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Niklas Schneider

Abstract

The horizontal scale dependences of in-phase and lagged imprints of ocean-mesoscale sea surface temperatures on surface winds are investigated using daily AMSR-E radiometer and QuikSCAT scatterometer observations in the Southern Ocean. Spectral transfer functions separate underlying processes dependent on large-scale winds, horizontal wavenumbers, and corresponding Rossby numbers. For Rossby numbers smaller than 1, winds reflect modulations of the Ekman layer by sea surface temperature–induced changes of hydrostatic pressure. Rossby numbers large compared to 1 suggest a balance of advection and modulations of vertical mixing. Impulse response functions reveal Southern Hemisphere, Doppler-shifted, near-inertial lee waves excited by warm ocean-mesoscale sea surface temperatures. On the right (left) flank of the downwind wake of warm air and low atmospheric pressure, winds are enhanced (diminished) due to constructive (destructive) interference of inertial turning, pressure gradient forces, and vertical mixing. Wind convergence over the warm wake is stronger compared to the upwind divergence. Time averaging smooths the response, and degrades the lee wave.

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Niklas Schneider and Bo Qiu

Abstract

The response of the atmospheric boundary layer to fronts of sea surface temperature (SST) is characterized by correlations between wind stress divergence and the downwind component of the SST gradient and between the wind stress curl and the crosswind component of the SST gradient. The associated regression (or coupling) coefficients for the wind stress divergence are consistently larger than those for the wind stress curl. To explore the underlying physics, the authors introduce a linearized model of the atmospheric boundary layer response to SST-induced modulations of boundary layer hydrostatic pressure and vertical mixing in the presence of advection by a background Ekman spiral. Model solutions are a strong function of the SST scale and background advection and recover observed characteristics. The coupling coefficients for wind stress divergence and curl are governed by distinct physics. Wind stress divergence results from either large-scale winds crossing the front or from a thermally direct, cross-frontal circulation. Wind stress curl, expected to be largest when winds are parallel to SST fronts, is reduced through geostrophic spindown and thereby yields weaker coupling coefficients.

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Kohei Takatama and Niklas Schneider

Abstract

The effect of ocean current drag on the atmosphere is of interest as a test case for the role of back pressure, because the response is independent of the thermally induced modulation of the boundary layer stability and hydrostatic pressure. The authors use a regional atmospheric model to investigate the impact of drag induced by the Kuroshio in the East China Sea on the overlying winter atmosphere. Ocean currents dominate the wind stress curl compared to the impacts of sea surface temperature (SST) fronts. Wind stress convergences and divergences are weakly enhanced even though the ocean current is almost geostrophic. These modifications change the linear relationships (coupling coefficients) between the wind stress curl/divergence and the SST Laplacian, crosswind, and downwind gradients. Clear signatures of the ocean current impacts are found beyond the sea surface: sea surface pressure (back pressure) decreases near the current axis, and precipitation increases over the downwind region. However, these responses are very small despite strong Ekman pumping due to the current. A linear reduced gravity model is used to explain the boundary layer dynamics. The linear vorticity equation shows that the oceanic influence on wind stress curl is balanced by horizontal advection decoupling the boundary layer from the interior atmosphere. Spectral transfer functions are used to explain the general response of back pressure to geostrophic ocean currents and sea surface height.

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Thomas Kilpatrick, Niklas Schneider, and Bo Qiu

Abstract

Satellite observations and modeling studies show that midlatitude SST fronts influence the marine atmospheric boundary layer (MABL) and atmospheric circulation. Here, the Weather Research and Forecasting (WRF) mesoscale model is used to explore the atmospheric response to a midlatitude SST front in an idealized, dry, two-dimensional configuration, with a background wind oriented in the alongfront direction.

The SST front excites an alongfront wind anomaly in the free atmosphere, with peak intensity just above the MABL. This response is nearly quasigeostrophic, in contrast to the inertia–gravity wave response seen for cross-front background winds. The free-atmosphere response increases with the background wind , in contrast to previously proposed SST frontal MABL models.

The MABL winds are nearly in Ekman balance. However, a cross-front wind develops in the MABL as a result of friction and rotation such that the MABL cross-front Rossby number ε ≈ 0.2. The MABL vorticity balance and scaling arguments indicate that advection plays an important role in the MABL dynamics. Surface wind convergence shows poor agreement with MABL depth-integrated convergence, indicating that the MABL mixed-layer assumption may not be appropriate for SST frontal zones with moderate to strong surface winds.

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