Warm pool thermodynamics from the Arabian Sea Monsoon Experiment (ARMEX)

Abstract

Before the onset of the south Asian summer monsoon, sea surface temperature (SST) of the north Indian Ocean warms to 30-32° C. Climatological mean mixed layer depth in spring (March-May) is 10-20 m, and net surface heat flux (Q _net ) is 80-100 W m^-2 into the ocean. Previous work suggests that observed spring SST warming is small mainly because of (1) penetrative flux of solar radiation through the base of the mixed layer (Q _pen ) and (2) advective cooling by upper ocean currents. We estimate the role of these two processes in SST evolution from a two-week Arabian Sea Monsoon Experiment process experiment in April-May 2005 in the southeastern Arabian Sea. The upper ocean is stratified by salinity and temperature, and mixed layer depth is shallow (6 to 12 m). Current speed at 2 m depth is high even under light winds. Currents within the mixed layer are quite distinct from those at 25 m. On subseasonal scales, SST warming is followed by rapid cooling, although the ocean gains heat at the surface: Q _net is about 105 W m^-2 in the warming phase and 25 W m^-2 in the cooling phase; penetrative loss Q _pen is 80 W m^-2 and 70 W m^-2. In the warming phase, SST rises mainly because of heat absorbed within the mixed layer, i.e., Q _net minus Q _pen ; Q _pen reduces the rate of SST warming by a factor of 3. In the second phase, SST cools rapidly because (1) Q _pen is larger than Q _net and (2) advective cooling is ~85 W m^-2. A calculation using time-averaged heat fluxes and mixed layer depth suggests that diurnal variability of fluxes and upper ocean stratification tends to warm SST on subseasonal timescale. Buoy and satellite data suggest that a typical premonsoon intraseasonal cooling event occurs under clear skies when the ocean is gaining heat through the surface. In this respect, premonsoon SST cooling in the north Indian Ocean is different from that due to the Madden-Julian oscillation or monsoon intraseasonal oscillation.