The Contribution of Warm SSTA to the Intensity of “Ida”

Highlight: The contribution of warm SSTA to the intensity of “Ida”

Fig. 1: The day before Ida went ashore in Louisiana North Atlantic SSTA analysis from the Climate Change Institute at the University of Maine.

Fig. 2: Early September 2021 SSTA analysis for the North Atlantic basin.

Discussion: The ferocity of “Ida” from landfall in Louisiana to the storm’s U.S. exit in the Northeast U.S. was unprecedented. Damage left behind caused by unusually high-intensity wind across southeast Louisiana and the attendant flooding from extreme rainfall and storm surge plus the historic rainfall some meteorologists identify as a 1-in-500-year event across the northern Mid-Atlantic States especially New York City will take weeks/months to repair.

What caused “Ida” to be so powerful over a long inland-stretch? A significant contributor was the western North Atlantic SSTA pattern. On August 28, the day before “Ida” went inland southeast Louisiana, the SST off the south and southeast coast was much warmer than normal (Fig. 1). Normally, late August SST in this zone is near 85F/29C. SSTA analysis as “Ida” moved into these waters indicated +2C to +3C anomalies implying actual SST were in the 87-89F/31-32C range. “Ida” drifting over this warm water just-prior to reaching the southeast Louisiana Coast enable intensification to a strong category-4 major hurricane.

“Ida” weakened after moving inland. However, as “Ida” transitioned into an extra-tropical cyclone crossing the northeast Tennessee Valley and heading toward the northern Mid-Atlantic States the energy release to the atmosphere related to that transition and caused by interaction with a cold mid-latitude upper trough was enhanced by the entrainment of very warm/moist air off the much warmer than normal western North Atlantic basin. SSTA off the Mid-Atlantic and Northeast U.S. Coast are in the +2C to +5C range. SST are well into the 80’s in the Gulf Stream and well into the 70’s off the New Jersey Coast toward southern New England. The much warmer-than-normal ocean surface contributed excessive low atmosphere moisture into “Ida” passing through the Northeast Corridor enhancing rainfall to historic levels (near 9 inches of rain from northeast New Jersey to western Connecticut and centered on NYC).

Ida’s passage through the Gulf of Mexico has up-welled cooler water although the western Gulf of Mexico remains unusually warm (Fig. 2). Waters off the East Coast remain unusually warm with excessive warmth off the Northeast Coast. Hurricane Larry may benefit the U.S. East Coast hurricane risk by up-welling cooler water as the storm approaches Bermuda and turns north and northeast from there next week.

Clearly, warm SSTA are contributing to the tendency of tropical cyclones to become unusually powerful and intensify at unusual speed. This characteristic has been known while tropical cyclones are over water. However, the contribution of warm SSTA to the intensity of “Ida” while inland is a new discovery.