Seasonal Cycle of Cross Equatorial Flow in the Central Equatorial Indian Ocean

This study investigates the seasonal cycle of cross equatorial flow in the upper layers of central equatorial Indian Ocean and its relation to monsoon forcing. Cross equatorial flow from the summer to the winter hemisphere in the Indian Ocean is an important regulator of the Earth’s energy balance on seasonal time scales. For this study we use the most comprehensive set of moored velocity measurements in the Indian Ocean to date, namely as set of acoustic Doppler current profiler (ADCP) measurements along 80.5°E for the period 2004-13. Cross equatorial transports during boreal summer and winter indicate that a quasi-steady Sverdrup transport balance dominates the seasonal cycle of upper-layer meridional currents. In addition, semi-annually varying westerly monsoon transition winds force Ekman convergence in the surface layer and set up transient zonal pressure gradients that drive seasonally enhanced meridional geostrophic divergence in the thermocline. Thus, a mean downwelling circulation prevails along the equator such that, in contrast to the trade wind driven Pacific and Atlantic Oceans, an equatorial cold tongue does not develop in the Indian Ocean. These results quantify expectations from ocean circulation theories with a high degree of confidence given the length of the data records. A unique feature of the circulation in the Indian Ocean, the so-called “equatorial roll” first identified in numerical model simulations, is also documented in the observations.