Satellite-Observed Sensitivity of World Land Ecosystems to El Niٌo/La Niٌa

The recent strong 1997–1998 El Niño and the following 1998–1999 La Niña disturbed weather, environment, economy, and human lives worldwide. The total impact of these events (ENSO) on societies is estimated in billions of dollars and consequences include famine, human health problems, loss of life, property damage, and destruction of the environment. Areas sensitive to ENSO have been identified in some world areas from climatic historical records and recently from the normalized difference vegetation index. This paper examines the last 15-year association between monthly sea surface temperature (SST) anomalies in the tropical Pacific and vegetation health (condition) index for the entire world. The vegetation health (condition) index, derived from a combination of Advanced Very High Resolution Radiometer-based normalized difference vegetation index and 10-μm to 11-μm thermal radiances, was designed for monitoring moisture and thermal impacts on vegetation health (greenness and vigor). Two types of responses were identified. In boreal winter, ecosystems of northern South America, southern Africa, and Southeast Asia experienced severe moisture and thermal stress during El Niño and favorable conditions during La Niña years. In Argentina and southwestern United States/Mexico regions, the response was the opposite. World ecosystems are less sensitive to SSTs during boreal summer, except for the areas in northern Brazil and Southeast Asia, where vegetation conditions deteriorate/improve during the warm/cold water cycle in the tropical Pacific. In some areas, ENSO impacts on ecosystems are clearly identified from satellites and not from in situ data (or vice versa). During the boreal winter, these discrepancies occurred in Central America, northern Brazil, and southeastern United States; during the boreal summer, the discrepancies occurred in the sub-Sahara Africa and in the northern part of South America. In the course of the 1997-1999 ENSOs, world vegetation generally responded in accordance with the vegetation health/SST teleconnection patterns. However, in southern Africa, January-March 1998 vegetation stress was not as severe and widespread as in other El Niño years. Similarly, no stress was recorded in Argentina during the 1998–1999 La Niña.