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

An analysis of the water mass structure of the Atlantic Ocean central layer is conducted by applying optimum multiparameter (OMP) analysis to an expan sive historical data set. This inverse method utilises hydrographic propert y fields to determine the spreading and mixing of water masses in the perma nent thermocline. An expanded form of OMP analysis is used, incorporating R edfield 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 form ation region in the Indian Ocean. The Atlantic Ocean is analysed on a fine three-dimensional grid so that at every grid point the relative contributio ns of each water mass and the pseudo-age are determined. The model is remarkably successful in verifying many accepted circulation f eatures in the Atlantic Ocean, including the large-scale circulations of th e subtropical gyres, the zonal flows of equatorial currents at the equator, and a cross-equatorial flow of the water masses formed in the southern hem isphere near the western boundary. The inter-hemisphere flow is so importan t that almost half of the thermocline waters in the Caribbean Sea and the G ulf of Mexico are supplied by the two water masses formed in the South Atla ntic and Indian Oceans. This provides support for an upper-layer replacemen t path for the formation of North Atlantic Deep Water. Further east, the sh arp front at about 15 degrees N between North and South Atlantic Central Wa ters is clearly discriminated throughout the thermocline. The central water s of the South Atlantic thermocline are found to be highly stratified, with central water formed in the Indian Ocean underlying the South Atlantic Cen tral Water. At around 5 degrees N a strong upwelling zone is identified in which the central water formed in the Indian Ocean penetrates towards the s urface. The pseudo-age results allow pathways for the flow of water masses to be inferred, and clearly identify circulation features such as the subtr opical gyres, the Equatorial Undercurrent, and the shadow zones in the east ern equatorial regions of the Atlantic Ocean. Water mass renewal in these s hadow zones occurs on considerably longer time scales than for the well-ven tilated subtropical gyres. (C) 1999 Elsevier Science Ltd. All rights reserv ed.