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Drought and Floods
Introduction
A recently assembled, long-term global AVHRR-NDVI dataset demonstrates that there is a connection between changing patterns of sea surface temperature and patterns of plant growth across the surface of the Earth. Sea surface temperature anomalies (Pathfinder SST data) (TRMM TMI SST data) like El Niño exert a profound influence on rainfall patterns (TOVS rain data) (TRMM TMI rain data) around the globe. Rainfall variations are reflected in variations in plant growth (Pathfinder NDVI data). A movie at the Earth Observatory Web Site shows that "...green waves of vigorous plant growth and creeping brown hues of drought wax and wane across the landscapes as if the ... vegetation dances in response to the rhythms of the ocean."
The severity of rainfall (TOVS rain data) (TRMM TMI rain data) and flooding in specific watersheds is very sensitive to low-level wind conditions, which vary with the phase of ENSO (Ralph et al. 2002). trade winds blow from east to west along the equator. Westerly winds blow from west to east at mid latitudes. Both carry moisture. Evaporation is slowed if the sea surface is cooler than normal, leading to decreased rainfall over downwind land. Conversely, more evaporation leads to excess rainfall when the sea surface temperature is higher than normal. As a result, the ENSO cycle of warm events and cold events is matched by a cycle of drought and floods.
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Anomalies During El Niño Winter |
Anomalies During El Niño Summer |
Rainfall
Precipitation anomalies (TOVS rain data) (TRMM TMI rain data) directly over the El Niño area are closely in phase with the El Niño signal. Thus precipitation over the eastern equatorial Pacific Ocean, Ecuador, and Peru increased in August 1997 compared to normal. At about the same time, precipitation over Indonesia decreased, and then intensified in September 1997. The Indonesia drought intensified and spread into the Bay of Bengal and India in October 1997. Southern Brazil also tended to have more rain (floods) during an El Niño episode and less rain (drought) during a La Niña period. During El Niño conditions, the southern states from Texas to Florida have wet winters.
The precipitation patterns vary with the seasons. During the strong El Niño of 1997 to 1998, autumn precipitation (TOVS rain data) (TRMM TMI rain data) was especially heavy over California, along the immediate Gulf Coast, over Florida, and throughout the Southeast and Mid-Atlantic States; winter and spring continued wet, except for southern California and the Southwest. Precipitation surpluses in sections of California and the Gulf Coast States have ranged from 300 to 600 mm (12 to 14 inches) since the beginning of December. More than a dozen storms have pounded communities throughout California in early 1998. More rain fell in California in 5 weeks than normally falls all year, setting all-time records. Florida, Maryland, Nevada, Rhode Island, and Virginia had their second wettest February since 1895. NOAA's National Climate Prediction Center forecast the possibility of "recurring periods of significant storm activity and precipitation across California and the southern tier of the United States" through March and possibly into April. By contrast, the summer was dry or very dry.
Drought
The precipitation patterns also vary with location. During the winter of 1997 to 1998, El Niño winds in the tropics dried out as they descended over Central America and Mexico. The storms that normally provided rainfall there were redirected (see above).
During the severe El Niño of 1982-83, the absence of the usual seasonal storms and rains caused Australia to suffer catastrophic agricultural and livestock losses that together cost billions of dollars of lost revenue and damage. Drought racked much of sub-Saharan Africa, forcing even normally food-exporting nations such as the Republic of South Africa and Zimbabwe to turn to the international community for help.
La Niña conditions can also bring droughts. In 1998, dry parts of Texas and Oklahoma suffered heavy losses of grains from drought and blights, followed by June floods as La Niña replaced El Niño. Parts of the Southwest that had been wet were dried out by hot scorching winds. The long La Niña of 1998-2001 set conditions for the Western U.S. wildfire seasons of 2000 and 2002 (U.S. Fire Season Statistics). Likewise, the Horn of Africa that had received abundant rainfall, leading to good crops and to an outbreak of Rift Valley Fever, in 1997-98, received insufficient rainfall during the rainy seasons of 2000, 2001, and 2002, leading to poor harvests and threats of malnutrition or starvation for some 14 million people.
Jump to 'Drought and Floods' List of Web Sites
Disclaimer: NASA offers these suggested sites for additional information regarding effects of El Niño and La Niña teleconnection events. Web access is required to reach these sites. Link existence and contents are not under the control of the EOSDIS Science Operations Office.
Drought and Floods Web Sites
