Ever since farmers sowed the first seasonal crops in the Fertile Crescent roughly 10,000 years ago, controlling and predicting the weather has been one of societys chief concerns. Weve forged weather vanes, consulted oracles, set up hundreds of weather stations, erected stone circles and launched advanced weather satellites. Despite our efforts, severe weather continues to catch us off guard. In the United States, the most technologically advanced nation on Earth, more than 16,500 deaths and $170 billion in damage occurred from severe weather over the past twenty years (Ross and Lott 1999). The essential problem forecasters face is more or less the same as it was in 8,000 BC. With a few exceptions, we still do not have the ability to predict the arrival of weather systems until we detect them. No one can tell if todays blue skies will continue for months on end or if the evening rains are the beginning of the worst flood in history.
However, ten millennia of failure may be coming to an end. Over the past thirty years researchers have recognized that many severe changes in our weather are due to massive, cyclical anomalies in air pressure and sea surface temperature across large tracts of ocean. Such fluctuations spawn the weather systems that cause huge floods, create droughts and even contribute to global warming. While the largest of these, El Niño, has received all the recent press, scientists have discovered other climate anomalies throughout the Earths oceans. Their names are generally unoriginalthe North Atlantic Oscillation, the Atlantic Intertropical Convergence Zone (ITCZ) oscillation, the warm pool oscillation and so on. But together with El Niño, they are responsible for well over fifty percent of climate variability on the Earth. If scientists ever get to the point where they understand all these climate cycles, they may be able to predict major weather patterns months in advance.
A Background of Anomalies
It wasnt until the 1960s that Jacob Bjerknes and a team of scientists at the University of California reopened the book on these climate anomalies. Bjerknes took Walkers idea one step further and suggested that many long-term variations in the worlds climate may be due to large-scale interactions between the oceans and the atmosphere. He then went on to prove that the South Pacific pressure undulations and the rains in western South America are all part of one large coupled atmosphere-ocean circulation in the equatorial Pacific (Elliasen 1998). The strength of this circulation oscillates roughly every five years. When the circulation slows, an El Niño occurs, and when it speeds up, we get La Niña.
Over the past three decades since Bjerkness discovery, researchers have located a number of climate anomalies across the globe. While all are driven by the same basic mechanisms, each has its own unique behavior and creates its own set of problems. The biggest and most influential of these second to El Niño is the North Atlantic Oscillation (NAO). A high-pressure system in the far Northern Atlantic and a low air pressure system just above the equator cause the anomaly. Roughly every five years, for reasons scientists dont fully understand, a tug-of-war between these two pressure zones redirects the path of the winter weather that crosses over from North America and Greenland into Europe. The results are periodic droughts and floods in Europe and Northern Africa. Another oscillation in the southern Atlantic works in much the same way. Here, two high-pressure systems on either side of the equator push and pull at the path tropical storms take as they travel from Africa toward South America. Every few years, when this conduit is shoved north, tropical storm after tropical storm pounds northeast Brazil, causing massive floods.
There are other cyclical climate anomalies that appear to cause relatively
little damage of their own, but contribute to bigger, more destructive
oscillations. In the far western Pacific, for instance, there is an area of the
ocean known as the warm pool. The warm pool grows warmer and cooler roughly
every twenty years like a slowly pulsating beacon. By itself, the oscillation
merely increases and decreases the humidity over the Indian Ocean and the
western Pacific. During El Niño years when the warm pool is cooler, El Niño hits
the midwestern United States and Australia much harder.
Now that scientists understand many of the Earths large climate cycles, the rush is on to build computer simulations of them from sea surface and atmospheric data. Already, computer models of the El Niño cycle have been constructed that can forecast its movements up to a year in advance, and similar models of the North Atlantic Oscillation are well underway. Once all these individual models are complete, researchers hope to combine them to create a global model of climate change. With such a tool, our society may finally know how much rain to expect each spring, how long a deadly heat wave will last, or whether global warming is just part of the Earths natural climate cycle.
But there is still one big obstacle in scientists way. Most ocean data only goes back a couple of hundred years, and only exists in areas that experienced heavy ship traffic. When talking about predicting trends that repeat themselves every ten to twenty years, scientists are concerned that two hundred years of data isnt enough. Without more data, the proposed models may never be accurate enough to give us any more than a slight impression of future weather patterns.
Over the next few months, the staff of the Earth Observatory will be speaking to the scientists who are tackling such problems and putting together these models. We will present the researchers work in a series of case studies centered on individual climate events. The North Atlantic Oscillation will be the first in the series, followed by the warm pool oscillation and the Intertropical Convergence Zone oscillation. We will conclude with a piece on the work underway to pull these models together into a global weather prediction system.
(Mayell, Hillary, 1997: History of El Nino: Tracking a Global Mystery, Environmental Network News.)
(Alliasen, Arnt, 1996: Jacob Aall Bonnevie Bjerknes, Biographical Memoirs, National Academy of Science.)
*All other information received from an interview with Vikram Mehta at Goddard Space Flight Center.