Link Between El Nino and Rainfall in Israel Discovered


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REHOVOT, Israel -- June 24, 1996 -- El Nino, the periodic abnormal warming of surface water in the eastern equatorial Pacific, has long been notorious for the devastation it often inflicts on South America. But now it appears that its effects reach as far as the Middle East -- and surprisingly, they may actually be positive.

By studying tree rings, satellite cloud images and rain water, an Israeli research team, led by the Weizmann Institute's Dr. Dan Yakir, has discovered a striking correlation between El Nino and rainfall in central Israel over the past 20 years. Their results, which may prove valuable in predicting rainfall, are reported in the current issue of Global Change Biology. "It is perhaps befitting that El Nino --Spanish for the Christ Child -- should have a link to the Holy Land," remarks Dr. Yakir.

He conducted this study together with Dr. S. Lev Yadun, then a post-doctoral fellow at the Weizmann Institute's Plant Genetics Department, and Prof. A. Zangvil of the Blaustein Institute for Desert Research in Sde Boker.

The scientists found that between 1975 and 1995, winters with above-average rainfall in central Israel coincided with El Nino events.

For example, the record rainfall in Israel for this century, in the winter of 1991/1992, coincided with one of the most devastating El Nino appearances in recent years. Conversely, relatively dry winters in Israel have coincided with below-average ocean surface temperatures in the eastern equatorial Pacific. El Nino events, which take place on average once in four to five years, are marked by a massive increase in ocean surface temperature, which leads to storms and floods that wreak havoc on the coasts of Ecuador and Peru and may cause other large-scale climatic upheavals, primarily in the tropics.

In Israel, by contrast, heavy rains are a welcome blessing. The researchers also found that El Nino associated variations in Israel's rainfall were significant for local plant growth. They revealed a strong correlation over the past 20 years between El Nino and the width of annual rings of pine trees growing near Jerusalem. (In the normally dry Middle East, even a minor increase in rainfall can produce marked increases in the width of tree rings.)

Going back in history, the scientists noted that no El Nino events were recorded during the crippling, decade long drought that affected Israel in the 1930s.

And even further back in time, "Could Pharaoh's seven bad years have reflected an ancient period devoid of El Nino events?" asks Dr. Yakir, referring to the major famine recounted in the Biblical book of Genesis. To offer an explanation for this long-distance climatic connection between the eastern Pacific and Israel, the scientists proposed a mechanism based on regional satellite cloud images and isotopic analysis of rain in Israel. The satellite images helped identify a previously unnoticed strip of clouds which, under certain conditions, form a nearly straight line connecting equatorial Africa, known to be influenced by El Nino, with Israel.

It is possible that, during El Nino winters, changes in atmospheric circulation over parts of Africa and the adjacent ocean allow more of the high-altitude clouds to cross the Sahara Desert and reach the Middle East, contributing to Israel's rainfall. This hypothesis is supported by the isotopic analysis of local rain water over the past 20 years. The results indicate that during El Nino years, changes in atmospheric circulation over the eastern Mediterranean allow moisture from Africa to penetrate the region and contribute to local rainfall.

If the findings of Yakir and colleagues are substantiated, it may be possible to take advantage of El Nino forecasts, based on advanced computer models developed in the United States, to predict rain patterns in Israel more than a year in advance. This is of great significance for Israel's water management and agriculture, which -- in a region plagued by water shortages -- are critically dependent on careful planning.

Dr. Yakir, a member of the Weizmann Institute's Environmental Sciences and Energy Research Department, holds the Rowland Schaefer Career Development Chair.

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.