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  • American Meteorological Society
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  • 1
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    American Meteorological Society
    In:  Bulletin of the American Meteorological Society, 78 (12). pp. 2771-2777.
    Publication Date: 2019-03-07
    Description: A review is given of the meaning of the term “El Niño” and how it has changed in time, so there is no universal single definition. This needs to be recognized for scientific uses, and precision can only be achieved if the particular definition is identified in each use to reduce the possibility of misunderstanding. For quantitative purposes, possible definitions are explored that match the El Niños identified historically after 1950, and it is suggested that an El Niño can be said to occur if 5-month running means of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°–170°W) exceed 0.4°C for 6 months or more. With this definition, El Niños occur 31% of the time and La Niñas (with an equivalent definition) occur 23% of the time. The histogram of Niño 3.4 SST anomalies reveals a bimodal character. An advantage of such a definition is that it allows the beginning, end, duration, and magnitude of each event to be quantified. Most El Niños begin in the northern spring or perhaps summer and peak from November to January in sea surface temperatures.
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  • 2
    Publication Date: 2018-04-06
    Description: On 24 and 25 October 1995, high-resolution oceanographic measurements were carried out in the Strait of Messina by using a towed conductivity-temperature-depth chain and a vessel-mounted acoustic Doppler current profiler. During the period of investigation the surface water of the Tyrrhenian Sea north of the strait sill was heavier than the surface water of the Ionian Sea south of the strait sill. As a consequence, during northward tidal flow surface water of the Ionian Sea spread as a surface jet into the Tyrrhenian Sea, whereas during southward tidal flow heavier surface water of the Tyrrhenian Sea spread, after having sunk to a depth of about 100 m, as a subsurface jet into the Ionian Sea. Both jets had the form of an internal bore, which finally developed into trains of internal solitary waves whose amplitudes were larger north than south of the strait sill. These measurements represent a detailed picture of the tidally induced internal dynamics in the Strait of Messina during the period of investigation, which contributes to elucidate several aspects of the general internal dynamics in the area: 1) Associated with the tidal flow are intense water jets whose equilibrium depth strongly depends on the horizontal density distribution along the Strait of Messina; 2) although climatological data show that a large horizontal density gradient in the near-surface layer along the Strait of Messina exists, its reversal can occur; 3) fluctuations in the larger-scale circulation patterns that determine the inflow of the modified Atlantic water into the Eastern Mediterranean Sea can be responsible for this reversal. As the tidally induced internal waves reflect the variability in the horizontal density distribution along the Strait of Messina, it is suggested that from the analysis of synthetic aperture radar imagery showing sea surface manifestations of internal waves in this area fluctuations of larger-scale circulation patterns in the Mediterranean Sea can be inferred.
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  • 3
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    American Meteorological Society
    In:  Journal of Atmospheric and Oceanic Technology, 15 . pp. 1051-1059.
    Publication Date: 2018-07-04
    Description: A new optical disdrometer has been developed that is optimized for use in high wind speeds, for example, on board ships. The minimal detectable size of droplets is 0.35 mm. Each drop is measured separately with regard to its size and residence time within the sensitive volume. From the available information, the drop size distribution can be calculated with a resolution of 0.05 mm in diameter either by evaluation of the residence time of drops or by drop counting knowing the local wind. Experience shows that using the residence time leads to better results. Rain rates can be determined from the droplet spectra by assuming terminal fall velocity of the drops according to their size. Numerical modeling of disdrometer measurements has been performed, allowing the study of the effect of multiple occupancy of the sensitive volume and grazing incidences on disdrometer measurements. Based on these studies an iterative procedure has been developed to eliminate the impact of these effects on the calculated drop size distributions. This technique may also be applied to any other kind of disdrometer. Long-term simultaneous measurements of the disdrometer and a conventional rain gauge have been used to validate this procedure.
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  • 4
    Publication Date: 2018-04-06
    Description: A new numerical two-layer model is presented, which describes the generation of internal tidal bores and their disintegration into internal solitary waves in the Strait of Messina. This model is used to explain observations made by the synthetic aperture radar (SAR) from the European Remote Sensing satellites ERS 1 and ERS 2. The analysis of available ERS 1/2 SAR data of the Strait of Messina and adjacent sea areas show that 1) northward as well as southward propagating internal waves are generated in the Strait of Messina, 2) southward propagating internal waves are observed more frequently than northward propagating internal waves, 3) sea surface manifestations of southward as well as northward propagating internal waves are stronger during periods where a strong seasonal thermocline is known to be present, 4) southward propagating internal bores are released from the sill between 1 and 5 hours after maximum northward tidal flow and northward propagating internal bores are released between 2 and 6 hours after maximum southward tidal flow, and 5) the spatial separation between the first two internal solitary waves of southward propagating wave trains is smaller in the period from July to September than in the period from October to June. The numerical two-layer model is a composite of two models consisting of 1) a hydrostatic “generation model,” which describes the dynamics of the water masses in the region close to the strait’s sill, where internal bores are generated, and 2) a weakly nonhydrostatic “propagation model,” which describes the dynamics of the water masses outside of the sill region where internal bores may disintegrate into internal solitary waves. Due to a technique for movable lateral boundaries, the generation model is capable of simulating the dynamics of a lower layer that may intersect the bottom topography. The proposed generation–propagation model depends on one space variable only, but it retains several features of a fully three-dimensional model by including a realistic channel depth and a realistic channel width. It is driven by semidiurnal tidal oscillations of the sea level at the two open boundaries of the model domain. Numerical simulations elucidate several observed characteristics of the internal wave field in the Strait of Messina, such as north–south asymmetry, times of release of the internal bores from the strait’s sill, propagation speeds, and spatial separations between the first two solitary waves of internal wave trains.
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  • 5
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 28 . pp. 1107-1129.
    Publication Date: 2018-04-06
    Description: On the basis of the collection of individual marine observations available from the Comprehensive Ocean–Atmosphere Data Set, major parameters of the sea state were evaluated. Climatological fields of wind waves and swell height and period, as well as significant wave height and resultant period are obtained for the North Atlantic Ocean for the period from 1964 to 1993. Validation of the results against instrumental records from National Data Buoy Center buoys and ocean weather station measurements indicate relatively good agreement for wave height and systematic biases in the visually estimated periods that were corrected. Wave age, which is important for wind stress estimates, was evaluated form wave and wind observations. The climatology of wave age indicates younger waves in winter in the North Atlantic midlatitudes and Tropics. Wave age estimates were applied to the calculations of the wind stress using parameterizations from field experiments. Differences between wave-age-based and traditional estimates are not negligible in wintertime in midlatitudes and Tropics where wave-induced stress contributes from 5% to 15% to the total stress estimates. Importance of the obtained effects for ocean circulation and climate variability is discussed.
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  • 6
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 29 . pp. 145-157.
    Publication Date: 2018-04-06
    Description: As a contribution to the WOCE Deep Basin Experiment, an array of current meters with individual record lengths exceeding ii years was set across the southern boundary of the Brazil Basin between early 1991 and early 1996. The array spanned the Santos Plateau, the Vema Channel, and the Hunter Channel, all areas believed to be important for transport of Antarctic Bottom Water between the Argentine and Brazil Basins. From the combination of geostrophic velocities computed from hydrographic stations and those directly measured, the total transport of bottom water (potential temperature below 2 degrees C) is estimated to be about 6.9 Sv (Sv = 10(6) m(3) s(-1)) northward, with about 4 Sv coming through the Vema Channel and the remainder through the Hunter Channel. Properties of the eddy field are also discussed. Eddy energy levels and their spatial distribution are similar to comparable regimes in the North Atlantic. Integral timescales vary from a few days to several weeks with distance from the Brazil Current and the western boundary. The eddy heat Bur is in the same direction as the heat advection by the mean flow but considerably smaller.
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  • 7
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 25 (10). pp. 2444-2457.
    Publication Date: 2018-04-05
    Description: Surface heat and freshwater fluxes from the Comprehensive 0cean-Atmosphere Data Set are revised and used diagnostically to compute air-sea transformation rates on density, temperature, and salinity classes over the domain of the data. Maximum rates occur over the warmest water and over mode waters, which are the dominant result of air-sea interaction. Transformation in different is accordingly distinguished by temperature and salinity, just as water masses in different oceans are so distinguished. Over the entire domain, to about 30°S, approximately 80×106 m3 s−1 of warm cool water are transformed by air-sea fluxes, on annual average. Calculations for several seas in the North Atlantic, where deep water is thought to originate, we also presented.
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  • 8
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 27 . pp. 381-402.
    Publication Date: 2018-04-06
    Description: Parametric representations of oceanic geostrophic eddy transfer of heat and salt are studied ranging fromhorizontal diffusion to the more physically based approaches of Green and Stone (GS) and Gent and McWilliams(GM). The authors argue for a representation that combines the best aspects of GS and GM: transfer coefficientsthat vary in space and time in a manner that depends on the large-scale density fields (GS) and adoption of atransformed Eulerian mean formalism (GM). Recommendations are based upon a two-dimensional (zonally orazimuthally averaged) model with parameterized horizontal and vertical fluxes that is compared to three-dimensional numerical calculations in which the eddy transfer is resolved. Three different scenarios are considered: 1) a convective “chimney” where the baroclinic zone is created by differential surface cooling; 2) spindownof a frontal zone due to baroclinic eddies; and 3) a wind-driven, baroclinically unstable channel. Guided bybaroclinic instability theory and calibrated against eddy-resolving calculations, the authors recommend a formfor the horizontal transfer coefficient given by where Ri = f2N2/M4 is the large-scale Richardson number and f is the Coriolis parameter; M2 and N2 are measuresof the horizontal and vertical stratification of the large-scale flow, l measures the width of the baroclinic zone,and α is a constant of proportionality. In the very different scenarios studied here the authors find α to be a“universal” constant equal to 0.015, not dissimilar to that found by Green for geostrophic eddies in the atmosphere. The magnitude of the implied k, however, varies from 300 m2 s−1 in the chimney to 2000 m2 s−1 inthe wind-driven channel.
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  • 9
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    American Meteorological Society
    In:  Journal of Atmospheric and Oceanic Technology, 16 . pp. 133-145.
    Publication Date: 2018-07-04
    Description: The reliability of the Comprehensive Ocean–Atmosphere Dataset (COADS) Release 1a 2° monthly winds is tested by comparing it with instrumental measurements in the northwest Atlantic from 1981 to 1991. The instrumental dataset contains anemometer measurements of a very high homogeneity and quality, which were taken by six research sister ships with known anemometer heights in the northwest Atlantic. Special data processing was made with instrumental samples to provide compatibility with the COADS winds. Comparison shows overestimation of the COADS winds in the low ranges and underestimation of the strong and moderate winds. Application of the alternative equivalent Beaufort scales does not remove this bias and makes it even more pronounced. Thus, the conclusion is made that the disagreement obtained results primarily from the uncertainties of anemometer measurements in COADS, especially from the incorrect evaluation of the true wind. Instrumental data also do not indicate significant long-term interannual changes, which are pronounced in the COADS dataset for the 1980s. Some regional features of the comparison are discussed.
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  • 10
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    American Meteorological Society
    In:  Bulletin of the American Meteorological Society, 79 (10). pp. 2033-2058.
    Publication Date: 2016-09-07
    Description: In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.
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  • 11
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    American Meteorological Society
    In:  Journal of Climate, 10 . pp. 2743-2763.
    Publication Date: 2017-07-20
    Description: Differences between “classical” and “sampling” estimates of mean climatological heat fluxes and their seasonal and interannual variability are considered on the basis of individual marine observations from the Comprehensive Ocean–Atmosphere Data Set. Calculations of fluxes were done for intramonthly averaging and for 1°–5° spatial averaging. Sampling estimates give in general 10% to 60% higher values of fluxes than do classical estimates. Spatial averaging has a larger effect than temporal averaging in the Tropics and subtropics, and temporal averaging is more effective than spatial averaging in midlatitudes. The largest absolute differences between sampling and classical estimates of fluxes are observed in middle latitudes, where they are 15 to 20 W m−2 for sensible heat flux and 50 to 70 W m−2 for latent heat flux. Differences between sampling and classical estimates can change the annual cycle of sea–air fluxes. There is a secular tendency of increasing “sampling- to-classical” ratios of 1% to 5% decade−1 over the North Atlantic. Relationships between sampling-to-classical ratios and parameters of the sea–air interface, the number of observations, and the spatial arrangement of samples are considered. Climatologically significant differences between sampling and classical estimates are analyzed in terms of the contribution from different covariances between individual variables. The influence of different parameterizations of the transfer coefficients on sampling minus classical differences is considered. Parameterizations that indicate growing transfer coefficients with wind speed give the larger sampling minus classical differences in comparison with those based on either constant or decreasing with wind coefficients. Nevertheless, over the North Atlantic midlatitudes, all parameterizations indicate significant sampling minus classical differences of about several tens of watts per square meter. The importance of differences between sampling and classical estimates for the evaluation of meridional heat transport shows that differences between sampling and classical estimates can lead to 0.5–1-PW differences in meridional heat transport estimates.
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  • 12
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 27 . pp. 1894-1902.
    Publication Date: 2018-04-06
    Description: The relative importance of the formation of different North Atlantic Deep Water masses on the meridional overturning is examined with a non-eddy-resolving version of the CME model. In contrast to a frequently held belief, convective deep-water formation south of the North Atlantic sill does not significantly force the large-scale overturning if an adequate overflow across the sill can be represented by the model. The sensitivity of the meridional transport to the surface thermohaline forcing is increased under alternate climatic conditions such as increased surface cooling or reduced overflow compared to the present-day situation. The results indicate that climate models may be too sensitive to decadal timescale variability of the surface forcing in subpolar regions.
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  • 13
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    American Meteorological Society
    In:  Journal of Atmospheric and Oceanic Technology, 27 (9). pp. 1533-1546.
    Publication Date: 2018-07-04
    Description: The eddy correlation technique is rapidly becoming an established method for resolving dissolved oxygen fluxes in natural aquatic systems. This direct and noninvasive determination of oxygen fluxes close to the sediment by simultaneously measuring the velocity and the dissolved oxygen fluctuations has considerable advantages compared to traditional methods. This paper describes the measurement principle and analyzes the spatial and temporal scales of those fluctuations as a function of turbulence levels. The magnitudes and spectral structure of the expected fluctuations provide the required sensor specifications and define practical boundary conditions for the eddy correlation instrumentation and its deployment. In addition, data analysis and spectral corrections are proposed for the usual nonideal conditions, such as the time shift between the sensor pair and the limited frequency response of the oxygen sensor. The consistency of the eddy correlation measurements in a riverine reservoir has been confirmed—observing a night–day transition from oxygen respiration to net oxygen production, ranging from −20 to +5 mmol m−2 day−1—by comparing two physically independent, eddy correlation instruments deployed side by side. The natural variability of the fluctuations calls for at least ∼1 h of flux data record to achieve a relative accuracy of better than ∼20%. Although various aspects still need improvement, eddy correlation is seen as a promising and soon-to-be widely applied method in natural waters.
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  • 14
    Publication Date: 2017-08-24
    Description: Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.
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  • 15
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    American Meteorological Society
    In:  Journal of Climate, 12 (8). pp. 2607-2624.
    Publication Date: 2018-07-24
    Description: The predictability of the coupled ocean–atmosphere climate system on interannual to decadal timescales has been studied by means of ensemble forecast experiments with a global coupled ocean–atmosphere general circulation model. Over most parts of the globe the model’s predictability can be sufficiently explained by damped persistence as expected from the stochastic climate model concept with damping times of considerably less than a year. Nevertheless, the tropical Pacific and the North Atlantic Ocean exhibit oscillatory coupled ocean–atmosphere modes, which lead to longer predictability timescales. While the tropical mode shares many similarities with the observed ENSO phenomenon, the coupled mode within the North Atlantic region exhibits a typical period of about 30 yr and relies on an interaction of the oceanic thermohaline circulation and the atmospheric North Atlantic oscillation. The model’s ENSO-like oscillation is predictable up to one-third to one-half (2–3 yr) of the oscillation period both in the ocean and the atmosphere. The North Atlantic yields considerably longer predictability timescales (of the order of a decade) only for quantities describing the model’s thermohaline circulation. For surface quantities and atmospheric variables only marginal predictability (of the order of a year) was obtained. The predictability of the coupled signal at the surface is destroyed by the large amount of internally generated (weather) noise. This is illustrated by means of a simple conceptual model for coupled ocean–atmosphere variability and predictability.
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  • 16
    Publication Date: 2018-07-24
    Description: The role of anomalous Indian Ocean sea surface temperature (SST) in forcing east African rainfall anomalies during December–January 1997/98 has been investigated by means of atmospheric model response experiments. It is shown that the strong precipitation anomalies that led to severe flooding over eastern equatorial Africa can be directly related to the contemporaneous changes in the Indian Ocean’s SST. The authors’ set of ensemble experiments prescribing SST anomalies in different ocean basins indicates further that the El Niño–related SST anomalies in the equatorial Pacific did not directly drive the changes in the climate over eastern Africa.
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  • 17
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    American Meteorological Society
    In:  Journal of Climate, 11 (4). pp. 602-624.
    Publication Date: 2018-07-24
    Description: The interdecadal variability as simulated by coupled ocean–atmosphere models is reviewed. Emphasis is given to that class of interdecadal variability that arises from ocean–atmosphere interactions. The interdecadal variability simulated can be classified roughly into four classes: tropical interdecadal variability, interdecadal variability that involves both the Tropics and the extratropics as active regions, midlatitudinal interdecadal variability involving the wind-driven ocean gyres, and midlatitudinal interdecadal variability involving the thermohaline circulation. Several coupled models predict the existence of different interdecadal climate cycles, with periods ranging from approximately 10–50 yr. This implies some inherent predictability at decadal timescales, provided that these interdecadal cycles exist in the real climate system.
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  • 18
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    American Meteorological Society
    In:  Journal of Climate, 11 (5). pp. 831-847.
    Publication Date: 2018-07-24
    Description: In this paper a decadal climate cycle in the North Atlantic that was derived from an extended-range integration with a coupled ocean–atmosphere general circulation model is described. The decadal mode shares many features with the observed decadal variability in the North Atlantic. The period of the simulated oscillation, however, is somewhat longer than that estimated from observations. While the observations indicate a period of about 12 yr, the coupled model simulation yields a period of about 17 yr. The cyclic nature of the decadal variability implies some inherent predictability at these timescales. The decadal mode is based on unstable air–sea interactions and must be therefore regarded as an inherently coupled mode. It involves the subtropical gyre and the North Atlantic oscillation. The memory of the coupled system, however, resides in the ocean and is related to horizontal advection and to the oceanic adjustment to low-frequency wind stress curl variations. In particular, it is found that variations in the intensity of the Gulf Stream and its extension are crucial to the oscillation. Although differing in details, the North Atlantic decadal mode and the North Pacific mode described by M. Latif and T. P. Barnett are based on the same fundamental mechanism: a feedback loop between the wind driven subtropical gyre and the extratropical atmospheric circulation.
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  • 19
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    American Meteorological Society
    In:  Journal of Climate, 10 (9). pp. 2221-2239.
    Publication Date: 2018-07-24
    Description: The dominant variability modes in the Tropics are investigated and contrasted with the anomalous situation observed during the last few years. The prime quantity analyzed is anomalous sea surface temperature (SST) in the region 30°S–60°N. Additionally, observed tropical surface wind stress fields were investigated. Further tropical atmospheric information was derived from a multidecadal run with an atmospheric general circulation model that was forced by the same SSTs. The tropical SST variability can be characterized by three modes: an interannual mode [the El Niño–Southern Oscillation (ENSO)], a decadal mode, and a trend or unresolved ultra-low-frequency variability. The dominant mode of SST variability is the ENSO mode. It is strongest in the eastern equatorial Pacific, but influences also the SSTs in other regions through atmospheric teleconnections, such as the Indian and North Pacific Oceans. The ENSO mode was strong during the 1980s, but it existed with very weak amplitude and short period after 1991. The second most energetic mode is characterized by considerable decadal variability. This decadal mode is connected with SST anomalies of the same sign in all three tropical oceans. The tropical Pacific signature of the decadal mode resembles closely that observed during the last few years and can be characterized by a horseshoe pattern, with strongest SST anomalies in the western equatorial Pacific, extending to the northeast and southeast into the subtropics. It is distinct from the ENSO mode, since it is not connected with any significant SST anomalies in the eastern equatorial Pacific, which is the ENSO key region. However, the impact of the decadal mode on the tropical climate resembles in many respects that of ENSO. In particular, the decadal mode is strongly linked to decadal rainfall fluctuations over northeastern Australia in the observations. It is shown that the anomalous 1990s were dominated by the decadal mode. Considerable SST variability can be attributed also to a linear trend or unresolved ultra-low-frequency variability. This trend that might be related to greenhouse warming is rather strong and positive in the Indian Ocean and western equatorial Pacific where it accounts for up to 30% of the total SST variability. Consistent with the increase of SST in the warm pool region, the trends over the tropical Pacific derived from both the observations and the model indicate a strengthening of the trade winds. This is inconsistent with the conditions observed during the 1990s. If the wind trends reflect greenhouse warming, it must be concluded that the anomalous 1990s are not caused by greenhouse warming. Finally, hybrid coupled ocean–atmosphere model experiments were conducted in order to investigate the sensistivity of ENSO to the low-frequency changes induced by the decadal mode and the trend. The results indicate that ENSO is rather sensitive to these changes in the background conditions.
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  • 20
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    American Meteorological Society
    In:  Journal of Climate, 10 (7). pp. 1488-1504.
    Publication Date: 2018-07-24
    Description: The El Niño–Southern Oscillation (ENSO) phenomenon is modeled as a stochastically driven dynamical system. This was accomplished by adding to a Hybrid Coupled Model (HCM) of the tropical Pacific ocean–atmosphere system a stochastic wind stress anomaly field that was derived from observations. The model exhibits irregular interannual fluctuations, whose space–time characteristics resemble those of the observed interannual climate variability in this region. To investigate the predictability of the model, the authors performed ensemble integrations with different realizations of the stochastic wind stress forcing. The ensembles were initialized at various phases of the model’s ENSO cycle simulated in a 120-yr integration with a particular noise realization. The numerical experiments indicate that the ENSO predictability is severely limited by the stochastic wind stress forcing. Linear stochastic processes were fitted to the restart ensembles in a reduced state space. A predictability measure based on a comparison of the stationary and the time-dependent probability distributions of the fitted linear models reveals an ENSO predictability limit of considerably less than an average cycle length.
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  • 21
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    American Meteorological Society
    In:  Journal of Climate, 9 (10). pp. 2407-2423.
    Publication Date: 2018-07-23
    Description: The dynamics and predictability of decadal climate variability over the North Pacific and North America are investigated by analyzing various observational datasets and the output of a state of the art coupled ocean–atmosphere general circulation model that was integrated for 125 years. Both the observations and model results support the picture that the decadal variability in the region of interest is based on a cycle involving unstable ocean–atmosphere interactions over the North Pacific. The period of this cycle is of the order of a few decades. The cycle involves the two major circulation regimes in the North Pacific climate system, the subtropical ocean gyre, and the Aleutian low. When, for instance, the subtropical ocean gyre is anomalously strong, more warm tropical waters are transported poleward by the Kuroshio and its extension, leading to a positive SST anomaly in the North Pacific. The atmospheric response to this SST anomaly involves a weakened Aleutian low, and the associated fluxes at the air–sea interface reinforce the initial SST anomaly, so that ocean and atmosphere act as a positive feedback system. The anomalous heat flux, reduced ocean mixing in response to a weakened storm track, and anonmalous Ekman heat transport contribute to this positive feedback. The atmospheric response, however, consists also of a wind stress curl anomaly that spins down the subtropical ocean gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The ocean adjusts with some time lag to the change in the wind stress curl, and it is this transient ocean response that allows continuous oscillations. The transient response can be expressed in terms of baroclinic planetary waves, and the decadal timescale of the oscillation is therefore determined to first order by wave timescales. Advection by the mean currents, however, is not negligible. The existence of such a cycle provides the basis of long-range climate forecasting over North America at decadal timescales. At a minimum, knowledge of the present phase of the decadal mode should allow a “now-cast” of expected climate “bias” over North America, which is equivalent to a climate forecast several years ahead.
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  • 22
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    American Meteorological Society
    In:  Journal of Climate, 9 (1). pp. 219-239.
    Publication Date: 2018-07-23
    Description: The physics of the Indo–Pacific warm pool are investigated using a coupled ocean atmosphere general circulation model. The model, developed at the Max-Planck-Institut fair Meteorologic, Hamburg, does not employ a flux correction and is used with atmospheres at T42 and T21 resolution. The simulations are compared with observations, and the model's mean and seasonal heat budgets and physics in the Indo–Pacific warm pool region are explored for the T42 resolution run. Despite the simulation of a split intertropical convergence zone, and of a cold tongue that extends too far to the west, simulated warm pool temperatures are consistent with observations at T42 resolution, while the T21 resolution yields a cold bias of 1K. At T42 resolution the seasonal migration of the warm pool is reproduced reasonably well, as are the surface heat fluxes, winds, and clouds. However, simulated precipitation is too small compared to observations, implying that the surface density flux is dominated by fluxes of heat. In the Pacific portion of the warm pool, the average net heat gain of the ocean amounts to 30–40 W m−2. In the northern branch, this heat gain is balanced by vertical advection, while in the southern branch, zonal, meridional, and vertical advection cool the ocean at approximately equal rates. At the equator, the surface heat flux is balanced by zonal and vertical advection and vertical mixing. The Indonesian and Indian Ocean portions of the warm pool receive from the atmosphere 30 and 50 W m−2, respectively, and this flux is balanced by vertical advection. The cooling due to vertical advection stems from numerical diffusion associated with the upstream scheme, the coarse vertical resolution of the ocean model, and near-inertial oscillations forced by high-frequency atmospheric variability. The seasonal migration of the warm pool is largely a result of the seasonal variability of the net surface heat flux, horizontal and vertical advections are of secondary importance and increase the seasonal range of surface temperature slightly everywhere in the warm pool, with the exception of its southern branch. There, advection reduces the effect of the surface flux. The seasonal variability of the surface heat flux in turn is mainly determined by the shortwave radiation, but evaporation modifies the signal significantly. The annual cycles of reduction of solar radiation due to clouds and SST evolve independently from each other in the Pacific portion of the warm pool; that is, clouds have little impact on SST. In the Indian Ocean, however, clouds limit the maximum SST attained during the annual cycle. In the western Pacific and Indonesian portion of the warm pool, penetrative shortwave radiation leads to convective mixing by heating deeper levels at a greater rate than the surface, which experiences heat losses due to turbulent and longwave heat fluxes. In the deeper levels, there is no mechanism to balance the heating due to penetrative radiation, except convection and its attendant mixing. In the Indian Ocean, however. the resulting vertical heating profile due to the surface fluxes decreases monotonically with depth and does not support convective mixing. Concurrently, the warm pool is shallower in the Indian Ocean compared with the western Pacific, indicating that convective mixing due to penetrative radiation is important in maintaining the vertical structure of the Pacific portion of the warm pool.
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  • 23
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    American Meteorological Society
    In:  Journal of Climate, 8 (4). pp. 952-964.
    Publication Date: 2018-07-23
    Description: The authors have investigated the interactions of the tropical oceans on interannual timescales by conducting a series of uncoupled atmospheric and oceanic general circulation experiments and hybrid-coupled model simulations. The results illustrate the key role of the El Niño/Southern Oscillation phenomenon in generating interannual variability in all three tropical ocean basins. Sea surface temperature anomalies in the tropical Pacific force SST anomalies of the same sign in the Indian Ocean and SST anomalies of the opposite sign in the Atlantic via a changed atmospheric circulation. However, although air-sea interactions in the Indian and Atlantic Oceans are much weaker than those in the Pacific, they contribute significantly to the variability in these two regions. The role of these air-sea interactions is mainly that of an amplifier by which the ENSO-induced signals are enhanced in the ocean and atmosphere. This process is particularly important in the tropical Atlantic region. The authors investigated, also, whether ENSO is part of a zonally propagating “wave,” which travels around the globe with a timescale of several years. Consistent with observations, the upper-ocean heat content in the various numerical simulators seems to propagate slowly around the globe. SST anomalies in the Pacific Ocean introduce a global atmospheric response, which in turn forces variations in the other tropical oceans. Since the different oceans exhibit different response characteristics to low-frequency wind changes, the individual tropical ocean responses can add up coincidentally to look like a global wave, and that appears to be the situation. In particular, no evidence is found that the Indian Ocean can significantly affect the ENSO cycle in the Pacific. Finally, the potential for climate forecasts in the Indian and Atlantic Oceans appears to be enhanced if one includes, in a coupled way, remote influences from the Pacific.
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  • 24
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    American Meteorological Society
    In:  Journal of Climate, 25 (6). pp. 1827-1846.
    Publication Date: 2017-08-25
    Description: Initial-value predictability measures the degree to which the initial state can influence predictions. In this paper, the initial-value predictability of six atmosphere–ocean general circulation models in the North Pacific and North Atlantic is quantified and contrasted by analyzing long control integrations with time invariant external conditions. Through the application of analog and multivariate linear regression methodologies, average predictability properties are estimated for forecasts initiated from every state on the control trajectories. For basinwide measures of predictability, the influence of the initial state tends to last for roughly a decade in both basins, but this limit varies widely among the models, especially in the North Atlantic. Within each basin, predictability varies regionally by as much as a factor of 10 for a given model, and the locations of highest predictability are different for each model. Model-to-model variations in predictability are also seen in the behavior of prominent intrinsic basin modes. Predictability is primarily determined by the mean of forecast distributions rather than the spread about the mean. Horizontal propagation plays a large role in the evolution of these signals and is therefore a key factor in differentiating the predictability of the various models.
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  • 25
    Publication Date: 2017-08-24
    Description: Continuous estimates of the oceanic meridional heat transport in the Atlantic are derived from the Rapid Climate Change–Meridional Overturning Circulation (MOC) and Heatflux Array (RAPID–MOCHA) observing system deployed along 26.5°N, for the period from April 2004 to October 2007. The basinwide meridional heat transport (MHT) is derived by combining temperature transports (relative to a common reference) from 1) the Gulf Stream in the Straits of Florida; 2) the western boundary region offshore of Abaco, Bahamas; 3) the Ekman layer [derived from Quick Scatterometer (QuikSCAT) wind stresses]; and 4) the interior ocean monitored by “endpoint” dynamic height moorings. The interior eddy heat transport arising from spatial covariance of the velocity and temperature fields is estimated independently from repeat hydrographic and expendable bathythermograph (XBT) sections and can also be approximated by the array. The results for the 3.5 yr of data thus far available show a mean MHT of 1.33 ± 0.40 PW for 10-day-averaged estimates, on which time scale a basinwide mass balance can be reasonably assumed. The associated MOC strength and variability is 18.5 ± 4.9 Sv (1 Sv ≡ 106 m3 s−1). The continuous heat transport estimates range from a minimum of 0.2 to a maximum of 2.5 PW, with approximately half of the variance caused by Ekman transport changes and half caused by changes in the geostrophic circulation. The data suggest a seasonal cycle of the MHT with a maximum in summer (July–September) and minimum in late winter (March–April), with an annual range of 0.6 PW. A breakdown of the MHT into “overturning” and “gyre” components shows that the overturning component carries 88% of the total heat transport. The overall uncertainty of the annual mean MHT for the 3.5-yr record is 0.14 PW or about 10% of the mean value.
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  • 26
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 42 (5). pp. 824-839.
    Publication Date: 2018-04-12
    Description: The mechanisms involved in setting the annual cycle of the Florida Current transport are revisited using an adjoint model approach. Adjoint sensitivities of the Florida Current transport to wind stress reproduce a realistic seasonal cycle with an amplitude of ~1.2 Sv (1 Sv ≡ 106 m3 s−1). The annual cycle is predominantly determined by wind stress forcing and related coastal upwelling (downwelling) north of the Florida Strait along the shelf off the North American coast. Fast barotropic waves propagate these anomalies southward and reach the Florida Strait within a month, causing an amplitude of ~1 Sv. Long baroclinic planetary Rossby waves originating from the interior are responsible for an amplitude of ~0.8 Sv but have a different phase. The sensitivities corresponding to the first baroclinic mode propagate westward and are highly influenced by topography. Considerable sensitivities are only found west of the Mid-Atlantic Ridge, with maximum values at the western shelf edge. The second baroclinic mode also has an impact on the Florida Current variability, but only when a mean flow is present. A second-mode wave train propagates southwestward from the ocean bottom on the western side of the Mid-Atlantic Ridge between ~36° and 46°N and at Flemish Cap, where the mean flow interacts with topography, to the surface. Other processes such as baroclinic waves along the shelf and local forcing within the Florida Strait are of minor importance.
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  • 27
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    American Meteorological Society
    In:  Journal of Climate, 26 (6). pp. 2137-2143.
    Publication Date: 2017-08-25
    Description: Quantile mapping is routinely applied to correct biases of regional climate model simulations compared to observational data. If the observations are of similar resolution as the regional climate model, quantile mapping is a feasible approach. However, if the observations are of much higher resolution, quantile mapping also attempts to bridge this scale mismatch. Here, it is shown for daily precipitation that such quantile mapping-based downscaling is not feasible but introduces similar problems as inflation of perfect prognosis ("prog") downscaling: the spatial and temporal structure of the corrected time series is misrepresented, the drizzle effect for area means is overcorrected, area-mean extremes are overestimated, and trends are affected. To overcome these problems, stochastic bias correction is required.
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  • 28
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 70 (7). pp. 2103-2118.
    Publication Date: 2018-04-16
    Description: The wintertime northern annular mode (NAM) at the surface is known to undergo slow intraseasonal variations in association with stratospheric variability, which leads the surface signal by up to several weeks. The relative contributions, however, of potentially relevant stratosphere–troposphere coupling mechanisms are not yet fully understood. In this study the relative roles of (i) the downward effect of the zonal-mean secondary circulation induced by quasigeostrophic (QG) adjustment to stratospheric wave drag and radiative damping and (ii) wave drag local to the troposphere are estimated. For this purpose, a spectral tendency equation of the QG zonal-mean zonal wind is derived and used, in a first step, to obtain the external mechanical forcing that, in the QG framework, drives exactly the observed stratospheric and tropospheric daily NAM. In a second step, the equation is then integrated in time to reconstruct the daily NAM, but with the forcing restricted to either stratospheric or tropospheric levels, each case leaving a characteristic NAM surface signal. The relative roles of the above-mentioned mechanisms are found to be of similar quantitative importance, but to differ in a qualitative sense. The downward effect of stratospheric QG adjustment is responsible for the initiation of the NAM surface signal, whereas subsequently local tropospheric wave drag actively maintains and persists the signal over several weeks. Furthermore, the downward effect of QG adjustment to stratospheric radiative damping is shown to have only a minor impact, compared to that from stratospheric wave drag. The robustness of these conclusions is demonstrated by a sensitivity study with respect to various model parameters.
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  • 29
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 44 (2). pp. 482-491.
    Publication Date: 2018-04-12
    Description: Sensible and latent heat fluxes were estimated from turbulence measurements gathered during several Atlantic transects of the R/V Polarstern. The inertial dissipation method was used to analyze the data. Resulting bulk transfer coefficients were then applied to the data from the ship’s meteorological system to get continuous time series of the heat fluxes. Combined to the measured downward solar and longwave radiation fluxes allows for an estimate of the total energy budget at the air-sea interface. Comparing these parameterized energy fluxes to ones based on the COARE 3.0 bulk flux algorithm show very strong agreement.
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  • 30
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 71 (6). pp. 2264-2279.
    Publication Date: 2018-04-16
    Description: The dynamical origin of the spectral and autocorrelation structure of annular variability in the troposphere is investigated by a deductive approach. Specifically, the structure of the power spectrum and autocorrelation function of the zonal-mean geopotential is analyzed for the case of a quasigeostrophic spherical atmosphere subject to a white noise mechanical forcing applied in a single Hough mode and concentrated at a particular level in the vertical, with vertically uniform Newtonian cooling and Rayleigh drag concentrated at a rigid lower boundary. Analytic expressions for the power spectrum are presented together with expressions for an approximate red noise (i.e., a Lorentzian-shaped) power spectrum. It is found that for an infinitely deep atmosphere the power spectrum can be well approximated by a red noise process for the first few Hough modes (associated with large Rossby heights), provided the distance from the forcing is not larger than about one Rossby height. When a frictional rigid lower boundary is included, however, the approximation is generally bad. The high-frequency part of the power spectrum exhibits near-exponential behavior and the autocorrelation function shows a transition from a rapid decay at short lags to a much slower decay at longer lags, if the thermal and mechanical damping time scales are sufficiently well separated. Since observed annular variability exhibits the same characteristics, the above results lead to the hypothesis that these characteristics may, to some extent, be intrinsic to the linear zonal-mean response problem—although the need for an additional contribution from eddy feedbacks is also implied by the results.
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  • 31
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    American Meteorological Society
    In:  Bulletin of the American Meteorological Society, 64 (9). ES123-ES126.
    Publication Date: 2015-01-28
    Description: European Polar Low Working Group This workshop summarized the current state of PL research in the Arctic and Antarctic. A couple of related projects are in the planning phase or already funded. The creation of a PL database for the Norwegian Sea in the frame of the Sea Surface Temperature and Altimeter Synergy (STARS) project (http://projects.met.no stars) will provide a valuable resource for future research and, potentially, predictability improvements. The maintenance of this database and the creation of similar databases for other polar areas including satellite and NWP data are highly recommended. There is also a need for free and timely access to satellite data, in particular to SAR data to fill the gap caused by the mission end of Envisat. With the increasing resolution of climate models, mesoscale processes such as polar MCs will have to be considered in international research programs such as the World Climate Research Programme (WCRP) Polar Climate Predictability Initiative and the World Weather Research Programme (WWRP) Polar Predictability Project.
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  • 32
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    American Meteorological Society
    In:  Journal of Climate, 27 (21). pp. 8135-8150.
    Publication Date: 2017-08-25
    Description: Sea surface temperature (SST) anomalies in the eastern equatorial Atlantic are connected to modulations in the strength of the South Atlantic subtropical high-pressure system, referred to as the South Atlantic Anticyclone (SAA). Using ocean and atmosphere reanalysis products we show here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic (EEA) via anomalous tropical wind power. This modulation of the timing and amplitude of the seasonal cold tongue development manifests as anomalous SST events peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA induced wind power variations south of the equator influence EEA SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variations
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  • 33
    Publication Date: 2017-08-24
    Description: Historical hydrographic data (1940s–2010) show a distinct cross-slope difference of the lower halocline water (LHW) over the Laptev Sea continental margins. Over the slope, the LHW is on average warmer and saltier by 0.2°C and 0.5 psu, respectively, relative to the off-slope LHW. The LHW temperature time series constructed from the on-slope historical records are related to the temperature of the Atlantic Water (AW) boundary current transporting warm water from the North Atlantic Ocean. In contrast, the on-slope LHW salinity is linked to the sea ice and wind forcing over the potential upstream source region in the Barents and northern Kara Seas, as also indicated by hydrodynamic model results. Over the Laptev Sea continental margin, saltier LHW favors weaker salinity stratification that, in turn, contributes to enhanced vertical mixing with underlying AW.
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  • 34
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 71 (12). pp. 4611-4620.
    Publication Date: 2017-10-24
    Description: Southern Hemisphere (SH) stratospheric variability is investigated with respect to chaotic behavior using time series from three different variables extracted from four different reanalysis products. The results are compared with the same analysis applied to the Northern Hemisphere (NH). The probability density functions (PDFs) for the SH show persistent deviations from a Gaussian distribution. The variability is given by white spectra for low frequencies, a slope of −1 for intermediate frequencies, and −3 slopes for high frequencies. Considering the time series for winter and summer separately, PDFs show a Gaussian distribution and the variability spectra change their slopes, indicating the role of the transition between winter and summer variability in shaping the time series. The correlation (D2) and the Kaplan–Yorke (DKY) dimensions are estimated. A finite value of the dimensions can be computed for each variable and data product, except for the NCEP zonal-mean zonal wind and temperature data, which violate the requirement D2 ≤ DKY, possibly owing to the presence of spurious trends and inconsistencies in the data. The value of D2 ranges between 2.6 and 3.9, while DKY ranges between 3.0 and 4.5. The results show that both D2 and DKY display large variability in their values both for different datasets and for different variables within the same dataset. The variability of the values of D2 and DKY thus leaves open the question about the existence of a low-dimensional attractor or if the finite dimensions of the system are the result of the projection of a larger attractor in a low-dimensional embedding space.
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  • 35
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 44 (1). pp. 202-219.
    Publication Date: 2015-07-24
    Description: The Arctic continental shelf seas hold a globally significant source of freshwater that impacts Arctic Ocean stratification, circulation, and climate. This freshwater can be injected below the surface mixed layer by intense turbulent kinetic energy dissipation events, as resolved by Laptev Sea microstructure observations. The tides provide a major source of energy that can be dissipated and hence drive diapycnal mixing in the Laptev Sea. Multiyear ADCP mooring records from locations across the shelf reveal that semidiurnal tides are dominated by theM2 and S2 constituents, with the largest amplitudes on the outer shelf. Throughoutmost of the shelf, tides are clockwise polarized and sheared by stratification, as characteristic near the M2 critical latitude. Interannual variations of the tidal and shear structures on the inner shelf aremainly determined by the stratification-setting Lena River freshwater plume. In all locations,M2 tides are enhanced under sea ice, and therefore changes in the seasonal ice cover may lead to changes in tides and water column structure. The main conclusions of this study are that (i) tides play a comparatively greater role year-round on the outer shelf relative to the inner shelf; (ii) a sea ice reduction will overall decrease the predictability of the currents, especially on the inner shelf; and (iii) the freshwater distribution directly impacts diapycnal mixing by setting the vertical tidal structure. These combined effects imply that future sea ice loss will increase the variability and vertical mixing of freshwater, particularly on the inner shelf, where the Lena River first enters the Laptev Sea.
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  • 36
    Publication Date: 2015-11-24
    Description: A surface diurnal warm layer is diagnosed from Seaglider observations, and develops on half the days in the CINDY/DYNAMO Indian Ocean experiment. The diurnal warm layer occurs on days of high solar radiation flux (〉 80 W m−2) and low wind speed (〈 6 m s−1), and preferentially in the inactive stage of the Madden–Julian Oscillation. Its diurnal harmonic has an exponential vertical structure with a depth scale of 4–5 m (dependent on chlorophyll concentration), consistent with forcing by absorption of solar radiation. The effective sea surface temperature (SST) anomaly due to the diurnal warm layer often reaches 0.8°C in the afternoon, with a daily mean of 0.2°C, rectifying the diurnal cycle onto longer time scales. This SST anomaly drives an anomalous flux of 4 W m−2 that cools the ocean. Alternatively, in a climate model where this process is unresolved, this represents an erroneous flux that warms the ocean. A simple model predicts a diurnal warm layer to occur on 30–50% of days across the tropical warm pool. On the remaining days, with low solar radiation and high wind speeds, a residual diurnal cycle is observed by the Seaglider, with a diurnal harmonic of temperature that decreases linearly with depth. As wind speed increases, this already weak temperature gradient decreases further, tending towards isothermal conditions.
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  • 37
    Publication Date: 2017-08-25
    Description: Precipitation is highly variable in space and time; hence, rain gauge time series generally exhibit additional random small-scale variability compared to area averages. Therefore, differences between daily precipitation statistics simulated by climate models and gauge observations are generally not only caused by model biases, but also by the corresponding scale gap. Classical bias correction methods, in general, cannot bridge this gap; they do not account for small-scale random variability and may produce artifacts. Here, stochastic model output statistics is proposed as a bias correction framework to explicitly account for random small-scale variability. Daily precipitation simulated by a regional climate model (RCM) is employed to predict the probability distribution of local precipitation. The pairwise correspondence between predictor and predictand required for calibration is ensured by driving the RCM with perfect boundary conditions. Wet day probabilities are described by a logistic regression, and precipitation intensities are described by a mixture model consisting of a gamma distribution for moderate precipitation and a generalized Pareto distribution for extremes. The dependence of the model parameters on simulated precipitation is modeled by a vector generalized linear model. The proposed model effectively corrects systematic biases and correctly represents local-scale random variability for most gauges. Additionally, a simplified model is considered that disregards the separate tail model. This computationally efficient model proves to be a feasible alternative for precipitation up to moderately extreme intensities. The approach sets a new framework for bias correction that combines the advantages of weather generators and RCMs.
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  • 38
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 43 (12). pp. 2611-2628.
    Publication Date: 2017-10-24
    Description: The Denmark Strait Overflow (DSO) supplies about one-third of the North Atlantic Deep Water and is critical to global thermohaline circulation. Knowledge of the pathways of DSO through the Irminger Basin and its transformation there is still incomplete, however. The authors deploy over 10 000 Lagrangian particles at the Denmark Strait in a high-resolution ocean model to study these issues. First, the particle trajectories show that the mean position and potential density of dense waters cascading over the Denmark Strait sill evolve consistently with hydrographic observations. These sill particles transit the Irminger Basin to the Spill Jet section (65.25°N) in 5–7 days and to the Angmagssalik section (63.5°N) in 2–3 weeks. Second, the dense water pathways on the continental shelf are consistent with observations and particles released on the shelf in the strait constitute a significant fraction of the dense water particles recorded at the Angmagssalik section within 60 days (~25%). Some particles circulate on the shelf for several weeks before they spill off the shelf break and join the overflow from the sill. Third, there are two places where the water density following particle trajectories decreases rapidly due to intense mixing: to the southwest of the sill and southwest of the Kangerdlugssuaq Trough on the continental slope. After transformation in these places, the overflow particles exhibit a wide range of densities.
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  • 39
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 44 (9). pp. 2524-2546.
    Publication Date: 2015-05-28
    Description: In this study, the authors discuss two different parameterizations for the effect of mixed layer eddies, one based on ageostrophic linear stability analysis (ALS) and the other one based on a scaling of the potential energy release by eddies (PER). Both parameterizations contradict each other in two aspects. First, they predict different functional relationships between the magnitude of the eddy fluxes and the Richardson number (Ri) related to the background state. Second, they also predict different vertical structure functions for the horizontal eddy fluxes. Numerical simulations for two different configurations and for a large range of different background conditions are used to evaluate the parameterizations. It turns out that PER is better suited to capture the Ri dependency of the magnitude of the eddy fluxes. On the other hand, the vertical structure of the meridional eddy fluxes predicted by ALS is more accurate than that of PER, while the vertical structure of the vertical eddy fluxes is well predicted by both parameterizations. Therefore, this study suggests the use of the magnitude of PER and the vertical structure functions of ALS for an improved parameterization of mixed layer eddy fluxes.
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  • 40
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    American Meteorological Society
    In:  Journal of Climate, 27 (3). pp. 977-993.
    Publication Date: 2019-01-08
    Description: Ammassalik in southeast Greenland is known for strong wind events that can reach hurricane intensity and cause severe destruction in the local town. Yet, these winds and their impact on the nearby fjord and shelf region have not been studied in detail. Here, data from two meteorological stations and the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) are used to identify and characterize these strong downslope wind events, which are especially pronounced at a major east Greenland fjord, Sermilik Fjord, within Ammassalik. Their local and regional characteristics, their dynamics and their impacts on the regional sea ice cover, and air–sea fluxes are described. Based on a composite of the events it is concluded that wind events last for approximately a day, and seven to eight events occur each winter. Downslope wind events are associated with a deep synoptic-scale cyclone between Iceland and Greenland. During the events, cold dry air is advected down the ice sheet. The downslope flow is accelerated by gravitational acceleration, flow convergence inside the Ammassalik valley, and near the coast by an additional thermal and synoptic-scale pressure gradient acceleration. Wind events are associated with a large buoyancy loss over the Irminger Sea, and it is estimated that they drive one-fifth of the net wintertime loss. Also, the extreme winds drive sea ice out of the fjord and away from the shelf.
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  • 41
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 43 (4). pp. 805-823.
    Publication Date: 2018-04-12
    Description: Mesoscale anticyclonic eddies in the Irminger Sea are observed using a mooring and a glider. Between 2002 and 2009, the mooring observed 53 anticyclones. Using a kinematic model, objective estimates of eddy length scales and velocity structure are made for 16 eddies. Anticyclones had a mean core diameter of 12 km, and their mean peak observed azimuthal speed was 0.1 m s(-1). They had core salinities and potential temperatures of 34.91-34.98 and 4.488-5.34 degrees C, respectively, making them warm and salty features. These properties represent a typical salinity anomaly of 0.03 and a temperature anomaly of 0.28 degrees C from noneddy values. All eddies had small (〈〈 1) Rossby numbers. In 2006, the glider observed two anticyclones having diameters of about 20 km and peak azimuthal speeds of about 0.3 m s(-1). Similar salinity anomalies were detected throughout the Irminger Sea by floats profiling in anticyclones. Two formation regions for the eddies are identified: one to the west of the Reykjanes Ridge and the other off the East Greenland Irminger Current near Cape Farewell close to the mooring. Observations indicate that eddies formed in the former region are larger than eddies observed at the mooring. A clear increase in eddy salinity is observed between 2002 and 2009. The observed breakup of these eddies in winter implies that they are a source of salt for the central gyre. The anticyclones are similar to those found in both the Labrador Sea and Norwegian Sea, making them a ubiquitous feature of the subpolar North Atlantic basins.
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  • 42
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 43 (10). pp. 2113-2131.
    Publication Date: 2018-04-12
    Description: The Agulhas Current plays a crucial role in the thermohaline circulation through its leakage into the South Atlantic. Under both past and present climates, the trade winds and westerlies could have the ability to modulate the amount of Indian-Atlantic inflow. Compelling arguments have been put forward suggesting that trade winds alone have little impact on the magnitude of Agulhas leakage. Here, employing three ocean models for robust analysis – a global coarse resolution, a regional eddy-permitting and a nested high-resolution eddy-resolving configuration – and systematically altering the position and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown that the westerlies, in particular their intensity, control the leakage. Leakage responds proportionally to the westerlies intensity up to a certain point. Beyond this, through the adjustment of the large-scale circulation, energetic interactions occur between the Agulhas Return Current and the Antarctic Circumpolar Current that result in a state where leakage no longer increases. This adjustment takes place within 1 to 2 decades. Contrary to previous assertions, our results further show that an equatorward (poleward) shift in westerlies increases (decreases) leakage. This occurs due to the redistribution of momentum input by the winds. It is concluded that the reported present-day leakage increase could therefore reflect an unadjusted oceanic response mainly to the strengthening westerlies over the last few decades.
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  • 43
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 70 (12). pp. 3959-3976.
    Publication Date: 2018-04-16
    Description: Accurate projections of stratospheric ozone are required because ozone changes affect exposure to ultraviolet radiation and tropospheric climate. Unweighted multimodel ensemble-mean (uMMM) projections from chemistry–climate models (CCMs) are commonly used to project ozone in the twenty-first century, when ozone-depleting substances are expected to decline and greenhouse gases are expected to rise. Here, the authors address the question of whether Antarctic total column ozone projections in October given by the uMMM of CCM simulations can be improved by using a process-oriented multiple diagnostic ensemble regression (MDER) method. This method is based on the correlation between simulated future ozone and selected key processes relevant for stratospheric ozone under present-day conditions. The regression model is built using an algorithm that selects those process-oriented diagnostics that explain a significant fraction of the spread in the projected ozone among the CCMs. The regression model with observed diagnostics is then used to predict future ozone and associated uncertainty. The precision of the authors’ method is tested in a pseudoreality; that is, the prediction is validated against an independent CCM projection used to replace unavailable future observations. The tests show that MDER has higher precision than uMMM, suggesting an improvement in the estimate of future Antarctic ozone. The authors’ method projects that Antarctic total ozone will return to 1980 values at around 2055 with the 95% prediction interval ranging from 2035 to 2080. This reduces the range of return dates across the ensemble of CCMs by about a decade and suggests that the earliest simulated return dates are unlikely.
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  • 44
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 44 (2). pp. 445-463.
    Publication Date: 2018-04-12
    Description: Mooring observations and model simulations point to an instability of the Labrador Current (LC) during winter, with enhanced eddy kinetic energy (EKE) at periods between 2 to 5 days, and much less EKE during other seasons. Linear stability analysis using vertical shear and stratification from the model reveals three dominant modes of instability in the LC: - a balanced interior mode with along-flow wavelengths of about 30–45 km, phase velocities of 0.3 m/s, maximal growth rates of 1 d−1 and surface intensified, but deep reaching amplitudes, - a balanced shallow mode with along-flow wavelengths of about 0.3–1.5 km, about three times larger phase speeds and growth rates, but amplitudes confined to the mixed layer (ML), - and an unbalanced symmetric mode with largest growth rates, vanishing phase speeds and along-flow structure, and very small cross-flow wavelengths, also confined to the ML. Both balanced modes are akin to baroclinic instability, but operate at moderate to small Richardson numbers Ri with much larger growth rates as for the quasi-geostrophic limit of Ri ≫ 1. The interior mode is found to be responsible for the instability of the LC during winter. Weak stratification and enhanced vertical shear due to local buoyancy loss and the advection of convective water masses from the interior result in small Ri within the LC, and to three times larger growth rates of the interior mode in March compared to summer and fall conditions. Both the shallow and the symmetric mode are not resolved by the model, but it is suggested that they might also play an important role for the instability in the LC and for lateral mixing.
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  • 45
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    American Meteorological Society
    In:  Journal of Atmospheric and Oceanic Technology, 30 . pp. 2820-2837.
    Publication Date: 2014-07-30
    Description: A large number of quantities have to be measured and processed to determine the atmospheric-state variables, which are the actual measurands, from aircraft-based measurements. A great part of the dependencies between these quantities depends on the aerodynamic state of the aircraft. Aircraft-based meteorological measurements, hence, require in-flight calibration. Most operators of research aircraft perform some kind of calibration, but the schemes used and the degree they are documented greatly vary. The flight maneuvers and calculation methods required, however, are published in a number of partly overlapping and partly contradictory publications. Some methods are only presented as a minor issue in publications mainly focused on atmospheric processes and are therefore hard to find. For an aircraft user, it is hence challenging to either perform or verify a calibration because of missing comprehensive guidance. This lack was stated on occasion of the in-flight calibration of the German research aircraft Polar5 carried out for the field experiment Investigation of Katabatic Winds and Polynyas during Summer (IKAPOS). In the present paper, a comprehensive review of the existing literature on this field and a practical guide to the wind calibration of a research aircraft to be used for turbulent flux measurements are given.
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  • 46
    Publication Date: 2015-04-27
    Description: Vertical mixing in the bottom boundary layer and pycnocline of the Laptev Sea is evaluated from a rapidly sampled 12-h time series of microstructure temperature, conductivity, and shear observations collected under 100% sea ice during October 2008. The bottom boundary turbulent kinetic energy dissipation was observed to be enhanced (ϵ ∼ 10−4 W m−3) beyond background levels (ϵ ∼ 10−6 W m−3), extending up to 10 m above the seabed when simulated tidal currents were directed on slope. Upward heat fluxes into the halocline-class waters along the Laptev Sea seabed peaked at ∼4–8 W m−2, averaging out to ∼2 W m−2 over the 12-h sampling period. In the Laptev Sea pycnocline, an isolated 2-h episode of intense dissipation (ϵ ∼ 10−3 W m−3) and vertical diffusivities was observed that was not due to a localized wind event. Observations from an acoustic Doppler current meter moored in the central Laptev Sea near the M2 critical latitude are consistent with a previous model in which mixing episodes are driven by an enhancement of the pycnocline shear resulting from the alignment of the rotating pycnocline shear vector with the under-ice stress vector. Upward cross-pycnocline heat fluxes from the Arctic halocline peaked at ∼54 W m−2, resulting in a 12-h average of ∼12 W m−2. These results highlight the intermittent nature of Arctic shelf sea mixing processes and how these processes can impact the transformation of Arctic Ocean water masses. The observations also clearly demonstrate that absence or presence of sea ice profoundly affects the availability of near-inertial kinetic energy to drive vertical mixing on the Arctic shelves.
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  • 47
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 71 (7). pp. 2674-2694.
    Publication Date: 2017-10-24
    Description: The sensitivities of the Brewer–Dobson circulation (BDC) to different distributions of tropical SST heating are investigated in an idealized aquaplanet model. It is found that an increase in tropical SSTs generally leads to an acceleration of tropical upwelling and an associated reduction in the age of air (AOA) in the polar stratosphere and that the AOA near the subtropical tropopause is correlated with local isentropic mixing of tropospheric air with stratospheric air. The zonal distribution of SST perturbations has a major impact on the vertical and meridional structure of the BDC as compared with other SST characteristics. Zonally localized SST heatings tend to generate a shallow acceleration of the stratospheric residual circulation, enhanced isentropic mixing associated with a weakened stratospheric jet, and a reduction in AOA mostly within the polar vortex. In contrast, SST heatings with a zonally symmetric structure tend to produce a deep strengthening of the stratospheric residual circulation, suppressed isentropic mixing associated with a stronger stratospheric jet, and a decrease of AOA in the entire stratosphere. The shallow versus deep strengthening of the stratospheric residual circulation change has been linked to wave propagation and dissipation in the subtropical lower stratosphere rather than wave generation in the troposphere, and the former can be strongly affected by the vertical position of the subtropical jet. These results suggest that, while the longitudinally localized SST trends under climate change may contribute to the change in the shallow branch of the BDC, the upward shift of the subtropical jet associated with the zonal SST heating can impact the deep branch of the BDC.
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  • 48
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 71 (4). pp. 1494-1507.
    Publication Date: 2017-10-24
    Description: Northern Hemisphere stratospheric variability is investigated with respect to chaotic behavior using time series from three different variables extracted from four different reanalysis products and two numerical model runs with different forcing. The time series show red spectra at all frequencies and the probability distribution functions show persistent deviations from a Gaussian distribution. An exception is given by the numerical model forced with perpetual winter conditions—a case that shows more variability and follows a Gaussian distribution, suggesting that the deviation from Gaussianity found in the observations is due to the transition between summer and winter variability. To search for the presence of a chaotic attractor the correlation dimension and entropy, the Lyapunov spectrum, and the associated Kaplan–Yorke dimension are estimated. A finite value of the dimensions can be computed for each variable and data product, with the correlation dimension ranging between 3.0 and 4.0 and the Kaplan–Yorke dimension between 3.3 and 5.5. The correlation entropy varies between 0.6 and 1.1. The model runs show similar values for the correlation and Lyapunov dimensions for both the seasonally forced run and the perpetual-winter run, suggesting that the structure of a possible chaotic attractor is not determined by the seasonality in the forcing, but must be given by other mechanisms.
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  • 49
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 71 (2). pp. 566-573.
    Publication Date: 2017-10-24
    Description: The authors test the hypothesis that recent observed trends in surface westerlies in the Southern Hemisphere are directly consequent on observed trends in the timing of stratospheric final warming events. The analysis begins by verifying that final warming events have an impact on tropospheric circulation in a simplified GCM driven by specified equilibrium temperature distributions. Seasonal variations are imposed in the stratosphere only. The model produces qualitatively realistic final warming events whose influence extends down to the surface, much like what has been reported in observational analyses. The authors then go on to study observed trends in surface westerlies composited with respect to the date of final warming events. If the considered hypothesis were correct, these trends would appear to be much weaker when composited with respect to the date of the final warming events. The authors find that this is not the case, and accordingly they conclude that the observed surface changes cannot be attributed simply to this shift toward later final warming events.
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  • 50
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 25 (11). pp. 2532-2546.
    Publication Date: 2018-04-05
    Description: In late austral summer 1991 a cyclonic thermocline eddy was detected in the subtropical western South Atlantic off the Brazilian shelf near the city of Vitória. This Vitória eddy was tracked for 55 days by surface drifters drogued at 100-m depth. The drifters had been deployed in the western boundary current regime by FS Meteor as part of a basinwide surface current study. The analysis of a combined CTD/XBT section across the Vitória eddy, together with drifter data and satellite images of the thermal surface structure revealed the unexpected complexity of the region. The eddy interacted not only with the local topography and the Brazil Current, located farther offshore, but also with an extended upwelling regime north of Cabo Frio. The hydrographic and kinematic properties and anomalies of the Vitóia eddy are analyzed and compared with similar vortices described elsewhere in literature.
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  • 51
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 26 . pp. 1142-1164.
    Publication Date: 2018-04-05
    Description: The authors use different versions of the model of the wind- and thermohaline-driven circulation in the North and Equatorial Atlantic developed under the WOCE Community Modeling Effort to investigate the mean flow pattern and deep-water formation in the subpolar region, and the corresponding structure of the basin-scale meridional overturning circulation transport. A suite of model experiments has been carded out in recent years, differing in horizontal resolution (1° × 1.2°, 1/3° × 0.4°, 1/6° × 0.2°), thermohaline boundary conditions, and parameterization of small-scale mixing. The mass transport in the subpolar gyre and the production of North Atlantic Deep Water (NADW) appears to be essentially controlled by the outflow of dense water from the Greenland and Norwegian Seas. in the present model simulated by restoring conditions in a buffer zone adjacent to the boundary near the Greenland–Scotland Ridge. Deep winter convection homogenizes the water column in the center of the Labrador Sea to about 2000 m. The water mass properties (potential temperature about 3°C, salinity about 34.9 psu) and the volume (1.1×1053 km3) of the homogenized water are in fair agreement with observations. The convective mixing has only little effect on the net sinking of upper-layer water in the subpolar gyre. Sensitivity experiments show that the export of NADW from the subpolar North Atlantic is more strongly affected by changes in the overflow conditions than by changes in the surface buoyancy fluxes over the Labrador and Irminger Seas, even if these suppress the deep convection completely. The host of sensitivity experiments demonstrates that realistic meridional overturning and heat transport distributions for the North Atlantic (with a maximum of 1 PW) can be obtained with NADW production rates of 15–16 Sv, provided the spurious upwelling of deep water that characterizes many model solutions in the Gulf Stream regime is avoided by adequate horizontal resolution add mixing parameterization.
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  • 52
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 28 . pp. 999-1002.
    Publication Date: 2018-04-06
    Description: New analytical, circular eddy solutions of the nonlinear, reduced-gravity, shallow-water equations in a rotating system are presented. While previous analytical solutions were limited to the description of pulsons, which are oscillating, frontal, warm-core eddies with paraboloidic shape and linear velocity components, the new solutions describe more general radial structures of eddy shape and azimuthal velocity. In particular, the new solutions, which contain as a subset the circular pulson solution, also allow for the description of circular, frontal, warm-core eddies with small azimuthal velocities at their periphery and/or with motionless cores, which are frequently observed characteristics of warm-core eddies in the World Ocean.
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  • 53
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    American Meteorological Society
    In:  Journal of Atmospheric and Oceanic Technology, 15 . pp. 380-386.
    Publication Date: 2018-07-04
    Description: A ship rain gauge has been developed that can be used under high wind speeds such as those experienced by ships at sea. The instrument has an improved aerodynamic design and an additional lateral collecting surface, which is effective especially with high wind speeds. The ship rain gauge has been calibrated at sea against a specially designed optical disdrometer. An accuracy of 2%–3% has been obtained for 6-hourly sums. The ship rain gauge has also successfully been tested at a test site of the German Weather Service and presently is used on research vessels and voluntary observing ship.
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  • 54
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 29 (6). pp. 1251-1264.
    Publication Date: 2018-04-06
    Description: A dynamic–thermodynamic sea ice–mixed layer model for the Weddell Sea is complemented by a simple, diagnostic model to account for local sea ice–atmosphere interaction. To consider the atmospheric influence on the oceanic mixed layer, the pycnocline upwelling velocity is calculated using the theory of Ekman pumping. In several experiments, formation and conservation of a polynya in the Weddell Sea are investigated. Intrusion of heat into the lower atmosphere above the polynya area is assumed to cause a thermal perturbation and a cyclonic thermal wind field. Superposed with daily ECMWF surface winds, this modified atmospheric forcing field intensifies oceanic upwelling and induces divergent ice drift. Simulation results indicate that in case of a weak atmospheric cross-polynya flow the formation of a thermal wind field can significantly extend the lifetime of a large polynya. The repeated occurrence of the Weddell polynya in the years 1974–76 thus appears to be an effect of feedback mechanisms between sea ice, atmosphere, and oceanic mixed layer.
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  • 55
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    American Meteorological Society
    In:  Journal of Applied Meteorology, 36 . pp. 919-930.
    Publication Date: 2017-07-03
    Description: A neural network is used to calculate the longwave net radiation (Lnet) at the sea surface from measurements of the Special Sensor Microwave/Imager (SSM/I). The neural network applied in this study is able to account largely for the nonlinearity between Lnet and the satellite-measured brightness temperatures (TB). The algorithm can be applied for instantaneous measurements over oceanic regions with the area extent of satellite passive microwave observations (30–60 km in diameter). Comparing with a linear regression method the neural network reduces the standard error for Lnet from 17 to 5 W m−2 when applied to model results. For clear-sky cases, a good agreement with an error of less than 5 W m−2 for Lnet between calculations from SSM/I observations and pyrgeometer measurements on the German research vessel Poseidon during the International Cirrus Experiment (ICE) 1989 is obtained. For cloudy cases, the comparison is problematic due to the inhomogenities of clouds and the low and different spatial resolutions of the SSM/I data. Global monthly mean values of Lnet for October 1989 are computed and compared to other sources. Differences are observed among the climatological values from previous studies by H.-J. Isemer and L. Hasse, the climatological values from R. Lindau and L. Hasse, the values of W. L. Darnell et al., and results from this study. Some structures of Lnet are similar for results from W. L. Darnell et al. and the present authors. The differences between both results are generally less than 15 W m−2. Over the North Atlantic Ocean the authors found a poleward increase for Lnet, which is contrary to the results of H.-J. Isemer and L. Hasse.
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  • 56
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    American Meteorological Society
    In:  Journal of Climate, 10 (11). pp. 2711-2724.
    Publication Date: 2018-07-24
    Description: Parameterization of turbulent wind stress and sensible and latent heat fluxes is reviewed in the context of climate studies and model calculations, and specific formulas based on local measurements are recommended. Wind speed is of key importance, and in applying experimental results, the differences between local and modeled winds must be considered in terms of their method of observation or calculation. Climatological wind data based on Beaufort wind force reports require correction for historical trends. Integrated long-term net turbulent and radiative heat fluxes at the sea surface, calculated from archived data, are consistent with meridional heat transport through oceanographic sections; this lends support to the methods used
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  • 57
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 55 (17). pp. 2874-2883.
    Publication Date: 2018-04-16
    Description: The roles of ice particle size distributions (SDs) and particle shapes in cirrus cloud solar radiative transfer are investigated by analyzing SDs obtained from optical array probe measurements (particle sizes larger than 20–40 μm) during intensive field observations of the International Cirrus Experiment, the European Cloud and Radiation Experiment, the First ISCCP Regional Experiment, and the Central Equatorial Pacific Experiment. It is found that the cloud volume extinction coefficient is more strongly correlated with the total number density than with the effective particle size. Distribution-averaged mean single scattering properties are calculated for hexagonal columns, hexagonal plates, and polycrystals at a nonabsorbing (0.5 μm), moderately absorbing (1.6 μm), and strongly absorbing (3.0 μm) wavelength. At 0.5 μm (1.6 μm) (3.0 μm), the spread in the resulting mean asymmetry parameters due to different SDs is smaller than (comparable to) (smaller than) the difference caused by applying different particle shapes to these distributions. From a broadband solar radiative transfer point of view it appears more important to use the correct particle shapes than to average over the correct size distributions.
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  • 58
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    American Meteorological Society
    In:  Journal of Applied Meteorology, 37 (8). pp. 832-844.
    Publication Date: 2017-07-03
    Description: A neural network (NN) has been developed in order to retrieve the cloud liquid water path (LWP) over the oceans from Special Sensor Microwave/Imager (SSM/I) data. The retrieval with NNs depends crucially on the SSM/I channels used as input and the number of hidden neurons—that is, the NN architecture. Three different combinations of the seven SSM/I channels have been tested. For all three methods an NN with five hidden neurons yields the best results. The NN-based LWP algorithms for SSM/I observations are intercompared with a standard regression algorithm. The calibration and validation of the retrieval algorithms are based on 2060 radiosonde observations over the global ocean. For each radiosonde profile the LWP is parameterized and the brightness temperatures (Tb’s) are simulated using a radiative transfer model. The best LWP algorithm (all SSM/I channels except T85V) shows a theoretical error of 0.009 kg m−2 for LWPs up to 2.8 kg m−2 and theoretical “clear-sky noise” (0.002 kg m−2), which has been reduced relative to the regression algorithm (0.031 kg m−2). Additionally, this new algorithm avoids the estimate of negative LWPs. An indirect validation and intercomparison is presented that is based upon SSM/I measurements (F-10) under clear-sky conditions, classified with independent IR-Meteosat data. The NN-based algorithms outperform the regression algorithm. The best LWP algorithm shows a clear-sky standard deviation of 0.006 kg m−2, a bias of 0.001 kg m−2, nonnegative LWPs, and no correlation with total precipitable water. The estimated accuracy for SSM/I observations and two of the proposed new LWP algorithms is 0.023 kg m−2 for LWP ⩽ 0.5 kg m−2.
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  • 59
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 29 . pp. 2303-2317.
    Publication Date: 2018-04-06
    Description: A primitive equation model to study the dynamics of the Agulhas system has been developed. The model domain covers the South Atlantic and the south Indian Ocean with a resolution of ⅓° in the Agulhas region while coarser outside. It is driven by a climatology of the European Centre for Medium-Range Weather Forecasts. It is shown that the model simulates the Agulhas Current, its retroflection, and the ring shedding successfully. The model results show baroclinic anticyclonic eddies in the Mozambique Channel and east of Madagascar, which travel toward the northern Agulhas Current. After the eddies reach the current they are advected southward with the mean flow. Due to the limited numerical resolution only a few eddies reach the retroflection region without much modification. These eddies are responsible for drastic enhancement of the heat transfer from the Indian Ocean to the South Atlantic and lead to periodicities in the interoceanic heat transport of about 50 days superimposed on the seasonal variability. Combined satellite data from TOPEX/Poseidon and ERS-1 show that the observed vortices in the Mozambique Channel are comparable to those seen in the model. In contrast to this the simulated eddies east of Madagascar seem not to be well reproduced. Analyses of the energy conversion terms between the mean flow and the eddies suggest that barotropic instability plays an important role in the generation of Mozambique Channel eddies. For the generation of Agulhas rings and other eddy structures in the model the barotropic instability mechanism seems to be minor, and baroclinic instability mechanisms are more likely.
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  • 60
    Publication Date: 2017-08-24
    Description: The Atlantic meridional overturning circulation (AMOC) makes the strongest oceanic contribution to the meridional redistribution of heat. Here, an observation-based, forty-eight-month-long time series of the vertical structure and strength of the AMOC at 26.5°N is presented. From April 2004 to April 2008 the AMOC had a mean strength of 18.7 ±2.1 Sv with fluctuations of 4.8 Sv rms. The best guess of the peak-to-peak amplitude of the AMOC seasonal cycle is 6.7 Sv, with a maximum strength in autumn and a minimum in spring. While seasonality in the AMOC was commonly thought to be dominated by the northward Ekman transport, this study reveals that fluctuations of the geostrophic mid-ocean and Gulf Stream transports of 2.2 Sv and 1.7 Sv rms, respectively, are substantially larger than those of the Ekman component (1.2 Sv rms). A simple model based on linear dynamics suggests that the seasonal cycle is dominated by wind stress curl forcing at the eastern boundary of the Atlantic. Seasonal geostrophic AMOC anomalies might represent an important and previously underestimated component of meridional transport and storage of heat in the subtropical North Atlantic. There is evidence that the seasonal cycle observed here is representative of much longer intervals. Previously, hydrographic snapshot estimates between 1957 and 2004 had suggested a long-term decline of the AMOC by 8 Sv. This study suggests that aliasing of seasonal AMOC anomalies might have accounted for a large part of the inferred slowdown.
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  • 61
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    American Meteorological Society
    In:  Journal of Climate, 11 (3). pp. 297-312.
    Publication Date: 2018-07-24
    Description: In this study, a hybrid coupled model (HCM) is used to investigate the physics of decadal variability in the North Pacific. This aids in an understanding of the inherent properties of the coupled ocean–atmosphere system in the absence of stochastic forcing by noncoupled variability. It is shown that the HCM simulates a self-sustained decadal oscillation with a period of about 20 yr, similar to that found in both the observations and coupled GCMs. Sensitivity experiments are carried out to determine the relative importance of wind stresses, net surface heat flux, and freshwater flux on the initiation and maintenance of the decadal oscillation in the North Pacific. It is found that decadal variability is a mode of the coupled system and involves interaction of sea surface temperature, upper-ocean heat content, and wind stress. This interaction is mainly controlled by the wind stress but can be strongly modified by the surface heat flux. The effect of the salinity is relatively small and is not necessary to generate the model decadal oscillation in the North Pacific. There are some limitations with this study. First, the effect of a stochastic forcing is not included. Second, a weak negative feedback is needed to run the control experiment for a longer time period. These two areas will be addressed in a future investigation.
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  • 62
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    American Meteorological Society
    In:  Journal of Climate, 25 (12). pp. 4294-4303.
    Publication Date: 2015-01-12
    Description: The tropical Atlantic wind response to El Niño forcing is robust, with weakened northeast trade winds north of the equator and strengthened southeast trade winds along and south of the equator. However, the relationship between sea surface temperature (SST) anomalies in the eastern equatorial Pacific and Atlantic is inconsistent, with El Niño events followed sometimes by warm and other times by cold boreal summer anomalies in the Atlantic cold tongue region. Using observational data and a hindcast simulation of the Nucleus for European Modeling of the Ocean (NEMO) global model at 0.5° resolution (NEMO-ORCA05), this inconsistent SST relationship is shown to be at least partly attributable to a delayed negative feedback in the tropical Atlantic that is active in years with a warm or neutral response in the eastern equatorial Atlantic. In these years, the boreal spring warming in the northern tropical Atlantic that is a typical response to El Niño is pronounced, setting up a strong meridional SST gradient. This leads to a negative wind stress curl anomaly to the north of the equator that generates downwelling Rossby waves. When these waves reach the western boundary, they are reflected into downwelling equatorial Kelvin waves that reach the cold tongue region in late boreal summer to counteract the initial cooling that is due to the boreal winter wind stress response to El Niño. In contrast, this initial cooling persists or is amplified in years in which the boreal spring northern tropical Atlantic warming is weak or absent either because of a positive North Atlantic Oscillation (NAO) phase or an early termination of the Pacific El Niño event.
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  • 63
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 43 . pp. 149-164.
    Publication Date: 2018-04-12
    Description: Previous attempts to derive the depth-dependent expression of the radiation stress have lead to a debate concerning (i) the applicability of Mellor’s approach to a sloping bottom, (ii) the introduction of the delta function at the mean sea surface in the later papers by Mellor, and (iii) a wave-induced pressure term derived in several recent studies. The authors use an equation system in vertically Lagrangian and horizontally Eulerian (VL) coordinates suitable for a concise treatment of the surface boundary, and obtain an expression for the depth-dependent radiation stress that is consistent with the vertically-integrated expression given by Longuet-Higgins and Stewart. Concerning (i)-(iii) in the above, the difficulty of handling a sloping bottom disappears when wave-averaged momentum equations in the VL coordinates are written for the development of (not the Lagrangian mean velocity but) the Eulerian mean velocity. There is also no delta function at the sea surface in the expression for the depth-dependent radiation stress. The connection between the wave-induced pressure term in the recent studies and the depth-dependent radiation stress term is easily shown by rewriting the pressure-based form stress term in the thickness-weighted-mean (TWM) momentum equations as a velocity-based term which contains the time derivative of the pseudomomentum in the TWM framework.
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  • 64
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    American Meteorological Society
    In:  Bulletin of the American Meteorological Society, 76 (1). pp. 5-11.
    Publication Date: 2019-01-21
    Description: Widespread and sustained in situ ocean measurements are essential to an improved understanding of the state of the ocean and its role in global change. Merchant marine vessels can play a major role in ocean monitoring, yet apart from routine weather observations and upper-ocean temperature measurements, they constitute a vastly underutilized resource due to lack of suitable instrumentation. Examples of ways in which vessels can assist include profiling techniques of physical properties, chemical sampling via automated water samplers, optical techniques to measure various biological parameters, and ground truth measurements for remote sensing from orbiting and geostationary satellites. Further, ships can act as relays between subsurface instrumentation and satellite communication services. To take advantage of the opportunities that the maritime industry can provide, two steps must be taken. The first is to initiate an instrumentation development program with emphasis on techniques optimized for highly automated use onboard ships at 15-20-kt speeds. The second is to forge partnerships or links between academic and government laboratories and the maritime industry for the institution and maintenance of such monitoring programs. No doubt significant resources will be required, but in the long run the improved ability to monitor the state of ocean in situ will make the effort more than worthwhile.
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  • 65
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 25 (8). pp. 1771-1787.
    Publication Date: 2018-04-05
    Description: The Cape Verde Frontal Zone separates the North and the South Atlantic Central Waters in the eastern North Atlantic. It also represents the boundary between the ventilated subtropical gyre and the quasi-stagnant shadow zone in the southeast. The thermohaline front is nearly compensated with respect to density, and density parameters RP, suggest the existence of double-diffusive processes. Datasets from three cruises to the region, approximately one year apart each, are used to determine the effects of double-diffusive diapycnal versus isopycnal mixing. For this purpose results from the usual temperature-salinity analysis assuming isopycnal mixing are compared to results from a multiparameter analysis where nutrient and oxygen data are also used. Significant diapycnal fluxes are found in the frontal zone between 200 and 300 m, with water mass contents being changed by more than 20% through diapycnal mixing. The associated buoyancy fluxes have a similar magnitude as surface fluxes in the area and thus represent an important contribution to the vertical balances of heat and salt.
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  • 66
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    American Meteorological Society
    In:  Journal of Climate, 26 (4). pp. 1387-1402.
    Publication Date: 2017-08-25
    Description: In this article we address the causes of the large-scale tropical sea level pressure (SLP) changes during climate change. The analysis we present is based on model simulations, observed trends and the seasonal cycle. In all three cases the regional changes of tropospheric temperature (Ttropos) and SLP are strongly related to each other (considerably stronger than (sea) surface temperature and SLP). This relationship basically follows the Bjerknes Circulation Theorem, with relatively low regional SLP where we have relatively high Ttropos and vice versa. A simple physical model suggests a tropical SLP response to horizontally inhomogeneous warming in the tropical Ttropos, with a sensitivity coefficient of about -1.7 hPa/K. This relationship explains a large fraction of observed and predicted changes in the tropical SLP. It is shown that in climate change model simulations the tropospheric land-sea warming contrast is the most significant structure in the regional Ttropos changes relative to the tropical mean changes. Since the land-sea warming contrast exists in the absent of any atmospheric circulation changes it can be argued that the large-scale response of tropical SLP changes is to first order a response to the tropical land-sea warming contrast. Further, as land-sea warming contrast is mostly available moisture dependent, the models predict a stronger warming and decreasing SLP in the drier regions from South America to Africa and a weaker warming and increasing SLP over the wetter Indo-Pacific warm pool region. This suggests an increase in the potential for deep convection conditions over the Atlantic Sector and a decrease over the Indo-Pacific warm pool region in the future.
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  • 67
    Publication Date: 2016-09-07
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  • 68
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    American Meteorological Society
    In:  Journal of Climate, 26 . pp. 7767-7782.
    Publication Date: 2017-08-25
    Description: Evidence is presented for the notion that some contribution to the recent decadal trends observed in the Southern Hemisphere, including the lack of a strong Southern Ocean surface warming, may have originated from longer-term internal centennial variability originating in the Southern Ocean. The existence of such centennial variability is supported by the instrumental sea surface temperatures (SSTs), a multimillennial reconstruction of Tasmanian summer temperatures from tree rings, and a millennial control integration of the Kiel Climate Model (KCM). The model variability was previously shown to be linked to changes in Weddell Sea deep convection. During phases of deep convection the surface Southern Ocean warms, the abyssal Southern Ocean cools, Antarctic sea ice extent retreats, and the low-level atmospheric circulation over the Southern Ocean weakens. After the halt of deep convection the surface Southern Ocean cools, the abyssal Southern Ocean warms, Antarctic sea ice expands, and the low-level atmospheric circulation over the Southern Ocean intensifies, consistent with what has been observed during the recent decades. A strong sensitivity of the time scale to model formulation is noted. In the KCM, the centennial variability is associated with global-average surface air temperature (SAT) changes of the order of a few tenths of a degree per century. The model results thus suggest that internal centennial variability originating in the Southern Ocean should be considered in addition to other internal variability and external forcing when discussing the climate of the twentieth century and projecting that of the twenty-first century.
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  • 69
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 26 . pp. 2251-2266.
    Publication Date: 2018-04-06
    Description: A simple point-vortex “heton” model is used to study localized ocean convection. In particular, the statistically steady state that is established when lateral buoyancy transfer, effected by baroclinic instability, offsets the localized surface buoyancy loss is investigated. Properties of the steady state, such as the statistically steady density anomaly of the convection region, are predicted using the hypothesis of a balance between baroclinic eddy transfer and the localized surface buoyancy loss. These predictions compare favorably with the values obtained through numerical integration of the heton model. The steady state of the heron model can be related to that in other convection scenarios considered in several recent studies by means of a generalized description of the localized convection. This leads to predictions of the equilibrium density anomalies in these scenarios, which concur with those obtained by other authors. Advantages of the heton model include its inviscid nature, emphasizing the independence of the fluxes affected by the baroclinic eddies from molecular processes, and its extreme economy, allowing a very large parameter space to be covered. This economy allows us to examine more complicated forcing scenarios: for example, forcing regions of varying shape. By increasing the ellipticity of the forcing region, the instability is modified by the shape and, as a result, no increase in lateral fluxes occurs despite the increased perimeter length. The parameterization of convective mixing by a redistribution of potential vorticity, implicit in the heton model, is corroborated; the heton model equilibrium state has analogous quantitative scaling behavior to that in models or laboratory experiments that resolve the vertical motions. The simplified dynamics of the heton model therefore allows the adiabatic advection resulting from baroclinic instability to be examined in isolation from vertical mixing and diffusive processes. These results demonstrate the importance of baroclinic instability in controlling the properties of a water mass generated by localized ocean convection. A complete parameterization of this process must therefore account for the fluxes induced by horizontal variations in surface buoyancy loss and affected by baroclinic instability.
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  • 70
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 26(10) . pp. 2281-2285.
    Publication Date: 2018-04-05
    Description: The compatibility of the Gent and McWilliams thickness mixing parameterization with perturbation thickness fluxes evaluated from eddy-resolving North Atlantic model results is investigated. After extensive spatial and temporal averaging, a linear correlation between the parameterized fluxes and those calculated directly from model fluctuations in the subtropics could be found. A direct estimate of a constant mixing parameter κ could be inferred in the order of 1.0 × 107 cm2 s−1.
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  • 71
    Publication Date: 2017-07-20
    Description: Many models of the large-scale thermohaline circulation in the ocean exhibit strong zonally integrated upwelling in the midlatitude North Atlantic that significantly decreases the amount of deep water that is carried from the formation regions in the subpolar North Atlantic toward low latitudes and across the equator. In an analysis of results from the Community Modeling Effort using a suite of models with different horizontal resolution, wind and thermohaline forcing, and mixing parameters, it is shown that the upwelling is always concentrated in the western boundary layer between roughly 30° and 40°N. The vertical transport across 1000 m appears to be controlled by local dynamics and strongly depends on the horizontal resolution and mixing parameters of the model. It is suggested that in models with a realistic deep-water formation rate in the subpolar North Atlantic, the excessive upwelling can be considered as the prime reason for the typically too low meridional overturning rates and northward heat transports in the subtropical North Atlantic. A new isopycnal advection and mixing parameterization of tracer transports by mesoscale eddies yield substantial improvements in these integral measures of the circulation.
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  • 72
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 29 (11). pp. 2785-2801.
    Publication Date: 2018-04-06
    Description: The Rio Grande Rise acts as a natural barrier for the equatorward flow of Antarctic Bottom Water in the subtropical South Atlantic. In addition to the Vema Channel, the Hunter Channel cuts through this obstacle and offers a separate route for bottom-water import into the southern Brazil Basin. On the occasion of the Deep Basin Experiment, a component of the World Ocean Circulation Experiment (WOCE), the expected deep flow through the Hunter Channel was directly observed for the first time by an array of moored current meters and thermistor chains from December 1992 to May 1994. Main results are (i) the Hunter Channel is, in fact, a conduit for bottom-water flow into the Brazil Basin. Our new mean transport from moored current meters [2.92 (±1.24) × 106 m3 s−1] is significantly higher than earlier estimates that were based on geostrophic calculations. (ii) During the WOCE observational period a tendency toward increased bottom-water temperatures was observed. This observation from the Hunter Channel is consistent with findings from the Vema Channel. (iii) The overflow through the Hunter Channel is highly variable and puts in perspective earlier synoptic geostrophic transport estimates
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  • 73
    Publication Date: 2018-04-06
    Description: During December 1991 to April 1992 measurements with moored acoustic Doppler current profiler (ADCP) stations and shipboard surveys were carried out in the convection regime of the Gulf of Lions, northwestern Mediterranean. First significant mixed layer deepening and generation of internal waves in the stratified intermediate layer occurred during a mistral cooling phase in late December. Mixed layer deepening to about 400 m, eroding the salinity maximum layer of saltier and warmer Levantine Intermediate Water and causing temporary surface-layer warming, followed during a second cooling period of late January. During a mistral cooling period from 18 to 23 February 1992, convection to 1500-m depth was observed, where the size of the convection regime was 50–100 km extent. Vertical velocities 40–640 m deep, recorded by four ADCPs of a triangular moored array of 2 km sidelength in the center of the convection regime, exceeded 5 cm s−1 and were not correlated over the separation of the moorings. Horizontal scales estimated from event duration and advection velocity were only around 500 m, in agreement with scaling arguments for convective plumes. Plume activity during nighttime cooling was larger than daytime daytime. Significant evidence for rotation of the plumes could not be found. Overall, plume energy, and the degree of mixing accomplished by them, was much lower than observed during a stronger mistral in February 1987. The mean vertical velocity over the mistral period, determined from the four ADCPs, was near zero, confirming the role of plumes as mixing agents rather than as part of a mean downdraft in a convection regime. The cyclonic rim current around the convection regime was confined to a strip of 〈20 km width with an average velocity of about 10 cm s−1, which is in agreement with near-zero vertical mean velocity in the interior based on potential vorticity conservation. A relation between variations of the larger-scale cyclonic North Mediterranean Current along the boundary and the deep convection could not be identified. An unexplained feature still is the cover of the convection regime by a shallow layer of light water that moves in rather quickly from the sides after the cooling ends.
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  • 74
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 26 . pp. 1721-1734.
    Publication Date: 2018-04-05
    Description: An initially resting ocean of stratification N is considered, subject to buoyancy loss at its surface of magnitude B0 over a circular region of radius r, at a latitude where the Coriolis parameter is f. Initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer. However, as deepening proceeds, horizontal density gradients at the edge of the forcing region support a geostrophic rim current, which develops growing meanders through baroclinic instability. Eventually finite-amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below, setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface. In this final state quasi-horizontal baroclinic eddy transfer dominates upright “plume” convection. By using “parcel theory” to consider the energy transformations taking place, it is shown that the depth, hfinal at which deepening by convective plumes is arrested by lateral buoyancy flux due to baroclinic eddies, and the time tfinal it takes to reach this depth, is given by both independent of rotation. Here γ and β are dimensionless constants that depend on the efficiency of baroclinic eddy transfer. A number of laboratory and numerical experiments are then inspected and carried out to seek confirmation of these parameter dependencies and obtain quantitative estimates of the constants. It is found that γ = 3.9 ± 0.9 and β = 12 ± 3. Finally, the implications of our stu