Ocean Atmosphere Interaction and Resolution Dependence over the Gulf Stream

Abstract

Part1 :The influence of horizontal resolution of an Atmospheric Global Circulation Model on the simulation of precipitation and atmospheric deep convection above the North Atlantic and especially the Gulf Stream region is studied. It is shown that mean precipitation increases with increasing resolution. This result is confirmed with re- analyses and weather forecasts using different resolutions, ranging from 125 to 16 km. Via an analysis of the position of the jetstream it is suggested that the differences in mean precipitation in the GCM are not caused by differences in large scale circula- tion, but mainly by local phenomena. Increasing resolution in the GCM especially leads to more extreme precipitation. This also occurs in reanalyses and operational analyses. An assessment whether the increase in extreme precipitation deteriorates or improves model performance appears impossible as it depends on which observational product is used. Furthermore, 10m wind convergence has been analyzed and it is shown that the higher resolution GCM shows more extreme wind convergence events and corresponds better to wind convergence derived from observations. Additionally, the number of deep convection events above the Gulf Stream increases with resolution in GCMs, reanalyses and operational forecasts, enhancing the communication of Sea Surface Temperatures up to the troposphere.

Part 2: In a controlled experiment with a regional climate model, we investigated the influence of strong SST-gradients in the Gulf Stream region on the development of a number of wintertime storms. We show that the removal of the SST-gradients significantly impacts the storms, with the number of weakening and strengthening storms roughly balancing each other. While one of the determining factors is the track of the storms with respect to the SST-front, we show that no single explanation is possible. Therefore, we propose two separate mechanisms that explain most of the responses. The first mechanism is related to latent heat flux, the second mechanism to baroclinicity.