Mesoscale Enhancement of Turbulent Air-Sea Heat Exchange in the Tropics

Abstract:

Heat exchange across the air-sea interface in the tropics plays a key role in global climate, with turbulent heat flux being a crucial component of the surface energy balance. The Global Tropical Moored Buoy Array (GTMBA), since its inception in the mid-1980s, has enabled the development of robust turbulent heat flux climatologies through the sustained measurement of bulk variables related to turbulent heat exchanges. In addition, the high temporal resolution buoy measurements at hourly intervals allow for the calculation of “mesoscale enhancement” (ME) of turbulent heat fluxes due to disturbed weather conditions throughout the tropics. Earlier studies of ME using buoy data were limited to a few years in the Pacific only, but the GTMBA now covers three ocean basins with decades of observations, providing an unprecedented view of ME in the tropics. In this study we focus on latent heat fluxes, which contribute the most to turbulent heat exchange. Using the COARE bulk algorithm, latent heat flux was found to typically range between 100-200 W m^-2 in all three tropical ocean basins. Large ME occurred most frequently at temperatures above 27°C, a well-known threshold for deep atmospheric convection. ME contributed most to total latent heat flux in the Indo-Pacific Warm Pool where winds were on average weak but SSTs were typically above 28°C. Buoys in areas with seasonally varying precipitation due to convergence zones showed similar seasonal ME cycles, while buoys in the Bay of Bengal exhibited an ME peak that aligned with the boreal spring cyclone season. There is also a clear variation of ME on interannual time scales associated with the El Niño and the Southern Oscillation (ENSO). Our results suggest that ME can significantly boost latent heat fluxes, especially in regions of deep convection, by contributing up to 100 W m^-2 on any given day.