Enhanced Wind-Driven Downwelling Flow in Warm Oceanic Eddy Features during the Intensification of Tropical Cyclone Isaac (2012): Observations and Theory

Abstract

ropical cyclones (TCs) typically produce intense oceanic upwelling underneath the storm?s center and weaker and broader downwelling outside upwelled regions. However, several cases of predominantly downwelling responses over warm, anticyclonic mesoscale oceanic features were recently reported, where the ensuing upper-ocean warming prevented significant cooling of the sea surface, and TCs rapidly attained and maintained major status. Elucidating downwelling responses is critical to better understanding TC intensification over warm mesoscale oceanic features. Airborne ocean profilers deployed over the Gulf of Mexico?s eddy features during the intensification of tropical storm Isaac into a hurricane measured isothermal downwelling of up to 60 m over a 12-h interval (5 m h?1) or twice the upwelling strength underneath the storm?s center. This displacement occurred over a warm-core eddy that extended underneath Isaac?s left side, where the ensuing upper-ocean warming was ~8 kW m?2; sea surface temperatures >28°C prevailed during Isaac?s intensification. Rather than with just Ekman pumping WE, these observed upwelling?downwelling responses were consistent with a vertical velocity Ws = WE ? Rogδ(Uh + UOML); Ws is the TC-driven pumping velocity, derived from the dominant vorticity balance that considers geostrophic flow strength (measured by the eddy Rossby number Rog = ?g/f), geostrophic vorticity ?g, Coriolis frequency f, aspect ratio δ = h/Rmax, oceanic mixed layer thickness h, storm?s radius of maximum winds Rmax, total surface stresses from storm motion Uh, and oceanic mixed layer Ekman drift UOML. These results underscore the need for initializing coupled numerical models with realistic ocean states to correctly resolve the three-dimensional upwelling?downwelling responses and improve TC intensity forecasting.