Enhanced Ahead-of-Eye Cooling of Stratified Coastal Oceans in Tropical Storms

Integrated ocean observations from Hurricane Irene (2011) reveal widespread and significant ahead-of-eye cooling (at least 5°C and up to 11°C) as it crossed the seasonally stratified continental shelf of the Mid-Atlantic Bight of North America. Buoys and gliders deployed in the storm allow the detailed evolution of the surface temperature to be examined at select points, revealing approximately 80% of the total cooling occurs before eye passage. A range of ocean models were used to diagnose the processes responsible for the observed cooling. In Irene, 1D vertical mixing models generate only 17% of the total cooling ahead of eye, while deepwater 3-D models forced by Irene’s nearly symmetrical offshore windfield produce an approximately 50-50 split in the cooling between the front and back side. A 3-D coastal ocean model (ROMS) generates a wind-forced two-layer circulation in the stratified Mid-Atlantic not present in the 1-D and 3-D deepwater models. The resultant shear-induced mixing more accurately reproduces both the magnitude and timing of the surface cooling with respect to eye passage. Atmospheric simulations establish that this cooling was the missing contribution required to reproduce Irene’s accelerated reduction in intensity over the Mid Atlantic Bight.

Historical buoys from 1985 to present show that ahead-of-eye cooling occurred beneath all 11 tropical cyclones that traversed along the Mid Atlantic Bight continental shelf during stratified summer conditions. The buoys also reveal that an average of about 74% of the cooling occurs ahead of eye. A Yellow Sea buoy array similarly revealed significant and rapid ahead-of-eye cooling during Typhoon Muifa (2011). These findings establish that including realistic 3D coastal ocean processes in forecasts of landfalling storm intensity and impacts will be increasingly critical to mid-latitude population centers as sea levels rise and tropical cyclone maximum intensities migrate poleward.