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21 January 2011

This La Niña: Typical, and Unusual at Once

This La Niña has had most of the typical characteristics of a La Niña , but there are a few stand out features that make this one different from your 'average' La Niña. The above image, from the Climate Prediction Center, is what a stereotypical La Niña is like. Note that the lines on this map are not to be taken literally, but instead as approximations of the boundaries between anomalies.

In order for meteorologists and climatologists to say the atmosphere is responding to La Niña conditions, you must have dryness in the central Pacific, near the dateline, and a horseshoe shaped area of excessive rainfall around it. Both of those conditions have been met for the last several months. Here is the evidence: Flooding reports from Vietnam, Thailand, the Philippines, Malaysia and parts of Indonesia over the last few months. The disastrous flooding in Queensland is also a sign of the La Niña. Things, however, get a bit hairy when we reach out towards Jakarta and southern parts of Sumatra, where for a couple of months now, rainfall anomalies have been negative. Continuing to look at Australasia, China and the Koreas, as well as Japan have been cold, at times dangerously so. The flooding in Sri Lanka can also be chalked up as being near the western edge of the horseshoe.

In Africa while we have ample proof of La Niña, with the destructive rains in southern Africa, as well as the ongoing drought in the Greater Horn, there is no evidence that temperatures have been cooler than normal in the Gulf of Guinea area, or the Sahel. Here local forcings in the Atlantic have kept temperatures near normal.

South America has also stepped out from a typical La Nina. Dangerous flooding has inundated southern Brazil and western Colombia, as the mouth of the Amazon River has had near normal precipitation. Meanwhile temperatures have remained near normal across the continent. Colombia may be something to keep an eye on. Since the flooding ended along the Pacific coast, there has been very dry weather, which would bring that area in line with typical La Niña.

While day-to-day variations will occur, La Niña has clearly exerted itself across Asia, and most of Africa. Local forcings, such as unusually warm water in much of the Atlantic, seems to be disrupting the typical pattern in South America and west Africa.

How much longer before this is over with? A while yet. The International Research Institute at Columbia University, compiles different model forecasts into a single image. This shows anomalies from normal sea surface temperatures in the Pacific. Remember that the actual definition of La Niña/El Niño status is determined by sea surface temperature anomalies of more than 1oC in a specific zone of the equatorial Pacific for three consecutive months. Looking at the image above, it does appear that models agree that temperatures will warm to withing the normal range sometime around the March-April-May three month period. After that happens, it will take some time for the atmosphere to react.

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