Numerical Investigation of Tropical Indian Ocean Barrier Layer Variability

The barrier layer (BL) is part of the surface ocean isolated from the local air-sea exchange processes despite it's great potential to store the heat beneath the mixed layer. The tropical oceans are known for the presence of BL. The interannual variability of the BL of the tropical Indian Ocean is examined in this study with a concomitant analysis of observations and outputs from an ocean general circulation model. The model faithfully reproduces the tropical variability of the dynamics and thermodynamics of the Indian Ocean when compared with observations and other studies. The BL thins (thickens) in the eastern (central-to-western) tropical Indian Ocean during the positive phase of the Indian Ocean Dipole Mode (IOD). The individual roles of five major forcing mechanisms on BL-IOD coupling are examined. The thinning in the east is caused by upward heaving thermocline due to upwelling Kelvin waves during the positive phase of the IOD, causing the isothermal layer to shoal steeper than the mixed layer. The surface freshwater fluxes substantiate the thinning of BL in the east during this time, while the net heat fluxes thin the BL in the entire tropical Indian Ocean. The basin-wide salinity (temperature) dynamics have widespread and complementary impacts, a BL thickening (thinning) in the equatorial and eastern tropical Indian Ocean during positive IOD years, with the former overwhelming the latter. The thickening of BL in the central-to-western tropical Indian Ocean during positive IOD is due to a combined action of salinity and freshwater forcing. The thermocline heaving, temperature and salinity dynamics and buoyancy forcing could explain the net variability of BL in the tropical Indian Ocean almost entirely in the model. The ocean-only response shows that a BL weakens the shoaling (deepening) of the ocean mixed layer during positive (negative) IOD years in the eastern tropical Indian Ocean. As barrier layers store heat beneath the mixed layer, their role in Indian Ocean weather and climate needs to be examined with coupled model experiments.