Isohaline Thermoclines in the Southeast Pacific Ocean

Abstract

A 300 m thick, nearly isohaline (34.2%) layer, reaching depths of 500 m, is found in the Subantarctic Zone of the southeast Pacific, the Drake Passage and the Scotia Sea. Within the nearly isohaline layer there is a very slight decrease in salinity with depth, but a marked change in temperature (4.5?2.0°C). Sometimes a temperature minimum occurs at the base of the nearly isohaline layer. The density ranges from σ0 = 27.15 at the top to σ0 = 27.3 at the bottom of the layer. Its formation in the Pacific is believed confined to the southeast portion because hydrographic stations in the Subantarctic Zone at 90°W do not reveal a nearly isohaline layer. The water column. represented by the 90°W stations, requires the addition of colder and fresher potential temperature (?) and salinity (S) characteristics at densities greater than 27.1 to account for formation of the nearly isohaline layer and underlying waters. Such characteristics are readily available in the uppermost few hundred meters of northern Antarctic Zone and southern Polar Front Zone waters in the southeast Pacific. It is suggested that cross Polar Front, isopycnal spreading of these Antarctic waters is sufficient to produce the observed nearly isohaline layer in the Subantarctic Zone, i.e., vertical mixing need not play a significant role in the original production. Geostrophic volume transport in the σ0 = 27.2?27.3 interval shows that in the area of formation subantarctic waters are carried to within 100 km of Antarctic waters. The ?,S properties of the nearly isohaline layer suggest it is a significant factor in the formation of Antarctic Intermediate Water (AAIW) in the Atlantic.