Water subducted into the Indian Ocean subtropical gyre

Subduction rates of the water into southern Indian Ocean permanent thermocline from 23.5 to 26.9 kg m−3 are calculated from a kinematic and a water-age approach and are compared with surface buoyancy flux calculations. The different estimates compare well within error margins, indicating that the effect of transient eddies on the subduction process is of second order. Considering only northward transport components, the overall transfer of water into the Indian Ocean thermocline is about 34 Sv (1 Sv=106 m3 s−1), with equal contributions from lateral and vertical (essentially Ekman pumping) components. Ekman pumping dominates the upper density range (<25.2 kg m−3), while for denser water the largest contribution stems from lateral input of Mode Waters. High silicate concentrations in the Mode Waters indicate that Southern Ocean waters participate in the formation process. The source water properties in salinity, oxygen, and nutrients along the surface of the deepest winter mixed layer are given.