Thermohaline steps off the northeast coast of South America

Abstract

The apparently permanent large-scale thermohaline staircases off the northeast coast of South America are thought to indicate vigorous double diffusive mixing due to salt fingering between two important water masses: the warm, salty Subtropical Underwater at about 150 m and the cooler, fresher Antarctic Intermediate Water at about 800 m. In summer 1983 and spring and fall 1985 the staircases occurred within large, coherent fields well bounded by historical composite location determined from reports from 1969 through early 1983. A detailed statistical analysis of the data sets showed distinct latitude-longitude relationships, preferred step and interface thicknesses, and preferred temperature jumps across the Interfaces. The staircases were shallowest in the southeast (180-360 m) and deepest in the northwest (420-650 m). Preferred sizes of the well-mixed layers were 2-6 m and 14-19 m and of the separating interfaces, 2 m, although the distributions were not strongly peaked. The preferred temperature change across an Interface was about 0.4??C, but again the distribution was fairly broad. The mean salinity and potential density changes across steps were 0.084 psu (practical salinity units) and 0.030, respectively. The core of the staircase region-- the area of largest number of steps, largest steps, and largest temperature changes between steps-- lay between 10-14??N, 52-57??W. Most staircases occurred between the 26.8 and 27.1 isopycnals. The mesoscale flow field in the upper 800 m deduced from depths of isotherms and from dynamic height anomalies relative to 700 dbar agreed fairly well with previous information from the area, and a number of surprising features were noted. A definite correlation between the flow field and the location of the staircases supported the hypothesis that the steps are related to the confluence of the Subtropical Underwater and the Antarctic Intermediate Water. Estimates of vertical fluxes of heat and salt through the staircases were large enough to suggest the process is playing a major role in vertical exchange across the thermocline in this region and, by extension, in other regions of similar staircases. However, much theoretical work remains to be done to explain staircase formation and action.