Scientia Marina, Vol 76, No S1 (2012)

Salinity intrusion and convective mixing in the Atlantic Equatorial Undercurrent


https://doi.org/10.3989/scimar.03611.19B

Mariona Claret
Departament d’Oceanografia Física, Institut de Ciències del Mar, CSIC , Spain

Rocío Rodríguez
Departament d’Oceanografia Física, Institut de Ciències del Mar, CSIC , Spain

Josep L. Pelegrí
Departament d’Oceanografia Física, Institut de Ciències del Mar, CSIC , Spain

Abstract


This study investigates the advection of positive-salinity anomalies by the Equatorial Undercurrent (EUC) and their potential importance in inducing vertical convective mixing. For this purpose we use hydrographic and velocity observations taken in April 2010 along the western Atlantic equatorial ocean (32 to 43°W). The high-salinity EUC core is a few tens of metres thick and occupies the base of the surface mixed layer and the upper portion of the surface thermocline. It leads to high positive values of the vertical salinity gradient, which in many instances cause statically unstable conditions in otherwise well-stratified regions. The unstable regions result in vertical convection, hence favouring the occurrence of step-like features. We propose that this combination of horizontal advection and vertical-instability leads to a sequence of downward-convective events. As a result the EUC salinity is diffused down to a potential density of 26.43, or about 200 m deep. This mechanism is responsible for water-mass and salt downwelling in the equatorial Atlantic Ocean, with a potentially large influence on the tropical and subtropical cells.

Keywords


Equatorial Undercurrent; high-salinity core; horizontal advection; convective mixing; step-like features

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References


Cromwell T., Montgomery R.B., Stroup E.D. 1954. Equatorial undercurrent in Pacific Ocean revealed by new methods. Science 119: 648-649. http://dx.doi.org/10.1126/science.119.3097.648 PMid:17732011

Flagg C.N., Gordon R.L., McDowell S. 1986. Hydrographic and current observations on the continental slope and shelf of the western equatorial Atlantic. J. Phys. Oceanogr. 16: 1412-1429. http://dx.doi.org/10.1175/1520-0485(1986)016<1412:HACOOT>2.0.CO;2

Fofonoff N.P., Montgomery R.B. 1955. The Equatorial Undercurrent in the light of the vorticity equation. Tellus 7: 518-521. http://dx.doi.org/10.1111/j.2153-3490.1955.tb01189.x

Gouriou Y., Reverdin G. 1992. Isopycnal and diapycnal circulation of the upper equatorial ocean in 1983-1984. J. Geophys. Res. 97: 3543-3572. http://dx.doi.org/10.1029/91JC02935

Hebert D., Moum J.N., Caldwell D.R. 1991. Does ocean turbulence peak at the equator?: Revisited. J. Phys. Oceanogr. 21: 1690-1698. http://dx.doi.org/10.1175/1520-0485(1991)021<1690:DOTPAT>2.0.CO;2

Hüttle-Kabus S., Böning C.W. 2008. Pathways and variability of the off-equatorial undercurrents in the Atlantic Ocean. J. Geophys. Res. 113: C10018.

Jackson L., Hallberg E., Legg S. 2008. A parametrization of shear-driven turbulence for ocean climate models. J. Phys. Oceanogr. 38: 1033-1053. http://dx.doi.org/10.1175/2007JPO3779.1

Metcalf W.G., Voorhis A.D., Stalcup M.C. 1962. The Atlantic Equatorial Undercurrent. J. Geophys. Res. 67: 2499-2508. http://dx.doi.org/10.1029/JZ067i006p02499

Metcalf W.G., Stalcup M.C. 1967. Origin of the Atlantic Equatorial Undercurrent. J. Geophys. Res. 72: 4959-4975. http://dx.doi.org/10.1029/JZ072i020p04959

Molinari R.L., Bauer S., Snowden D., Johnson G.C., Bourles B., Gouriou Y., Mercier H. 2003. A comparison of kinematic evidence for tropical cells in the Atlantic and Pacific Oceans. In: Interhemispheric Water Exchange in the Atlantic Ocean. Elsevier Oceanogr. Ser. 68: 269-286.

Neumann G. 1960. Evidence for an equatorial undercurrent in the Atlantic Ocean. Deep-Sea Res. 6: 328-334.

Pacanowsky R.C., Philander S.G.H. 1981. Parametrization of vertical mixing in numerical models of tropical oceans. J. Phys. Oceanogr. 11: 1443-1451. http://dx.doi.org/10.1175/1520-0485(1981)011<1443:POVMIN>2.0.CO;2

Peters H., Gregg M.C., Toole J.M. 1988. On the parametrization of equatorial turbulence. J. Geophys. Res. 93: 1199-1218. http://dx.doi.org/10.1029/JC093iC02p01199

Peters H., Gregg M.C., Sanford T.B. 1995. Detail and scaling of turbulent overturns in the Pacific Equatorial Undercurrent. J. Geophys. Res. 100: 18349-18368. http://dx.doi.org/10.1029/95JC01360

Philander S.G.H, Pacanowsky R.C. 1986. A model of the seasonal cycle in the tropical Atlantic Ocean. J. Geophys. Res. 91:14192-14206. http://dx.doi.org/10.1029/JC091iC12p14192

Schott F.A., Fischer J., Stramma L. 1998. Transports and pathways of the upper-layer circulation in the western tropical Atlantic. J. Phys. Oceanogr. 28: 1904-1928. http://dx.doi.org/10.1175/1520-0485(1998)028<1904:TAPOTU>2.0.CO;2

Silva A.C., Bourles B., Araujo M. 2009. Circulation of the thermocline salinity maximum waters off the Northern Brazil as inferred from in situ measurements and numerical results. Ann. Geophys. 27: 1861-1873. http://dx.doi.org/10.5194/angeo-27-1861-2009

Stramma L., Schott F. 1999. The mean flow field of the tropical Atlantic Ocean. Deep-Sea Res. II 46: 279-303. http://dx.doi.org/10.1016/S0967-0645(98)00109-X

Stramma L., Rhein M., Brandt P., Dengler M., Böning C., Walter M. 2005. Upper ocean circulation in the western tropical Atlantic in boreal fall 2000. Deep-Sea Res. I 52: 221-240. http://dx.doi.org/10.1016/j.dsr.2004.07.021

Voorhis A.D. 1961. Evidence of an eastward equatorial undercurrent in the Atlantic from measurements of current shear. Nature 191: 157-158. http://dx.doi.org/10.1038/191157b0

Wang C. 2005. Subthermocline tropical cells and equatorial subsurface currents. Deep-Sea Res. I 52: 123-135. http://dx.doi.org/10.1016/j.dsr.2004.08.009

Wang D., Müller P. 2002. Effects of equatorial undercurrent shear on upper-ocean mixing and internal waves. J. Phys. Oceanogr. 32: 1041-1057. http://dx.doi.org/10.1175/1520-0485(2002)032<1041:EOEUSO>2.0.CO;2

Wyrtki K., Kilonsky B. 1984. Mean water and current structure during the Hawaii-to-Tahiti shuttle experiment. J. Phys. Oceanogr. 14: 242–254. http://dx.doi.org/10.1175/1520-0485(1984)014<0242:MWACSD>2.0.CO;2

Zaron E.D., Moum J.N. 2009. A new look at Richardson number mixing schemes for equatorial ocean modeling. J. Phys. Oceanogr. 39: 2652-2664. http://dx.doi.org/10.1175/2009JPO4133.1

Zhang D., McPhaden M.J., Johns W.E. 2003. Observational evidence for flow between the subtropical and tropical Atlantic: The Atlantic subtropical cells. J. Phys. Oceanogr. 33: 1783-1797. http://dx.doi.org/10.1175/2408.1




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