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• 1. [Article] Coupling between Sea Surface Temperature and Low-Level Winds in Mesoscale Numerical Models

  This study evaluates the impacts of sea surface temperature (SST) specification and grid resolution on numerical simulations of air–sea coupling near oceanic fronts through analyses of surface winds from ...

  Citation × Citation Citation Abstract Copy Title: Coupling between Sea Surface Temperature and Low-Level Winds in Mesoscale Numerical Models Author: Chelton, Dudley B., Thum, Nicolai, Esbensen, Steven K., O’Neill, Larry W., Song, Qingtao This study evaluates the impacts of sea surface temperature (SST) specification and grid resolution on numerical simulations of air–sea coupling near oceanic fronts through analyses of surface winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The 9 May 2001 change of the boundary condition from the Reynolds SST analyses to the NOAA Real-Time Global (RTG) SST in the ECMWF model resulted in an abrupt increase in mesoscale variance of the model surface winds over the ocean. In contrast, the 21 November 2000 change of the grid resolution resulted in an abrupt increase in mesoscale variability of surface winds over mountainous regions on land but had no significant effect on winds over the ocean. To further investigate model sensitivity to the SST boundary condition and grid resolution, a series of simulations were made with the Weather Research and Forecasting (WRF) model over a domain encompassing the Agulhas return current (ARC: also called “retroflection”) region in the south Indian Ocean. Results from three WRF simulations with SST measured by the Advanced Microwave Scanning Radiometer on the Earth Observing System Aqua satellite (AMSR-E) and the Reynolds and RTG SST analyses indicate the vital importance of the resolution of the SST boundary condition for accurate simulation of the air–sea coupling between SST and surface wind speed. WRF simulations with grid spacings of 40 and 25 km show that the latter increased energy only on scales shorter than 250 km. In contrast, improved resolution of SST significantly increased the mesoscale variability for scales up to 1000 km. Further sensitivity studies with the WRF model conclude that the weak coupling of surface wind speeds from the ECMWF model to SST is likely attributable primarily to the weak response of vertical turbulent mixing to SST-induced stability in the parameterization of boundary layer turbulence, with an overestimation of vertical diffusion by about 60% on average in stable conditions and an underestimation by about 40% in unstable conditions Title: Coupling between Sea Surface Temperature and Low-Level Winds in Mesoscale Numerical Models Author: Chelton, Dudley B., Thum, Nicolai, Esbensen, Steven K., O’Neill, Larry W., Song, Qingtao This study evaluates the impacts of sea surface temperature (SST) specification and grid resolution on numerical simulations of air–sea coupling near oceanic fronts through analyses of surface winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The 9 May 2001 change of the boundary condition from the Reynolds SST analyses to the NOAA Real-Time Global (RTG) SST in the ECMWF model resulted in an abrupt increase in mesoscale variance of the model surface winds over the ocean. In contrast, the 21 November 2000 change of the grid resolution resulted in an abrupt increase in mesoscale variability of surface winds over mountainous regions on land but had no significant effect on winds over the ocean. To further investigate model sensitivity to the SST boundary condition and grid resolution, a series of simulations were made with the Weather Research and Forecasting (WRF) model over a domain encompassing the Agulhas return current (ARC: also called “retroflection”) region in the south Indian Ocean. Results from three WRF simulations with SST measured by the Advanced Microwave Scanning Radiometer on the Earth Observing System Aqua satellite (AMSR-E) and the Reynolds and RTG SST analyses indicate the vital importance of the resolution of the SST boundary condition for accurate simulation of the air–sea coupling between SST and surface wind speed. WRF simulations with grid spacings of 40 and 25 km show that the latter increased energy only on scales shorter than 250 km. In contrast, improved resolution of SST significantly increased the mesoscale variability for scales up to 1000 km. Further sensitivity studies with the WRF model conclude that the weak coupling of surface wind speeds from the ECMWF model to SST is likely attributable primarily to the weak response of vertical turbulent mixing to SST-induced stability in the parameterization of boundary layer turbulence, with an overestimation of vertical diffusion by about 60% on average in stable conditions and an underestimation by about 40% in unstable conditions Close

• 2. [Article] The Effects of SST-Induced Surface Wind Speed and Direction Gradients on Midlatitude Surface Vorticity and Divergence

  The effects of surface wind speed and direction gradients on midlatitude surface vorticity and divergence fields associated with mesoscale sea surface temperature (SST) variability having spatial scales ...

  Citation × Citation Citation Abstract Copy Title: The Effects of SST-Induced Surface Wind Speed and Direction Gradients on Midlatitude Surface Vorticity and Divergence Author: Chelton, Dudley B., Esbensen, Steven K., O’Neill, Larry W. The effects of surface wind speed and direction gradients on midlatitude surface vorticity and divergence fields associated with mesoscale sea surface temperature (SST) variability having spatial scales of 100–1000 km are investigated using vector wind observations from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite and SST from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) Aqua satellite. The wind–SST coupling is analyzed over the period June 2002–August 2008, corresponding to the first 6+ years of the AMSR-E mission. Previous studies have shown that strong wind speed gradients develop in response to persistent mesoscale SST features associated with the Kuroshio Extension, Gulf Stream, South Atlantic, and Agulhas Return Current regions. Midlatitude SST fronts also significantly modify surface wind direction; the surface wind speed and direction responses to typical SST differences of about 2°–4°C are, on average, about 1–2 m s⁻¹ and 4°–8°, respectively, over all four regions. Wind speed perturbations are positively correlated and very nearly collocated spatially with the SST perturbations. Wind direction perturbations, however, are displaced meridionally from the SST perturbations, with cyclonic flow poleward of warm SST and anticyclonic flow poleward of cool SST. Previous observational analyses have shown that small-scale perturbations in the surface vorticity and divergence fields are related linearly to the crosswind and downwind components of the SST gradient, respectively. When the vorticity and divergence fields are analyzed in curvilinear natural coordinates, the wind speed contributions to the SST-induced vorticity and divergence depend equally on the crosswind and downwind SST gradients, respectively. SST-induced wind direction gradients also significantly modify the vorticity and divergence fields, weakening the vorticity response to crosswind SST gradients while enhancing the divergence response to downwind SST gradients. Title: The Effects of SST-Induced Surface Wind Speed and Direction Gradients on Midlatitude Surface Vorticity and Divergence Author: Chelton, Dudley B., Esbensen, Steven K., O’Neill, Larry W. The effects of surface wind speed and direction gradients on midlatitude surface vorticity and divergence fields associated with mesoscale sea surface temperature (SST) variability having spatial scales of 100–1000 km are investigated using vector wind observations from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite and SST from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) Aqua satellite. The wind–SST coupling is analyzed over the period June 2002–August 2008, corresponding to the first 6+ years of the AMSR-E mission. Previous studies have shown that strong wind speed gradients develop in response to persistent mesoscale SST features associated with the Kuroshio Extension, Gulf Stream, South Atlantic, and Agulhas Return Current regions. Midlatitude SST fronts also significantly modify surface wind direction; the surface wind speed and direction responses to typical SST differences of about 2°–4°C are, on average, about 1–2 m s⁻¹ and 4°–8°, respectively, over all four regions. Wind speed perturbations are positively correlated and very nearly collocated spatially with the SST perturbations. Wind direction perturbations, however, are displaced meridionally from the SST perturbations, with cyclonic flow poleward of warm SST and anticyclonic flow poleward of cool SST. Previous observational analyses have shown that small-scale perturbations in the surface vorticity and divergence fields are related linearly to the crosswind and downwind components of the SST gradient, respectively. When the vorticity and divergence fields are analyzed in curvilinear natural coordinates, the wind speed contributions to the SST-induced vorticity and divergence depend equally on the crosswind and downwind SST gradients, respectively. SST-induced wind direction gradients also significantly modify the vorticity and divergence fields, weakening the vorticity response to crosswind SST gradients while enhancing the divergence response to downwind SST gradients. Close

• 3. [Article] Dynamical Analysis of the Boundary Layer and Surface Wind Responses to Mesoscale SST Perturbations

  The dynamical response of the marine atmospheric boundary layer (MABL) to mesoscale sea surface temperature (SST) perturbations is investigated over the Agulhas Return Current during winter from a 1-month, ...

  Citation × Citation Citation Abstract Copy Title: Dynamical Analysis of the Boundary Layer and Surface Wind Responses to Mesoscale SST Perturbations Author: Chelton, Dudley B., Thum, Nicolai, Samelson, Roger M., O’Neill, Larry W., Esbensen, Steven K. The dynamical response of the marine atmospheric boundary layer (MABL) to mesoscale sea surface temperature (SST) perturbations is investigated over the Agulhas Return Current during winter from a 1-month, high-resolution, three-dimensional simulation using the Weather Research and Forecasting (WRF) mesoscale model. A steady lower boundary condition for July 2002 is obtained using SST measurements from the Advanced Microwave Scanning Radiometer on the Earth Observing System (EOS)–Aqua satellite (AMSR-E). The WRF models’ ability to accurately simulate the SST-induced surface wind response is demonstrated from a comparison with satellite surface wind observations from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite. Relevant features of this simulation include a quasi-periodic distribution of mesoscale SST perturbations with spatial scales ~200 km and strong winds that lead to a large surface sensible heat flux response, whose broad range of 80–100 W m⁻² between warm and cool SST perturbations is much larger than seen in most previous simulations of mesoscale wind–SST coupling. This simulation provides the first realistic example of vertical turbulent redistribution of momentum driven by the SST-induced surface heating perturbations acting in concert with the SST-induced pressure gradients to accelerate near-surface flow toward warm water and decelerate near-surface flow toward cool water. This simulation is also the first example of a near-surface wind speed response to mesoscale SST perturbations that differs qualitatively and substantially from the vertically averaged MABL wind response. In the vertically averaged MABL momentum budget, the surface wind stress acts as a drag on the SST-induced perturbation flow as it is being accelerated by SST-induced pressure gradients. However, only in the middle and upper reaches of the MABL does the turbulent stress divergence act as a drag on the SST-induced winds perturbations in this simulation. These mesoscale SST perturbations are also shown to modify the wind direction within the MABL. Dynamically, this is accomplished through SST-induced perturbations to the crosswind components of the pressure gradient, turbulent stress divergence, and the Coriolis force. Title: Dynamical Analysis of the Boundary Layer and Surface Wind Responses to Mesoscale SST Perturbations Author: Chelton, Dudley B., Thum, Nicolai, Samelson, Roger M., O’Neill, Larry W., Esbensen, Steven K. The dynamical response of the marine atmospheric boundary layer (MABL) to mesoscale sea surface temperature (SST) perturbations is investigated over the Agulhas Return Current during winter from a 1-month, high-resolution, three-dimensional simulation using the Weather Research and Forecasting (WRF) mesoscale model. A steady lower boundary condition for July 2002 is obtained using SST measurements from the Advanced Microwave Scanning Radiometer on the Earth Observing System (EOS)–Aqua satellite (AMSR-E). The WRF models’ ability to accurately simulate the SST-induced surface wind response is demonstrated from a comparison with satellite surface wind observations from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite. Relevant features of this simulation include a quasi-periodic distribution of mesoscale SST perturbations with spatial scales ~200 km and strong winds that lead to a large surface sensible heat flux response, whose broad range of 80–100 W m⁻² between warm and cool SST perturbations is much larger than seen in most previous simulations of mesoscale wind–SST coupling. This simulation provides the first realistic example of vertical turbulent redistribution of momentum driven by the SST-induced surface heating perturbations acting in concert with the SST-induced pressure gradients to accelerate near-surface flow toward warm water and decelerate near-surface flow toward cool water. This simulation is also the first example of a near-surface wind speed response to mesoscale SST perturbations that differs qualitatively and substantially from the vertically averaged MABL wind response. In the vertically averaged MABL momentum budget, the surface wind stress acts as a drag on the SST-induced perturbation flow as it is being accelerated by SST-induced pressure gradients. However, only in the middle and upper reaches of the MABL does the turbulent stress divergence act as a drag on the SST-induced winds perturbations in this simulation. These mesoscale SST perturbations are also shown to modify the wind direction within the MABL. Dynamically, this is accomplished through SST-induced perturbations to the crosswind components of the pressure gradient, turbulent stress divergence, and the Coriolis force. Close

• 4. [Article] High-Resolution Satellite Measurements of the Atmospheric Boundary Layer Response to SST Variations along the Agulhas Return Current

  The marine atmospheric boundary layer (MABL) response to sea surface temperature (SST) perturbations with wavelengths shorter than 30° longitude by 10° latitude along the Agulhas Return Current (ARC) is ...

  Citation × Citation Citation Abstract Copy Title: High-Resolution Satellite Measurements of the Atmospheric Boundary Layer Response to SST Variations along the Agulhas Return Current Author: Chelton, Dudley B., Esbensen, Steven K., Wentz, Frank J., O’Neill, Larry W. The marine atmospheric boundary layer (MABL) response to sea surface temperature (SST) perturbations with wavelengths shorter than 30° longitude by 10° latitude along the Agulhas Return Current (ARC) is described from the first year of SST and cloud liquid water (CLW) measurements from the Advanced Microwave Scanning Radiometer (AMSR) on the Earth Observing System (EOS) Aqua satellite and surface wind stress measurements from the QuikSCAT scatterometer. AMSR measurements of SST at a resolution of 58 km considerably improves upon a previous analysis that used the Reynolds SST analyses, which underestimate the short-scale SST gradient magnitude over the ARC region by more than a factor of 5. The AMSR SST data thus provide the first quantitatively accurate depiction of the SST-induced MABL response along the ARC. Warm (cold) SST perturbations produce positive (negative) wind stress magnitude perturbations, leading to short-scale perturbations in the wind stress curl and divergence fields that are linearly related to the crosswind and downwind components of the SST gradient, respectively. The magnitudes of the curl and divergence responses vary seasonally and spatially with a response nearly twice as strong during the winter than during the summer along a zonal band between 40° and 50°S. These seasonal variations closely correspond to seasonal and spatial variability of large-scale MABL stability and surface sensible heat flux estimated from NCEP reanalysis fields. SST-induced deepening of the MABL over warm water is evident in AMSR measurements of CLW. Typical annual mean differences in cloud thickness between cold and warm SST perturbations are estimated to be about 300 m Title: High-Resolution Satellite Measurements of the Atmospheric Boundary Layer Response to SST Variations along the Agulhas Return Current Author: Chelton, Dudley B., Esbensen, Steven K., Wentz, Frank J., O’Neill, Larry W. The marine atmospheric boundary layer (MABL) response to sea surface temperature (SST) perturbations with wavelengths shorter than 30° longitude by 10° latitude along the Agulhas Return Current (ARC) is described from the first year of SST and cloud liquid water (CLW) measurements from the Advanced Microwave Scanning Radiometer (AMSR) on the Earth Observing System (EOS) Aqua satellite and surface wind stress measurements from the QuikSCAT scatterometer. AMSR measurements of SST at a resolution of 58 km considerably improves upon a previous analysis that used the Reynolds SST analyses, which underestimate the short-scale SST gradient magnitude over the ARC region by more than a factor of 5. The AMSR SST data thus provide the first quantitatively accurate depiction of the SST-induced MABL response along the ARC. Warm (cold) SST perturbations produce positive (negative) wind stress magnitude perturbations, leading to short-scale perturbations in the wind stress curl and divergence fields that are linearly related to the crosswind and downwind components of the SST gradient, respectively. The magnitudes of the curl and divergence responses vary seasonally and spatially with a response nearly twice as strong during the winter than during the summer along a zonal band between 40° and 50°S. These seasonal variations closely correspond to seasonal and spatial variability of large-scale MABL stability and surface sensible heat flux estimated from NCEP reanalysis fields. SST-induced deepening of the MABL over warm water is evident in AMSR measurements of CLW. Typical annual mean differences in cloud thickness between cold and warm SST perturbations are estimated to be about 300 m Close