Date of Award

Winter 1993

Project Type


Program or Major

Earth Sciences: Oceanography

Degree Name

Doctor of Philosophy

First Advisor

Wendell S Brown


Water mass flow patterns within the 150-1300 m intermediate layer of the western tropical Atlantic were determined from observations during four 1990-1991 Pegasus velocity and CTD hydrographic surveys. Non-divergent flows were analyzed to determine transports.

A survey-average 29 Sverdrups (1 Sv = 10$\sp6$m$\sp3$/s) of southern hemisphere water masses flowed northwestward across 44$\sp\circ$W near the equator as part of the subsurface South Equatorial Current (SEC). 21 Sv retroflected anticyclonically around the Amazon Eddy (2$\sp\circ$N 45$\sp\circ$W) and flowed eastward toward the North Equatorial Undercurrent (NEUC). The other 8 Sv moved northwestward within a series of anticyclonic retroflection eddies which periodically separate from the Amazon Eddy. 61 Sv of a northern/southern water mass mixture flowed southeastward across 9$\sp\circ$N as part of the North Equatorial Current (NEC) and bifurcated: 32 Sv turned anticyclonically northwestward, and 29 Sv turned cyclonically around a low (6$\sp\circ$N 44$\sp\circ$W), merged with the southern water, and flowed toward the NEUC. The 50 Sv NEUC, a mixture of mostly southern water, flowed eastward into the interior. The remaining NEC and SEC branches blended to form a 40 Sv northwestward transport across 53$\sp\circ$W. These observed transports were generally about three times larger than the wind-driven equatorial and tropical gyre return transports (Mayer and Weisberg 1993); suggesting the existance of subgyre-scale recirculation eddies in the western tropics.

One of the retroflection eddies was measured during September 1990. Its structure was approximated by a 300 km diameter cylinder which extended vertically through the surface and intermediate layers to 1300 m. During two other surveys, retroflection eddies were identified at the region's northwestern corner. Instantaneous transport associated with an eddy which translated at 10 cm/s was about 30 Sv. Assuming three were generated each year, retroflection eddy translation could account for the 8 Sv alongshore flow through the region.

According to water mass budgets, 21 Sv of northern water masses were transformed by mixing within the region. Temperature-salinity relationships indicated this mixing effectively transports the cool, low salinity Antarctic Intermediate Water into the North Atlantic by as yet unknown pathways.