Mechanisms of heat, freshwater, oxygen and nutrient transports and budgets at 24.5°N in the subtropical North Atlantic

Lavin, A.M., Bryden, H.L. and Parrilla, G. (2003) Mechanisms of heat, freshwater, oxygen and nutrient transports and budgets at 24.5°N in the subtropical North Atlantic. Deep Sea Research Part I: Oceanographic Research Papers, 50 (9), 1099-1128. (doi:10.1016/S0967-0637(03)00095-5).

Abstract

Hydrographic data from a cruise at 24.5°N in the subtropical North Atlantic are used to calculate and examine the fluxes across the section. The components of the fluxes of heat, freshwater, oxygen and nutrients through the section are analysed. After the variables are separated into section average, baroclinic profile and anomalies, the fluxes are separated into an Ekman component and its barotropic compensation, baroclinic and horizontal components. The baroclinic contribution due to the meridional overturning circulation (MOC) is responsible for the largest amount of heat transport with warmer waters flowing poleward and cooler deep waters flowing equatorward. The Ekman component and its barotropic compensation is the second-most important component of heat transport. The MOC transports about 25% of the combined meridional heat transport by atmosphere and ocean at 24°N. The Ekman and baroclinic components are responsible for the northward salt transport. Salt conservation for the Atlantic north of 24.5°N suggests that there is a net precipitation between the Bering Strait and the 24.5°N section of about 0.5 Sv. Horizontal transport is the major contribution to the southward oxygen flux, as oxygen-poor water flows northward in the Florida Straits and oxygen-rich water flows southward in mid-ocean. Baroclinic transport is the main factor in the southward transport of all nutrients: low concentration upper water flows northward and higher concentration deep water flows southward.
The zonally integrated meridional cell carries a poleward heat flux of 1.51±0.39 PW, 0.14 PW of which is due to seasonal sampling, and equatorward transports of -2621±705 kmol s-1 of oxygen, -254±176 kmol s-1 of silicate, -130±95 kmol s-1 of nitrate and -12.6±6.3 kmol s-1 of phosphate.
Finally we have explored the changes in ocean circulation implied by changes in silicate transport through the section. Strict silicate conservation through the section leads to a great increase in the northward deep circulation in the eastern basin that is not in accord with the present understanding of the circulation. Taking an intermediate silicate transport through the section, according to estimates of silicate sources, produces a reasonable increase in the southward Deep Western Boundary Current (DWBC) flow and an increase in the deep northward transport in the eastern deep basin, as well as a decrease in silicate and nitrate transports through the section. Fluxes of heat, salt and oxygen are not significantly affected by changes in the conditions of silicate conservation.

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Contributors

Author: H.L. Bryden

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