Air-Sea Interaction in the Gulf Stream Region

Abstract

The Gulf Stream (GS) is a region with large mesoscale activity, as strong sea surface temperature gradients and oceanic eddies are developed in the area. These mesoscale features force the atmosphere by affecting the near surface wind. In this study, we investigate this mesoscale air-sea interaction using several air-sea coupled configurations of the Global Climate Models: ECMWF-IFS, HadGEM3 and MPI-ESM. The climatology and the internal variability of variables such as sea surface temperature (SST), turbulent heat fluxes, precipitation and near surface wind divergence are studied for the time period between 1951 and 2049. The horizontal resolution of the oceanic component of the models seems to be determinant factor for the accurate representation of the air-sea coupling. We find that the models with eddy parameterized oceanic component are incapable of accurately describing several atmospheric fields over this region. Also, only the eddy resolving oceanic component is capable of describing correctly the separation of GS at Cape Hatteras, whereas the rest of the configurations shift the GS path northwards. Even though the atmospheric resolution does not seem to be important for the climatology, it is significant for the variability of many atmospheric fields. Next, we focus on the two main mechanisms which explain the wind response to the SST fronts. These are: the Vertical Mixing (VMM) and Pressure Adjustment (PAM) mechanisms. In accordance with the VMM, the destabilization of the atmosphere over the warmer SSTs leads to intensification of the vertical mixing in the boundary layer. Thus, large momentum from aloft is transported downwards resulting in acceleration of the near surface wind. On the other hand, the PAM suggests that the differential heating of the atmosphere at each side of the SST front generates pressure gradient forces. These pressure anomalies are responsible for the convergence and divergence zones on the warm and cold flank of GS respectively. The strength of the two aforementioned mechanisms is investigated for all the configurations. For the VMM, we find that the coupling between the downwind SST gradients and the wind divergence becomes stronger by increasing the resolution. However, the coupling between the crosswind SST gradients and the wind curl is not statistically significant for all the configurations. Regarding the PAM, we find that the mean sea level pressure adjustment due to SST forcing is stronger in the higher atmospheric resolution configurations. In contrast, the coupling between the mean sea level pressure Laplacian and the wind convergence gets slightly weaker by increasing the resolution. This fact is likely due to the large scale processes that can contribute to the generation of secondary circulations. For the strength of the air-sea coupling, both oceanic and atmospheric resolutions are significant, as a too coarse atmospheric component is not capable of capturing the mesoscale features produced by the oceanic one.