La Niña, westerly current

Lively Data : January 22, 2008

A La Niña episode is occuring in the Equatorial Pacific since the beginning of 2007. After an interruption in March-May 2007, this event continues during this early winter 2007-2008 (see bulletin El Niño) and is expected to continue until spring 2008. The Lively Data have often echoed the events El Niño / La Niña with maps of Sea Level Anomalies. The LAS also allows the visualization of this phenomenon in terms of geostrophic currents. The geostrophic current is divided into 2 components : the zonal component (called U) along the parallels (East-West) and the meridian component (called V) along the meridians (North-South).

La Niña conditions are particularly notable with the zonal component of current geostrophic (called U) at the end of fall 2007. The central equatorial pacific region is showing strong negative values, describing westerly geostrophic current. On the contrary, during the El Niño event in 2002, the zonal component of geostrophic current was marked by positive values, describing easterly current.

 

Zonal component of geostrophic current (in cm/s) averaged over September-December 2007 (above, during La Niña conditions) and over September-December 2002 (at the bottom, during El Niño conditions) in The equatorial Pacific. To create these maps with the LAS, choose NRT, Global, (Maps of) Geostrophic Velocity Anomalies, and the variable U (Zonal Velocity (cm/s)). Define the period in Define variable (here September-December 2007 and 2002) and select "T" in Apply to these axes and "Average" in Select analysis type. Select the geographic area Equatorial Pacific. To overlay colors and lines, select Color filled and lines in the Options Contour style. The colors are defined in Color filled levels (-70,70,10)(85) and lines in Contour levels : 10C.

The Coriolis force (which is included in the calculation of geostrophic currents) is cancelled at the equator. To get around this problem, between latitudes 5°N and 5°S, current grids used in the Ssalto/Duacs products are estimated from Lagerloef and al. method.

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