[clarification needed] Typically, it takes about two days for the Ekman velocity to develop before it is directed at right angles to the wind.
Through vertical eddy viscosity, winds act directly and frictionally on the Ekman layer, which typically is the upper 50–100m in the ocean.
[clarification needed] When the contributions from all the vertical layers are added up – the integration of the velocity over depth, from the bottom to the top of the Ekman layer – the total "Ekman transport" is exactly 90 degrees to the right of the wind direction in the Northern Hemisphere and left in the Southern Hemisphere.
Suppose geostrophic balance is achieved in the Ekman layer, and wind stress is applied at the water surface: where
The SI unit of Ekman transport is: m2·s−1, which is the horizontal velocity integrated in the vertical direction.
The surface velocity is defined as the motion of a standard World Ocean Circulation Experiment/Tropical Ocean-Global Atmosphere (WOCE/TOGA) 15m drogue drifter.
Near-surface Ekman velocity can be estimated with variables which best represent the ageostrophic motion of the WOCE/TOGA 15m drogue drifters relative to the surface wind stress.
[5] A negative sea surface temperature (SST) anomaly prevails in the eastern equatorial Pacific from October to January.
A zone of strong easterly Ekman flow propagates westward into the central Pacific basin near the date line during December–February.
Relaxation of trade winds in the eastern coincided with the eastward propagation of the geostrophic flow east of 240°E (particularly in February), while westward currents dominated in the central and western equatorial region, which then reversal in the east, with weak local trade winds and weak upwelling along the coast, coincided with the onset of warm SST anomaly.
Geostrophic current anomaly, like a Kelvin wave signature propagating eastward to South America between December and April can be easily discerned, and this arrival to South America also coincided with the above-mentioned SST anomaly onset.
As the El Niño SST anomaly developed during May and June, this eastward geostrophic flow persisted.