While much attention has been given to the potential global impact of climate change, less has been paid to how a warmer planet would affect regional climates. The Intergovernmental Panel on Climate Change (IPCC) projects that the global average temperature will rise about 1°C by the middle of the century, but the global average does not tell us anything about what will happen to regional climates, for example rainfall in the western United States or Hawaiian Islands.

Analyzing warming projections in models used by the IPCC, a team of scientists claim that ocean temperature patterns in the tropics and subtropics will change in ways that will lead to significant changes in rainfall patterns. The study will be published in the Journal of Climate this month.

Most experts have assumed that the surfaces of Earth's oceans will warm rather evenly in the tropics. This assumption has led to "wetter-gets-wetter" and "drier-gets-drier" regional rainfall projections. But authors of the new study have gathered evidence that, although ocean surface temperatures can be expected to increase mostly everywhere by the middle of the century, the increase may differ by up to 1.5°C depending upon the region.

This diagram shows global warming pattern formation in sea surface temperature and rainfall.

(Photo Credit: Original publication: Xie, S.-P., C. Deser, G.A. Vecchi, J. Ma, H. Teng, and A.T. Wittenberg, 2010.)

"Compared to the mean projected rise of 1°C, such differences are fairly large and can have a pronounced impact on tropical and subtropical climate by altering atmospheric heating patterns and therefore rainfall," explains University of Hawaii meteorologist Shang-Ping Xie. "Our results broadly indicate that regions of peak sea surface temperature will get wetter, and those relatively cool will get drier."

Two patterns stand out. First, the maximum temperature rise in the Pacific is along a broad band at the equator. Already today the equatorial Pacific sets the rhythm of a global climate oscillation as shown by the world-wide impact of El Niño. This broad band of peak temperature on the equator changes the atmospheric heating in the models. By anchoring a rainband similar to that during an El Nino, it influences climate around the world through atmospheric teleconnections.

A second ocean warming pattern with major impact on rainfall noted by the researchers occurs in the Indian Ocean and would affect the lives of billions of people. Overlayed on Indian Ocean warming for part of the year is what scientists call the Indian Ocean Dipole that occasionally occurs today once every decade or so. Thus, the models show that warming in the western Indian Ocean is amplified, reaching 1.5°C, while the eastern Indian Ocean it is dampened to around 0.5°C.

"Should this pattern come about," Xie predicts, "it can be expected to dramatically shift rainfall over eastern Africa, India, and Southeast Asia. Droughts could then beset Indonesia and Australia, whereas regions of India and regions of Africa bordering the Arabian Sea could get more rain than today."

Patterns of sea surface temperature warming and precipitation change in 2050 as compared with 2000. Annual mean precipitation change is shown in green/gray shade and white contours in mm/month. Precipitation tends to increase over regions with ocean warming above the tropical mean (contours of warm colors in oC), and to decrease where ocean warming is below the tropical mean (contours of cool colors).