Interannual and Intraseasonal Variability of Mixed Layer Depth in Tropical Indian Ocean

Oceanic mixed layer depth (MLD) variations are known to modulate air-sea interactions and primary productivity. Most studies discussing MLD variations in the Indian Ocean (IO) have focussed to date on the seasonal timescale. Here, we investigate IO MLD intraseasonal and interannual variations from an eddy-permitting numerical simulation and observational datasets. We show that intraseasonal and interannual MLD variability have roughly the same amplitude, two to four times smaller than the seasonal cycle, except in the equatorial IO where non-seasonal variations dominate. The Indian Ocean Dipole is responsible for most interannual MLD variations in the 10°N-10°S band, with positive phases being associated with a shallow MLD near the coast of Java-Sumatra and a deepening between 5° and 10°S in the central IO. El Niño and anomalous summer monsoons have comparatively weak signatures. Active/break phases of the summer monsoon are responsible for most summer MLD intraseasonal variations in the Bay of Bengal and eastern equatorial IO. Southern Arabian Sea summer MLD variations are largely driven by seemingly independent intraseasonal fluctuations of the Findlater jet. During winter, active phases of the Madden-Julian Oscillation deepen the MLD in the 0-10°S band, especially near Sumatra. Large intraseasonal MLD signals develop in the northern Arabian Sea in winter, in response to advection of anomalously warm or cold air from the adjacent continental areas. The mechanisms driving the MLD variability (generally a mix of wind stirring and buoyancy fluxes) and the impact of these MLD variations on surface layer chlorophyll are finally discussed.