A Description of COADS Surface Meteorological Fields and the Implied Sverdrup Transports for the Atlantic Ocean from 30°S to 60°N

Abstract

Using COADS data spanning 1947?1988, we describe the regional nature of the Atlantic Ocean wind-driven circulation between 30°8 and 60°N and its annual and interannual variability. The Sverdrup streamfunction defines the circulation gyres. Our focus is on three central gyres: the Northern Hemisphere anticyclonic subtropical gyre, the cyclonic tropical gyre just north of the equator, and the clockwise equatorial gyre straddling the equator. This rendition of the Sverdrup streamfunction, computed with constant drag coefficient and air density, compares favorably with that from other climatologies. In the Straits of Florida, analyses suggest that differences between the annual cycle in Sverdrup transport and observations may be due to regional winds farther north. In the tropical gyre, the Sverdrup circulation argues against a continuous western boundary current transporting water from the equatorial region into the Caribbean in boreal winter, bringing to question the mechanisms for the known interhemisphere and intergyre exchanges of heat and mass. A conceptual model is proposed involving two stages. First, the western boundary current closing the clockwise equatorial gyre is instrumental in storing heat and mass between the North Equatorial Countercurrent ridge and the North Equatorial Current trough in boreal summer. Transport farther north, across the tropical gyre and into the subtropical gyre, in boreal winter is then accomplished by Ekman transport, as the seasonal change in wind-stress torque deepens the thermocline, thus allowing for vortex stretching and northward Sverdrup transport over the region of warmest waters. Once in the subtropical gyre, the Ekman transport continues to be northward despite the fact that the Sverdrup transport reverses to be southward. Annual and interannual variability is addressed by examining the spectrum of curl and its regional distribution. Outside the tropics and the Sargasso Sea, interannual exceeds annual variability by at least a factor of 1.5. A pentadal analysis in the subtropical gyre indicates that wind-stress curl was not a major factor in the density structure differences reported between 1955?1959 and 1970?1974; hence, these require other explanations.