Sensitivity of East Coast winter storms to sea surface temperature gradients

Mid-latitude cyclones, known as Nor'easters when they track northward along the East Coast of North America, have a long history of producing severe, and sometimes catastrophic, blizzard conditions along the eastern seaboard. The coastal region east of the Carolinas, in association with the warm Gulf Stream current, has been identified as an epicenter of extratropical cyclogenesis in previous climatological studies; this is due in part to the semi-permanent thermal gradient found along the western edge of the Gulf Stream.

A two-part study was conducted on the sensitivity of lower-tropospheric cyclogenesis to the sea surface thermal gradient associated with the Gulf Stream. The first part is carried out by systematically reducing the magnitude of the sea surface temperature (SST) gradient by 50% for three consecutive mesoscale model simulations of the 24-25 January 2000 winter storm to verify a pre-storm baroclinic index (PSBI). This is done to test the hypothesis that numerical simulations of this case will follow deepening rates predicted by the PSBI. Results for each consecutive simulation show a significant decrease in deepening rate of 0.45 mb h-1 and 0.17 mb h-1 with each respective reduction in SST. The combined results are also in agreement with the regression fits from the Atlantic Surface Cyclone Intensification Index (ASCII) climatology.

In the second part, the Gulf Stream was shifted to the east by 1° and 2.5° of longitude for two respective experimental numerical simulations, while leaving the unique features such as curvature of the Gulf Stream and the SST values unchanged. The objectives of the second part were to (i) isolate the contribution of surface-level forcing based on the position of the Gulf Stream without changing the magnitude of the SST, and (ii) to verify ASCII from the Gulf Stream front (GSF) position parameter, as well as to test the hypothesis that by altering the track of the surface low, the feedback link to the upper-level trough will be weakened, thus reducing the surface-level cyclogenesis. Results of the second part show that the surface low pressure tends to track along the frontal boundary formed over the GSF as a result of the preexisting vorticity. These results also reveal an ASCII limitation within the GSF position parameter, and show that by altering the track of the surface low, the feedback link to the upper-level trough is weakened, thus reducing the storm intensification.