Date of Award

Winter 1995

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

Gabriel T. Csanady

Committee Member

Chester Grosch

Committee Member

A. D. Kirwin, Jr.

Committee Member

Paul S. Schopf

Abstract

A simple 2.5 layer numerical model was developed and used to illustrate the seasonal variability of heat and mass transports in the upper tropical Atlantic Ocean, associated with the seasonal movement of the Inter-Tropical Convergence Zone (ITCZ). The model ocean was forced by seasonally varying climatological wind and heat flux fields. The entrainment at the base of the mixed layer was scaled by wind stress and shear at the bottom of the mixed layer. On an annual average, the northward transport of the tropical warm water is about 11 Sv, with roughly 10 Sv associated with entrainment of upper thermocline water and the other 1 Sv executing a cross-equatorial path continuously from the South Atlantic. Out of the total 10 Sv of the needed upper thermocline water, 9 Sv enters the equatorial belt from the South Atlantic.

The seasonal response to the ITCZ movement was most striking in the entrainment rate and the warm water escape rate across the northern edge of the equatorial cell. The entrainment rate was found to be significant during May/December and ceased between January and April. The locally forced equilibrium response between the interfacial shear and the zonal wind stress east of 30°W appears to be responsible for this cycle. The warm water escape toward the North Atlantic takes place mainly between December and May and stops during July/September. Further investigation suggested that the seasonal intensification of the North Equatorial Countercurrent (NECC) serves as a major obstacle to the warm water escape: during July/September the strong negative wind stress curl north of the equator intensifies the NECC, which requires a source of mass at its origin in the west. The North Brazil Current (NBC), therefore, veers offshore completely and provides the mass, terminating the warm water escape via the NBC-Guiana Current route. Strengthening of the NECC also steepens the mixed layer floor, forming a strong potential vorticity front along the northern edge of the NECC. The northward warm water escape via the eastern leg of the cyclonic gyre is therefore limited. In addition, the northward Ekman transport also reaches a minimum during this period. As a result of seasonal variations of the two key processes, namely the entrainment and the meridional transport across the northern edge of the equatorial cell, the tropical warm water pool experiences heat storage during May/October and heat escape in November/April.

The heat budget study of the equatorial mixed layer revealed that, on an annual average, heat gain at the sea surface between 8°S and 8°N is about 0.35 PW, and is used exclusively to warm up the cold water entrained from the upper thermocline layer. The heat-anomaly fluxes across the zonal boundaries are negligibly small. Further analysis showed that the sea surface temperature variation in the tropical Atlantic Ocean is determined by the local heat balance between the atmospheric heating and the entrainment cooling. The model results suggested that the ITCZ movement is the primary cause of the net northward heat transport and its seasonal variation observed in the upper tropical Atlantic Ocean.

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DOI

10.25777/n4vd-t015

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