The climatic phenomenon known as El Nino became a household name during 1997.  The El Nino is a swing in the Earth's climate that occurs every 3-7 years originating in the Pacific ocean. Normally, as a result of the rotation of the earth, the winds along the equator blow from the east to the west (known as the easterlies or trade winds). In the Pacific ocean this results in waters being pushed from the eastern parts, near South America, to the western boundary, near Indonesia and Australia, causing the sea level to be higher in the west than in the east.  In the east the water from the deep ocean rises to replace the surface waters that are transported westward. This results in the waters off the coast of Peru and Ecuador being very cold and the waters off the coast of Indonesia and Borneo being very warm. The warm water results in large amounts of rainfall in the west supporting the existense of rain forests in this region. In the east the cold waters have the opposite effect causing dry desert conditions in parts of Peru and Chile. So the normal sitution is warm wet conditions in the western tropical Pacific and cool dry conditions in the eastern tropical Pacific.

During El Nino years , the easterly winds along the equator weaken and even reverse. This results in a chain reaction where the warm waters from the western Pacific migrate to the east, taking the rainfall with them, causing heavy rainfall and flooding in the east Pacific (Peru, Ecuador, Chile) and drought conditions in the west (Australia, Indonesia, Borneo). The warming (animation of 1982/3 and 1997/8 warming) of the waters in the eastern Pacific normally reaches its peak around Christmas time, and hence the name "El Nino" which means Christ child (actually "the boy") in Spanish.

The weakening of the easterlies is part of a continuous cycle driven by very long slow-moving waves in the ocean that take anywhere from months to years to propagate from one side of the Pacific to the other, and back again (similar to waves sloshing back and forth in a bathtub). The return of the El Nino every few years is related to the size of the Pacific Ocean, and a larger Pacific would perhaps cause the El Nino to return at longer time periods. Between El Ninos we often get the other phase of the cycle, known as La Nina ("the girl"), when ocean waters off the coast of Peru and Ecuador are colder than normal.

The El Nino phenomenon has always existed, although only recently have scientists around the world started to make connections between the eastern Pacific warming and other weather-related phenomenon around the world . Drought in Africa, rainfall in California, hurricanes in the Atlantic, the monsoon in India, .... have all been shown to be connected to the El Nino cycle. Even here in Israel we have detected a connection between the El Nino years and rainfall patterns during our winter.  We have looked at rainfall and snowfall data here in Israel to try to see whether there is any relationship to the El Nino cycle.

What we have found is that since the 1970s there is a statistically significant connection between El Nino years (positive NINO4 SST anomalies) and above average rainfall/streamflow in northern Israel (see Figure below). The strange thing is that this connection is less clear before the 1970s. There are some El Nino years before 1970 that have high rainfall here in Israel, but there are also years with below average rainfall. Why do we see this strong connection primarily in the last 30-35 years? It is not clear, although there are suggestions by other scientists that the climate of the earth shifted into a new regime in the 1970s, which may have resulted in the connections between the tropics and midlatitudes being stronger since then. Whatever the reason, most of the winters (in Israel) since 1975 associated with at least a 1 degree warming in the eastern Pacific have been associated with above average rainfall in northern Israel.  However, the size of the rainfall anomaly (how much above the mean) does not appear to be directly related to the intensity of the El Nino warming. To address some of these relationships we are investigating the physical connections between the tropical Pacific warmings and atmospheric flow patterns in the eastern editteranean during the winter rainy season in Israel.  It is interesting to note that the driest years in the last 30 years in Israel are often La Nina years. Coincidence?
 

Related websites:

   Introductory EL Nino report
   NOAA El Nino information
   USA Today El Nino information