Oceanic Surface Mixed Layer Heat and Salinity Budgets from Mooring Observations in the Central Equatorial Indian Ocean

The oceanic surface mixed layer heat and salinity budgets are evaluated during the stretch of DYNAMO field campaign (September 2011 – January 2012) in the central equatorial Indian Ocean, with particular interests at the intraseasonal band. The budgets estimate the time rate change of mixed layer heat/ salinity, surface heat/ fresh water flux, horizontal advection fluxes and turbulent fluxes at the base of the oceanic surface mixed layer. Throughout the four months of observation, three Madden–Julian Oscillation (MJO) events were recorded at the mooring site, 0N 79E. The surface mixed layer cools down during MJO wet phases and warms up during MJO dry phases. The mixed layer salinity increases (at a rate of +0.5 psu/month) in fall following its typical seasonal trend until the arrival of a strong MJO in late November 2011. It is then followed by a series of large salinity oscillations with amplitude of ~0.4 psu at ~7-day period until the end of December.

Both the atmosphere-ocean interactions at the sea surface and the oceanic interior variabilities play important parts in regulating the surface mixed layer heat and salinity on sub-monthly to intraseasonal time scales in the central equatorial Indian Ocean. The surface heat flux and the oceanic turbulent heat flux at the base of the surface mixed layer are the two dominant terms that dictate the variation of the mixed layer heat content associated with MJOs. As for the salinity variation, the surface fresh water flux and horizontal advective flux are the two critical components, comparable in magnitude but opposite in sign. The former is largely associated with MJOs, whereas the latter exhibits noticeable bi-weekly frequencies. The turbulent salinity flux at the base of surface mixed layer is nontrivial and generally entrains saltier water to the mixed layer; therefore, resulting in a gradual increase of mixed layer salinity over the fall to early winter season. 

The central equatorial Indian Ocean is an interesting area where distinct water masses meet in both zonal and meridional directions, e.g. by the semiannual eastward zonal jet centered at the equator during monsoon transitional seasons and by oceanic equatorial waves, which complicates the oceanic surface mixed layer budgets. The quasi-permanent oceanic barrier layer that is suggested to retard the cooling of SST during surface wind events in fall leads to further complexities of the upper ocean dynamics at the central equatorial Indian Ocean.