The predictability of the Indian Ocean monsoon circulation is limited due to incomplete understanding of physical processes operating on the monsoon and other time scales, particularly interannual and intraseasonal.  In the Arabian Sea (AS), the ocean circulation reverses in accordance with the monsoon winds. Regional observations have been scant there over the past fifteen years due to piracy, so we use the Accelerated Climate Model for Energy (ACME) V0.1 with enhanced horizontal resolution in each of its components relative to standard coupled climate model resolution, to better understand the variability of the AS circulation.

The model simulated the seasonally reversing AS circulation, however the Somali Current (SC), the western boundary current off northeast Africa, was seen to reverse polewards in March/April prior to the onset in May/June of the southwest monsoon. This result is in agreement with recent altimeter-derived observations, and both the observations and the model show the early SC reversal between 5° and 10°N to be associated with the arrival of an annual period downwelling westward propagating Rossby wave, excited at the tip of southern India. In addition, the model also showed that coastal onshore winds induced a coastal poleward flow between the equator and 10°N during the inter-monsoon period. Positive Indian Ocean Dipole (IOD) mode events were found to impact the upper ocean stratification in the southern AS, through which the annual downwelling/upwelling Rossby waves propagate. In April, following the onset of a positive IOD event, anomalously warm upper ocean temperatures were found relative to normal years in both the model and Argo data. It is hypothesized that the fall upwelling (cooling) Rossby wave that usually propagates across the southern AS did not materialize due to anomalous wind stress curl conditions in the eastern AS. Hence, the upper ocean heat content of the southern AS was preconditioned to be anomalously high in the spring, likely impacting regional air-sea interaction and possibly the western boundary circulation.