The role of Gulf Stream Warm Core Eddies on East Coast Winter Storms

Efforts to relate the deepening rate of East Coast extratropical cyclones to the surface forcing through pre-storm boundary layer baroclinicity has resulted in the creation of the Atlantic Surface Cyclone Intensification Index (ASCII). An improved method, I-ASCII developed recently relates the deepening rate to an additional parameter, the strength of the associated upper level vorticity maximum. An examination of the performance of the index against independent reanalysis data from 2002 through 2004 was undertaken in order to investigate the influence of warm core eddies present along the western edge of the Gulf Stream on storm intensification.

Two case studies demonstrate the ability of I-ASCII to perform well, one in which a warm core eddy is present (December 25, 2002) and one in which no warm core eddy exists (March 17, 2004). Using I-ASCII for these storms, the predicted deepening for 12 hours are 15mb and 4mb respectively, both of which are within 1mb of the observed deepening. The strength of the associated upper level vorticity maximum for the each case are similar (21s-1 for December 25 and 16 s-1 for March 17) suggesting that much of the variation between the two deepening rates could be the result of another mechanism.

The role of the Gulf Stream warm core eddy in the intensification of the storms is investigated using high resolution mesoscale numerical simulations of these two cases. Weather Research and Forecasting (WRF) Model is used for this study. Results from the sensitivity tests involving the warm core eddy will be presented. Experiments in which the warm core eddies are removed from the SST field will be compared to the control runs in order to quantify the impact of these features on storm structure and evolution.