Mechanism of Interannual Variability in Western Boundary Currents along Madagascar and their Relation with the ENSO

The South Equatorial Current (SEC) in the Indian Ocean bifurcates at the east coast of Madagascar into the Northeast and Southeast Madagascar Currents (NEMC and SEMC, respectively). The NEMC and SEMC transport anomalies influence eddy activities in the Mozambique Channel and the southwest of Madagascar. These eddies not only influence the ecosystems in the Mozambique Channel, but may also modulate the global thermohaline circulation by affecting water mass exchange between the Indian and Atlantic Oceans through their influence on the Agulhas Ring shedding. Therefore, to enhance understanding of the dynamical connection between NEMC and SEMC variability and physical and biological variability in their downstream, the dynamical mechanism of interannual variations in the NEMC and SEMC transport is investigated.

The NEMC (SEMC) transport in reanalysis data undergoes interannual variations with the peak-to-peak amplitude of about 15 (6) Sv. The time-dependent Island Rule indicates that the above interannual variations are due to the responses to anomalous meridional interior transport. This is a result of westward popagating Rossby waves generated by Ekman pumping anomalies over 60°E – 90°E. It is shown that the NEMC and SEMC transports are correlated with the Niño 3.4 index with 5-15 months lag and ENSO-related diabatic heating anomalies over the western tropical Pacific generate wind stress curl anomalies over the southern Indian Ocean. These wind stress anomalies are also modified by local SST anomalies in the southeastern Indian Ocean. The above influences from the local and remote SST anomalies are confirmed by atmospheric general circulation model experiments.