ALBERT

Description: Shortly after the advent of the first imaging passive microwave sensor on board a research satellite an anomalous climate feature was observed within the Weddell Sea. During the years 1974–1976, a 250 × 103 km2 area within the seasonal sea ice cover was virtually free of winter sea ice. This feature, the Weddell Polynya, was created as sea ice formation was inhibited by ocean convection that injected relatively warm deep water into the surface layer. Though smaller, less persistent polynyas associated with topographically induced upwelling at Maud Rise frequently form in the area, there has not been a reoccurrence of the Weddell Polynya since 1976. Archived observations of the surface layer salinity within the Weddell gyre suggest that the Weddell Polynya may have been induced by a prolonged period of negative Southern Annular Mode (SAM). During negative SAM the Weddell Sea experiences colder and drier atmospheric conditions, making for a saltier surface layer with reduced pycnocline stability. This condition enables Maud Rise upwelling to trigger sustained deep-reaching convection associated with the polynya. Since the late 1970s SAM has been close to neutral or in a positive state, resulting in warmer, wetter conditions over the Weddell Sea, forestalling repeat of the Weddell Polynya. A contributing factor to the Weddell Polynya initiation may have been a La Niña condition, which is associated with increased winter sea ice formation in the polynya area. If the surface layer is made sufficiently salty due to a prolonged negative SAM period, perhaps aided by La Niña, then Maud Rise upwelling meets with positive feedback, triggering convection, and a winter persistent Weddell Polynya.

Description: A generalization of the transformed Eulerian and temporal residual means is presented. The new formulation uses rotational fluxes of buoyancy, and the full hierarchy of statistical density moments, to reduce the cross-isopycnal eddy flux to the physically relevant component associated with the averaged water mass properties. The resulting eddy-induced diapycnal diffusivity vanishes for adiabatic, statistically steady flow, and is related to either the growth or decay of mesoscale density variance and/or the covariance between small-scale forcing (mixing) and density fluctuations, such as that associated with the irreversible removal of density variance by dissipation. The relationship between the new formulation and previous approaches is described and is illustrated using results from an eddying channel model. The formalism is quite general and applies to all kinds of averaging and to any tracer (not just density).

Description: Using the same approach as in Part I, here it is shown how sampling problems in voluntary observing ship (VOS) data affect conclusions about interannual variations and secular changes of surface heat fluxes. The largest uncertainties in linear trend estimates are found in relatively poorly sampled regions like the high-latitude North Atlantic and North Pacific as well as the Southern Ocean, where trends can locally show opposite signs when computed from the regularly sampled and undersampled data. Spatial patterns of shorter-period interannual variability, quantified through the EOF analysis, also show remarkable differences between the regularly sampled and undersampled flux datasets in the Labrador Sea and northwest Pacific. In particular, it is shown that in the Labrador Sea region, in contrast to regularly sampled NCEP–NCAR reanalysis fluxes, VOS-like sampled NCEP–NCAR reanalysis fluxes neither show significant interannual variability nor significant trends. These regions, although quite localized covering small parts of the globe, play a crucial role for the coupled atmosphere–ocean system. In the Labrador Sea, for instance, interannual and decadal-scale changes of the surface net heat fluxes are known to affect oceanic convection and, thus, the meridional overturning circulation of the Atlantic Ocean. From a discussion of current atmospheric data assimilation systems it is argued that in poorly sampled regions reanalysis products are superior to VOS-based products for studying interannual and interdecadal variations of atmosphere–ocean interaction. In well-sampled regions, on the other hand, conclusions about surface heat flux variations are relatively insensitive to the choice of the flux products used (VOS versus reanalysis data). The results are confirmed for two different datasets, that is, ECMWF 40-yr Re-Analysis (ERA-40) data and seasonal integrations with a recent version of the ECMWF model in which no actual data were assimilated.

Description: On 19 October 2000, Hurricane Michael merged with an approaching baroclinic trough over the western North Atlantic Ocean south of Nova Scotia. As the hurricane moved over cooler sea surface temperatures (SSTs; less than 25°C), it intensified to category-2 intensity on the Saffir–Simpson hurricane scale [maximum sustained wind speeds of 44 m s−1 (85 kt)] while tapping energy from the baroclinic environment. The large “hybrid” storm made landfall on the south coast of Newfoundland with maximum sustained winds of 39 m s−1 (75 kt) causing moderate damage to coastal communities east of landfall. Hurricane Michael presented significant challenges to weather forecasters. The fundamental issue was determining which of two cyclones (a newly formed baroclinic low south of Nova Scotia or the hurricane) would become the dominant circulation center during the early stages of the extratropical transition (ET) process. Second, it was difficult to predict the intensity of the storm at landfall owing to competing factors: 1) decreasing SSTs conducive to weakening and 2) the approaching negatively tilted upper-level trough, favoring intensification. Numerical hindcast simulations using the limited-area Mesoscale Compressible Community model with synthetic vortex insertion (cyclone bogus) prior to the ET of Hurricane Michael led to a more realistic evolution of wind and pressure compared to running the model without vortex insertion. Specifically, the mesoscale model correctly simulates the hurricane as the dominant circulation center early in the transition process, versus the baroclinic low to its north, which was the favored development in the runs not employing vortex insertion. A suite of experiments is conducted to establish the sensitivity of the ET to various initial conditions, lateral driving fields, domain sizes, and model parameters. The resulting storm tracks and intensities fall within the range of the operational guidance, lending support to the possibility of improving numerical forecasts using synthetic vortex insertion prior to ET in such a model.

Description: In this paper, a version of the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model is used to (i) diagnose the diabatic heating associated with the winter North Atlantic Oscillation (NAO) and (ii) assess the role of this heating in the dynamics of the NAO in the model. Over the North Atlantic sector, the NAO-related diabatic heating is dominated above the planetary boundary layer by the latent heat release associated with precipitation, and within the boundary layer by vertical diffusion associated with sensible heat flux from the ocean. An association between La Niña–El Niño–type conditions in the tropical Pacific and the positive/negative NAO is found in model runs using initial conditions and sea surface temperature (SST) lower boundary conditions from the period 1982–2001, but not in a companion set of model runs for the period 1962–81. Model experiments are then described in which the NAO-related diabatic heating diagnosed from the 1982–2001 control run is applied as a constant forcing in the model temperature equation using both 1982–2001 and 1962–81 model setups. To assess the local feedback from the diabatic heating, the specified forcing is first restricted to the North Atlantic sector alone. In this case, the model response (in an ensemble mean sense) is suggestive of a weak negative feedback, but exhibits more baroclinic structure and has its centers of action shifted compared to those of the NAO. On the other hand, forcing with only the tropical Pacific part of the diabatic heating leads to a robust model response in both the 1982–2001 and 1962–81 model setups. The model response projects on to the NAO with the same sign as that used to diagnose the forcing, arguing that the link between the tropical Pacific and the NAO is real in the 1982–2001 control run. The missing link in the corresponding run for 1962–81 is a result of a change in the tropical forcing between the two periods, and not the extratropical flow regime.

Description: The depth of winter convection in the central Labrador Sea is strongly influenced by the prevailing stratification in late summer. For this late summer stratification salinity is as important as temperature, and in the upper water layers salinity even dominates. To analyze the source of the spring and summer freshening in the central region, seasonal freshwater cycles have been constructed for the interior Labrador Sea, the West Greenland Current, and the Labrador Current. It is shown that none of the local freshwater sources is responsible for the spring–summer freshening in the interior, which appears to occur in two separate events in April to May and July to September. Comparing the timing and volume estimates of the seasonal freshwater cycles of the boundary currents with the central Labrador Sea helps in understanding the origin of the interior freshwater signals. The first smaller pulse cannot be attributed clearly to either of the boundary currents. The second one is about three times stronger and supplies 60% of the seasonal summer freshwater. Transport estimates and calculated mixing properties provide evidence that its source is the West Greenland Current. The finding implies a connection also on interannual time scales between Labrador Sea surface salinity and freshwater sources in the West Greenland Current and farther upstream in the East Greenland Current. The freshwater input from the West Greenland Current thus also is the likely pathway for the known modulation of Labrador Sea Water mass formation by freshwater export from the Arctic (via the East Greenland Current), which implies some predictability on longer time scales.

Description: Multichannel singular spectrum analysis (MSSA) of surface zonal wind, sea surface temperature (SST), 20° isotherm depth, and surface zonal current observations (between 1990 and 2004) identifies three coupled ocean–atmosphere modes of variability in the tropical Pacific: the El Niño–Southern Oscillation (ENSO), the annual cycle, and a mode with a 14–18-month period, which is referred to as sub-ENSO in this study. The sub-ENSO mode accounts for the near 18-month (near annual) variability prior to (following) the 1997/98 El Niño event. It was strongest during this El Niño event, with SST anomalies exceeding 1°C. Sub-ENSO peak SST anomalies are ENSO-like in structure and are associated with eastward propagating heat content variations. However, the SST anomalies are preceded by and in near quadrature with relatively strong remotely forced westward propagating zonal current variations, suggesting the sub-ENSO mode arises from the zonal-advective feedback. The sub-ENSO mode is found to exist also in an intermediate complexity model (ICM) of the tropical Pacific. A heat budget analysis of the model’s sub-ENSO mode shows it indeed arises from the zonal-advective feedback. In the model, both ENSO and sub-ENSO modes coexist, but there is a weak nonlinear interaction between them. Experiments also show that the observed changes in sub-ENSO’s characteristics may be explained by changes in the relative importance of zonal and vertical advection SST tendencies.

Description: A coordinated set of global coupled climate model [atmosphere–ocean general circulation model (AOGCM)] experiments for twentieth- and twenty-first-century climate, as well as several climate change commitment and other experiments, was run by 16 modeling groups from 11 countries with 23 models for assessment in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Since the assessment was completed, output from another model has been added to the dataset, so the participation is now 17 groups from 12 countries with 24 models. This effort, as well as the subsequent analysis phase, was organized by the World Climate Research Programme (WCRP) Climate Variability and Predictability (CLIVAR) Working Group on Coupled Models (WGCM) Climate Simulation Panel, and constitutes the third phase of the Coupled Model Intercomparison Project (CMIP3). The dataset is called the WCRP CMIP3 multimodel dataset, and represents the largest and most comprehensive international global coupled climate model experiment and multimodel analysis effort ever attempted. As of March 2007, the Program for Climate Model Diagnostics and Intercomparison (PCMDI) has collected, archived, and served roughly 32 TB of model data. With oversight from the panel, the multimodel data were made openly available from PCMDI for analysis and academic applications. Over 171 TB of data had been downloaded among the more than 1000 registered users to date. Over 200 journal articles, based in part on the dataset, have been published so far. Though initially aimed at the IPCC AR4, this unique and valuable resource will continue to be maintained for at least the next several years. Never before has such an extensive set of climate model simulations been made available to the international climate science community for study. The ready access to the multimodel dataset opens up these types of model analyses to researchers, including students, who previously could not obtain state-of-the-art climate model output, and thus represents a new era in climate change research. As a direct consequence, these ongoing studies are increasing the body of knowledge regarding our understanding of how the climate system currently works, and how it may change in the future.

Description: Sampling uncertainties in the voluntary observing ship (VOS)-based global ocean–atmosphere flux fields were estimated using the NCEP–NCAR reanalysis and ECMWF 40-yr Re-Analysis (ERA-40) as well as seasonal forecasts without data assimilation. Air–sea fluxes were computed from 6-hourly reanalyzed individual variables using state-of-the-art bulk formulas. Individual variables and computed fluxes were subsampled to simulate VOS-like sampling density. Random simulation of the number of VOS observations and simulation of the number of observations with contemporaneous sampling allowed for estimation of random and total sampling uncertainties respectively. Although reanalyses are dependent on VOS, constituting an important part of data assimilation input, it is assumed that the reanalysis fields adequately reproduce synoptic variability at the sea surface. Sampling errors were quantified by comparison of the regularly sampled (i.e., 6 hourly) and subsampled monthly fields of surface variables and fluxes. In poorly sampled regions random sampling errors amount to 2.5°–3°C for air temperature, 3 m s−1 for the wind speed, 2–2.5 g kg−1 for specific humidity, and 15%–20% of the total cloud cover. The highest random sampling errors in surface fluxes were found for the sensible and latent heat flux and range from 30 to 80 W m−2. Total sampling errors in poorly sampled areas may be higher than random ones by 60%. In poorly sampled subpolar latitudes of the Northern Hemisphere and throughout much of the Southern Ocean the total sampling uncertainty in the net heat flux can amount to 80–100 W m−2. The highest values of the uncertainties associated with the interpolation/extrapolation into unsampled grid boxes are found in subpolar latitudes of both hemispheres for the turbulent fluxes, where they can be comparable with the sampling errors. Simple dependencies of the sampling errors on the number of samples and the magnitude of synoptic variability were derived. Sampling errors estimated from different reanalyses and from seasonal forecasts yield qualitatively comparable spatial patterns, in which the actual values of uncertainties are controlled by the magnitudes of synoptic variability. Finally, estimates of sampling uncertainties are compared with the other errors in air–sea fluxes and the reliability of the estimates obtained is discussed.

Description: Metabolomics offers great potential for understanding the metabolic consequences of exposure to chemicals and other stressors in the environment. Biological systems encompass humans exposed to chemicals in the environment as well as the diverse organisms inhabiting an ecosystem and exposed to environmental contaminants. An overall goal of environmental metabolomics is to understand what is metabolomically “normal” or adaptive and what constitutes a threat to human health and the environment.

Description: Output from an eddy-resolving model of the North Atlantic Ocean is used to estimate values for the thickness diffusivity κ appropriate to the Gent and McWilliams parameterization. The effect of different choices of rotational eddy fluxes on the estimated κ is discussed. Using the raw fluxes (no rotational flux removed), large negative values (exceeding −5000 m2 s−1) of κ are diagnosed locally, particularly in the Gulf Stream region and in the equatorial Atlantic. Removing a rotational flux based either on the suggestion of Marshall and Shutts or the more general theory of Medvedev and Greatbatch leads to a reduction of the negative values, but they are still present. The regions where κ 〈 0 correspond to regions where eddies are acting to increase, rather than decrease (as in baroclinic instability) the mean available potential energy. In the subtropical gyre, κ ranges between 500 and 2000 m2 s−1, rapidly decreasing to zero below the thermocline in all cases. Rotational fluxes and κ are also estimated using an optimization technique. In this case, |κ| can be reduced or increased by construction, but the regions where κ 〈 0 are still present and the optimized rotational fluxes also remain similar to a priori values given by the theoretical considerations. A previously neglected component (ν) of the bolus velocity is associated with the horizontal flux of buoyancy along, rather than across, the mean buoyancy contours. The ν component of the bolus velocity is interpreted as a streamfunction for eddy-induced advection, rather than diffusion, of mean isopycnal layer thickness, showing up when the lateral eddy fluxes cannot be described by isotropic diffusion only. All estimates show a similar large-scale pattern for ν, implying westward advection of isopycnal thickness over much of the subtropical gyre. Comparing ν with a mean streamfunction shows that it is about 10% of the mean in midlatitudes and even larger than the mean in the Tropics.

Description: Anthecularin (1), a minor sesquiterpene lactone with a novel ring system was isolated from Greek Anthemis auriculata (Asteraceae). Its structure was elucidated by means of NMR, HRMS, and X-ray crystallography. Anthecularin showed antitrypanosomal (IC50 = 10.1 μg/mL) and antiplasmodial activity (IC50 = 23.3 μg/mL) and inhibited two key enzymes of the plasmodial type II fatty acid biosynthesis pathway, PfFabI and PfFabG (IC50 values = 14 and 28.3 μg/mL, respectively). A probable biogenesis of 1 is also proposed and discussed.

Description: The water-soluble part of the methanolic extract from the aerial parts of Scrophularia crypthophila, through chromatographic methods, yielded three new resin glycosides, crypthophilic acids A−C (1−3). Compounds 1−3 are tetraglycosides of (+)-3S,12S-dihydroxypalmitic acid. The structures of these and 10 known compounds were elucidated by spectroscopic and chemical means. All natural resin glycosides known so far have been obtained from Convolvulaceae plants; this is the first report of such glycosides from another, taxonomically unrelated family (Scrophulariaceae).

Description: Significant changes have occurred in the anthropogenic emissions of many compounds related to the Kyoto and Montreal Protocols within the past 20 years and many of their atmospheric abundances have responded dramatically. Additionally, there are a number of related natural compounds with underdetermined source or sink budgets. A new instrument, Medusa, was developed to make the high frequency in situ measurements required for the determination of the atmospheric lifetimes and emissions of these compounds. This automated system measures a wide range of halocarbons, hydrocarbons, and sulfur compounds involved in ozone depletion and/or climate forcing, from the very volatile perfluorocarbons (PFCs, e.g., CF4 and CF3CF3) and hydrofluorocarbons (HFCs, e.g., CH3CF3) to the higher-boiling point solvents (such as CH3CCl3 and CCl2CCl2) and CHBr3. A network of Medusa systems worldwide provides 12 in situ ambient air measurements per day of more than 38 compounds of part per trillion mole fractions and precisions up to 0.1% RSD at the five remote field stations operated by the Advanced Global Atmospheric Gases Experiment (AGAGE). This custom system couples gas chromatography/mass spectrometry (GC/MSD) with a novel scheme for cryogen-free low-temperature preconcentration (−165 °C) of analytes from 2 L samples in a two-trap process using HayeSep D adsorbent.

Description: Combined simultaneous satellite observations are used to evaluate the performance of parameterizations of the microphysical and optical properties of cirrus clouds used for radiative flux computations in climate models. Atmospheric and cirrus properties retrieved from Television and Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) observations are given as input to the radiative transfer model developed for the Met Office climate model to simulate radiative fluxes at the top of the atmosphere (TOA). Simulated cirrus shortwave (SW) albedos are then compared to those retrieved from collocated Scanner for Radiation Budget (ScaRaB) observations. For the retrieval, special care has been given to angular direction models. Three parameterizations of cirrus ice crystal optical properties are represented in the Met Office radiative transfer model. These parameterizations are based on different physical approximations and different hypotheses on crystal habit. One parameterization assumes pristine ice crystals and two ice crystal aggregates. By relating the cirrus ice water path (IWP) retrieved from the effective infrared emissivity to the cirrus SW albedo, differences between the parameterizations are amplified. This study shows that pristine crystals seem to be plausible only for cirrus with IWP less than 30 g m−2. For larger IWP, ice crystal aggregates lead to cirrus SW albedos in better agreement with the observations. The data also indicate that climate models should allow the cirrus effective ice crystal diameter (De) to increase with IWP, especially in the range up to 30 g m−2. For cirrus with IWP less than 20 g m−2, this would lead to SW albedos that are about 0.02 higher than the ones of a constant De of 55 μm.

Description: A model of the subpolar North Atlantic Ocean is used to study different aspects of ventilation and water mass transformation during a year with moderate convection intensity in the Labrador Sea. The model realistically describes the salient features of the observed hydrographic structure and current system, including boundary currents and recirculations. Ventilation and transformation rates are defined and compared. The transformation rate of Labrador Sea Water (LSW), defined in analogy to several observational studies, is 6.3 Sv (Sv ≡ 106 m3 s−1) in the model. Using an idealized ventilation tracer, mimicking analyses based on chlorofluorocarbon inventories, an LSW ventilation rate of 10 Sv is found. Differences between both rates are particularly significant for those water masses that are partially transformed into denser water masses during winter. The main export route of the ventilated LSW is the deep Labrador Current (LC). Backward calculation of particle trajectories demonstrates that about one-half of the LSW leaving the Labrador Sea within the deep LC originates in the mixed layer during that same year. Near the offshore flank of the deep LC at about 55°W, the transformation of LSW begins in January and is at a maximum in February/March. While the export of transformed LSW out of the central Labrador Sea continues for several months, LSW generated near the boundary current is exported more rapidly, with maximum transport rates during March/April within the deep LC.

Description: Climate models predict a gradual weakening of the North Atlantic meridional overturning circulation (MOC) during the twenty-first century due to increasing levels of greenhouse gas concentrations in the atmosphere. Using an ensemble of 16 different coupled climate models performed for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), the evolution of the MOC during the twentieth and twenty-first centuries is analyzed by combining model simulations for the IPCC scenarios Twentieth-Century Climate in Coupled Models (20C3M) and Special Report on Emission Scenarios, A1B (SRESA1B). Earlier findings are confirmed that even for the same forcing scenario the model response is spread over a large range. However, no model predicts abrupt changes or a total collapse of the MOC. To reduce the uncertainty of the projections, different weighting procedures are applied to obtain “best estimates” of the future MOC evolution, considering the skill of each model to represent present day hydrographic fields of temperature, salinity, and pycnocline depth as well as observation-based mass transport estimates. Using different methods of weighting the various models together, all produce estimates that the MOC will weaken by 25%–30% from present day values by the year 2100; however, absolute values of the MOC and the degree of reduction differ among the weighting methods.

Description: An observational-based analysis of coupled variability in the equatorial Atlantic and its seasonality is presented. Regression analysis shows that the three elements of the Bjerknes positive feedback exist in the Atlantic and are spatially similar to those of the Pacific. The cross-correlation functions of the elements of the Bjerknes feedback are also similar and consistent with an ocean–atmosphere coupled mode. However, the growth rate in the Atlantic is up to 50% weaker, and explained variance is significantly lower. The Bjerknes feedback in the Atlantic is strong in boreal spring and summer, and weak in other seasons, which explains why the largest sea surface temperature anomalies (SSTAs) occur in boreal summer. Its seasonality is determined by seasonal variations in both atmospheric sensitivity to SSTA and SSTA sensitivity to subsurface temperature anomalies.

Description: This study investigates the influence of El Niño on the upper-ocean circulation in the tropical Atlantic Ocean (via changes in the Atlantic trade winds) by analyzing observed sea surface temperature (SST) together with an ocean general circulation model integration forced by the NCEP–NCAR reanalysis. During periods with anomalously warm (cold) eastern equatorial Pacific SST, the southern Atlantic tropical cell is strengthened (weakened). The difference of the cell strength between El Niño and La Niña years is about 20% of the mean cell strength. However, the variability of the cell is not dominated by the remote forcing from the eastern equatorial Pacific but seems to be caused by intrinsic tropical Atlantic variability. A strengthening (weakening) for periods with anomalously warm (cold) eastern equatorial Pacific SST is also found for the zonal surface and subsurface currents. TOPEX/Poseidon altimetry data are used to validate the results based on the OGCM integration.