Was the record Amazon drought caused by warm seas?

On December 11, 2005, The New York Times ran a story on record drought conditions in the Amazonas region of Brasil, linking it to global warming, and specifically the warm ocean temperatures in the North Atlantic that have also been linked to the ferocity of the 2005 Atlantic hurricane season. This prompted a response from Chris Mooney, calling for a comment from RealClimate about whether such an assertion is valid, as we earlier made it very clear that it is impossible to say whether one single extreme event in a very noisy environment – such as Hurricane Katrina – is related to climate change. So we decided to take a look at this phenomena, and address why there might be a connection and what it takes to make an attribution.

The background to this story is that the annual northward-soutward migration of the Inter Tropical Convergence Zone (ITCZ) is related to the north-south sea surface temperature (SST) gradients. The ITCZ tends to follow the peak summer sun with rainy seasons where the sun is directly overhead in the tropics and dry seasons otherwise. The ITCZ is not, however, perfectly symmetrical about the equator because, especially in the Atlantic, the northwards cross-equatorial transport of heat means that the maximum SSTs are shifted towards the north.

The rain is associated with deep convection caused by the peak in received visible sunlight and the underlying temperatures. It is important to keep in mind that the magnitude of the Atlantic convection may be linked with both Walker circulation and the ITCZ. The Walker Circulation is connected with the El Nino Southern Oscillation (ENSO) phenomenon. The rainfall over the Nordeste region has shown some associations with ENSO. There is a good deal of confidence in this link between ENSO and rainfall over northeastern Brazil, since this empirical relationship also appears to be supported by model studies. The wettest months in the Amazon region is December-February, whereas the June–November months are relatively dry.

Updated rainfall deficits for a number of location in northeastern Brazil can be found from the US Climate Prediction Center. The deficit of rain can be seen on on the CPC web pages of NCEP, who also provide maps showing the regional extent of the drought (Note: these data have not yet been quality checked!). These regions don’t appear to coincide with the drought conditions that have been reported unfortunately, which are more concentrated to the south and west of Manaus (up the Madiera river, Acre province (on the border with Peru) etc.). However, more to the east, the deficit in Maraba was mainly caused by the failure of the March–May rains.

The attribution of any event to a cause (whether it is global warming or a long term natural cycle or a recurring phenomena like El Nino) simply relies on a probabilistic argument – what is the probablity of such an event occuring in the absence of the supposed cause (P0), and how much more likely is it with the cause present (P). The attribution (in percent) is then 100*(1-P0/P) 100*(1-P/P0) (see this preprint for more details). However, the estimates of the probability are affected both by statistical sampling for recurring events, and by the lack of an identical planet without global warming to act as a control for the climate change case. So these probabilities often have to be estimated from a model. These problems mean that attributions can change as models get better, or when statistics get more significant.

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