Cold and warm eddies pair off Vietnam

Image of the Month - September 2011

Sea level anomalies just East of Vietnam on June 1st, July 15, September 2, 2011. In June, the pair of eddies (one cold -- below average sea surface height -- North, at about 12°N, 111°E, one warm eddy, just below) is barely visible. Both eddies  strengthen throughout the Summer, their feature more or less obvious depending on the date. (Credits CLS/Cnes).

Latitude-time diagram for the 111°E longitude between September 2010 and September 2011. Last year the eddy pair was clearly missing, while in June-September 2011 it developed nicely (cold eddy at about 12°N, warm eddy a little more variable below) since beginning of July (Julian day 22470 is July 10, 2011). (credits CLS/Cnes)

The oceanic circulation in the South China Sea depends quite strongly on the monsoon. In Summer, in particular, eddies are born from the interaction of the monsoon winds with the reliefs of the coasts (East Vietnam). Every two to four years, an eddy pair (an anti-cyclonic southern eddy and a cyclonic northern eddy) appears at about 111°E, 13°N in June/July and stays till October, with a variability related to local winds. These two eddies interact with the thermocline structure and thus the local physical conditions in the ocean.

Studying such features with altimetry, now over nearly twenty years, enable to see the variability and changes. With the next altimetry missions, even longer time series will be available, thus showing long-term changes.

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