Deep Meridional Overturning Circulation in the Indian Ocean and Its Relation to Indian Ocean Dipole

The Indian deep ocean has rarely been discussed in relation to the Indian Ocean dipole mode (IOD) albeit its pronounced interannual variability. This study characterizes the Indian meridional overturning circulation (DMOC) during IOD events based on 42-year GECCO synthesis data. To assess contributions of various dynamical processes, the DMOC is decomposed into Ekman component, geostrophic current, the external mode accounting for zonal topographic gradients, and a residual term. The first three terms successfully describe the DMOC with only a marginal contribution from the residual term. Our analysis concludes the following results. First, the seasonal cycle of the DMOC is mainly determined by the Ekman component. Exception is during the transitional season (March-April and September-October) in the northern Indian Ocean basin, where the geostrophic component dominates. Second, IOD influences the deep ocean through wind and ocean temperature anomalies. At the beginning phase of IOD (May-June), the Ekman component dominates the DMOC structure; at and after the peak phase of IOD (September-December), the DMOC structure is primarily determined by the geostrophic component in correspondence with the well-developed ocean temperature anomalies; the wind (thus the Ekman component) plays a positive but secondary role south of 10°S and contributes negatively within the zonal band of 10 degrees both sides of the equator. Therefore, there exists a surface-to-deep ocean connection in the Indian Ocean, through which what occurs on the ocean surface is transferred down, possibly by the aid of the prominent annual wind-driven Rossby waves.