Basin-wide sea level coherency in the tropical Indian Ocean driven by Madden–Julian Oscillation

Abstract

Changes in sea level may be attributed either to barotropic (involving the entire water column) or baroclinic processes (governed by stratification). It has been widely accepted that barotropic sea level changes in the tropics are insignificant at intraseasonal time scales (periods of 30–80 days). Based on bottom pressure records, we present evidence for significant basin-wide barotropic sea level variability in the tropical Indian Ocean during December–April with standard deviations amounting to ∼30–60% of the standard deviation in total intraseasonal sea level variability. The origin of this variability is linked to a small patch of wind over the Eastern Indian Ocean, associated with boreal winter Madden–Julian Oscillations (MJO). These large fluctuations are likely to play a prominent role in the intraseasonal sea level and mass budgets. Because of their much faster propagation than baroclinic processes, they allow the basin to adjust to climatic perturbations much more rapidly than was previously thought.

The intraseasonal barotropic response of the ocean has been assumed to be negligible in tropical regions. We show that boreal winter MJOs, aided by ocean stratification, trigger a significant basin-wide barotropic sea level response in the tropical Indian Ocean at intraseasonal time scales.