SST Reveals Pacific Equatorial Current During 1998 La Niña

Date Added: 
Thursday, May 6, 2010

The plot below shows images of TMI SST in the Equatorial Pacific. South America is apparent in grey on the right hand side of all images and black indicates missing data. The images begin on June 1, 1998 and continue, every 10 days, until August 30, 1998. The color has been restricted to emphasize the edge of the Equatorial Current.

As you see here, June 1, 1998 shows the onset of the cold tongue typical of late summer circulation patterns. It is stronger than normal due to the La Niña conditions. Only 10 days later, in the June 6th plot, tropical instability waves are already apparent. Individual waves can be identified and followed. Our analysis shows that the phase speed of the waves on the northern edge of the jet averages .5 m/s (43.2 km/day) to the west. Waves traveling along the southern edge are harder to pick out, but are clearer in the August images.

Many of the images clearly contain eddy-like structures in the wave troughs on both the northern and southern edges. Colder (yellow) water is curling back to the East from the tips of the waves. On July 31, 1998, a complete ring of cold water curls off the top of a wave on the northern edge. These eddies provide evidence to Philander's (1978) suggestion that as the equatorial jet shoots out into the West Pacific, the Equatorial Counter-Current shears the tops of the waves, creating vortices in the troughs. Drifting buoy tracks (Legeckis, 1983) indicate the presence of anticyclonic circulation in the troughs of the waves. This is consistent with the pattern that the cold water is making on both the Northern and Southern edges of the jet.

SST in Equatorial Pacific 
SST in Equatorial Pacific

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