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

This paper uses the 42-yr German Estimating the Circulation and Climate of the Ocean (GECCO) synthesis data to analyze and examine the relationship of the Indian Ocean deep meridional overturning circulation (DMOC) with the Indian Ocean dipole mode (IOD). Contributions of various dynamical processes are assessed by decomposing the DMOC into the Ekman and geostrophic transport, the external mode, and a residual term. The first three terms successfully describe the DMOC with a marginal residual term. The following conclusions are obtained. First, the seasonal cycle of the DMOC is mainly determined by the Ekman component. The exception is during the transitional seasons (March-April and September-October) in the northern Indian Ocean Basin, where the geostrophic component dominates. Second, at the beginning phase of the IOD (May-June), the Ekman component dominates the DMOC structure; at and after the peak phase of the IOD (September-December), the DMOC structure is primarily determined by the geostrophic component in correspondence with the well-developed sea surface temperature anomalies, while the wind (and thus the Ekman component) plays a secondary role south of 10 degrees S and contributes negatively within the zonal band of 10 on both sides of the equator. Therefore, there exists a surface to deep-ocean connection through which IOD-related surface wind and ocean temperature anomalies are transferred down to the deep ocean. Westward-propagating signals are observed even in the deep ocean, suggesting possible roles of Rossby waves in transferring the surface signal to the deep ocean.