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The Equator against the current

Lively Data : Januray 23, 2007

The LAS provide two kinds of sea surface currents; in both cases these currents are computed from the sea surface heights grids, considering the geostrophic balance.
Geostrophic assumption is not valid on the Equator where the coriolis force is zero; actually, for Ssalto/Duacs products, the Lagerloef et al. method is used to compute the current grids in a band of 5° towards the Equator.

opposite: zonal component of geostrophic current; meridian component of geostrophic current; corresponding geostrophic current vector, December 20th, 2006.
Data used: SSALTO/DUACS near-real time Absolute Gesotrophic Velocity; variables: U (Zonal Velocity), V (Meridian Velocity), Vector (U,V).
Geographic area is defined by following coordinates: (0°N-7°N, 20°W-40°W).
On the upper figure U component have strong positive values (i.e eastward) towards 7°N and then decrease. Towards the Equator the values are negative (westward flow). For the V component, the changing red and blue areas highlights some successive northward/eastward flow around 7°N. The velocity vector plot confirm it.



On the Equator the surface waters flow westward because of Trade Winds, as underlined by the LAS figures; however some waters seem to flow eastward. This current, the North Equatorial Counter Current, is usually located between 4°N and 10°N. It is due to the upper surface slope towards the west at the Equator.

More information:

Lagerloef, G.S.E.; G.T. Mitchum, R.B. Lukas, P.P. Niiler, 1999: Tropical Pacific near-surface currents estimated from altimeter, wind, and drifter data, J. Geophys. Res., 104, 23313-23326.

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