Surface mixed layer heat and salinity budget in the Central Equatorial Indian Ocean

Abstract

The oceanic surface mixed layer (ML) heat and salinity budgets in the central equatorial Indian Ocean are estimated using measurements taken from two DYNAMO and two RAMA moorings during September 2011 to January 2012. The broader central to eastern tropical ML salinity (MLS) budget in boreal fall and winter between 2011 to 2015 is also evaluated using satellite and data products. The ML heat variation is mostly modulated by 1-D processes while MLS is modulated by 3-D processes at intraseasonal to seasonal time scales. During MJO active phases, ML is cooled by the net surface heat flux and the turbulent heat flux at the bottom of the ML. During MJO suppressed phases, ML is heated by the net surface heat flux. MLS does not vary greatly between different phases of MJOs since 1-D processes largely compensate each other and leaves the MLS increasing by horizontal advection. A barrier layer exerts control on the turbulent fluxes. This study provides a new measure of barrier layer strength by a “barrier layer potential energy (BLPE),” which depends on the thickness of the barrier layer, the thickness of the ML, and the density stratification across the isothermal layer. In boreal fall the MLS increases due to horizontal advection and turbulent mixing, yet the precipitation that often associated with MJOs events freshens the MLS. In boreal winter the mooring MLS decreases due to zonal advection and precipitation at the equator, and by meridional advection and precipitation at 1.5°S. The MLS budget estimated using multi-year satellite and data products yields qualitatively consistent results as that computed using mooring measurements. The maximal increase (decrease) of MLS at central and eastern equatorial Indian Ocean resembles the eastward development (westward retreat) of the equatorial salty tongue in boreal fall (winter). The horizontal advection acts to transport spicier, i.e., warmer and saltier, water mass from the northwest. The vertical cooling dominates the ML heat budget so that the ML becomes cooler and saltier and therefore denser. The advective and turbulent fluxes at the base of ML vary greatly in the central equatorial Indian Ocean at O (100 km) horizontal scales, and between different phases of MJOs and seasons, suggesting the important and complicated roles of oceanic processes in the surface ML heat and salinity budgets. Therefore, models should properly estimate the oceanic BLPE, background shear and surface forcing for ML heat variation, and the strength and timing of the equatorial currents as well as the salinity gradients at least on 100-km resolution every few days for MLS variation.