Extreme weather to increase around Indian Ocean

In austral spring, southeasterly trade winds blow across the Indian Ocean from Northern Australia past Indonesia to Kenya and the countries of eastern Equatorial Africa. In most years, the patterns of temperature and rainfall associated with the winds are steady, but in some years an anomaly known as the positive Indian Ocean Dipole (pIOD) develops. Triggered by a lowering of the sea surface temperature off Java and Sumatra, a pIOD strengthens the easterly winds in that region and along the equator. The winds deprive Australia and Indonesia of moisture, but when they reach the waters off East Africa, they boost evaporation and, ultimately, rainfall. The consequences can be devastating. The pIOD event of 1997 led to thousands of flooding deaths in East Africa and to costly wildfires in Australia, Borneo, and other Indonesian islands (see satellite image). As Earth’s climate warms, the frequency and severity of pIOD events could conceivably change. Although climate records suggest that pIOD events are indeed increasing, whether global warming is responsible is unclear. To assess future pIOD behavior, Wenju Cai of CSIRO Marine and Atmospheric Research in Aspendale, Australia, and his collaborators ran 23 climate simulations for the years 1900–2099. According to the models, if the concentration of greenhouse gases continues to rise as expected, the frequency of pIOD events will rise from one every 17.3 years over the 20th century to one every 6.3 years over the 21st century. (W. Cai et al., Nature 510, 254, 2014 doi:10.1038/nature13327 .)