At the end of 2006, East Africa was bombarded with unusually heavy rainfall. Floods swept across the region, affecting up to 1.5 million people in Somalia, Ethiopia, Kenya, and parts of surrounding countries. At the same time, Indonesia and Australia fought back widespread fires, fueled in part by unusually dry conditions. What’s up with the weather? Part of it may be that El Niño was back, and the Eastern Hemisphere was probably feeling the impact in November and December 2006.
This image shows rainfall anomalies (difference from normal) over the South Pacific and Indian Oceans that may be related to El Niño. The image was made by comparing rainfall totals in November 2006 to the average November rainfall between 1999 and 2005. The data come in part from measurements taken by the Tropical Rainfall Measuring Mission (TRMM) satellite. Areas of dark brown over Indonesia and Southeast Asia show that these regions received much less rain than normal during November 2006, while blue over East Africa reveals higher-than-average rainfall totals. Australia is light brown, indicating shortages of a few millimeters per day. Though the country was in extreme drought at the time, the deviation from normal rainfall levels doesn’t appear exceptionally large, probably because November is already a dry month in Australia.
The rainfall patterns illustrated in this image are typical during an El Niño event. El Niño happens when trade winds weaken, and the eastern Pacific Ocean heats up. These changes in the atmosphere and the ocean set off a string of unusual weather patterns around the globe, including drought in Australia, Southeast Asia, and Indonesia, and high rainfall in East Africa and parts of North and South America. El Niño also suppresses hurricanes in the Atlantic Ocean and increases the number of typhoons that form in the Pacific.
La Niña, the counterpart to El Niño, alters rainfall patterns over the Pacific and Indian Ocean basins. La Niña develops when stronger-than-average trade winds push the warm surface waters of the equatorial Pacific west. Since cold water rises to replace the warm water, La Niña leaves the eastern and central Pacific Ocean much cooler than normal, while the western Pacific is much warmer than normal. These anomalies in sea surface temperature are mirrored in rainfall patterns, with warmer-than-normal temperatures resulting in enhanced rainfall. In general, La Niña brings unusually heavy rain to the West Pacific, Indonesia, parts of Southeast Asia, and northern Australia.
For many people, El Niño and La Niña mean floods or drought, but the events are actually a warming or cooling of the eastern Pacific Ocean that impacts rainfall. These sea surface temperature and rainfall anomaly images show the direct correlation between ocean temperatures and rainfall during El Niño and La Niña events.