The monsoon circulation of the Indian Ocean

In this paper, we review observations, theory and model results on the monsoon circulation of the Indian Ocean. We begin with a general overview, discussing wind-stress forcing fields and their anomalies, climatological distributions of stratification, mixed-layer depths, altimetric sea-level distributions, and seasonal circulation patterns (Section 2). The three main monsoon circulation sections deal with the equatorial regime (Section 3), the Somali Current and western Arabian Sea (Section 4), and the Bay of Bengal, seasonally reversing monsoon currents south of India and Sri Lanka, and the eastern and central Arabian Sea (Section 5). For the equatorial regime, we discuss equatorial jets and undercurrents, their interactions with the eastern and western boundaries, and intraseasonal and vertically propagating signals. In the Somali Current section, we describe the ocean’s responses to the summer and winter monsoon winds, and outline the modelling efforts that have been carried out to understand them. In the Bay of Bengal section, we present observational and modeling evidence showing the importance of remote forcing from the east, which to a large extent originates along the equator. In the following three sections, we review the southern-hemisphere subtropical regime and its associated boundary currents (Section 6), the Indonesian Throughflow (Section 7), the Red Sea and Persian Gulf circulations (Section 8), and discuss aspects of their interactions with other Indian-Ocean circulations. Next, we describe the Indian Ocean’s deep and shallow meridional overturning cells (Section 9). Model results show large seasonal variability of the meridional overturning streamfunction and heat flux, and we discuss possible physical mechanisms behind this variability. While the monsoon-driven variability of the deep cell is mostly a sloshing motion affecting heat storage, interesting water-mass transformations and monsoonal reversals occur in the shallow crossequatorial cell. In the mean, the shallow cell connects the subduction areas in the southern subtropics and parts of the Indonesian Throughflow waters with the upwelling areas of the northern hemisphere via the cross-equatorial Somali Current. Its near-surface branch includes a shallow equatorial roll that is seasonally reversing. We close by looking at coupled ocean-climate anomalies, in particular the large events that were observed in the tropical and subtropical Indian Ocean in 1993/94 and 1997/98. These events have been interpreted as an independent Indian-Ocean climate mode by some investigators and as an ENSO-forced anomaly by others.  2001 Elsevier Science Ltd. All rights reserved.