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Dudley B. Chelton - One of the best experts on this subject based on the ideXlab platform.
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summertime coupling between sea surface temperature and Wind Stress in the california current system
Journal of Physical Oceanography, 2007Co-Authors: Dudley B. Chelton, Michael G Schlax, Roger M. SamelsonAbstract:Satellite observations of Wind Stress and sea surface temperature (SST) are analyzed to investigate ocean–atmosphere interaction in the California Current System (CCS). As in regions of strong SST fronts elsewhere in the World Ocean, SST in the CCS region is positively correlated with surface Wind Stress when SST fronts are strong, which occurs during the summertime in the CCS region. This ocean influence on the atmosphere is apparently due to SST modification of stability and mixing in the atmospheric boundary layer and is most clearly manifest in the derivative Wind Stress fields: Wind Stress curl and divergence are linearly related to, respectively, the crossWind and downWind components of the local SST gradient. The dynamic range of the Ekman upwelling velocities associated with the summertime SST-induced perturbations of the Wind Stress curl is larger than that of the upwelling velocities associated with the mean summertime Wind Stress curl. This suggests significant feedback effects on the ocean, which likely modify the SST distribution that perturbed the Wind Stress curl field. The atmosphere and ocean off the west coast of North America must therefore be considered a fully coupled system. It is shown that the observed summertime ocean– atmosphere interaction is poorly represented in the NOAA North American Mesoscale Model (formerly called the Eta Model). This is due, at least in part, to the poor resolution and accuracy of the SST boundary condition used in the model. The sparse distribution of meteorological observations available over the CCS for data assimilation may also contribute to the poor model performance.
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On the Coupling of Wind Stress and Sea Surface Temperature
Journal of Climate, 2006Co-Authors: Roger M. Samelson, Dudley B. Chelton, Steven K. Esbensen, Eric D. Skyllingstad, Larry W. O’neill, Nicolai ThumAbstract:A simple quasi-equilibrium analytical model is used to explore hypotheses related to observed spatial correlations between sea surface temperatures and Wind Stress on horizontal scales of 50–500 km. It is argued that a plausible contributor to the observed correlations is the approximate linear relationship between the surface Wind Stress and Stress boundary layer depth under conditions in which the Stress boundary layer has come into approximate equilibrium with steady free-atmospheric forcing. Warmer sea surface temperature is associated with deeper boundary layers and stronger Wind Stress, while colder temperature is associated with shallower boundary layers and weaker Wind Stress. Two interpretations of a previous hypothesis involving the downward mixing of horizontal momentum are discussed, and it is argued that neither is appropriate for the warm-to-cold transition or quasi-equilibrium conditions, while one may be appropriate for the cold-to-warm transition. Solutions of a turbulent large-eddy simulation numerical model illustrate some of the processes represented in the analytical model. A dimensionless ratio A is introduced to measure the relative influence of lateral momentum advection and local surface Stress on the boundary layer Wind profile. It is argued that when A 1, and under conditions in which the thermodynamically induced lateral pressure gradients are small, the boundary layer depth effect will dominate lateral advection and control the surface Stress.
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scatterometer and model Wind and Wind Stress in the oregon northern california coastal zone
Monthly Weather Review, 2004Co-Authors: Natalie Perlin, R M Samelson, Dudley B. CheltonAbstract:Measurements of surface Wind Stress by the SeaWinds scatterometer on NASA’s Quick Scatterometer (QuikSCAT) satellite are analyzed and compared with several different atmospheric model products, from an operational model and two high-resolution nested regional models, during two summer periods, June through September 2000 and 2001, in the coastal region west of Oregon and northern California. The mean summer Wind Stress had a southward component over the entire region in both years. Orographic intensifications of both the mean and fluctuating Wind Stress occurred near Cape Blanco, Cape Mendocino, and Point Arena. Substantial differences between the model products are found for the mean, variable, and diurnal Wind Stress fields. Temporal correlations with the QuikSCAT observations are highest for the operational model, and are not improved by either nested model. The highest-resolution nested model most accurately reproduced the mean observed Stress fields, but slightly degrades the temporal correlations due to incoherent high-frequency (0.5‐2 cpd) fluctuations. The QuikSCAT data reveal surprisingly strong diurnal fluctuations that extend offshore 150 km or more with magnitudes that are a significant fraction of the mean Wind Stress. Wind Stress curl fields from QuikSCAT and the models show local cyclonic and anticyclonic maxima associated with the orographic Wind intensification around the capes. The present results are consistent with the hypothesis of a Wind-driven mechanism for coastal jet separation and cold water plume and anticyclonic eddy formation in the California Current System south of Cape Blanco.
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observations of coupling between surface Wind Stress and sea surface temperature in the eastern tropical pacific
Journal of Climate, 2001Co-Authors: Dudley B. Chelton, Michael H. Freilich, Michael J. Mcphaden, Steven K. Esbensen, Nicolai Thum, Michael G Schlax, Frank J Wentz, Chelle L Gentemann, Paul S SchopfAbstract:Satellite measurements of surface Wind Stress from the QuikSCAT scatterometer and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission Microwave Imager are analyzed for the three-month period 21 July‐20 October 1999 to investigate ocean‐atmosphere coupling in the eastern tropical Pacific. Oceanic tropical instability waves (TIWs) with periods of 20‐40 days and wavelengths of 1000‐2000 km perturb the SST fronts that bracket both sides of the equatorial cold tongue, which is centered near 1 8S to the east of 1308W. These perturbations are characterized by cusp-shaped features that propagate systematically westward on both sides of the equator. The space‐time structures of these SST perturbations are reproduced with remarkable detail in the surface Wind Stress field. The Wind Stress divergence is shown to be linearly related to the downWind component of the SST gradient with a response on the south side of the cold tongue that is about twice that on the north side. The Wind Stress curl is linearly related to the crossWind component of the SST gradient with a response that is approximately half that of the Wind Stress divergence response to the downWind SST gradient. The perturbed SST and Wind Stress fields propagate synchronously westward with the TIWs. This close coupling between SST and Wind Stress supports the Wallace et al. hypothesis that surface Winds vary in response to SST modification of atmospheric boundary layer stability.
Renato M. Castelao - One of the best experts on this subject based on the ideXlab platform.
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Sea Surface Temperature and Wind Stress Curl Variability near a Cape
Journal of Physical Oceanography, 2012Co-Authors: Renato M. CastelaoAbstract:AbstractThe coupling between sea surface temperature (SST), SST gradients, and Wind Stress curl variability near a cape off Brazil is investigated using satellite observations and several different SST high-resolution analyses. The cape is characterized by strong SST fronts year-round, associated with upwelling and advection of warm water offshore in a western boundary current. Observations reveal a strong coupling between crossWind SST gradients and Wind Stress curl variability, with the predominantly negative crossWind gradients leading to negative, upwelling favorable Wind Stress curl anomalies. The spatial correlation between empirical orthogonal functions (EOF) of those variables is ~0.6, while the correlation between the EOF amplitude time series of the Wind Stress curl and crossWind SST gradients is larger than 0.7. The coupling occurs during summer and winter and is strongly modulated by variations in the Wind Stress directional steadiness. The intensity of the coupling is weaker than around capes...
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The Role of Wind Stress Curl in Jet Separation at a Cape
Journal of Physical Oceanography, 2007Co-Authors: Renato M. Castelao, John A. BarthAbstract:Abstract A high-resolution numerical model is used to study the importance of spatial variability in the Wind forcing to the separation of a coastal upwelling jet at a cape. An idealized topography and Wind field based on observations from the Cape Blanco (Oregon) region are used. Several simulations are investigated, with both the intensity and the spatial structure of the Wind forcing varied to isolate the importance of the observed intensification in the Wind Stress and Wind Stress curl magnitudes to the separation process. A simulation using a straight coast confirms that the presence of the cape is crucial for separation. Wind Stress intensification by itself, with zero curl, does not aid separation. The Wind Stress curl intensification south of the cape, on the other hand, is important for controlling details of the process. Because the positive Wind Stress curl drives upwelling, isotherms in the offshore region tilt upward, creating a pressure gradient that sustains an intensification of the southw...
Dmitry Beletsky - One of the best experts on this subject based on the ideXlab platform.
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Relative effects of Wind Stress curl, topography, and stratification on large‐scale circulation in Lake Michigan
Journal of Geophysical Research, 2003Co-Authors: David J. Schwab, Dmitry BeletskyAbstract:[1] This paper uses the results from two multiseason numerical model simulations of Lake Michigan hydrodynamics to examine the relative effects of Wind Stress curl, topography, and stratification on large-scale circulation. The multiseason simulations provide a period long enough to encompass the full range of atmospheric and thermal conditions that can occur in the lake. The purpose of this paper is to diagnose the relative importance of various mechanisms responsible for the large-scale circulation patterns by analyzing the vorticity balance in the lake on a monthly timescale. Five different model scenarios are used to isolate the predominant mechanisms: (1) baroclinic lake, spatially variable Wind Stress; (2) barotropic lake, spatially variable Wind Stress; (3) baroclinic lake, spatially uniform Wind Stress; (4) barotropic lake, spatially uniform Wind Stress; and (5) barotropic lake, linearized equations, spatially uniform Wind Stress. By comparing the results of these five model scenarios it is shown that the cyclonic Wind Stress curl in the winter and the effect of baroclinicity in the summer are primarily responsible for the predominantly cyclonic flow in the lake. Topographic effects are also important but are not as significant as Wind Stress curl and baroclinic effects. Nonlinear effects are much smaller.
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relative effects of Wind Stress curl topography and stratification on large scale circulation in lake michigan
Journal of Geophysical Research, 2003Co-Authors: David J. Schwab, Dmitry BeletskyAbstract:[1] This paper uses the results from two multiseason numerical model simulations of Lake Michigan hydrodynamics to examine the relative effects of Wind Stress curl, topography, and stratification on large-scale circulation. The multiseason simulations provide a period long enough to encompass the full range of atmospheric and thermal conditions that can occur in the lake. The purpose of this paper is to diagnose the relative importance of various mechanisms responsible for the large-scale circulation patterns by analyzing the vorticity balance in the lake on a monthly timescale. Five different model scenarios are used to isolate the predominant mechanisms: (1) baroclinic lake, spatially variable Wind Stress; (2) barotropic lake, spatially variable Wind Stress; (3) baroclinic lake, spatially uniform Wind Stress; (4) barotropic lake, spatially uniform Wind Stress; and (5) barotropic lake, linearized equations, spatially uniform Wind Stress. By comparing the results of these five model scenarios it is shown that the cyclonic Wind Stress curl in the winter and the effect of baroclinicity in the summer are primarily responsible for the predominantly cyclonic flow in the lake. Topographic effects are also important but are not as significant as Wind Stress curl and baroclinic effects. Nonlinear effects are much smaller.
David R. Munday - One of the best experts on this subject based on the ideXlab platform.
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Southern Ocean carbon-Wind Stress feedback
Climate Dynamics, 2018Co-Authors: Ben Bronselaer, David R. Munday, Laure Zanna, Jason LoweAbstract:The Southern Ocean is the largest sink of anthropogenic carbon in the present-day climate. Here, Southern Ocean $$p\hbox {CO}_{2}$$ and its dependence on Wind forcing are investigated using an equilibrium mixed layer carbon budget. This budget is used to derive an expression for Southern Ocean $$p\hbox {CO}_{2}$$ sensitivity to Wind Stress. Southern Ocean $$p\hbox {CO}_{2}$$ is found to vary as the square root of area-mean Wind Stress, arising from the dominance of vertical mixing over other processes such as lateral Ekman transport. The expression for p\hbox {CO}_{2} is validated using idealised coarse-resolution ocean numerical experiments. Additionally, we show that increased (decreased) stratification through surface warming reduces (increases) the sensitivity of the Southern Ocean $$p\hbox {CO}_{2}$$ to Wind Stress. The scaling is then used to estimate the Wind-Stress induced changes of atmospheric $$p\hbox {CO}_2$$ in CMIP5 models using only a handful of parameters. The scaling is further used to model the anthropogenic carbon sink, showing a long-term reversal of the Southern Ocean sink for large Wind Stress strength.
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Mean, Variability, and Trend of Southern Ocean Wind Stress: Role of Wind Fluctuations
Journal of Climate, 2018Co-Authors: Xia Lin, Xiaoming Zhai, Zhaomin Wang, David R. MundayAbstract:AbstractThe Southern Ocean (SO) surface westerly Wind Stress plays a fundamental role in driving the Antarctic Circumpolar Current and the global meridional overturning circulation. Here, the authors investigate the contributions of atmospheric Wind fluctuations to the mean, variability, and trend of SO Wind Stress over the last four decades using NCEP reanalysis and ERA-Interim products. Including Wind variability at synoptic frequencies (2–8 days) and higher in the Stress calculation is found to increase the strength of the mean SO Wind Stress by almost 40% in both reanalysis products. The southern annular mode index is found to be a good indicator for the strength of the mean Wind and mean Wind Stress, but not as good an indicator for Wind fluctuations, at least for the chosen study period. Large discrepancies between reanalysis products emerge regarding the contributions of Wind fluctuations to the strengthening trend of SO Wind Stress. Between one-third and one-half of the Stress trend in NCEP can be...
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The impact of atmospheric storminess on the sensitivity of Southern Ocean circulation to Wind Stress changes
Ocean Modelling, 2017Co-Authors: David R. Munday, Xiaoming ZhaiAbstract:The influence of changing the mean Wind Stress felt by the ocean through alteration of the variability of the atmospheric Wind, as opposed to the mean atmospheric Wind, on Southern Ocean circulation is investigated using an idealised channel model. Strongly varying atmospheric Wind is found to increase the (parameterised) near-surface viscous and diffusive mixing. Analysis of the kinetic energy budget indicates a change in the main energy dissipation mechanism. For constant Wind Stress, dissipation of the power input by surface Wind work is always dominated by bottom kinetic energy dissipation. However, with time-varying atmospheric Wind, near surface viscous dissipation of kinetic energy becomes increasingly important as mean Wind Stress increases. This increased vertical diffusivity leads to thicker mixed layers and higher sensitivity of the residual circulation to increasing Wind Stress, when compared to equivalent experiments with the same Wind Stress held constant in time. This may have implications for Southern Ocean circulation in different climate change scenarios should the variability of the atmospheric Wind change rather than the mean atmospheric Wind.
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Sensitivity of Southern Ocean circulation to Wind Stress changes: Role of relative Wind Stress
Ocean Modelling, 2015Co-Authors: David R. Munday, Xiaoming ZhaiAbstract:The influence of different Wind Stress bulk formulae on the response of the Southern Ocean circulation to Wind Stress changes is investigated using an idealised channel model. Surface/mixed layer properties are found to be sensitive to the use of the relative Wind Stress formulation, where the Wind Stress depends on the difference between the ocean and atmosphere velocities. Previous work has highlighted the surface eddy damping effect of this formulation, which we find leads to increased circumpolar transport. Nevertheless the transport due to thermal Wind shear does lose sensitivity to Wind Stress changes at sufficiently high Wind Stress. In contrast, the sensitivity of the meridional overturning circulation is broadly the same regardless of the bulk formula used due to the adiabatic nature of the relative Wind Stress damping. This is a consequence of the steepening of isopycnals offsetting the reduction in eddy diffusivity in their contribution to the eddy bolus overturning, as predicted using a residual mean framework.
Michael J. Mcphaden - One of the best experts on this subject based on the ideXlab platform.
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Wind Stress and near surface shear in the equatorial atlantic ocean
Geophysical Research Letters, 2014Co-Authors: Jacob O. Wenegrat, Michael J. Mcphaden, Ren-chieh LienAbstract:The upper ocean response to Wind Stress is examined using 8 months of unique near-surface moored velocity, temperature, and salinity data at 0°N, 23°W in the equatorial Atlantic. The effects of Wind Stress and shear on the time-varying eddy viscosity are inferred using the surface shear-Stress boundary condition. Parameterizations of eddy viscosity as a function of Wind Stress and shear versus Wind Stress alone are then examined. In principle, eddy viscosity should be proportional to the inverse shear, but how it is represented implicitly or explicitly can affect estimates of the near-surface flow field. This result may explain some discrepancies that have arisen from using parameterizations based only on Wind Stress to characterize the effects of turbulent momentum mixing.
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Wind Stress and near‐surface shear in the equatorial Atlantic Ocean
Geophysical Research Letters, 2014Co-Authors: Jacob O. Wenegrat, Michael J. Mcphaden, Ren-chieh LienAbstract:The upper ocean response to Wind Stress is examined using 8 months of unique near-surface moored velocity, temperature, and salinity data at 0°N, 23°W in the equatorial Atlantic. The effects of Wind Stress and shear on the time-varying eddy viscosity are inferred using the surface shear-Stress boundary condition. Parameterizations of eddy viscosity as a function of Wind Stress and shear versus Wind Stress alone are then examined. In principle, eddy viscosity should be proportional to the inverse shear, but how it is represented implicitly or explicitly can affect estimates of the near-surface flow field. This result may explain some discrepancies that have arisen from using parameterizations based only on Wind Stress to characterize the effects of turbulent momentum mixing.
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observations of coupling between surface Wind Stress and sea surface temperature in the eastern tropical pacific
Journal of Climate, 2001Co-Authors: Dudley B. Chelton, Michael H. Freilich, Michael J. Mcphaden, Steven K. Esbensen, Nicolai Thum, Michael G Schlax, Frank J Wentz, Chelle L Gentemann, Paul S SchopfAbstract:Satellite measurements of surface Wind Stress from the QuikSCAT scatterometer and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission Microwave Imager are analyzed for the three-month period 21 July‐20 October 1999 to investigate ocean‐atmosphere coupling in the eastern tropical Pacific. Oceanic tropical instability waves (TIWs) with periods of 20‐40 days and wavelengths of 1000‐2000 km perturb the SST fronts that bracket both sides of the equatorial cold tongue, which is centered near 1 8S to the east of 1308W. These perturbations are characterized by cusp-shaped features that propagate systematically westward on both sides of the equator. The space‐time structures of these SST perturbations are reproduced with remarkable detail in the surface Wind Stress field. The Wind Stress divergence is shown to be linearly related to the downWind component of the SST gradient with a response on the south side of the cold tongue that is about twice that on the north side. The Wind Stress curl is linearly related to the crossWind component of the SST gradient with a response that is approximately half that of the Wind Stress divergence response to the downWind SST gradient. The perturbed SST and Wind Stress fields propagate synchronously westward with the TIWs. This close coupling between SST and Wind Stress supports the Wallace et al. hypothesis that surface Winds vary in response to SST modification of atmospheric boundary layer stability.
