Optimum multiparameter analysis of the water mass structure in the Atlantic Ocean thermocline

Abstract

An analysis of the water mass structure of the Atlantic Ocean central layer is conducted by applying optimum multiparameter (OMP) analysis to an expansive historical data set. This inverse method utilises hydrographic property fields to determine the spreading and mixing of water masses in the permanent thermocline. An expanded form of OMP analysis is used, incorporating Redfield ratios and pseudo-age to correct for the non-conservative behaviour of oxygen and nutrients over large oceanic areas. Three water masses are considered to contribute to the central layer of the Atlantic Ocean. One of these is formed in each hemisphere of the Atlantic Ocean and the other advects around the southern tip of Africa from its formation region in the Indian Ocean. The Atlantic Ocean is analysed on a fine three-dimensional grid so that at every grid point the relative contributions of each water mass and the pseudo-age are determined. The model is remarkably successful in verifying many accepted circulation features in the Atlantic Ocean, including the large-scale circulations of the subtropical gyres, the zonal flows of equatorial currents at the equator, and a cross-equatorial flow of the water masses formed in the southern hemisphere near the western boundary. The inter-hemisphere flow is so important that almost half of the thermocline waters in the Caribbean Sea and the Gulf of Mexico are supplied by the two water masses formed in the South Atlantic and Indian Oceans. This provides support for an upper-layer replacement path for the formation of North Atlantic Deep Water. Further east, the sharp front at about 15°N between North and South Atlantic Central Waters is clearly discriminated throughout the thermocline. The central waters of the South Atlantic thermocline are found to be highly stratified, with central water formed in the Indian Ocean underlying the South Atlantic Central Water. At around 5°N a strong upwelling zone is identified in which the central water formed in the Indian Ocean penetrates towards the surface. The pseudo-age results allow pathways for the flow of water masses to be inferred, and clearly identify circulation features such as the subtropical gyres, the Equatorial Undercurrent, and the shadow zones in the eastern equatorial regions of the Atlantic Ocean. Water mass renewal in these shadow zones occurs on considerably longer time scales than for the well-ventilated subtropical gyres.