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  • Climate Implications of Frontal Scale Air–Sea Interaction x
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A. Foussard, G. Lapeyre, and R. Plougonven

Abstract

The response of the atmospheric boundary layer to mesoscale sea surface temperature (SST) is often characterized by a link between wind stress divergence and downwind SST gradients. In this study, an idealized simulation representative of a storm track above a prescribed stationary SST field is examined in order to determine in which background wind conditions that relationship occurs. The SST field is composed of a midlatitude large-scale frontal zone and mesoscale SST anomalies. It is shown that the divergence of the surface wind can correlate either with the Laplacian of the atmospheric boundary layer temperature or with the downwind SST gradient. The first case corresponds to background situations of weak winds or of unstable boundary layers, and the response is in agreement with an Ekman balance adjustment in the boundary layer. The second case corresponds to background situations of stable boundary layers, and the response is in agreement with downward mixing of momentum. Concerning the divergence of the wind stress, it generally resembles downwind SST gradients for stable and unstable boundary layers, in agreement with past studies. For weak winds, a correlation with the temperature Laplacian is, however, found to some extent. In conclusion, our study reveals the importance of the large-scale wind conditions in modulating the surface atmospheric response with different responses in the divergences of surface wind and wind stress.

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Larry W. O’Neill, Tracy Haack, Dudley B. Chelton, and Eric Skyllingstad

Abstract

The distribution of surface divergence in the northwest Atlantic is investigated using 10 years of satellite wind observations from QuikSCAT and a 1-yr simulation from the COAMPS atmospheric model. A band of time-mean surface convergence overlies the Gulf Stream [called here the Gulf Stream convergence zone (GSCZ)] and has been attributed previously to a local boundary layer response to Gulf Stream SST gradients. However, this analysis shows that the GSCZ results mainly from the aggregate impacts of strong convergence anomalies associated with storms propagating along the storm track, which approximately overlies the Gulf Stream. Storm surface convergence anomalies are one to two orders of magnitude greater than the time-mean convergence and produce a highly asymmetric divergence distribution skewed toward convergent winds. The sensitivity of the sign and magnitude of the time-mean divergence to extreme weather events is demonstrated through analysis using an extreme-value filter, conditional sampling based on rain occurrence, and comparison to its median and mode. Vertical velocity and surface pressure are likewise affected by strong storms, which are characterized by upward velocity and low surface pressure. Storms are thus an important process in shaping the mean state of the atmosphere in the northwest Atlantic. These results are difficult to reconcile with the prevailing view that SST “anchors” surface convergence, upward vertical velocity, and increased rain over the Gulf Stream through a local boundary layer adjustment mechanism.

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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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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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