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

Abstract

[1] 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 À2 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 À2 in the warming phase and 25 W m À2 in the cooling phase; penetrative loss Q pen is 80 W m À2 and 70 W m À2 . 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 À2 . 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.

The Role of Salinity in the Southeastern Arabian Sea in Determining Monsoon Onset and Strength

“…As ERA5's moisture flux and surface latent heat fluxes are strongly related, it follows that surface fluxes, especially moisture and latent heat fluxes, are largely responsible for the drop off of monsoon strength. This is consistent with previous studies of the ASMWP, which found surface heat fluxes to place a critical role in the development of the ASMWP, especially in increasing the strength of the ASMWP (Rao et al, ; Sengupta et al, ). It is worth noting that the BLTA also decreases at the same time as the moisture flux trend decreases, which may have contributed to the decrease in monsoon strength.…”

“…These results are consistent with those of previous studies, which found a deeper freshwater pool and barrier layer to be associated with a stronger ASMWP (Rao et al, ). Observational studies based on the ARMEX program found that ASMWP temperatures could exceed 30 °C (Sengupta et al, ). As their study and our salt budget analysis also have connections to surface fluxes, it is useful now to look at the relationship between the barrier layer, net surface heat fluxes, and OHC in the upper 45‐m layer (Figure ).…”

“…A number of field experiments have been conducted during the southwest monsoon seasons in the north Indian Ocean to understand the air‐sea interaction processes and upper ocean variability and their coupling, such as the Bay of Bengal Monsoon Experiment (Bhat et al, ), the Joint Air‐Sea Monsoon Interaction Experiment (Webster, Bradley et al, ), and the Arabian Sea Monsoon Experiment (ARMEX‐I) during June–July 2002 and ARMEX‐II during March–June 2003 (Sengupta et al, ). ARMEX‐I had the goal of studying intense convective precipitation off the west coast of India associated with the monsoon.…”

Observed Seasonal Variations of the Upper Ocean Structure and Air‐Sea Interactions in the Andaman Sea

“…Not all of the surface shortwave radiation can be absorbed by the mixed layer. The solar penetration at the bottom of the mixed layer (Q b ) is nonnegligible when the mixed layer is very thin (Sengupta et al, 2002(Sengupta et al, , 2008. The solar penetration decays with the depth, which can be expressed by the following empirical formula (Paulson & Simpson, 1977;Qiu & Kelly, 1993):…”

“…Changes in the mixed layer temperature reflect not only the air‐sea interaction but also about the upper ocean stratification. The mixed layer heat budget analysis is a useful tool to identify the dominant oceanic and/or atmospheric processes that governs the sea surface temperature (SST) changes (de Boyer Montégut et al, ; Dong et al, ; Sengupta et al, ). To our best knowledge, however, few studies have focused on the upper ocean stratification and mixed layer heat budget in the AS due to the scarcity of observations.…”