Mechanisms shaping wind convergence under extreme synoptic situations over the gulf stream region

Abstract

Air sea interaction processes over the Gulf Stream have received particular attention over the last decade. It has been shown that sea surface temperature (SST) gradients over the Gulf Stream can alter the near-surface wind divergence through changes in the marine atmospheric boundary layer (MABL). Two mechanisms have been proposed to explain the response: The vertical mixing mechanism (VMM) and the pressure adjustment mechanism (PAM). However, their respective contribution is still under debate. It has been argued that the synoptic perturbations over the Gulf Stream can provide more insight into the MABL response to SST fronts. We analyze the VMM and PAM under different atmospheric conditions obtained from a classification method that is based on the deciles of the statistical distribution of winter turbulent heat fluxes over the Gulf Stream. The lowest deciles are associated with weak air sea interactions and anticyclonic atmospheric circulation over the Gulf Stream, whereas the highest deciles are related to strong air sea interactions and a cyclonic circulation. Our analysis includes the low-and high-resolution versions of the ARPEGEv6 atmospheric model forced by observed SST, and the recently released ERA5 global reanalysis. We find that the occurrence of anticyclonic and cyclonic perturbations associated with different anomalous wind regimes can locally modulate the activation of the VMM and the PAM. In particular, the PAM is predominant in anticyclonic conditions, whereas both mechanisms are equally present in most of the cyclonic conditions. Our results highlight the role of the atmospheric circulation and associated anomalous winds in the location, strength, and occurrence of both mechanisms.