On the formation of central water and thermocline ventilation in the southern hemisphere

Abstract

A statistical method is used to measure the similarity between the surface T-S characteristics of the region 30°–45°S with that of the vertical T-S relationship between 10° and 20°S. This follows Iselin's notion, first proposed in 1939 ( Transactions of the American Geophysical Union, 3, 565–599) that the Central Water of the subtropical oceans originates in the Subtropical Convergence (STC), where it is formed by subduction of the fluid through Ekman pumping out of the mixed layer, and propagates towards the tropics y isopycnal spreading. His classical schematic diagram shows how the surface T-S characteristics along a meridional track across the STC coincide with the T-S characteristics of a vertical profile through the thermocline at lower latitudes. Iselin's original concept depicts, in the geostrophic regime, a strictly zonal movement of water and neglects any reference to gyre-scale oceanic circulation within a basin. The technique described here accounts for subduction along the isopycnals, and includes both meridional and zonal circulation within and between the basins. Results are presented for each of the four seasons in the three major oceanic basins using the 1° square Levitus ( Climatological Atlas of the World Ocean, National Oceanic and Atmospheric Administration, Rockville, 1982) data set. An RMS statistics is calculated based on minimising the difference in spiciness between the T-S points that lie along the same isopycnal at the surface and at depth. (Spiciness is a quantity that compares the relative contributions of temperature and salinity to density change, and is defined by a certain orthogonality condition with density.) Results show that the water most likely to be subducted to form the thermocline waters of the entire tropical South Pacific (10°–20°S) from 155°E to 100°W is the surface water of the STC in the western South Pacific between 155°E and 130°W. Formation occurs mainly during winter, but evidence for subduction is found in other seasons. The results indicate the presence of a “shadow zone” in the southeastern section of the South Pacific gyre. For the Indian Ocean, no shadow zone is present, and highest correlations again occured during winter in the STC from 72°E to 110°E. In the South Atlantic Ocean, Central Water formation occurs both in the Brazil/Malvinas Current confluence region, and also in the subtropical western Indian Ocean, indicating leakage from the Agulhas retroflection.