Observed Extreme Air-Sea Heat Flux Variations during Three Tropical Cyclones in the Tropical Southeastern Indian Ocean

Six-month buoy-based heat flux observations from the poorly sampled tropical southeastern Indian Ocean are examined to document the extremes during three tropical cyclones (TCs) from December 2018 to May 2019. The most striking feature at the mooring site (16.9 degrees S, 115.2 degrees E) during the TCs is the extensively suppressed diurnal cycle of the net surface flux (Qnet), with a mean daytime (nighttime) reduction of 470 (131) W m(-2), a peak decrease at approximately noon of 695 W m(-2) and an extreme drop during TC Riley of 800 W m(-2). The mean surface cooling in the daytime is primarily contributed by the 370 W m(-2) decrease in shortwave radiation associated with the increased cloudiness. The air-sea turbulent heat fluxes increase by approximately 151 W m(-2) in response to the enhanced wind speed under near-neutral boundary conditions. The daily mean rainfall-induced cooling is 8 W m(-2), with a maximum magnitude of 90 W m(-2). The mean values, seasonal variation, and synoptic variability of the characteristic heat fluxes are used to assess the new reanalysis data from ERA5 and MERRA2 and the analyzed OAFlux. The overall performance of the high-frequency net heat flux estimates at the synoptic scale is satisfactory, but the four flux components exhibit different quality levels. A serious error is that ERA5 and MERRA2 poorly represent TCs, and they show significant daily mean Qnet biases with opposite directions, -59 W m(-2) (largely due to the overestimated latent heat with a bias of -76 W m(-2)) and 50 W m(-2) (largely due to the overestimated shortwave radiation with a bias of 41 W m(-2)), respectively.

Automated summary

The study examines buoy-based heat flux observations during tropical cyclones in the southeastern Indian Ocean, highlighting the significant diurnal variations in net surface flux primarily influenced by shortwave radiation and wind speed changes. Additionally, rainfall-induced cooling also plays a role in the surface cooling during these extreme weather events.