephemeral nature of fronts and deficiencies in the usual analyses of cold fronts. One type of misanalyzed case involves post-cold-frontal boundary layer air that is warmer than the prefrontal air. A second type is usually nocturnal, with a rise of local temperature during disruption of an inversion and a wind shift with later cooling that accompanies advection of a climatological gradient of temperature.

Description: The advent of the polar front theory of cyclones in Norway early in the last century held that the development of fronts and air masses is central to understanding midlatitude weather phenomena. While work on fronts continues to this day, the concept of air masses has been largely forgotten, superseded by the idea of a continuum. The Norwegians placed equal emphasis on the thermodynamics of airmass formation and on the dynamical processes that moved air masses around; today, almost all the emphasis is on dynamics, with little published literature on diabatic processes acting on a large scale. In this essay, the author argues that a lack of understanding of large-scale diabatic processes leads to an incomplete picture of the atmosphere and contributes to systematic errors in medium- and long-range weather forecasts. At the same time, modern concepts centered around potential vorticity conservation and inversion lead one to a redefinition of the term "air mass" that may have some utility in conceptualizing atmospheric physics and in weather forecasting.

Description: Historically, the atmospheric sciences have tended to treat problems of weather and climate separately. The real physical system, however, is a continuum, with short-term (minutes to days) “weather” fluctuations influencing climate variations and change, and, conversely, more slowly varying aspects of the system (typical time scales of a season or longer) affecting the weather that is experienced. While this past approach has served important purposes, it is becoming increasingly apparent that in order to make progress in addressing many socially important problems, an improved understanding of the connections between weather and climate is required. This overview summarizes the progress over the last few decades in the understanding of the phenomena and mechanisms linking weather and climate variations. The principal emphasis is on developments in understanding key phenomena and processes that bridge the time scales between synoptic-scale weather variability (periods of approximately 1 week) and climate variations of a season or longer. Advances in the ability to identify synoptic features, improve physical understanding, and develop forecast skill within this time range are reviewed, focusing on a subset of major, recurrent phenomena that impact extratropical wintertime weather and climate variations over the Pacific–North American region. While progress has been impressive, research has also illuminated areas where future gains are possible. This article concludes with suggestions on near-term directions for advancing the understanding and capabilities to predict the connections between weather and climate variations.

Description: Fred Sanders' career extended over 55 yr, touching upon many of the revolutionary transformations in the field of meteorology during that period. In this paper, his contributions to the transformation of synoptic meteorology, his research into the nature of explosive cyclogenesis, and related advances in the ability to predict these storms are reviewed. In addition to this review, the current status of forecasting oceanic cyclones 4.5 days in advance is presented, illustrating the progress that has been made and the challenges that persist, especially for forecasting those extreme extratropical cyclones that are marked by surface wind speeds exceeding hurricane force. Last, Fred Sanders' participation in a forecast for the historic 1947 snowstorm (that produced snowfall amounts in the New York City area that set records at that time) is reviewed along with an attempt to use today's operational global model to simulate this storm using data that were available at the time. The study reveals the predictive limitations involved with this case based on the scarcity of upper-air data in 1947, while confirming Fred Sanders' forecasting skills when dealing with these types of major storm events, even as a young aviation forecaster at New York's LaGuardia Airport.

Description: A case study of a double dryline on 22 May 2002 is presented. Mobile, 3-mm-wavelength Doppler radars from the University of Massachusetts and the University of Wyoming (Wyoming cloud radar) were used to collect very fine resolution vertical-velocity data in the vicinity of each of the moisture gradients associated with the drylines. Very narrow (50–100 m wide) channels of strong upward vertical velocity (up to 8 m s–1) were measured in the convergence zone of the easternmost dryline, larger in magnitude than reported with previous drylines. Distinct areas of descending motion were evident to the east and west of both drylines. Radar data are interpreted in the context of other observational platforms available during the International H2O Project (IHOP-2002). a variational ground-based mobile radar data processing technique was developed and applied to pseudo-dual-Doppler data collected during a rolling range-height indicator deployment. It was found that there was a secondary (vertical) circulation normal to the easternmost moisture gradient; the circulation comprised an easterly component near-surface flow to the east, a strong upward vertical component in the convergence zone, a westerly return, flow above the convective boundary layer, and numerous regions of descending motion, the most prominent approximately 3–5 km to the east of the surface convergence zone.

Description: Fred Sanders' teaching and research contributions in the area of quasigeostrophic theory are highlighted in this paper. The application of these contributions is made to the topic of extreme cold-season precipitation events in the Saint Lawrence valley in the northeastern United States and southern Quebec. This research focuses on analyses of Saint Lawrence valley heavy precipitation events. Synoptic- and planetary-scale circulation anomaly precursors are typically identified several days prior to these events. These precursors include transient upper-level troughs, strong moisture transports into the region, and anomalously large precipitable water amounts. The physical insight of Fred Sanders' work is used in the analysis of these composite results. Further details of this insight are provided in analyses of one case of heavy precipitation.

Description: Sanders designed a barotropic tropical cyclone (TC) track prediction model for the North Atlantic TC basin that became known as the Sanders barotropic (SANBAR) model. It predicted the streamfunction of the deeplayer mean winds (tropical circulation vertically averaged from 1000 to 100 hPa) that represents the vertically averaged tropical circulations. Originally, the wind input for the operational objective analysis (OA) consisted of winds measured by radiosondes and 44 bogus winds provided by analysis at the National Hurricane Center (NHC), which corresponded to the vertically averaged flow over sparsely observed tropical, subtropical, and midlatitude oceanic regions. The model covered a fixed regional area and had a grid size of ~ 154 km. It estimated the initial storm motion solely on the basis of the large-scale flow from the OA, not taking into account the observed storm motion. During 1970, the SANBAR model became the first dynamical TC track model to be run operationally at NHC. Track forecasts of SANBAR were verified from the 1971 TC season when track model verifications began at NHC until its retirement after the 1989 Atlantic TC season. The average annual SANBAR forecast track errors were verified relative to Climatology and Persistence (CLIPER), the standard no-skill track forecast. Comparison with CLIPER determines the skill of track forecast methods. Verifications are presented for two different versions of the SANBAR model system used operationally during 1973–84 and 1985–89. In homogeneous comparisons (i.e., includes only forecasts for the same initial times) for the former period, SANBAR's track forecasts were slightly better than CLIPER at 24–48-h forecast intervals; however, from 1985 to 1989 the average SANBAR track forecast errors from 24–72 h were ~10% more skillful than homogeneous CLIPER track forecasts.

Description: In the last decade, Fred Sanders was often critical of current surface analysis techniques. This led to his promoting the use of surface potential temperatures to distinguish between fronts, baroclinic troughs, and non-frontal baroclinic zones, and to the development of a climatology of surface baroclinic zones. In this paper, criticisms of current surface analysis techniques and the usefulness of surface potential temperature analyses are discussed. Case examples are used to compare potential temperature analyses and current National Centers for Environmental Prediction analyses. The 1-yr climatology of Sanders and Hoffman is reconstructed using a composite technique. Annual and seasonal mean potential temperature analyses over the continental United States, southern Canada, northern Mexico, and adjacent coastal waters are presented. In addition, gridpoint frequencies of moderate and strong potential temperature gradients are calculated. The results of the mean potential temperature analyses show that moderate and strong surface baroclinic zones are favored along the coastlines and the slopes of the North American cordillera. Additional subsynoptic details, not found in Sanders and Hoffman, are identified. The availability of the composite results allows for the calculation of potential temperature gradient anomalies. It is shown that these anomalies can be used to identify significant frontal baroclinic zones that are associated with weak potential temperature gradients. Together the results and reviews in this paper show that surface potential temperature analyses are a valuable forecasting and analysis tool allowing analysts to distinguish and identify fronts, baroclinic troughs, and nonfrontal baroclinic zones.

Description: The nature of the different types of surface boundaries that appear in the southern plains of the United States during the convectively active season is reviewed. The following boundaries are discussed: fronts, the dryline, troughs, and outflow boundaries, The boundaries are related to their environment and to local topography. The role these boundaries might play in the initiation of convective storms is emphasized. The various types of boundary-related vertical circulations and their dynamics are discussed. In particular, quasigeostrophic and semigeostrophic dynamics, and the dynamics of solenoidal circulations, density currents, boundary layers, and gravity waves are considered. Miscellaneous topics pertinent to convective storms and their relationship to surface boundaries such as along-the-boundary variability, boundary collisions, and the role of vertical shear are also discussed. Although some cases of storm initiation along surface boundaries have been well documented using research datasets collected during comprehensive field experiments, much of what we know is based only on empirical forecasting and nowcasting experience. It is suggested that many problems relating to convective-storm formation need to be explored in detail using real datasets with new observing systems and techniques, in conjunction with numerical simulation studies, and through climatological studies.

Description: This paper begins with a review of basic surface frontogenesis concepts with an emphasis on fronts located over sloping terrain adjacent to mountain barriers and fronts located in large-scale baroclinic zones close to coastlines. The impact of cold-air damming and differential diabatic heating and cooling on frontogenesis is considered through two detailed case studies of intense surface fronts. The first case, from 17 to 18 April 2002, featured the westward passage of a cold (side-door) front across coastal eastern New England in which 15°–20°C temperature decreases were observed in less than one hour. The second case, from 28 February to 4 March 1972, featured a long-lived front that affected most of the United States from the Rockies to the Atlantic coast and was noteworthy for a 50°C temperature contrast between Kansas and southern Manitoba, Canada. In the April 2002 case most of New England was initially covered by an unusually warm, dry air mass. Dynamical anticyclogenesis over eastern Canada set the stage for a favorable pressure gradient to allow chilly marine air to approach coastal New England from the east. Diabatic cooling over the chilly (5°–8°C) waters of the Gulf of Maine allowed surface pressures to remain relatively high offshore while diabatic heating over the land (31°–33°C temperatures) enabled surface pressures to fall relative to over the ocean. The resulting higher pressures offshore resulted in an onshore cold push. Frontal intensity was likely enhanced prior to leaf out and grass green-up as virtually all of the available insolation went into sensible heating. The large-scale environment in the February–March 1972 case favored the accumulation of bitterly cold arctic air in Canada. Frontal formation occurred over northern Montana and North Dakota as the arctic air moved slowly southward in conjunction with surface pressure rises east of the Canadian Rockies. The arctic air accelerated southward subsequent to lee cyclogenesis–induced pressure falls ahead of an upstream trough that crossed the Rockies. The southward acceleration of the arctic air was also facilitated by dynamic anticyclogenesis in southern Canada beneath a poleward jet-entrance region. Frontal intensity varied diurnally in response to differential diabatic heating. Three types of cyclogenesis events were observed over the lifetime of the event: 1) low-amplitude frontal waves with no upper-level support, 2) low-amplitude frontal waves that formed in a jet-entrance region, and 3) cyclones that formed ahead of advancing upper-level troughs. All cyclones were either nondeveloping or weak developments despite extreme baroclinicity, likely the result of large atmospheric static stability in the arctic frontal zone and unfavorable alongfront stretching deformation. Significant frontal–mountain interactions were observed over the Rockies and the Appalachians.

Description: Over 50 yr have passed since the publication of Sanders' 1955 study, the first quantitative study of the structure and dynamics of a surface cold front. The purpose of this chapter is to reexamine some of the results of that study in light of modern methods of numerical weather prediction and diagnosis. A simulation with a resolution as high as 6-km horizontal grid spacing was performed with the fifth-generation-Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5), given initial and lateral boundary conditions from the National Centers for Environmental Precipitation-National Center for Atmospheric Research (NCEP-NCAR) reanalysis project data from 17 to 18 April 1953. The MM5 produced a reasonable simulation af the front, albeit its strength was not as intense and its movement was not as fast as was analyzed by Sanders. The vertical structure of the front differed from that analyzed by Sanders in several significant ways. First, the strongest horizontal temperature gradient associated with the cold front in the simulation occurred above a surface-based inversion, not at the earth's surface. Second, the ascent plume at the leading edge of the front was deeper and more intense than that analyzed by Sanders. The reason was an elevated mixed layer that had moved over the surface cold front in the simulation, allowing a much deeper vertical circulation than was analyzed by Sanders. This structure is similar to that of Australian cold fronts with their deep, well-mixed, prefrontal surface layer. These two differences between the model simulation and the analysis by Sanders may be because upper-air data from Fort Worth, Texas, was unavailable to Sanders. Third, the elevated mixed layer also meant that isentropes along the leading edge of the front extended vertically. Fourth, the field of frontogenesis of the horizontal temperature gradient calculated from the three-dimensional wind differed in that the magnitude of the maximum of the deformation term was larger than the magnitude of the maximum of the tilting term in the simulation, in contrast to Sanders' analysis and other previously published cases. These two discrepancies may be attributable to the limited horizontal resolution of the data that Sanders used in constructing his cross section. Last, a deficiency of the model simulation was that the postfrontal surface superadiabatic layer in the model did not match the observed well-mixed boundary layer. This result raises the question of the origin of the well-mixed postfrontal boundary layer behind cold fronts. To address this question, an additional model simulation without surface fluxes was performed, producing a well-mixed, not superadiabatic, layer. This result suggests that surface fluxes were not necessary for the development of the well-mixed layer, in agreement with previous research. Analysis of this event also amplifies two research themes that Sanders returned to later in his career, First, a prefrontal wind shift occurred in both the observations and model simulation at stations in western Oklahoma. This prefrontal wind shift was caused by a lee cyclone departing the leeward slopes of the Rockies slightly equatorward of the cold front, rather than along the front as was the case farther eastward. Sanders' later research showed how the occurrence of these prefrontal wind shifts leads to the weakening of fronts. Second, this study shows the advantage of using surface potential temperature, rather than surface temperature, for determining the locations of the surface fronts on sloping terrain.

Description: Today, even with state-of-the-art observational, data assimilation, and modeling systems run routinely on supercomputers, there are often surprises in the prediction of snowstorms, especially the “big ones,” affecting coastal regions of the mid-Atlantic and northeastern United States. Little did the author know that lessons from Fred Sanders' synoptic meteorology class at the Massachusetts Institute of Technology (1967) would later (late 1980s) inspire him to pursue practical issues of predictability in the context of the development of ensemble prediction systems, strategies, and applications for providing information on the inevitable case-dependent uncertainties in forecasts. This paper is a brief qualitative and somewhat colloquial overview, based upon this author's personal involvement and experiences, intended to highlight some basic aspects of the source and nature of uncertainties in forecasts and to illustrate the sort of value added information ensembles can provide in dealing with uncertainties in predictions of East Coast snowstorms.

Description: One characteristic of Fred Sanders' research is his ability to take a topic that is believed to be well understood by the research community and show that interesting research problems still exist. Among Sanders' considerable contributions to synoptic meteorology, those concerned with surface cold fronts have been especially influential. After a brief historical review of fronts and frontal analysis, this chapter presents three stages in Sanders' career when he performed research on the structure, dynamics, and analysis of surface cold fronts. First, his 1955 paper, "An investigation of the structure and dynamics of an intense surface frontal zone," was the first study to discuss quantitatively the dynamics of a surface cold front. In the 1960s, Sanders and his students further examined the structure of cold fronts, resulting in the unpublished 1967 report to the National Science Foundation, "Frontal structure and the dynamics of frontogenesis." For a third lime in his career, Sanders published several papers (1995–2005) revisiting the structure and dynamics of cold fronts. His 1967 and 1995–2005 work raises the question of the origin and dynamics of the surface pressure trough and/or wind shift that sometimes precedes the temperature gradient (hereafter called a prefrontal trough or prefrontal wind shift, respectively). Sanders showed that the relationship between this prefrontal feature and the temperature gradient is fundamental to the strength of the front. When the wind shift is coincident with the temperature gradient, frontogenesis (strengthening of the front) results; when the wind shift lies ahead of the temperature gradient, frontolysis (weakening of the front) results. a number of proposed mechanisms for the formation of prefrontal troughs and prefrontal wind shifts exist. Consequently, much research remains to be performed on these topics.

Description: Multilag singular value decomposition (MLSVD) analysis is developed and applied to diagnosing the impact of interannual variations of outgoing longwave radiation (OLR) on tropical stratospheric temperature changes. MLSVD is designed to analyze simultaneously variations at multiple levels and for a large number of temporal lags and leads. The two dominant MLSVDs are strongly related to El Niño–Southern Oscillation (ENSO). The associated patterns of tropical OLR are similar to the canonical ENSO SST patterns with strong negative sign regions stretching along the equator in the eastern and central Pacific. These dominant modes are strongly linked to temperature perturbations at a wide range of lags. At the lowest analyzed level (200 hPa) and zero lag positive temperatures anomalies are in the region of low OLR. In the lower stratosphere near 100 hPa, strong negative temperature perturbations replace the positive values of the lowest level. Higher in the stratosphere near 20 hPa, equatorial temperature perturbations are again positive, but with a more zonally elongated spatial pattern. Overall, the equatorial temperature anomalies propagate slowly to the east, at a speed strongly related to ocean–atmosphere coupling of less than 1 m s−1, and vertically and westward into the stratosphere by Rossby waves with a speed in the range of 30 m s−1.

Description: The authors present initial results of a new pan-European and international storminess since 1800 as interpreted from European and North Atlantic barometric pressure variability (SENABAR) project. This first stage analyzes results of a new daily pressure variability index, dp(abs)24, from long-running meteorological stations in Denmark, the Faroe Islands, Greenland, Iceland, the United Kingdom, and Ireland, some with data from as far back as the 1830s. It is shown that dp(abs)24 is significantly related to wind speed and is therefore a good measure of Atlantic and Northwest European storminess and climatic variations. The authors investigate the temporal and spatial consistency of dp(abs)24, the connection between annual and seasonal dp(abs)24 and the North Atlantic Oscillation Index (NAOI), as well as dp(abs)24 links with historical storm records. The results show periods of relatively high dp(abs)24 and enhanced storminess around 1900 and the early to mid-1990s, and a relatively quiescent period from about 1930 to the early 1960s, in keeping with earlier studies. There is little evidence that the mid- to late nineteenth century was less stormy than the present, and there is no sign of a sustained enhanced storminess signal associated with “global warming.” The results mark the first step of a project intending to improve on earlier work by linking barometric pressure data from a wide network of stations with new gridded pressure and reanalysis datasets, GCMs, and the NAOI. This work aims to provide much improved spatial and temporal coverage of changes in European, Atlantic, and global storminess.

Description: Although climate models have steadily improved their ability to reproduce the observed climate, over the years there has been little change to the wide range of sensitivities exhibited by different models to a doubling of atmospheric CO2 concentrations. Stochastic optimization is used to mimic how six independent climate model development efforts might use the same atmospheric general circulation model, set of observational constraints, and model skill criteria to choose different settings for parameters thought to be important sources of uncertainty related to clouds and convection. Each optimized model improved its skill with respect to observations selected as targets of model development. Of particular note were the improvements seen in reproducing observed extreme rainfall rates over the tropical Pacific, which was not specifically targeted during the optimization process. As compared to the default model sensitivity of 2.4°C, the ensemble of optimized model configurations had a larger and narrower range of sensitivities around 3°C but with different regional responses related to the uncertain choice in optimized parameter settings. These results suggest current generation models, if similarly optimized, may become more convergent in their measure of global sensitivity to greenhouse gas forcing. However, this exploration of the possible sources of modeling and observational uncertainty is not exhaustive. The optimization process illustrates an objective means for selecting an ensemble of plausible climate model configurations that quantify a portion of the uncertainty in the climate model development process.

Description: This paper examines the controls on global precipitation that are evident in the transient experiments conducted using coupled climate models collected for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The change in precipitation, water vapor, clouds, and radiative heating of the atmosphere evident in the 1% increase in carbon dioxide until doubled (1pctto2x) scenario is examined. As noted in other studies, the ensemble-mean changes in water vapor as carbon dioxide is doubled occur at a rate similar to that predicted by the Clausius–Clapeyron relationship. The ratio of global changes in precipitation to global changes in water vapor offers some insight on how readily increased water vapor is converted into precipitation in modeled climate change. This ratio ɛ is introduced in this paper as a gross indicator of the global precipitation efficiency under global warming. The main findings of this paper are threefold. First, increases in the global precipitation track increase atmospheric radiative energy loss and the ratio of precipitation sensitivity to water vapor sensitivity is primarily determined by changes to this atmospheric column energy loss. A reference limit to this ratio is introduced as the rate at which the emission of radiation from the clear-sky atmosphere increases as water vapor increases. It is shown that the derived efficiency based on the simple ratio of precipitation to water vapor sensitivities of models in fact closely matches the sensitivity derived from simple energy balance arguments involving changes to water vapor emission alone. Second, although the rate of increase of clear-sky emission is the dominant factor in the change to the energy balance of the atmosphere, there are two important and offsetting processes that contribute to ɛ in the model simulations studied: One involves a negative feedback through cloud radiative heating that acts to reduce the efficiency; the other is the global reduction in sensible heating that counteracts the effects of the cloud feedback and increases the efficiency. These counteracting feedbacks only apply on the global scale. Third, the negative cloud radiative heating feedback occurs through reductions of cloud amount in the middle troposphere, defined as the layer between 680 and 440 hPa, and by slight global cloud decreases in the lower troposphere. These changes act in a manner to expose the warmer atmosphere below to high clouds, thus resulting in a net warming of the atmospheric column by clouds and a negative feedback on the precipitation.

Description: Observations show snowpack has declined across much of the western United States over the period 1950–99. This reduction has important social and economic implications, as water retained in the snowpack from winter storms forms an important part of the hydrological cycle and water supply in the region. A formal model-based detection and attribution (D–A) study of these reductions is performed. The detection variable is the ratio of 1 April snow water equivalent (SWE) to water-year-to-date precipitation (P), chosen to reduce the effect of P variability on the results. Estimates of natural internal climate variability are obtained from 1600 years of two control simulations performed with fully coupled ocean–atmosphere climate models. Estimates of the SWE/P response to anthropogenic greenhouse gases, ozone, and some aerosols are taken from multiple-member ensembles of perturbation experiments run with two models. The D–A shows the observations and anthropogenically forced models have greater SWE/P reductions than can be explained by natural internal climate variability alone. Model-estimated effects of changes in solar and volcanic forcing likewise do not explain the SWE/P reductions. The mean model estimate is that about half of the SWE/P reductions observed in the west from 1950 to 1999 are the result of climate changes forced by anthropogenic greenhouse gases, ozone, and aerosols.

Description: The possible role that tropical Pacific SSTs played in driving the megadroughts over North America during the medieval period is addressed. Fossil coral records from the Palmyra Atoll are used to derive tropical Pacific SSTs for the period from a.d. 1320 to a.d. 1462 and show overall colder conditions as well as extended multidecadal La Niña–like states. The reconstructed SSTs are used to force a 16-member ensemble of atmosphere GCM simulations, each with different initial conditions, with the atmosphere coupled to a mixed layer ocean outside of the tropical Pacific. Model results are verified against North American tree ring reconstructions of the Palmer Drought Severity Index. A singular value decomposition analysis is performed using the soil moisture anomaly simulated by another 16-member ensemble of simulations forced by global observed SSTs for 1856–2004 and tree ring reconstructions of the Palmer Drought Severity Index for the same period. This relationship is used to transfer the modeled medieval soil moisture anomaly (relative to the modern simulation) into a model-estimated Palmer Drought Severity Index. The model-estimated Palmer Drought Severity Index reproduces many aspects of both the interannual and decadal variations of the tree ring reconstructions, in addition to an overall drier climate that is drier than the tree ring records suggest. The model-estimated Palmer Drought Severity Index simulates two previously identified “megadroughts,” a.d. 1360–1400 and a.d. 1430–60, with a realistic spatial pattern and amplitude. In contrast, the model fails to produce a period of more normal conditions in the early fifteenth century that separated these two megadroughts. The dynamical link between tropical SSTs and the North American megadroughts is akin to that operating in modern droughts. The model results are used to argue that the tropical Pacific played an active role in driving the megadroughts. However, the match between simulated and reconstructed hydroclimate is such that it is likely that both the coral-reconstructed SST anomalies contain significant errors and that SST anomalies in other basins also played a role in driving hydroclimate variations over North America during the late medieval period.

Description: The regularized expectation maximization (RegEM) method has been used in recent studies to derive climate field reconstructions of Northern Hemisphere temperatures during the last millennium. Original pseudoproxy experiments that tested RegEM [with ridge regression regularization (RegEM-Ridge)] standardized the input data in a way that improved the performance of the reconstruction method, but included data from the reconstruction interval for estimates of the mean and standard deviation of the climate field—information that is not available in real-world reconstruction problems. When standardizations are confined to the calibration interval only, pseudoproxy reconstructions performed with RegEM-Ridge suffer from warm biases and variance losses. Only cursory explanations of this so-called standardization sensitivity of RegEM-Ridge have been published, but they have suggested that the selection of the regularization (ridge) parameter by means of minimizing the generalized cross validation (GCV) function is the source of the effect. The origin of the standardization sensitivity is more thoroughly investigated herein and is shown not to be associated with the selection of the ridge parameter; sets of derived reconstructions reveal that GCV-selected ridge parameters are minimally different for reconstructions standardized either over both the reconstruction and calibration interval or over the calibration interval only. While GCV may select ridge parameters that are different from those that precisely minimize the error in pseudoproxy reconstructions, RegEM reconstructions performed with truly optimized ridge parameters are not significantly different from those that use GCV-selected ridge parameters. The true source of the standardization sensitivity is attributable to the inclusion or exclusion of additional information provided by the reconstruction interval, namely, the mean and standard deviation fields computed for the complete modeled dataset. These fields are significantly different from those for the calibration period alone because of the violation of a standard EM assumption that missing values are missing at random in typical paleoreconstruction problems; climate data are predominantly missing in the preinstrumental period when the mean climate was significantly colder than the mean of the instrumental period. The origin of the standardization sensitivity therefore is not associated specifically with RegEM-Ridge, and more recent attempts to regularize the EM algorithm using truncated total least squares could theoretically also be susceptible to the problems affecting RegEM-Ridge. Nevertheless, the principal failure of RegEM-Ridge arises because of a poor initial estimate of the mean field, and therefore leaves open the possibility that alternative methods may perform better.

Description: Climatological surface wind speed probability density functions (PDFs) estimated from observations are characterized and used to evaluate, for the first time, contemporaneous wind PDFs predicted by a GCM. The observations include NASA’s global Quick Scatterometer (QuikSCAT) dataset, the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2) 6-hourly reanalysis, and the Tropical Atmosphere Ocean (TAO)/Triangle Trans-Ocean Buoy Network (TRITON) moored buoy data, all from 2000 to 2005. Wind speed mean, 90th percentile, standard deviation, and Weibull shape parameter climatologies are constructed from these data. New features that emerge from the analysis include the identification of a stationary pattern to the wind speed variance in the equatorial Pacific. Interestingly, a distinct wind speed shape anomaly migrates with the ITCZ across this stationary background. The GCM despite its coarser spatial and temporal resolution predicts wind speed PDFs in general agreement with observations. Relative to QuikSCAT, the NCAR Community Atmosphere Model, version 3 (CAM3) GCM has a globally averaged positive mean wind speed bias of about 0.2 m s−1 originating primarily within the trades and Southern Hemisphere storm track. Global standard deviation biases are largest in the winter hemisphere storm tracks. The largest shape biases occur along the equatorial peripheries of the Northern Hemisphere and southern Indian Ocean anticyclones. Year-round negative shape and mean wind speed biases persist along the ITCZ. The GCM’s overactive tropical convection and slight subtropical anticyclone displacement contribute to positive mean speed, standard deviation, and shape trade biases. Surface heat and energy fluxes depend nonlinearly on wind speed magnitude, are sensitive to the tails of the wind distribution, and hence vary significantly on spatiotemporal scales not resolved by GCMs. Limited computing resources force the use of coarse-resolution GCMs, which do not resolve finer-scale wind speed fluctuations. Rather, surface fluxes are determined from the mean wind speed computed by averaging spatially and temporally over subgrid-scale features. Some surface flux routines account for gustiness during low mean winds resulting from thermally driven convection. The authors hypothesize that GCMs systematically underestimate surface momentum flux nonlinearities and that this biases surface wind predictions most in regions of strong winds with high variability. To test this, climate simulations that account for surface fluxes due to subgrid-scale GCM winds are performed. This significantly improves climatological surface wind speed statistics, particularly in the Southern Hemisphere storm track, consistent with the hypothesis. These wind speed improvements can be attributed to a reduction in GCM sea level pressure biases throughout the globe.

Description: Via its impact on surface fluxes, subseasonal variability in soil moisture has the potential to feed back on regional atmospheric circulations, and thereby rainfall. An understanding of this feedback mechanism in the climate system has been hindered by the lack of observations at an appropriate scale. In this study, passive microwave data at 10.65 GHz from the Tropical Rainfall Measuring Mission satellite are used to identify soil moisture variability during the West African monsoon. A simple model of surface sensible heat flux is developed from these data and is used, alongside atmospheric analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF), to provide a new interpretation of monsoon variability on time scales of the order of 15 days. During active monsoon periods, the data indicate extensive areas of wet soil in the Sahel. The impact of the resulting weak surface heat fluxes is consistent in space and time with low-level variations in atmospheric heating and vorticity, as depicted in the ECMWF analyses. The surface-induced vorticity structure is similar to previously documented intraseasonal variations in the monsoon flow, notably a westward-propagating vortex at low levels. In those earlier studies, the variability in low-level flow was considered to be the critical factor in producing intraseasonal fluctuations in rainfall. The current analysis shows that this vortex can be regarded as an effect of the rainfall (via surface hydrology) as well as a cause.

Description: A previous study by Mikolajewicz suggested that the wind stress feedback stabilizes the Atlantic thermohaline circulation. This result was obtained under modern climate conditions, for which the presence of the massive continental ice sheets characteristic of glacial times is missing. Here a coupled ocean–atmosphere–sea ice model of intermediate complexity, set up in an idealized spherical sector geometry of the Atlantic basin, is used to show that, under glacial climate conditions, wind stress feedback actually reduces the stability of the meridional overturning circulation (MOC). The analysis reveals that the influence of the wind stress feedback on the glacial MOC response to an external source of freshwater applied at high northern latitudes is controlled by the following two distinct processes: 1) the interactions between the wind field and the sea ice export in the Northern Hemisphere (NH), and 2) the northward Ekman transport in the tropics and upward Ekman pumping in the core of the NH subpolar gyre. The former dominates the response of the coupled system; it delays the recovery of the MOC, and in some cases even stabilizes collapsed MOC states achieved during the hosing period. The latter plays a minor role and mitigates the impact of the former process by reducing the upper-ocean freshening in deep-water formation regions. Hence, the wind stress feedback delays the recovery of the glacial MOC, which is the opposite of what occurs under modern climate conditions. Close to the critical transition threshold beyond which the circulation collapses, the glacial MOC appears to be very sensitive to changes in surface wind stress forcing and exhibits, in the aftermath of the freshwater pulse, a nonlinear dependence upon the wind stress feedback magnitude: a complete and irreversible MOC shutdown occurs only for intermediate wind stress feedback magnitudes. This behavior results from the competitive effects of processes 1 and 2 on the midlatitude upper-ocean salinity during the shutdown phase of the MOC. The mechanisms presented here may be relevant to the large meltwater pulses that punctuated the last glacial period.

Description: This work presents a description of the 1979–2002 tropical Atlantic (TA) SST variability modes coupled to the anomalous West African (WA) rainfall during the monsoon season. The time-evolving SST patterns, with an impact on WA rainfall variability, are analyzed using a new methodology based on maximum covariance analysis. The enhanced Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) dataset, which includes measures over the ocean, gives a complete picture of the interannual WA rainfall patterns for the Sahel dry period. The leading TA SST pattern, related to the Atlantic El Niño, is coupled to anomalous precipitation over the coast of the Gulf of Guinea, which corresponds to the second WA rainfall principal component. The thermodynamics and dynamics involved in the generation, development, and damping of this mode are studied and compared with previous works. The SST mode starts at the Angola/Benguela region and is caused by alongshore wind anomalies. It then propagates westward via Rossby waves and damps because of latent heat flux anomalies and Kelvin wave eastward propagation from an off-equatorial forcing. The second SST mode includes the Mediterranean and the Atlantic Ocean, showing how the Mediterranean SST anomalies are those that are directly associated with the Sahelian rainfall. The global signature of the TA SST patterns is analyzed, adding new insights about the Pacific–Atlantic link in relation to WA rainfall during this period. Also, this global picture suggests that the Mediterranean SST anomalies are a fingerprint of large-scale forcing. This work updates the results given by other authors, whose studies are based on different datasets dating back to the 1950s, including both the wet and the dry Sahel periods.

Description: The impact of coupled air–sea feedbacks on the simulation of tropical intraseasonal variability is investigated in this study using the National Centers for Environmental Prediction Climate Forecast System. The simulation of tropical intraseasonal variability in a freely coupled simulation is compared with two simulations of the atmospheric component of the model. In one experiment, the uncoupled model is forced with the daily sea surface temperature (SST) from the coupled run. In the other, the uncoupled model is forced with climatological SST from the coupled run. Results indicate that the overall intraseasonal variability of precipitation is reduced in the coupled simulation compared to the uncoupled simulation forced by daily SST. Additionally, air–sea coupling is responsible for differences in the simulation of the tropical intraseasonal oscillation between the coupled and uncoupled models, specifically in terms of organization and propagation in the western Pacific. The differences between the coupled and uncoupled simulations are due to the fact that the relationships between precipitation and SST and latent heat flux and SST are much stronger in the coupled model than in the uncoupled model. Additionally, these relationships are delayed by about 5 days in the uncoupled model compared to the coupled model. As demonstrated by the uncoupled simulation forced with climatological SST, some of the intraseasonal oscillation can be simulated by internal atmospheric dynamics. However, the intraseasonally varying SST appears to be important to the amplitude and propagation of the oscillation beyond the Maritime Continent.

Description: Large changes in the hydrology of the western United States have been observed since the mid-twentieth century. These include a reduction in the amount of precipitation arriving as snow, a decline in snowpack at low and midelevations, and a shift toward earlier arrival of both snowmelt and the centroid (center of mass) of streamflows. To project future water supply reliability, it is crucial to obtain a better understanding of the underlying cause or causes for these changes. A regional warming is often posited as the cause of these changes without formal testing of different competitive explanations for the warming. In this study, a rigorous detection and attribution analysis is performed to determine the causes of the late winter/early spring changes in hydrologically relevant temperature variables over mountain ranges of the western United States. Natural internal climate variability, as estimated from two long control climate model simulations, is insufficient to explain the rapid increase in daily minimum and maximum temperatures, the sharp decline in frost days, and the rise in degree-days above 0°C (a simple proxy for temperature-driven snowmelt). These observed changes are also inconsistent with the model-predicted responses to variability in solar irradiance and volcanic activity. The observations are consistent with climate simulations that include the combined effects of anthropogenic greenhouse gases and aerosols. It is found that, for each temperature variable considered, an anthropogenic signal is identifiable in observational fields. The results are robust to uncertainties in model-estimated fingerprints and natural variability noise, to the choice of statistical downscaling method, and to various processing options in the detection and attribution method.

Description: In this study, the method of self-organizing maps (SOMs) is used with NCEP–NCAR reanalysis data to advance the continuum perspective of Northern Hemisphere teleconnection patterns and to shed light on the secular eastward shift of the North Atlantic Oscillation (NAO) that began in the late 1970s. A 20-pattern SOM analysis of daily, wintertime, Northern Hemisphere sea level pressure reveals a continuum of patterns that correspond closely with well-known teleconnection patterns. This analysis also reveals that interdecadal variability of the hemispheric sea level pressure field may be understood in terms of changes in the frequency distribution within the continuum of sea level pressure patterns described by the SOM. Based on the continuum perspective illustrated with the SOM, the above secular shift of the NAO may be understood as a change in dominance from westward-displaced, negative NAO-like patterns to eastward-displaced, positive NAO-like patterns, though westward- and eastward-displaced NAO-like patterns existed during all time periods and for both phases.

Description: It is generally agreed that changing climate variability, and the associated change in climate extremes, may have a greater impact on environmentally vulnerable regions than a changing mean. This research investigates rainfall variability, rainfall extremes, and their associations with atmospheric and oceanic circulations over southern Africa, a region that is considered particularly vulnerable to extreme events because of numerous environmental, social, and economic pressures. Because rainfall variability is a function of scale, high-resolution data are needed to identify extreme events. Thus, this research uses remotely sensed rainfall data and climate model experiments at high spatial and temporal resolution, with the overall aim being to investigate the ways in which sea surface temperature (SST) anomalies influence rainfall extremes over southern Africa. Extreme rainfall identification is achieved by the high-resolution microwave/infrared rainfall algorithm dataset. This comprises satellite-derived daily rainfall from 1993 to 2002 and covers southern Africa at a spatial resolution of 0.1° latitude–longitude. Extremes are extracted and used with reanalysis data to study possible circulation anomalies associated with extreme rainfall. Anomalously cold SSTs in the central South Atlantic and warm SSTs off the coast of southwestern Africa seem to be statistically related to rainfall extremes. Further, through a number of idealized climate model experiments, it would appear that both decreasing SSTs in the central South Atlantic and increasing SSTs off the coast of southwestern Africa lead to a demonstrable increase in daily rainfall and rainfall extremes over southern Africa, via local effects such as increased convection and remote effects such as an adjustment of the Walker-type circulation.

Description: The structure and variability of convectively coupled Kelvin waves during the boreal summer are explored using satellite-observed brightness temperature data and ECMWF reanalyses. Kelvin wave activity is most prominent between the central and eastern Pacific, across Africa, and the Indian Ocean. Composite analysis shows that over sub-Saharan Africa Kelvin wave convection is peaked north of the equator, while the dynamical fields tend to be symmetric with respect to the equator. Convectively coupled Kelvin waves propagate faster over the Pacific and western Atlantic (∼24 m s−1), and slow down over tropical Africa (∼14 m s−1), consistent with stronger coupling between the dynamics and convection over tropical Africa. The Kelvin waves observed over Africa generally propagate into the region from anywhere between the eastern Pacific and the Atlantic, and decay over the eastern Indian Ocean basin. Results show marked interannual variability of Kelvin wave activity over Africa. Anomalously high Kelvin wave variance tends to occur during dry years, while low variance occurs during wet years. African Kelvin wave activity is positively correlated with SST anomalies in the equatorial east Pacific. The same warm SST anomalies that are favorable for enhanced Kelvin wave activity suppress the mean rainfall over tropical Africa via a more slowly varying teleconnection and associated subsidence. A brief analysis of an intense Kelvin wave in August 1987 (a dry year) shows a clear impact of the wave on convective development and daily rainfall over tropical Africa. This Kelvin wave was also associated with a series of easterly wave initiations over tropical Africa.

Description: The geographic-dependence characteristics of the energy spectrum of the boreal summer intraseasonal oscillation (BSISO; May–October) over the Indo–western Pacific region were analyzed using 25-yr (1979–2003) observational data. The BSISO energy spectrum distribution exhibits a distinctive regional characteristic. The stationary and eastward-propagating modes are most pronounced at the equator (5°S–5°N), while the westward-propagating modes are dominant in the off-equatorial region (10°–20°N). While the eastward intraseasonal oscillation (ISO) spectrum agglomerates on the 30–60-day period and zonal wavenumber 1, the westward mode covers wider spatial (wavenumber) and temporal (period) range. Along the Arabian Sea, Bay of Bengal, and South China Sea (SCS) latitudes, the dominant wavenumber 1 mode is the eastward (westward) propagation at the 30–60-day (10–20 day) period; for zonal wavenumber 2, the dominant mode is the westward propagation at both the 30–60-day and 10–20-day periods. Compared to the absolute amplitude of both zonal and meridional mode energy spectrum, northward propagation is the most predominant mode in boreal summer over the Indo–western Pacific regions. The strongest northward-propagating BSISO signal appears in the eastern tropical Indian Ocean. The variation of BSISO differs significantly in the El Niño and La Niña developing and decaying phases. During the El Niño (La Niña) developing summer, the eastward propagation is enhanced (weakened) at the equator, while the northward propagation is also strengthened (weakened) over the western Pacific (east of 140°E). During the El Niño (La Niña) decaying summer, the eastward propagation weakens (strengthens) at the equator, opposite to that in the developing summer; the westward propagation off the equator and the northward propagation over SCS and the western Pacific are suppressed (enhanced). The amplitude of the BSISO variation is stronger in the decaying summer than that in the developing summer. This asymmetry in BSISO variations is primarily attributed to the asymmetry of the background mean flow change associated with the developing and decaying phases of ENSO.

Description: The ways in which deep convection over the tropical Atlantic affects the midlatitude climate variability through meridional circulation, planetary wave teleconnection, and wave–mean flow interaction is examined for the 1979–2002 period, by following the North Atlantic anomalous rainfall evolution from summer to late winter. In this way, the first two covariability modes between anomalous summer tropical Atlantic sea surface temperature (SST) and anomalous summer–late-winter precipitation over the North Atlantic basin are analyzed using the same methodology of extended maximum covariance analysis developed for Part I. This work updates the results given by other authors, whose studies are based on different datasets dating back to the 1950s. To this end, the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) dataset, which includes measures over the ocean, is used to give a complete picture of the interannual rainfall patterns for the last decades. The first mode, which accounts for more than 40% of the squared covariance fraction (SCF), involves SST anomalies related to the equatorial mode or Atlantic Niño. Its atmospheric response shows variations of the Atlantic Hadley and Ferrel circulations, reinforcing the direct and indirect circulation cells, respectively, displacements of the Atlantic Walker circulation, and the excitation of Rossby waves, which are trapped in the North African–Asian jet. The second mode, which accounts for 15% of the SCF, is associated with the summer horseshoe and winter tripole SST patterns. The related atmospheric circulation anomalies include direct thermal forcing (altering the local Hadley cell), perturbations in the ITCZ, and wavelike responses from the Caribbean region. The method used in this work highlights the seasonal dependence of the modes, in contrast to previous work that neglects to take into account the month-to-month evolution of these modes. The results add new and valuable information to the understanding of these modes from the important period back to the 1980s.

Description: This paper examines nine analyses of global ocean 0-/700-m temperature and heat content during the 43-yr period of warming, 1960–2002. Among the analyses are two that are independent of any numerical model, six that rely on sequential data assimilation, including an ocean general circulation model, and one that uses four-dimensional variational data assimilation (4DVAR), including an ocean general circulation model and its adjoint. Most analyses show gradual warming of the global ocean with an ensemble trend of 0.77 × 108 J m−2 (10 yr)−1 (=0.24 W m−2) as the result of rapid warming in the early 1970s and again beginning around 1990. One proposed explanation for these variations is the effect of volcanic eruptions in 1963 and 1982. Examination of this hypothesis suggests that while there is an oceanic signal, it is insufficient to explain the observed heat content variations. A second potential cause of decadal variations in global heat content is the uncorrelated contribution of heat content variations in individual ocean basins. The subtropical North Atlantic is warming at twice the global average, with accelerated warming in the 1960s and again beginning in the late 1980s and extending through the end of the record. The Barents Sea region of the Arctic Ocean and the Gulf of Mexico have also warmed, while the western subpolar North Atlantic has cooled. Heat content variability in the North Pacific differs significantly from the North Atlantic. There the spatial and temporal patterns are consistent with the decadal variability previously identified through observational and modeling studies examining SST and surface winds. In the Southern Hemisphere large heat content anomalies are evident, and while there is substantial disagreement among analyses on average the band of latitudes at 30°–60°S contribute significantly to the global warming trend. Thus, the uncorrelated contributions of heat content variations in the individual basins are a major contributor to global heat content variations. A third potential contributor to global heat content variations is the effect of time-dependent bias in the set of historical observations. This last possibility is examined by comparing the analyses to the unbiased salinity–temperature–depth dataset and finding a very substantial warm bias in all analyses in the 1970s relative to the latter decades. This warm bias may well explain the rapid increase in analysis heat content in the early 1970s, but not the more recent increase, which began in the early 1990s. Finally, this study provides information about the similarities and differences between analyses that are independent of a model and those that use sequential assimilation and 4DVAR. The comparisons provide considerable encouragement for the use of the sequential analyses for climate research despite the presence of erroneous variability (also present in the no-model analyses) resulting from instrument bias. The strengths and weaknesses of each analysis need to be considered for a given application.

Description: Intraseasonal equatorial Kelvin wave activity (IEKW) at a low frequency in the Pacific is investigated using the Simple Ocean Data Assimilation (SODA) oceanic reanalyses. A vertical and horizontal mode decomposition of SODA variability allows estimation of the Kelvin wave amplitude according to the most energetic baroclinic modes. A wavenumber–frequency analysis is then performed on the time series to derive indices of modulation of the IEKW at various frequency bands. The results indicate that the IEKW activity undergoes a significant modulation that projects onto baroclinic modes and is not related in a straightforward manner to the low-frequency climate variability in the Pacific. Linear model experiments corroborate that part of the modulation of the IEKW is tightly linked to change in oceanic mean state rather than to the low-frequency change of atmospheric equatorial variability.

Description: The Arctic region has exhibited dramatic changes in recent times. Many of these are intimately tied up with synoptic activity, but little research has been undertaken on how the characteristics of Arctic cyclones have changed. This paper presents a comprehensive analysis of Arctic (here defined as the domain north of 70°N) cyclones diagnosed with the Melbourne University cyclone tracking scheme applied to the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP–NCAR (NCEP1) and NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP)-II (NCEP2) reanalysis sets (the last two extending to the end of 2006). A wide variety of cyclone characteristics is presented as befits these complex features. In winter the highest density of cyclones is found between Norway and Svalbard and to the east to the Barents and Kara Seas, and significant numbers are found in the central Arctic. In summer the greatest frequencies are found in the central Arctic. The total number of cyclones identified in the ERA-40 record exceeds those in the two NCEP compilations. The mean size of cyclones shows similar maxima in the central Arctic in both winter and summer. By contrast, the greatest mean system depth in winter (in excess of 8 hPa) is found to the southeast of Greenland, although average depths exceed 6 hPa over a considerable portion of the basin. In summer the deepest cyclones are found in the central portion of the Arctic. The analysis shows that the total number of cyclones in winter exceeds that in summer, a result in contrast to earlier studies. This difference comes about primarily due to the greater numbers of “open strong” systems in winter in all reanalyses. Cyclones in this category are associated with very active synoptic situations; it is of importance that they be included in cyclone counts but would not be considered in many cyclone identification schemes. Since 1979 neither the ERA-40 nor the NCEP2 sets show significant trends in any of the cyclone variables considered. However, over the entire record starting in 1958 the NCEP1 reanalysis exhibits a significant increase in summer cyclone frequency (due mainly to the increase in closed strong systems). Both NCEP1 and ERA-40 also reveal significant increases in the number of summer closed strong cyclones, as well as in their mean depth and intensity in that season. Interannual variations in Arctic cyclone numbers are closely related to the Arctic Oscillation (AO) index in the full reanalyses records. An even stronger relationship is found between the AO and the number of deep cyclones. These relationships have still held in the last decade when the AO has returned to more normal values but the summer and fall sea ice extent has continued to decrease.

Description: The Indian summer monsoon rainfall has complex, regionally heterogeneous, interannual variations with huge socioeconomic impacts, but the underlying mechanisms remain uncertain. The upwelling along the Somalia and Oman coasts starts in late spring, peaks during the summer monsoon, and strongly cools the sea surface temperature (SST) in the western Arabian Sea. They restrict the westward extent of the Indian Ocean warm pool, which is the main moisture source for the monsoon rainfall. Thus, variations of the Somalia–Oman upwelling can have significant impacts on the moisture transport toward India. Here the authors use both observations and an advanced coupled atmosphere–ocean general circulation model to show that a decrease in upwelling strengthens monsoon rainfall along the west coast of India by increasing the SST along the Somalia–Oman coasts, and thus local evaporation and water vapor transport toward the Indian Western Ghats (mountains). Further observational analysis reveals that such decreases in upwelling are caused by anomalously weak southwesterly winds in late spring over the Arabian Sea that are due to warm SST/increased precipitation anomalies over the Seychelles–Chagos thermocline ridge of the southwestern Indian Ocean (and vice versa for years with strong upwelling/weak west Indian summer monsoon rainfall). The latter SST/precipitation anomalies are often related to El Niño conditions and the strength of the Indonesian–Australian monsoon during the previous winter. This sheds new light on the ability to forecast the poorly predicted Indian monsoon rainfall on a regional scale, helped by a proper ocean observing/forecasting system in the western tropical Indian Ocean.

Description: Long-term trends in the heat content of the Southern Hemisphere ocean are evaluated by comparing temperature profiles collected during the 1990s with profiles collected starting in the 1930s. Data are drawn both from ship-based hydrographic surveys and from autonomous floats. Results show that the upper 1000 m of the Southern Hemisphere ocean has warmed substantially during this time period at all depths. Warming is concentrated within the Antarctic Circumpolar Current (ACC). On a global scale, this warming trend implies that the ocean has gained heat from the atmosphere over the last 50 to 70 years. Although the data do not preclude the possibility that the Southern Ocean has warmed as a result of increased heat fluxes, either into the ocean or within the ocean, in general the strong trend in the Southern Ocean appears regionally consistent with a poleward migration of the ACC, possibly driven by long-term poleward shifts in the winds of the region, as represented by the southern annular mode.

Description: Analysis of the monthly NCEP–NCAR reanalysis data and station data reveals a teleconnection pattern (NAULEA) that links climate changes over the North Atlantic and Eurasia. The NAULEA pattern has five action centers. It extends from the North Atlantic to northwestern Europe, and then stretches eastward to the Urals, with its eastern end over North China. Certain climate changes over East Asia, such as the cooling changes in both the upper troposphere and surface in the last few decades of the twentieth century, can be attributed to the NAULEA pattern anomalies and traced upstream to the North Atlantic. The NAULEA pattern is suggested to be another NAO-related teleconnection pattern. Compared with the pattern with the Asian jet waveguide path, which leads to temperature anomalies over northeastern Asia, the NAULEA pattern with a high-latitude path exerts stronger influences on the climate of southwestern China.

Description: This study examines the interdecadal variability of the tropical cyclone (TC) tracks over the western North Pacific (WNP) during the 1960–2005 period. An empirical orthogonal function analysis of the 10-yr Gaussian-filtered annual frequency of TC occurrence shows three leading modes of TC occurrence patterns. The first mode is related to the variation of TC activity in the areas near Japan and its east. The second mode is characterized by a northeast–southwest dipole of TC occurrence anomalies along the southeast coast of China and an east–west dipole near Japan and its east. The third mode is similar to the second mode, except for the absence of the east–west dipole. These patterns are shown to be related to the decadal changes in the prevailing TC tracks. Two characteristic flow patterns related to the first and third modes of TC occurrence pattern are identified. The first pattern is characterized by a north–south dipole of 500-hPa geopotential anomalies over the WNP. Such a pattern may affect the strength and westward extension of the subtropical high and the midlevel steering flow and hence the TC occurrence pattern. The Pacific decadal oscillation (PDO) is found to display a similar dipole-like structure. The decadal variability of TC tracks may therefore be partly attributed to the PDO signal. The second characteristic pattern shows a series of anomalous midlevel atmospheric circulations extending from the sea east of Japan to the south coast of China, which may explain the other part of the decadal variations.

Description: The partitioning of the earth radiation budget (ERB) between its atmosphere and surface components is of crucial interest in climate studies as it has a significant role in the oceanic and atmospheric general circulation. An analysis of the present-day climate simulation of the surface radiation budget in the atmospheric component of the new Hadley Centre Global Environmental Model version 1 (HadGEM1) is presented, and the simulations are assessed by comparing the results with fluxes derived from satellite data from the International Satellite Cloud Climatology Project (ISCCP) and ground measurements from the Baseline Surface Radiation Network (BSRN). Comparisons against radiative fluxes from satellite and ground observations show that the model tends to overestimate the surface incoming solar radiation (Ss,d). The model simulates Ss,d very well over the polar regions. Consistency in the comparisons against BSRN and ISCCP-FD suggests that the ISCCP-FD database is a good test for the performance of the surface downwelling solar radiation in climate model simulations. Overall, the simulation of downward longwave radiation is closer to observations than its shortwave counterpart. The model underestimates the downward longwave radiation with respect to BSRN measurements by 6.0 W m−2. Comparisons of land surface albedo from the model and estimates from the Moderate Resolution Imaging Spectroradiometer (MODIS) show that HadGEM1 overestimates the land surface albedo over deserts and over midlatitude landmasses in the Northern Hemisphere in January. Analysis of the seasonal cycle of the land surface albedo in different regions shows that the amplitude and phase of the seasonal cycle are not well represented in the model, although a more extensive validation needs to be carried out. Two decades of coupled model simulations of the twentieth-century climate are used to look into the model’s simulation of global dimming/brightening. The model results are in line with the conclusions of the studies that suggest that global dimming is far from being a uniform phenomenon across the globe.

Description: Defining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979–2006 period. The existing indices can be classified into five categories: the east–west thermal contrast, north–south thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Niño, while the first category reflects the second mode that corresponds to the developing El Niño. The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring. The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon.

Description: The authors quantitatively describe the changes in the characteristics of ice phenology including the flow rate and freeze/breakup dates of the Yellow River based on observations of the past 50 yr. In both the upper and lower reaches of the Yellow River, increasing temperature delays the freeze date and advances the breakup date, thus decreasing the number of freeze days and the expanse of river freeze. From 1968 to 2001, the freeze duration has shortened significantly by 38 days at Bayangaole and 25 days at Sanhuhe, respectively. From the early 1950s to the early 2000s, the changes in freeze and breakup dates have shortened the freeze duration in the lower reach of the Yellow River by 12 days. The flow rate has reduced from 500 to 260 m3 s−1, and the expanse of river freeze has also decreased significantly by about 310 km. In addition, in the lower reach of the river, the location of earliest ice breakup has shifted downstream significantly in the last 50 yr, although the location of earliest freeze exhibits little change.

Description: Links are examined between time-averaged cloud radiative properties, particularly the longwave and shortwave components of cloud radiative forcing (CRF), and properties of the long-term averages of atmospheric soundings, in particular upper-tropospheric humidity (UTH), lower-tropospheric precipitable water (PW), and static stability (SS). The joint distributions of moisture measures and the composite or conditional mean CRF for different moisture and stability combinations are computed. This expands on previous studies that have examined cloud properties versus vertical velocity and surface temperature. These computations are done for satellite observations and for three representative coupled climate models from major modeling centers. Aside from mean biases reported previously, several departures are identified between the modeled and observed joint distributions that are qualitative and significant. Namely, the joint distribution of PW and UTH is very compact in observations but less so in models, cloud forcings are tightly related to PW in the data but to UTH in the models, and strong negative net CRF in marine stratocumulus regions occurs only for high SS and low UTH in the data but violates one or both of these restrictions in each of the models. All three errors are preliminarily interpreted as symptoms of inadequate dependence of model convective development on ambient humidity above the boundary layer. In any case, the character of the errors suggests utility for model testing and future development. A set of scalar metrics for quantifying some of the problems is presented; these metrics can be easily applied to standard model output. Finally, an examination of doubled-CO2 simulations suggests that the errors noted here are significantly affecting cloud feedback in at least some models. For example, in one model a strong negative feedback is found from clouds forming in model conditions that never occur in the observations.

Description: The response of air temperatures to widespread irrigation may represent an important component of past and/or future regional climate changes. The quantitative impact of irrigation on daily minimum and maximum temperatures (Tmin and Tmax) in California was estimated using historical time series of county irrigated areas from agricultural censuses and daily climate observations from the U.S. Historical Climatology Network. Regression analysis of temperature and irrigation changes for stations within irrigated areas revealed a highly significant (p 〈 0.01) effect of irrigation on June–August average Tmax, with no significant effects on Tmin (p 〉 0.3). The mean estimate for Tmax was a substantial 5.0°C cooling for 100% irrigation cover, with a 95% confidence interval of 2.0°–7.9°C. As a result of small changes in Tmin compared to Tmax, the diurnal temperature range (DTR) decreased significantly in both spring and summer months. Effects on percentiles of Tmax within summer months were not statistically distinguishable, suggesting that irrigation’s impact is similar on warm and cool days in California. Finally, average trends for stations within irrigated areas were compared to those from nonirrigated stations to evaluate the robustness of conclusions from previous studies based on pairwise comparisons of irrigated and nonirrigated sites. Stronger negative Tmax trends in irrigated sites were consistent with the inferred effects of irrigation on Tmax. However, Tmin trends were significantly more positive for nonirrigated sites despite the apparent lack of effects of irrigation on Tmin from the analysis within irrigated sites. Together with evidence of increases in urban areas near nonirrigated sites, this finding indicates an important effect of urbanization on Tmin in California that had previously been attributed to irrigation. The results therefore demonstrate that simple pairwise comparisons between stations in a complex region such as California can lead to misinterpretation of historical climate trends and the effects of land use changes.

Description: The northward-propagating intraseasonal (30–40 day) oscillation (NPISO) between active and break monsoon phases exerts a critical control on summer-season rainfall totals over India. Advances in diagnosing these events and comprehending the physical mechanisms behind them may hold the potential for improving their predictability. While previous studies have attempted to extract active and break events from reanalysis data to elucidate a composite life cycle, those studies have relied on first isolating the intraseasonal variability in the record (e.g., through bandpass filtering, removing harmonics, or empirical orthogonal function analysis). Additionally, the underlying physical processes that previous studies have proposed have varied, both among themselves and with studies using general circulation models. A simple index is defined for diagnosing NPISO events in observations and reanalysis, based on lag correlations between outgoing longwave radiation (OLR) over India and over the equatorial Indian Ocean. This index is the first to use unfiltered OLR observations and so does not specifically isolate intraseasonal periods. A composite NPISO life cycle based on this index is similar to previous composites in OLR and surface winds, demonstrating that the dominance of the intraseasonal variability in the monsoon climate system eliminates the need for more complex methods (e.g., time filtering or EOF analysis) to identify the NPISO. This study is also among the first to examine the NPISO using a long-period record of high-resolution sea surface temperatures (SSTs) from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager. Application of this index to those SSTs demonstrates that SST anomalies exist in near quadrature with convection, as suggested by recent coupled model studies. Analysis of the phase relationships between atmospheric fields and SSTs indicates that the atmosphere likely forced the SST anomalies. The results of this lag-correlation analysis suggest that the oscillation serves as its own most reliable—and perhaps only—predictor, and that signals preceding an NPISO event appear first over the Indian subcontinent, not the equatorial Indian Ocean where the events originate.

Description: Meridional structure and transports of energy in the atmosphere, ocean, and land are evaluated holistically for the mean and annual cycle zonal averages over the ocean, land, and global domains, with discussion and assessment of uncertainty. At the top of the atmosphere (TOA), adjusted radiances from the Earth Radiation Budget Experiment (ERBE) and Clouds and Earth’s Radiant Energy System (CERES) are used along with estimates of energy storage and transport from two global reanalysis datasets for the atmosphere. Three ocean temperature datasets are used to assess changes in the ocean heat content (OE) and their relationship to the net upward surface energy flux over ocean (FoS), which is derived from the residual of the TOA and atmospheric energy budgets. The surface flux over land is from a stand-alone simulation of the Community Land Model forced by observed fields. In the extratropics, absorbed solar radiation (ASR) achieves a maximum in summer with peak values near the solstices. Outgoing longwave radiation (OLR) maxima also occur in summer but lag ASR by 1–2 months, consistent with temperature maxima over land. In the tropics, however, OLR relates to high cloud variations and peaks late in the dry monsoon season, while the OLR minima in summer coincide with deep convection in the monsoon trough at the height of the rainy season. Most of the difference between the TOA radiation and atmospheric energy storage tendency is made up by a large heat flux into the ocean in summer and out of the ocean in winter. In the Northern Hemisphere, the transport of energy from ocean to land regions is substantial in winter, and modest in summer. In the Southern Hemisphere extratropics, land − ocean differences play only a small role and the main energy transport by the atmosphere and ocean is poleward. There is reasonably good agreement between FoS and observed changes in OE, except for south of 40°S, where differences among several ocean datasets point to that region as the main source of errors in achieving an overall energy balance. The winter hemisphere atmospheric circulation is the dominant contributor to poleward energy transports outside of the tropics [6–7 PW (1 petawatt = 1015 W)], with summer transports being relatively weak (∼3 PW)—slightly more in the Southern Hemisphere and slightly less in the Northern Hemisphere. Ocean transports outside of the tropics are found to be small (

Description: The physical reality of the Arctic Oscillation (AO; or northern annular mode) is considered. The data used are mainly the monthly mean sea level pressure (SLP). A schematic figure is first presented to illustrate the relationship between the North Atlantic Oscillation (NAO)–Pacific–North American Oscillation (PNA) system and the AO–negative correlation mode between the Atlantic and the Pacific (AO–NCM) system. Although the NAO–PNA (apparent AO–NCM) and true AO–NCM systems give rise to the same EOFs, the probability density functions for the time coefficients of the two leading modes are different. Therefore, the discrimination of the two systems is possible. Several pieces of evidence indicate that, in the real world, the NAO–PNA and the AO–NCM are located on almost the same plane in phase space. This means that the NAO–PNA and AO–NCM systems have the same variations on the plane in common, implying that when the NAO–PNA system is real, the AO–NCM is unlikely to be real. Simple independent component analysis is carried out to distinguish between the true and apparent AO–NCM systems, indicating that the NAO and PNA are independent oscillations, that is, true ones. The analysis is extended to the winter mean SLP field, for which the EOF shows the NAO–PNA but not the AO–NCM. This may be due to the fact that the winter mean NAO and PNA patterns have little spatial correlation. Calculations using randomly selected samples also indicate that when the NAO and PNA patterns have little spatial correlation, the AO never appears as EOF1. All the preceding results show that almost all characteristics of the AO–NCM can be explained from those of the NAO–PNA. Hence it is concluded that the AO, which is extracted by EOF analysis from the temporarily independent but spatially overlapping variations of the NAO and PNA, is almost apparent.

Description: Clouds have an important controlling influence on the radiation balance, and hence surface melting, over the Greenland ice sheet and need to be classified to derive reliable albedo estimates from visible imagery. Little is known, however, about the true cloud cover characteristics for the largest island on Earth, Greenland. Here, an attempt is made to address this knowledge gap by examining cloud characteristics, as determined by three complementary satellites sensors: the Advanced Very High Resolution Radiometer (AVHRR), the Along Track Scanning Radiometer-2 (ATSR-2), and the Moderate Resolution Imaging Spectroradiometer (MODIS). The first provides a multidecadal time series of clouds, albedo, and surface temperature, and is available, in the form of the extended AVHRR Polar Pathfinder dataset (APP-x), as a homogeneous, consistent dataset from 1982 until 2004. APP-x data, however, are also the most challenging to cloud classify over snow-covered terrain, due to the limited spectral capabilities of the instrument. ATSR-2 permits identification and classification using stereophotogrammetric techniques and MODIS has enhanced spectral sampling in the visible and thermal infrared but over more limited time periods. The spatial cloud fractions from the three sensors are compared and show good agreement in terms of both magnitude and spatial pattern. The cloud fractions, and inferred patterns of accumulation, are then assessed from three commonly used reanalysis datasets: NCEP–NCAR, the second NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II), and the 40-yr ECMWF Re-Analysis (ERA-40). Poor agreement between the reanalysis datasets is found. NCEP–DOE AMIP-II produces a cloud fraction similar to that observed by the satellites. NCEP–NCAR and ERA-40, however, bear little similarity to the cloud fractions derived from the satellite observations. This suggests that they may produce poor accumulation estimates over the ice sheet and poor estimates of radiation balance. Using these reanalysis data to force a mass balance model of the ice sheet, without appropriate downscaling and correction for the substantial biases present, may, therefore, produce substantial errors in surface melt rate estimates.

Description: Variability of the Great Plains low-level jet (GPLLJ) is analyzed from the perspective of larger-scale, lower-frequency influences and regional hydroclimate impacts as opposed to the usual analysis of its frequency, diurnal variability, and mesoscale structure. The circulation-centric core analysis is conducted with monthly data from the high spatiotemporal resolution, precipitation-assimilating North American Regional Reanalysis, and the 40-yr ECMWF Re-Analysis (ERA-40) (as necessary) to identify the recurrent patterns of GPLLJ variability and their large-scale circulation links. The links are first investigated from regressions of an index representing meridional wind speed in the climatological jet-core region; the core region itself is defined from analysis of seasonal and diurnal variability of the jet structure and moisture fluxes. The analysis reveals that GPLLJ variability is, indeed, linked to coherent, large-scale, upper-level height patterns over the Pacific and North Atlantic Oscillation (NAO) variability in the Atlantic. A Rossby wave source analysis shows the Pacific height pattern to be potentially linked to tropical diabatic heating anomalies in the west-central basin and in the eastern Pacific sector. EOF analysis of GPLLJ variability shows it to be composed of three modes that, together, account for ∼75% of the variance. The modes represent the strengthening/expansion of the jet core (38%), with a strong precipitation impact on the northern Great Plains, and linked to post-peak-phase ENSO variability; meridional shift of the GPLLJ (23%), with a Gulf states precipitation focus, and linked to pre-peak-phase ENSO variability; and in-place strengthening of the GPLLJ (12%), with dipolar influence on Great Plains and Gulf states precipitation, and linked to summer NAO variability.

Description: Transport of momentum by convection is an important process affecting global circulation. Owing to the lack of global observations, the quantification of the impact of this process on the tropospheric climate is difficult. Here an implementation of two convective momentum transport parameterizations, presented by Schneider and Lindzen and Gregory et al., in the Community Atmosphere Model, version 3 (CAM3) is presented, and their effect on global climate is examined in detail. An analysis of the tropospheric zonal momentum budget reveals that convective momentum transport affects tropospheric climate mainly through changes to the Coriolis torque. These changes result in improvement of the representation of the Hadley circulation: in December–February, the upward branch of the circulation is weakened in the Northern Hemisphere and strengthened in the Southern Hemisphere, and the lower northerly branch is weakened. In June–August, similar improvements are noted. The inclusion of convective momentum transport in CAM3 reduces many of the model’s biases in the representation of surface winds, as well as in the representation of tropical convection. In an annual mean, the tropical easterly bias, subtropical westerly bias, and the bias in the 60°S jet are improved. Representation of convection is improved along the equatorial belt with decreased precipitation in the Indian Ocean and increased precipitation in the western Pacific. The improvements of the representation of tropospheric climate are greater with the implementation of the Schneider and Lindzen parameterization.

Description: The large regional summer warming on the east coast of the northern Antarctic Peninsula (AP), which has taken place since the mid-1960s, has previously been proposed to be caused by a trend in the Southern Hemisphere Annular Mode (SAM). The authors utilize a high-resolution regional atmospheric model climatology (14-km grid spacing) to study the mechanisms that determine the response of the near-surface temperature to an increase in the SAM (ΔT/ΔSAM). Month-to-month variations in near-surface temperature and surface pressure are well represented by the model. It is found that north of ∼68°S, ΔT/ΔSAM is much larger on the eastern (lee) side than on the western (windward) side of the barrier. This is because of the enhanced westerly flow of relatively warm air over the barrier, which warms (and dries) further as it descends down the lee slope. The downward motion on the eastern side of the barrier causes a decrease in surface-mass balance and cloud cover. South of ∼68°S, vertical deflection across the barrier is greatly reduced and the contrast in ΔT/ΔSAM between the east and west sides of the barrier vanishes. In the northeastern part of the AP, the modeled ΔT/ΔSAM distribution is similar to the distribution derived from satellite infrared radiometer data. The region of strongest modeled temperature sensitivity to the SAM is where ice shelf collapse has recently taken place and does not extend farther south over the Larsen-C Ice Shelf.

Description: River runoff to the Arctic Ocean has increased over the last century, primarily during the winter and spring and primarily from the major Eurasian rivers. Some recent studies have suggested that the additional runoff is due to increased northward transport of atmospheric moisture (and associated increased precipitation), but other studies show inconsistencies in long-term runoff and precipitation trends, perhaps partly due to biases in the observational datasets. Through trend analysis of precipitation, temperature, and streamflow data, the authors investigate the extent to which Eurasian Arctic river discharge changes are attributable to precipitation and temperature changes as well as to reservoir construction and operation between the years of 1936 and 2000. Two new datasets are applied: a gridded precipitation product, in which the low-frequency variability is constrained to match that of long-term bias-corrected precipitation station data, and a reconstructed streamflow product, in which the effects of reservoirs have been minimized using a physically based reservoir model. It is found that reservoir operations have primarily affected streamflow seasonality, increasing winter discharge and decreasing summer discharge. To understand the influences of climate on streamflow changes, the authors hypothesize three cases that would cause precipitation trends to be inconsistent with streamflow trends: first, for the coldest basins in northeastern Siberia, streamflow should be sensitive to warming primarily as a result of the melting of excess ground ice, and for these basins positive streamflow trends may exceed precipitation trends in magnitude; second, evapotranspiration (ET) in the warmer regions of western Siberia and European Russia is sensitive to warming and increased precipitation, therefore observed precipitation trends may exceed streamflow trends; and third, streamflow from the central Siberian basins should respond to both effects. It is found that, in general, these hypotheses hold true. In the coldest basins, streamflow trends diverged from precipitation trends starting in the 1950s to 1960s, and this divergence accelerated thereafter. In the warmest basins, precipitation trends consistently exceeded streamflow trends, suggesting that increased precipitation contributed to increases in both ET and streamflow. In the central basins, permafrost degradation and ET effects appear to be contributing to long-term streamflow trends in varying degrees for each basin. The results herein suggest that the extent and state of the permafrost underlying a basin is a complicating factor in understanding long-term changes in Eurasian Arctic river discharge.

Description: The observation and representation in general circulation models (GCMs) of cloud vertical overlap are the objects of active research due to their impacts on the earth’s radiative budget. Previous studies have found that vertically contiguous cloudy layers show a maximum overlap between layers up to several kilometers apart but tend toward a random overlap as separations increase. The decorrelation length scale that characterizes the progressive transition from maximum to random overlap changes from one location and season to another and thus may be influenced by large-scale vertical motion, wind shear, or convection. Observations from the U.S. Department of Energy Atmospheric Radiation Measurement program ground-based radars and lidars in midlatitude and tropical locations in combination with reanalysis meteorological fields are used to evaluate how dynamics and atmospheric state influence cloud overlap. For midlatitude winter months, strong synoptic-scale upward motion maintains conditions closer to maximum overlap at large separations. In the tropics, overlap becomes closer to maximum as convective stability decreases. In midlatitude subsidence and tropical convectively stable situations, where a smooth transition from maximum to random overlap is found on average, large wind shears sometimes favor minimum overlap. Precipitation periods are discarded from the analysis but, when included, maximum overlap occurs more often at large separations. The results suggest that a straightforward modification of the existing GCM mixed maximum–random overlap parameterization approach that accounts for environmental conditions can capture much of the important variability and is more realistic than approaches that are only based on an exponential decay transition from maximum to random overlap.

Description: Atmosphere–ocean general circulation models (AOGCMs) employ very different land surface schemes (LSSs) and, as a result, their predictions of land surface quantities are often difficult to compare. Some of the disagreement in quantities such as soil moisture is likely due to differences in the atmospheric component; however, previous intercomparison studies have determined that different LSSs can produce very different results even when supplied with identical atmospheric forcing. A simple off-line LSS is presented that can reproduce the soil moisture simulations of various AOGCMs, based on their modeled temperature and precipitation. The scheme makes use of the well-established Thornthwaite method for estimating potential evapotranspiration combined with a variation of the Manabe “bucket” model. The model can be tuned to reproduce the control climate soil moisture of an AOGCM by adjusting the ease with which runoff and evapotranspiration continue as the moisture level in the bucket goes down. This produces a set of parameter values that provides a good fit to each of several AOGCM control climates. In addition, the parameter values can be set to imitate the LSS from one AOGCM while the model is forced with atmospheric data from another, thus providing an estimate of the magnitude of variation caused by the differences in land surface parameterization and by differences in atmospheric forcing. In general, the authors find that differences in LSSs account for about half of the difference in soil moisture as simulated by different AOGCMs, and the differences in atmospheric forcing account for the other half of the difference. However, the LSS can be more important than differences in atmospheric forcing in some regions (such as the United States) and less important in others (such as East Africa).

Description: An ensemble of seasonal Atlantic hurricane simulations is conducted using The Florida State University/Center for Ocean–Atmospheric Prediction Studies (FSU–COAPS) global spectral model (Cocke and LaRow) at a resolution of T126L27 (a Gaussian grid spacing of 0.94°). Four integrations comprising the ensembles were generated using the European Centre for Medium-Range Weather Forecasts (ECMWF) time-lagged initial atmospheric conditions centered on 1 June for the 20 yr from 1986 to 2005. The sea surface temperatures (SSTs) were updated weekly using the Reynolds et al. observed data. An objective-tracking algorithm obtained from the ECMWF and modified for this model’s resolution was used to detect and track the storms. It was found that the model’s composite storm structure and track lengths are realistic. In addition, the 20-yr interannual variability was well simulated by the ensembles with a 0.78 ensemble mean rank correlation. The ensembles tend to overestimate (underestimate) the numbers of storms during July (September) and produced only one CAT4–level storm on the Saffir–Simpson scale. Similar problems are noted in other global model simulations. All ensembles did well in simulating the large number of storms forming in the Atlantic basin during 1995 and showed an increase in the number of storms during La Niña and a decrease during El Niño events. The results are found to be sensitive to the choices of convection schemes and diffusion coefficients. The overall conclusion is that models such as the one used here are needed to better hindcast the interannual variability; however, going to an even higher resolution does not guarantee better interannual variability, tracks, or intensity. Improved physical parameterizations, such as using an explicit convection scheme and better representation of surface roughness at high wind speeds, are likely to more accurately represent hurricane intensity.

Description: Controversy exists over the role of the recent rise in sea surface temperatures (SST) and the frequency of tropical cyclones or hurricanes. Here, 135 yr of observational records are used to demonstrate how sea surface temperature, sea level pressure, and cyclone numbers are linked. A novel wavelet-lag coherence method is used to study cause and effect relations over a large space of time scales, phase lags, and periods. It is found that SST and cyclones are not merely correlated, but are in a negative feedback loop, where rising SST causes increased numbers of cyclones, which reduce SST. This is statistically most significant at decadal and not at longer periods, which is contrary to expectations if long-period natural cycles are important in driving cyclone numbers. Spatial relationships are examined using phase-aware teleconnections, which at the dominant decadal period show the in-phase behavior of the Atlantic SST in the Gulf Stream region, reflecting the role of the transportion of heat northward from the tropical Atlantic. At 5-yr periods there is significant coherence when SST leads cyclones by 2 yr, and this is associated with tropical ENSO activity such that, as predicted, increasing numbers of El Niños cause fewer Atlantic cyclones. The pattern of coherence existing since 1970 strongly favors the decadal coherence band, and despite growing coherence at higher frequencies, there is none at the 5-yr band, perhaps explaining why the observed sensitivity between SST and cyclones is larger than that from general circulation model (GCM) predictions and becoming greater.

Description: In this study, data from three atmospheric models are analyzed to investigate the existence of atmospheric flow regimes despite nearly Gaussian statistics of the planetary waves in these models. A hierarchy of models is used, which describes the atmospheric circulation with increasing complexity. To systematically identify atmospheric regimes, the presence of metastable states in the data is searched for by fitting so-called hidden Markov models (HMMs) to the time series. A hidden Markov model is designed to describe the situation in which part of the information of the system is unknown or hidden and another part is observed. Within the context of this study, some representative variable of planetary-scale flow (e.g., mean zonal flow or leading principal component) is known (“observed”), but its dynamics may depend crucially on the overall flow configuration, which is unknown. The behavior of this latter, “hidden” variable is described by a Markov chain. If the Markov chain possesses metastable (or quasi persistent) states, they are identified as regimes. In this perspective, regimes can be present even though the observed data have a nearly Gaussian probability distribution. The parameters of the HMMs are fit to the time series using a maximum-likelihood approach; well-established and robust numerical methods are available to do this. Possible metastability of the Markov chain is assessed by inspecting the eigenspectrum of the associated transition probability matrix. The HMM procedure is first applied to data from a simplified model of barotropic flow over topography with a large-scale mean flow. This model exhibits regime behavior of its large-scale mean flow for sufficiently high topography. In the case of high topography, the authors find three regimes, two of which correspond to zonal flow and the third to blocking. Next, a three-layer quasigeostrophic model is used as a prototype atmospheric general circulation model (GCM). Its first empirical orthogonal function (EOF) is similar to the Arctic Oscillation (AO) and exhibits metastability. For this model, two regime states are found: one corresponding to the positive phase of the AO with large amplitude and decreased variability of the streamfunction field, and another corresponding to the negative AO phase with small amplitude and increased variability. Finally, the authors investigate a comprehensive GCM. The leading four EOFs of this model show no signs of metastability. The results of the barotropic flow over topography and of the quasigeostrophic model suggest that the observed small skewness of planetary wave probability density functions (PDFs) is an imprint of blocked circulation states.

Description: A simulation of the present-day climate by the Community Climate System Model version 3 (CCSM3) that uses a Finite Volume (FV) numerical method for solving the equations governing the atmospheric dynamics is presented. The simulation is compared to observations and to the well-documented simulation by the standard CCSM3, which uses the Eulerian spectral method for the atmospheric dynamics. The atmospheric component in the simulation herein uses a 1° latitude × 1.25° longitude grid, which is a slightly finer resolution than the T85-grid used in the spectral transform. As in the T85 simulation, the ocean and ice models use a nominal 1-degree grid. Although the physical parameterizations are the same and the resolution is comparable to the standard model, substantial testing and slight retuning were required to obtain an acceptable control simulation. There are significant improvements in the simulation of the surface wind stress and sea surface temperature. Improvements are also seen in the simulations of the total variance in the tropical Pacific, the spatial pattern of ice thickness distribution in the Arctic, and the vertically integrated ocean circulation in the Antarctic Circumpolar Current. The results herein demonstrate that the FV version of the CCSM coupled model is a state-of-the-art climate model whose simulation capabilities are in the class of those used for Intergovernmental Panel on Climate Change (IPCC) assessments. The simulated climate is very similar to that of the T85 version in terms of its biases, and more like the T85 model than the other IPCC models.

Description: The author analyzes the impact of 13 major stratospheric aerosol producing volcanic eruptions since 1870 on the large-scale variability modes of sea level pressure in the Northern Hemisphere winter. The paper focuses on the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) to address the question about the physical nature of these modes. The hypothesis that the phase of the El Niño–Southern Oscillation (ENSO) may control the geographical extent of the dominant mode in the Northern Hemisphere is also investigated, as well as the related possibility that the impact of the eruptions may be different according to the phase of ENSO. The author finds that both the AO and the NAO are excited in the first winter after the eruptions with statistical significance at the 95% level. Both the signal and the significance are larger for the NAO than for the AO. The excitation of the AO and the NAO is connected with the excitation of a secondary mode, which resembles an augmented Pacific–North American pattern. This mode has opposite polarity in the Atlantic and the Pacific and interferes negatively with the AO in the Pacific and positively in the Atlantic in the first winter after the eruptions, giving the superposition a strong NAO resemblance. Some evidence is found that the correlations between the Atlantic and the Pacific are stronger in the negative ENSO phase than in the positive phase, although this difference is not statistically significant when all data since 1870 are considered. The author does not find any evidence that the impact of the volcanic eruptions is more hemispheric in the negative than in the positive ENSO phase.

Description: This study combines geostationary water vapor imagery with optical cloud property retrievals and microwave sea surface observations in order to investigate, in a Lagrangian framework, (i) the importance of cirrus anvil sublimation on tropical upper-tropospheric humidity and (ii) the sea surface temperature dependence of deep convective development. Although an Eulerian analysis shows a strong spatial correlation of ∼0.8 between monthly mean cirrus ice water path and upper-tropospheric humidity, the Lagrangian analysis indicates no causal link between these quantities. The maximum upper-tropospheric humidity occurs ∼5 h after peak convection, closely synchronized with the maximum cirrus ice water path, and lagging behind it by no more than 1.0 h. Considering that the characteristic e-folding decay time of cirrus ice water is determined to be ∼4 h, this short time lag does not allow for significant sublimative moistening. Furthermore, a tendency analysis reveals that cirrus decay and growth, in terms of both cloud cover and integrated ice content, is accompanied by the drying and moistening of the upper troposphere, respectively, a result opposite that expected if cirrus ice were a primary water vapor source. In addition, it is found that an ∼2°C rise in sea surface temperature results in a measurable increase in the frequency, spatial extent, and water content of deep convective cores. The larger storms over warmer oceans are also associated with slightly larger anvils than their counterparts over colder oceans; however, anvil area per unit cumulus area, that is, cirrus detrainment efficiency, decreases as SST increases.

Description: A dataset of 282 meteorological stations including all of the ordinary and national basic/reference surface stations of north China is used to analyze the urbanization effect on surface air temperature trends. These stations are classified into rural, small city, medium city, large city, and metropolis based on the updated information of total population and specific station locations. The significance of urban warming effects on regional average temperature trends is estimated using monthly mean temperature series of the station group datasets, which undergo inhomogeneity adjustment. The authors found that the largest effect of urbanization on annual mean surface air temperature trends occurs for the large-city station group, with the urban warming being 0.16°C (10 yr)−1, and the effect is the smallest for the small-city station group with urban warming being only 0.07°C (10 yr)−1. A similar assessment is made for the dataset of national basic/reference stations, which has been widely used in regional climate change analyses in China. The results indicate that the regional average annual mean temperature series, as calculated using the data from the national basic/reference stations, is significantly impacted by urban warming, and the trend of urban warming is estimated to be 0.11°C (10 yr)−1. The contribution of urban warming to total annual mean surface air temperature change as estimated with the national basic/reference station dataset reaches 37.9%. It is therefore obvious that, in the current regional average surface air temperature series in north China, or probably in the country as a whole, there still remain large effects from urban warming. The urban warming bias for the regional average temperature anomaly series is corrected. After that, the increasing rate of the regional annual mean temperature is brought down from 0.29°C (10 yr)−1 to 0.18°C (10 yr)−1, and the total change in temperature approaches 0.72°C for the period analyzed.

Description: This study identifies interdecadal natural climate variability in global surface temperatures by subtracting, from the observed temperature evolution, multimodel ensemble mean based on the World Climate Research Programme's (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset. The resulting signal resembles the so-called Atlantic multidecadal oscillation (AMO) and is presumably associated with intrinsic dynamics of the oceanic thermohaline circulation (THC). While certain phases of the oscillation are dominated by the anomalies in the North Atlantic region, other phases are characterized by global teleconnections to the North Pacific Ocean, tropical Atlantic Ocean, as well as the Southern Ocean. In particular, natural variability of sea surface temperature in the Atlantic hurricanes’ main development region has a peak-to-peak amplitude comparable in magnitude to this region’s surface temperature increase over the past century, for all seasons. Evidence suggests that the AMO influence on secular trends in the global-mean surface temperature can arise via direct, regional contribution to the surface temperature evolution, as well as via an indirect route linked to variability of the oceanic uptake of CO2 associated with AMO-related THC changes.

Description: Radiation schemes in general circulation models currently make a number of simplifications when accounting for clouds, one of the most important being the removal of horizontal inhomogeneity. A new scheme is presented that attempts to account for the neglected inhomogeneity by using two regions of cloud in each vertical level of the model as opposed to one. One of these regions is used to represent the optically thinner cloud in the level, and the other represents the optically thicker cloud. So, along with the clear-sky region, the scheme has three regions in each model level and is referred to as “Tripleclouds.” In addition, the scheme has the capability to represent arbitrary vertical overlap between the three regions in pairs of adjacent levels. This scheme is implemented in the Edwards–Slingo radiation code and tested on 250 h of data from 12 different days. The data are derived from cloud retrievals using radar, lidar, and a microwave radiometer at Chilbolton, southern United Kingdom. When the data are grouped into periods equivalent in size to general circulation model grid boxes, the shortwave plane-parallel albedo bias is found to be 8%, while the corresponding bias is found to be less than 1% using Tripleclouds. Similar results are found for the longwave biases. Tripleclouds is then compared to a more conventional method of accounting for inhomogeneity that multiplies optical depths by a constant scaling factor, and Tripleclouds is seen to improve on this method both in terms of top-of-atmosphere radiative flux biases and internal heating rates.

Description: The role of the Atlantic summer and autumn SSTs on the predictability of the winter Iberian Peninsula river flows is analyzed during the period 1923–2004. The analysis is based on the results of an interannual predictability experiment, using autoregressive-moving-average (ARMA) models, carried out in the first part of this work. A standard principal component analysis (PCA) was applied to the summer and autumn SST fields for the entire Atlantic Ocean. Then, the association between the resulting principal component (PC) series and the Iberian Peninsula streamflow series was analyzed, in order to use the PC series as additional predictor variables in a seasonal forecasting regression model. Results proved, first, that during autumn, the SST variability in the so-called North Atlantic horseshoe pattern has a statistically significant linear influence in the following winter streamflow values. In particular, the use of this SST information considerably improves the skill of the linear forecast (improvements against climatology range from 61% to 90%) compared to the ARMA-alone model (51%–53%). These improvements are mostly related to the ability of the SST information to provide better estimates of extreme streamflow values. Additionally, results showed that the summer tropical Atlantic and the autumn southwestern Atlantic SST variability have a significant nonlinear influence on the following winter streamflow values. In particular, there is a tendency for negative streamflow anomalies following tropical Atlantic summer negative SST anomalies and following southwestern Atlantic autumn SST positive anomalies. It is finally concluded that the linear interannual predictability of the Iberian Peninsula winter streamflow is greater (two-thirds of the total predictability) than the predictability associated with the previous season autumn SSTs (one-third).

Description: The authors provide a detailed examination of observed ocean–atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM), and the El Niño–Southern Oscillation (ENSO). Results are examined separately for the warm (November–April) and cold (May–October) seasons and for monthly and weekly time scales. It is shown that the signatures of the SAM and ENSO in the SH SST field vary as a function of season, both in terms of their amplitudes and structures. The role of surface turbulent and Ekman heat fluxes in driving seasonal variations in the SAM- and ENSO-related SST anomalies is investigated. Analyses of weekly data reveal that variability in the SAM tends to precede anomalies in the SST field by ∼1 week, and that the e-folding time scale of the SAM-related SST field anomalies is at least 4 months. The persistence of the SAM-related SST anomalies is consistent with the passive thermal response of the Southern Ocean to variations in the SAM, and seasonal variations in the persistence of the SAM-related SST anomalies are consistent with the seasonal cycle in the depth of the ocean mixed layer.

Description: Droughts and persistent wet spells over the United States and northwest Mexico have preferred regions of occurrence and persistence. Wet or dry conditions that persist more than 1 yr tend to occur over the interior United States west of 90°–95°W and northwest Mexico. In contrast, events over the eastern United States are less likely to occur and often last less than 6 months. The long persistent drought and wet spells are often modulated by low-frequency sea surface temperature anomalies (SSTAs). The persistent dry or wet conditions over northwest Mexico and the Southwest are associated with decadal variability of SSTAs over the North Pacific. Persistent events over the northwestern mountains are associated with two decadal SSTA modes. One mode has loadings over three southern oceans and another one is an El Niño–Southern Oscillation (ENSO) like decadal mode. Wet and dry conditions over the Pacific Northwest and the Great Plains are often associated with ENSO. The seasonal cycle of precipitation over the central-eastern United States, the East Coast, and the Ohio Valley is weak. Drought and wet spells over these regions are less persistent because the ENSO events have opposite impacts on precipitation for summer and winter.

Description: The recently reported increase in the proportion of intense hurricanes is considerably larger than those projected by the maximum potential intensity (MPI) theory and the results of numerical simulation. To reconcile this discrepancy, the authors examined the best-track datasets for the North Atlantic (NA), western North Pacific (WNP), and eastern North Pacific (ENP) basins. It was found that the changes in the tropical cyclone formation locations and prevailing tracks may have contributed to the changes in the proportion of the intense hurricanes over the past 30 yr. The authors suggest that the changes in the formation locations and prevailing tracks have a profound impact on the basinwide tropical cyclone intensity. Thus, how the atmospheric circulation in the tropical cyclone basins responds to the global warming may be a critical factor in understanding the impacts of global warming on tropical cyclone intensity.

Description: In this study, droughts are analyzed using the standardized precipitation index (SPI) at different time scales for all of Europe over the period 1901–2000. The SPI is calculated at different time scales (1–12 months), as are the average values that correspond to negative and positive phases of the North Atlantic Oscillation (NAO). The responses of droughts to the phases of the NAO vary spatially, but the response also depends on the month of the year and the time scale of the analysis. During the positive/negative phases, negative/positive SPI values are generally recorded in southern Europe, with the opposite pattern recorded in northern Europe. In certain regions, significant differences in the SPI are also recorded during spring, summer, and even autumn. In several regions, the magnitude of the average SPI anomalies is noticeably different for the positive and negative phases of the NAO, indicating the asymmetric response of droughts to the NAO. The unstable response of drought occurrence is also demonstrated, at different time scales, to positive and negative phases of the NAO throughout the twentieth century. During the second half of the twentieth century, there is a strengthening of the influence of the positive phases of the NAO on droughts. In contrast, the negative phases show a weaker influence on the SPI during the second half of the twentieth century. This pattern is related to changes in the wintertime sea level pressure fields associated with positive and negative phases of the NAO.

Description: The authors develop statistical data models to combine ensembles from multiple climate models in a fashion that accounts for uncertainty. This formulation enables treatment of model specific means, biases, and covariance matrices of the ensembles. In addition, the authors model the uncertainty in using computer model results to estimate true states of nature. Based on these models and principles of decision making in the presence of uncertainty, this paper poses the problem of superensemble experimental design in a quantitative fashion. Simple examples of the resulting optimal designs are presented. The authors also provide a Bayesian climate modeling and forecasting analysis. The climate variables of interest are Northern and Southern Hemispheric monthly averaged surface temperatures. A Bayesian hierarchical model for these quantities is constructed, including time-varying parameters that are modeled as random variables with distributions depending in part on atmospheric CO2 levels. This allows the authors to do Bayesian forecasting of temperatures under different Special Report on Emissions Scenarios (SRES). These forecasts are based on Bayesian posterior distributions of the unknowns conditional on observational data for 1882–2001 and climate system model output for 2002–97. The latter dataset is a small superensemble from the Parallel Climate Model (PCM) and the Community Climate System Model (CCSM). After summarizing the results, the paper concludes with discussion of potential generalizations of the authors’ strategies.

Description: Prescribing sea surface temperature (SST) for the atmospheric general circulation models (GCM) may not lead to underestimation of the coupled variability. In this study, a set of SST-driven atmospheric GCM experiments, starting from slightly different multiple initial conditions, is performed. The SST used here is prepared by a coupled GCM, which has the same atmospheric GCM component as the AGCM used in the experiment with the prescribed SST. The results indicate that prescribing SST leads to underestimation of the coupled air temperature variance only in subtropics. Meanwhile, in midlatitudes, prescribing SST may result in the overestimation of the coupled air temperature variance, where the major role of ocean–atmosphere contrast is to provide damping for SST. The simple stochastically driven coupled model is revisited with an extension to the direct wind-driven forcing for SST. In addition to the previous setup relying exclusively on the stochastic perturbation for air temperature, the ocean temperature is also forced by the pure random wind. By this extended model, it is speculated that the coupled air temperature variance can be overestimated by prescribing SST, depending on the sensitivity of SST to the wind-driven heat flux. The midlatitude is the most probable place for the overestimation since the wind-driven ocean dynamics can enhance the wind-driven surface heat flux due to the dominant zonal wind anomaly.

Description: Hurricanes cause drastic social problems as well as generate huge economic losses. A reliable forecast of the level of hurricane activity covering the next several seasons has the potential to mitigate against such losses through improvements in preparedness and insurance mechanisms. Here a statistical algorithm is developed to predict North Atlantic hurricane activity out to 5 yr. The algorithm has two components: a time series model to forecast average hurricane-season Atlantic sea surface temperature (SST), and a regression model to forecast the hurricane rate given the predicted SST value. The algorithm uses Monte Carlo sampling to generate distributions for the predicted SST and model coefficients. For a given forecast year, a predicted hurricane count is conditional on a sampled predicted value of Atlantic SST. Thus forecasts are samples of hurricane counts for each future year. Model skill is evaluated over the period 1997–2005 and compared against climatology, persistence, and other multiseasonal forecasts issued during this time period. Results indicate that the algorithm will likely improve on earlier efforts and perhaps carry enough skill to be useful in the long-term management of hurricane risk.

Description: The Met Office unified forecast–climate model is used to compare the properties of simulated climatological cloud regimes with those produced in short-range forecasts initialized from operational analyses. The regimes are defined as principal clusters of joint cloud-top pressure–optical depth histograms. In general, the cloud regime properties are found to be similar at all forecast times, including the climatological mean. This suggests that weaknesses in the representation of fast local processes are responsible for errors in the simulation of the cloud regimes. The increased horizontal resolution of the model used for numerical weather prediction generally has little impact on the cloud regimes, although the simulation of tropical shallow cumulus is improved, while the relative frequency of tropical deep convection and cirrus compare less favorably with observations. Analysis of the initial temperature tendency profiles for each cloud regime indicates that some of the initial temperature tendency, which leads to a systematic bias in the model climatology, is associated with a particular cloud regime.

Description: Several observational studies suggest that the vertical diffusivity in the Indonesian marginal seas is an order of magnitude larger than in the open ocean and what is used in most ocean general circulation models. The experiments described in this paper show that increasing the background diffusivity in the Banda Sea from the commonly used value of 0.1 cm2 s−1 to the observed value of 1 cm2 s−1 improves the watermass properties there by reproducing the observed thick layer of Banda Sea Water. The resulting reduced sea surface temperatures lead to weaker convection and a redistribution of precipitation, away from the Indonesian seas toward the equatorial Indian and Pacific Oceans. In particular, the boreal summer precipitation maximum of the Indonesian seas shifts northward from the Banda Sea toward Borneo, which reduces a longstanding bias in the simulation of the Austral–Asian Monsoon in the Community Climate System Model. Because of the positive feedback mechanisms inherent in tropical atmosphere dynamics, a reduction in Banda Sea heat loss of only 5% leads locally to a reduction in convection of 20%.

Description: A new spectral-based approach is presented to find orthogonal patterns from gridded weather/climate data. The method is based on optimizing the interpolation error variance. The optimally interpolated patterns (OIP) are then given by the eigenvectors of the interpolation error covariance matrix, obtained using the cross-spectral matrix. The formulation of the approach is presented, and the application to low-dimension stochastic toy models and to various reanalyses datasets is performed. In particular, it is found that the lowest-frequency patterns correspond to largest eigenvalues, that is, variances, of the interpolation error matrix. The approach has been applied to the Northern Hemispheric (NH) and tropical sea level pressure (SLP) and to the Indian Ocean sea surface temperature (SST). Two main OIP patterns are found for the NH SLP representing respectively the North Atlantic Oscillation and the North Pacific pattern. The leading tropical SLP OIP represents the Southern Oscillation. For the Indian Ocean SST, the leading OIP pattern shows a tripole-like structure having one sign over the eastern and north- and southwestern parts and an opposite sign in the remaining parts of the basin. The pattern is also found to have a high lagged correlation with the Niño-3 index with 6-months lag.

Description: Long-term trends in the climate system are always partly obscured by naturally occurring interannual variability. All else being equal, the larger the natural variability, the less precisely one can estimate a trend in a time series of data. Measurement uncertainty, though, also obscures long-term trends. The way in which measurement uncertainty and natural interannual variability interact in inhibiting the detection of climate trends using simple linear regression is derived and the manner in which the interaction between the two can be used to formulate accuracy requirements for satellite climate benchmark missions is shown. It is found that measurement uncertainty increases detection times, but only when considered in direct proportion to natural variability. It is also found that detection times depend critically on the correlation time of natural variability and satellite lifetime. As a consequence, requirements on satellite climate benchmark accuracy and mission lifetime must be directly related to the natural variability of the climate system and its associated correlation times.

Description: A method is presented to assess the influence of changes in Arctic cloud cover on the surface temperature trend, allowing for a more robust diagnosis of causes for surface warming or cooling. Seasonal trends in satellite-derived Arctic surface temperature under clear-, cloudy-, and all-sky conditions are examined for the period 1982–2004. The satellite-derived trends are in good agreement with trends in the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis product and surface-based weather station measurements in the Arctic. Surface temperature trends under clear and cloudy conditions have patterns similar to the all-sky trends, though the magnitude of the trends under cloudy conditions is smaller than those under clear-sky conditions, illustrating the negative feedback of clouds on the surface temperature trends. The all-sky surface temperature trend is divided into two parts: the first part is a linear combination of the surface temperature trends under clear and cloudy conditions; the second part is caused by changes in cloud cover as a function of the clear–cloudy surface temperature difference. The relative importance of these two components is different in the four seasons, with the first part more important in spring, summer, and autumn, but with both parts being equally important in winter. The contribution of biases in satellite retrievals is also evaluated.

Description: The basic effect of extratropical atmosphere–ocean thermal coupling is to enhance the variance of both anomalous sea surface temperatures (SSTs) and air temperatures (AIRT) due to a decreased energy flux between the atmosphere and ocean, called reduced thermal damping. In this paper it is shown that rapidly varying surface winds, through their influence upon the turbulent surface heat fluxes that drive this coupling, act to effectively weaken the coupling and thus partially counteract the reduced thermal damping. In effect, rapid fluctuations in wind speed somewhat insulate the atmosphere and ocean from each other. The nonlinear relationship between the rapidly varying wind speed anomalies and SST and AIRT anomalies results in a rapidly varying component of the surface heat fluxes. The clear separation between the dynamical time scales of the ocean and atmosphere allows this rapidly varying flux to be simply approximated by a stochastic process in which rapidly varying wind speed is represented as Gaussian white noise whose amplitude is modulated by the more slowly evolving thermal anomalies. Such state-dependent (multiplicative) noise can alter the dynamics of atmosphere–ocean coupling because it induces an additional heat flux term, the noise-induced drift, that effectively acts to weaken both coupling and dissipation. Another key implication of the outlined theory is that air–sea coupling includes both deterministic and stochastic components. The theory is tested by examining daily observations during extended winter (November–April) at several ocean weather stations (OWSs). Two important results are found. First, multiplicative noise at OWS P effectively decreases the coupling by about one-third, with about a 10% (20%) decrease in the damping of SST (AIRT). This suggests that multiplicative noise may be responsible for roughly half of the AIRT variability at OWS P on subseasonal time scales. Second, OWS observations reveal that joint probability distribution functions of daily averaged SST and AIRT anomalies are significantly non-Gaussian. It is shown that treating the rapidly varying boundary layer heat fluxes as state-dependent noise can reproduce this observed non-Gaussianity. It is concluded that the effect of state-dependent noise is crucial to understand and model atmosphere–ocean coupling.

Description: The Colorado State University (CSU) Multiscale Modeling Framework (MMF) is a new type of general circulation model (GCM) that replaces the conventional parameterizations of convection, clouds, and boundary layer with a cloud-resolving model (CRM) embedded into each grid column. The MMF has been used to perform a 19-yr-long Atmospheric Model Intercomparison Project–style simulation using the 1985–2004 sea surface temperature (SST) and sea ice distributions as prescribed boundary conditions. Particular focus has been given to the simulation of the interannual and subseasonal variability. The annual mean climatology is generally well simulated. Prominent biases include excessive precipitation associated with the Indian and Asian monsoon seasons, precipitation deficits west of the Maritime Continent and over Amazonia, shortwave cloud effect biases west of the subtropical continents due to insufficient stratocumulus clouds, and longwave cloud effect biases due to overestimation of high cloud amounts, especially in the tropics. The geographical pattern of the seasonal cycle of precipitation is well reproduced, although the seasonal variance is considerably overestimated mostly because of the excessive monsoon precipitation mentioned above. The MMF does a good job of reproducing the interannual variability in terms of the spatial structure and magnitude of major anomalies associated with El Niño–Southern Oscillation (ENSO). The subseasonal variability of tropical climate associated with the Madden–Julian oscillation (MJO) and equatorially trapped waves are particular strengths of the simulation. The wavenumber–frequency power spectra of the simulated outgoing longwave radiation (OLR), precipitation rate, and zonal wind at 200 and 850 mb for time scales in the range of 2–96 days compare very well to the spectra derived from observations, and show a robust MJO and Kelvin and Rossby waves with phase speeds similar to those observed. The geographical patterns of the MJO and Kelvin wave–filtered OLR variance for summer and winter seasons are well simulated; however, the variances tend to be overestimated by as much as 50%. The observed seasonal and interannual variations of the strength of the MJO are also well reproduced. The physical realism of the simulated marine stratocumulus clouds is demonstrated by an analysis of the composite diurnal cycle of cloud water content, longwave (IR) cooling, vertical velocity variance, rainfall, and subcloud vertical velocity skewness. The relationships between vertical velocity variance, IR cooling, and negative skewness all suggest that, despite the coarse numerical grid of the CRM, the simulated clouds behave in a manner consistent with the understanding of the stratocumulus dynamics. In the stratocumulus-to-cumulus transition zone, the diurnal cycle of the inversion layer as simulated by the MMF also bears a remarkable resemblance to in situ observations. It is demonstrated that in spite of the coarse spacing of the CRM grid used in the current version of MMF, the bulk of vertical transport of water in the MMF is carried out by the circulations explicitly represented on the CRM grid rather than by the CRM’s subgrid-scale parameterization.

Description: The modulation of El Niño and La Niña responses by the long-term sea surface temperature (SST) warming trend in the Indian–Western Pacific (IWP) Oceans has been investigated using a large suite of sensitivity integrations with an atmospheric general circulation model. These model runs entail the prescription of anomalous SST conditions corresponding to composite El Niño or La Niña episodes, to SST increases associated with secular warming in IWP, and to combinations of IWP warming and El Niño/La Niña. These SST forcings are derived from the output of coupled model experiments for climate settings of the 1951–2000 and 2001–50 epochs. Emphasis is placed on the wintertime responses in 200-mb height and various indicators of surface climate in the North American sector. The model responses to El Niño and La Niña forcings are in agreement with the observed interannual anomalies associated with warm and cold episodes. The wintertime model responses in North America to IWP warming bear a distinct positive (negative) spatial correlation with the corresponding responses to La Niña (El Niño). Hence, the amplitude of the combined responses to IWP warming and La Niña is notably higher than that to IWP warming and El Niño. The model projections indicate that, as the SST continues to rise in the IWP sector during the twenty-first century, the strength of various meteorological anomalies accompanying La Niña (El Niño) will increase (decrease) with time. The response of the North American climate and the zonal mean circulation to the combined effects of IWP forcing and La Niña (El Niño) is approximately equal to the linear sum of the separate effects of IWP warming and La Niña (El Niño). The summertime responses to IWP warming bear some similarity to the meteorological anomalies accompanying extended droughts and heat waves over the continental United States.

Description: The Dustbowl Era drought in the 1930s was the principal Midwest drought of the twentieth century, occurring primarily in late spring–summer [April–August (AMJJA)] when 〉70% of annual rainfall normally occurred. Another major Midwest drought occurred in the 1950s but primarily in fall–early winter [September–December (SOND)] when normal rainfall was ∼1/2 as much. Optimized canonical correlation analysis (CCA) is applied to forecast AMJJA and SOND Midwest rainfall variability in cross-validated fashion from antecedent DJF and JJA sea surface temperature (SST) variability in the surrounding oceans. These CCA models simulate (i.e., hindcast, not forecast) the Dustbowl Era drought of the 1930s and four of seven secondary AMJJA droughts (≥3-yr duration) during the twentieth century, and the principal Midwest drought of the 1950s and one of three secondary SOND droughts. Diagnosing the model canonical correlations finds the superposition of tropical Pacific cool phases of the quasi-decadal oscillation (QDO) and interdecadal oscillation (IDO) responsible for secondary droughts in AMJJA when ENSO was weak and finds the eastern equatorial Pacific cool phase of the ENSO responsible for secondary droughts during SOND when ENSO was strong. These explain why secondary droughts in AMJJA occurred more often (nearly every decade) and were of longer duration than secondary droughts in SOND when decadal drought tendencies were usually interrupted by ENSO. These diagnostics also find the AMJJA Dustbowl Era drought in the 1930s and the principal SOND drought in the 1950s driven primarily by different phases (i.e., in quadrature) of the pentadecadal signal in the Pacific decadal oscillation (PDO).

Description: This study examines the predictability of seasonal mean Great Plains precipitation using an ensemble of century-long atmospheric general circulation model (AGCM) simulations forced with observed sea surface temperatures (SSTs). The results show that the predictability (intraensemble spread) of the precipitation response to SST forcing varies on interannual and longer time scales. In particular, this study finds that pluvial conditions are more predictable (have less intraensemble spread) than drought conditions. This rather unexpected result is examined in the context of the physical mechanisms that impact precipitation in the Great Plains. These mechanisms include El Niño–Southern Oscillation’s impact on the planetary waves and hence the Pacific storm track (primarily during the cold season), the role of Atlantic SSTs in forcing changes in the Bermuda high and low-level moisture flux into the continent (primarily during the warm season), and soil moisture feedbacks (primarily during the warm season). It is found that the changes in predictability are primarily driven by changes in the strength of the land–atmosphere coupling, such that under dry conditions a given change in soil moisture produces a larger change in evaporation and hence precipitation than the same change in soil moisture would produce under wet soil conditions. The above changes in predictability are associated with a negatively skewed distribution in the seasonal mean precipitation during the warm season—a result that is not inconsistent with the observations.

Description: In this paper the authors report on a study of cloud and surface flux processes in the tropical eastern Pacific Ocean based on a series of ship-based cloud and flux measurements made during fall (1999–2002) and spring (2000–02) maintenance cruises along the 95° and 110°W Tropical Atmosphere Ocean (TAO) buoy lines between 8°S and 12°N. The year-to-year and seasonal variabilities of many of the meteorological and oceanic means are relatively small. However, notable seasonal variability is found in the northern branch of the intertropical convergence zone, the north–south sea surface temperature gradient, and heat fluxes north of the equator. In the fall, the strengthening of the north–south SST contrast enhances convective activity (more and deeper clouds, precipitation, southerly inflow) in the area around 6°N, 95°W; diurnal variations of low cloud fraction were weak. Spring cloud fraction varied significantly over the diurnal cycle with substantially lower cloud fraction during the day south of 5°N. Relatively low average cloud-base heights around the equator are due to chilling of the marine boundary layer over the cold tongue. Cloud radiative forcing strongly correlates with cloud fraction; clouds in the observation region cool the surface by about 40 W m−2 in both seasons. Cloud forcing estimates from the ship data, the TAO buoys, and International Satellite Cloud Climatology Project (ISCCP) products were combined to form a consensus observation dataset that is compared with the second NCEP reanalysis (NCEP-2) and 40-yr ECMWF Re-Analysis (ERA-40) cloud forcing values. The reanalysis products were within 10 W m−2 of the observations for IR cloud forcing but substantially overestimated the solar cloud forcing, particularly in spring.

Description: The North Pacific subtropical front (STF) is a zone of high sea surface temperature (SST) gradients located around 25°N in the western basin and is most pronounced in spring. The STF’s atmospheric effects are investigated using satellite observations and an atmospheric reanalysis. During April–May along the STF, surface wind stress curl turns weakly cyclonic in the general background of anticyclonic curls. Atmospheric column-integrated water vapor displays a pronounced meridional maximum along this surface trough, suggesting a deep vertical structure. Cyclonic wind curls occur intermittently at intervals of a few days along the STF in subsynoptic low pressure systems accompanying larger, synoptic highs in the main storm track to the north. In the subsynoptic surface lows, convective rain takes place with deep upward motion moistening the entire troposphere. The lows are enhanced by condensational heating, leading to the formation of weak cyclonic wind curls. The lows display vertical structure characteristic of baroclinic instability, suggesting that they are triggered by the passage of synoptic migratory highs and grow on the baroclinicity anchored by the SST front. The cyclonic wind curls appear to be related to a cloud/rainband associated with the so-called pre-baiu/meiyu front in May.

Description: The NCAR Community Climate System Model, version 3 (CCSM3) exhibits persistent errors in its simulation of the El Niño–Southern Oscillation (ENSO) mode of coupled variability. The amplitude of the oscillation is too strong, the dominant 2-yr period too regular, and the width of the sea surface temperature response in the Pacific too narrow, with positive anomalies extending too far into the western Pacific. Two changes in the parameterization of deep convection result in a significant improvement to many aspects of the ENSO simulation. The inclusion of convective momentum transport (CMT) and a dilution approximation for the calculation of convective available potential energy (CAPE) are used in development integrations, and a striking improvement in ENSO characteristics is seen. An increase in the periodicity of ENSO is achieved by a reduction in the strength of the existing “short-circuited” delayed-oscillator mode. The off-equatorial response is weaker and less tropically confined, largely as a result of the CMT and an associated redistribution of zonal momentum. The Pacific east–west structure is improved in response to the presence of convective dilution and cooling provided by increased surface fluxes. The initiation of El Niño events is fundamentally different. Enhanced intraseasonal surface stress variability leads to absolute surface westerlies and a cooling–warming dipole between the Philippine Sea and western Pacific. Lag-regression analysis shows that intraseasonal variability may play a significant role in event initiation and maintenance as opposed to being a benign response to increased SSTs. Recent observational evidence appears to support such a leading relationship.

Description: The influence of the Indian Ocean dipole (IOD) on the poleward propagation of boreal summer intraseasonal oscillations (BSISOs) is examined using observed datasets. This study finds that coherent (incoherent) poleward propagation of precipitation anomalies from 5°S to 25°N are observed during negative (positive) IOD years. Disorganized poleward propagation of BSISO in the south equatorial Indian Ocean is observed during positive IOD years. The rationale behind such an anomaly in the poleward propagation of BSISO in contrasting IOD years is identified based on the theory of northward-propagating BSISO, which suggests the influential role of air–sea interaction on the genesis and propagation of BSISO. It is found that the mean structure of moisture convergence and meridional specific humidity distribution undergoes radical changes in contrasting IOD years, which in turn influences the meridional propagation of BSISO. This study assumes significance, considering the critical role of BSISO in modulating the seasonal mean summer monsoon rainfall.

Description: A time-varying warm bias in the global XBT data archive is demonstrated to be largely due to changes in the fall rate of XBT probes likely associated with small manufacturing changes at the factory. Deep-reaching XBTs have a different fall rate history than shallow XBTs. Fall rates were fastest in the early 1970s, reached a minimum between 1975 and 1985, reached another maximum in the late 1980s and early 1990s, and have been declining since. Field XBT/CTD intercomparisons and a pseudoprofile technique based on satellite altimetry largely confirm this time history. A global correction is presented and applied to estimates of the thermosteric component of sea level rise. The XBT fall rate minimum from 1975 to 1985 appears as a 10-yr “warm period” in the global ocean in thermosteric sea level and heat content estimates using uncorrected data. Upon correction, the thermosteric sea level curve has reduced decadal variability and a larger, steadier long-term trend.

Description: Hemispherically and temporally asymmetric tropical precipitation responses to global warming are evaluated in 13 different coupled atmosphere–ocean climate model simulations. In the late boreal summer, hemispherical averages of the tropical precipitation anomalies from the multimodel ensemble show a strong positive trend in the Northern Hemisphere and a weak negative trend in the Southern Hemisphere. In the late austral summer, on the other hand, the trends are reversed. This implies that the summer hemisphere becomes wetter and the winter hemisphere becomes a little drier in the tropics. Thus, the seasonal range of tropical precipitation, differences between wet and dry seasons, is increased. Zonal averages of the precipitation anomalies from the multimodel ensemble also reveal a meridional movement, which basically follows the seasonal migration of the main convection zone. Similar asymmetric features can be found in all 13 climate model simulations used in this study. Based on the moisture budget analysis, the vertical moisture advection associated with mean circulation is the main contribution for the robustness of the asymmetric distribution of the tropical precipitation anomalies. Under global warming, tropospheric water vapor increases as the temperature rises and most enhanced water vapor is in the lower troposphere. The ascending motion of the Hadley circulation then transports more water vapor upward, that is, anomalous moisture convergence, and enhances precipitation over the main convection zones. On the other hand, the thermodynamic effect associated with the descending motion of the Hadley circulation, that is, anomalous moisture divergence, reduces the precipitation over the descending regions.

Description: Multidecadal variability of the Atlantic meridional overturning circulation (MOC) is investigated diagnostically in the NCAR Community Climate System Model version 3 (CCSM3) present-day simulations, using the highest (T85 × 1) resolution version. This variability has a 21-yr period and is present in many other ocean fields in the North Atlantic. In MOC, the oscillation amplitude is about 4.5 Sv (1 Sv ≡ 106 m3 s−1), corresponding to 20% of the mean maximum MOC transport. The northward heat transport (NHT) variability has an amplitude of about 0.12 PW, representing 10% of the mean maximum NHT. In sea surface temperature (SST) and sea surface salinity (SSS), the peak-to-peak changes can be as large as 6°–7°C and 3 psu, respectively. The Labrador Sea region is identified as the deep-water formation (DWF) site associated with the MOC oscillations. In contrast with some previous studies, temperature and salinity contributions to the total density in this DWF region are almost equal and in phase. The heat and freshwater budget analyses performed for the DWF site indicate a complex relationship between the DWF, MOC, North Atlantic Oscillation (NAO), and subpolar gyre circulation anomalies. Their complicated interactions appear to be responsible for the maintenance of this multidecadal oscillation. In these interactions, the atmospheric variability associated with the model’s NAO plays a prominent role. In particular, the NAO modulates the subpolar gyre strength and contributes to the formation of the temperature and salinity anomalies that lead to positive/negative density anomalies at the DWF site. In addition, the wind stress curl anomalies occurring during the transition phase between the positive and negative NAO states produce fluctuations of the subtropical–subpolar gyre boundary, thus creating midlatitude SST and SSS anomalies. Comparisons with observations show that neither the pattern nor the magnitude of this dominant SST variability is realistic.

Description: This study investigates whether air–sea interactions contribute to differences in the predictability of the boreal winter tropical intraseasonal oscillation (TISO) using the NCEP operational climate model. A series of coupled and uncoupled, “perfect” model predictability experiments are performed for 10 strong model intraseasonal events. The uncoupled experiments are forced by prescribed SST containing different types of variability. These experiments are specifically designed to be directly comparable to actual forecasts. Predictability estimates are calculated using three metrics, including one that does not require the use of time filtering. The estimates are compared between these experiments to determine the impact of coupled air–sea interactions on the predictability of the tropical intraseasonal oscillation and the sensitivity of the potential predictability estimates to the different SST forcings. Results from all three metrics are surprisingly similar. They indicate that predictability estimates are longest for precipitation and outgoing longwave radiation (OLR) when the ensemble mean from the coupled model is used. Most importantly, the experiments that contain intraseasonally varying SST consistently predict the control events better than those that do not for precipitation, OLR, 200-hPa zonal wind, and 850-hPa zonal wind after the first 10 days. The uncoupled model is able to predict the TISO with similar skill to that of the coupled model, provided that an SST forecast that includes these intraseasonal variations is used to force the model. This indicates that the intraseasonally varying SSTs are a key factor for increased predictability and presumably better prediction of the TISO.

Description: The association between Southern Hemisphere cyclones and anticyclones and the El Niño–Southern Oscillation (ENSO), southern annular mode (SAM), Antarctic sea ice extent (SIE), and rainfall in Perth and Melbourne is explored. Those cities are, respectively, located in the southwestern and southeastern corners of Australia, where substantial decreasing rainfall trends have been observed over the last decades. The need for a more unified understanding of large-scale anomalies in storm indicators associated with the climate features itemized above has motivated this study. The main aim is to identify cyclone-anomalous areas that are potentially important in characterizing continental rainfall anomalies from a hemispheric perspective, focusing on midlatitude Australia. The study covers the “satellite era” from 1979 to 2003 and was conducted for the southern winter when midlatitude rainfall is predominantly baroclinic. The results indicate a well-organized hemispheric cyclone pattern associated with ENSO, SAM, SIE, and rainfall anomalies. There is a moderate large-scale, high-latitude resemblance between La Niña, negative SAM, and reduced SIE in some sectors. In particular, there is a suggestion that SIE anomalies over the Indian Ocean and Western Australia sectors are associated with a large-scale pattern of cyclone/anticyclone anomalies that is more pronounced over the longitudes of Australia and New Zealand. Spatial correlation analysis suggests a robust link between cyclone density over the sectors mentioned above and rainfall in Perth and Melbourne. Statistical analyses of rainfall and SIE show modest correlations for Perth and weak correlations for Melbourne, generally corroborating the above. It is proposed that SAM and SIE are part of a complex physical system that is best understood as a coupled mechanism, and that their impacts on the circulation can be seen as partially independent of ENSO. While SAM and SIE have greater influence on the circulation affecting rainfall in the western side of Australia, ENSO is the dominant influence on the eastern half of the country. A contraction of the sea ice seems to be accompanied by a southward shift of high-latitude cyclones, which is also hypothesized to increase downstream cyclone density at midlatitudes via conservation of mass, similarly to what is observed during the extreme positive phase of the SAM. These associations build on previous developments in the literature. They bring a more unified view on high-latitude climate features, and may also help to explain the declining trends in Australian rainfall.