Thermocline circulation and ventilation in the Indian Ocean derived from water mass analysis

Abstract

A mixing model, which combines cluster analysis with optimum multiparameter (OMP) analysis, is used to determine the spreading and mixing of water masses in the thermocline of the Indian Ocean, focusing on the ventilation process for the thermocline in the northern hemisphere. Mixing ratios are quantified and plotted on five isopycnal surfaces covering the depht range 150–800 m, two meridional sections along 60°E and 90°E, and one zonal section along 10°S. Three water masses are identified in the thermocline by cluster analysis. Indian Central Water (ICW) is subducted at the Subtropical Front in the southern Indian Ocean and advected with the southern subtropical gyre. Australasian Mediterranean Water (AAMW) enters from the Indonesian seas as the result of throughflow from the Pacific Ocean. Red Sea Water (RSW) combines with water from the Persian Gulf to provide a minor source. A fourth water mass identified by cluster analysis occurs in the Bay of Bengal; it is labelled North Indian Central Water (NICW) and interpreted as aged ICW. Cluster analysis did not produce evidence for the existence of Equatorial Water, a water mass often referred to in the literature. Mixing ratios and pathways of the thermocline water masses are established using OMP analysis. The input of RSW is insufficient to renew the thermocline waters of the northern Indian Ocean, which therefore has to be ventilated by advection from the south. The jet-like of AAMW produces one of the strongest frontal systems of the world ocean's thermocline, suppressing meridional motion across 10–15°S east of 50°E. This leaves the western boundary currents as the only region for advective transfer of thermocline water between the hemispheres. ICW is shown to advect along this path on the isopyenal surface σ θ =26.7 (depth range 300–400 m). Above and below this surface ICW movement into the northern hemisphere is accompanied by significant diapycnal mixing. The annual mean distribution of ICW shows that it ages rapidly as it crosses the equator. This is interpreted as the result of seasonally varying advection, producing very little mean net transport across the equator. As a consequence, ICW in the northern hemisphere is extremely low in oxygen and high in nutrients. AAMW can be traced to 70°E, but is mixed into the ICW background and no longer recognizable as AAMW by the time it leaves the Indian Ocean with Agulhas Current eddies. It is argued that the so-called Equatorial Water is the result of the mixing process, i.e. a mixture of two well-defined water masses that should not be considered a water mass in its own right.