A 2D coupled atmosphere-ocean model study of air-sea interactions during a cold air outbreak over the Gulf Stream

Abstract

The two-dimensional, Advanced Regional Prediction System (ARPS) has been coupled with the Princeton Ocean Model to study air-sea interaction processes during an extreme cold air outbreak over the Gulf Stream off the southeastern United States. Emphases have been placed on the development of the mesoscale front and local winds in the lower atmosphere due to differential fluxes over the land, the cold shelf water, and the warm Gulf Stream, and on how the mesoscale front and the local winds feed back to the ocean and modify the upper-ocean temperature and current fields. Model results show that a shallow mesoscale atmospheric front is generated over the Gulf Stream and progresses eastward with the prevailing airflow. Behind the front, the wind intensifies by as much as 75% and a northerly low-level wind maximum with speeds near 5 m s-1 appears. The low-level northerly winds remain relatively strong even after the front has progressed past the Gulf Stream. The total surface heat flux in the coupled experiment is about 10% less than the total surface heat flux in the experiment with fixed SST, suggesting that the oceanic feedback to the atmosphere might not be of leading importance. On the other hand, the response of the upper-ocean velocity field to the local winds is on the order of 20 cm s-1, dominating over the response to the synoptic winds. This suggests the modification in the atmosphere by air-sea fluxes, which induces the locally enhanced winds, has considerable impact on the ocean. That is, there is significant atmospheric feedback to the ocean through the heat-flux-enhanced surface winds.