Indian Ocean Intraseasonal Variability in an Ocean General Circulation Model

Abstract

The impact of atmospheric intraseasonal variability on the tropical Indian Ocean is examined with an ocean general circulation model (OGCM). The model is forced by observation-based wind stresses and surface heat fluxes from an atmospheric boundary layer model. Composites of 26 well-defined boreal spring and summer intraseasonal events from 1985 to 1994 are used to explore surface and subsurface impacts of intraseasonal oscillations in the ocean. The phase and amplitude of simulated intraseasonal sea surface temperature (SST) variations agree well with observations. The net surface heat flux dominates the composite mixed layer heat budget on intraseasonal timescales, while entrainment through the base of the mixed layer contributes locally. Horizontal advection is of secondary importance in the composite heat balance. However, inspection of individual events suggests that in individual intraseasonal events different processes may control their dynamics. A characteristic feature of equatorial intraseasonal variability is the formation of a shallow mixed layer caused by a surface freshwater cap associated with strong freshwater fluxes into the ocean. This ?barrier-layer? formation in association with mean temperature inversions significantly impacts the heat transfer across the bottom of the mixed layer during the transition from calm and clear to windy and cloudy conditions of an event, such that strong entrainment at the peak of an intraseasonal event warms rather than cools the surface. The intraseasonal mixed layer salinity budget is about equally determined by entrainment, surface freshwater fluxes, and horizontal advection. The latter is due to notable horizontal salinity gradients in the central and eastern Indian Ocean in combination with equatorial jetlike velocity anomalies that develop in response to the intraseasonal atmospheric wind forcing. Use of equatorial mooring data in 1994 was useful for understanding model phenomena on several timescales. However, the observations contained no representative intraseasonal events.