When you think of monsoons, floods (like those in Pakistan in 2010) might be the first thing to come to mind. But in some areas, malaria epidemics can follow the monsoon rains. Malaria is caused by the Plasmodium protozoa carried by mosquitoes. These mosquitoes hatch from standing water. In some of the drier parts of India, the conditions necessary for malaria are only present during the monsoon season—the wetter the year, the greater the risk.

Being able to forecast a likely malaria epidemic far enough ahead of time to mobilize help for those regions could save a lot of lives. The best we can do at present is watch the rainfall totals. Recognizing particularly wet weather gives about a month’s notice before the malaria cases will start to climb. A new paper in Nature Climate Change seeks to increase that lead time by using sea surface temperatures in the South Atlantic to predict the rainfall before it happens.

The researchers looked for correlations between malaria in India and sea surface temperatures around the globe, which affect atmospheric circulation and weather patterns. The tightest correlation was between malaria in the drier regions of northwest India and temperatures in the South Atlantic.

Since correlations can be coincidental, the researchers analyzed the relationship using a climate model. This allowed them to experimentally test the connections by warming or cooling one portion of the ocean at a time, while holding sea surface temperatures constant everywhere else. They were also able to run the experiments repeatedly—another luxury that isn’t available when examining real-world data.

The model simulations displayed the same correlations, with cool surface temperatures in the South Atlantic linked to higher rainfall in northwest India. The simulations also allowed the researchers to see why that would be the case. The pattern of air circulation in the region is altered by temperatures in the South Atlantic. Cold sea surface temperatures there mean high pressure and sinking air. In response, the rising motion of air over the warmer seas of the Indian Ocean is strengthened. As the moist air over the Indian Ocean rises, it cools, generating precipitation.

Because these conditions develop over the course of the monsoon season, the temperature pattern in the South Atlantic can be identified four months before the malaria season it influences.

The researchers also found a moisture threshold, beyond which the number of malaria cases in northwest India jumps up considerably. Those are the epidemic years that it would help to be better prepared for. Just using July sea surface temperatures in the South Atlantic, the researchers show they could have correctly predicted whether or not an epidemic would occur in 21 of the last 26 years.

Perhaps the most surprising aspect of the study is the finding that a major sea surface event isn't a factor. The El Niño Southern Oscillation is considered to have the largest impact on the monsoon rains in India and Southeast Asia. For Northwest India, however, it’s the South Atlantic that appears to be in the driver’s seat.

Nature Climate Change, 2013. DOI: 10.1038/NCLIMATE1834 (About DOIs).