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 society’s chief concerns. We’ve 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 today’s blue skies will continue for months on end or if the evening rains are the beginning of the worst flood in history.

Climate Oscillations:
Introduction: El Niño’s Extended Family
Searching for Atlantic Rhythms
Reverberations of the Pacific Warm Pool

Coming Soon:
Intertropical Convergence Zone

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 Earth’s oceans. Their names are generally unoriginal—the 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
Though these discoveries are relatively new, the idea that global climate cycles exist is over three-quarters of a century old. The earliest theories of their existence began with British atmospheric scientist Sir Gilbert Walker in 1923. After pouring over global weather data for twenty years, he noticed a connection between increased rainfall in South America and air pressure undulations in the Western Pacific. He believed that the circulation in the atmosphere and the ocean were linked across the Pacific. Due to a lack of climate records and computing power, he could do no more than voice his opinion. The scientific community simply scoffed at Walker’s findings, refusing to believe that two such distant weather events could be connected (Mayell, 1997).

It wasn’t 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 Walker’s idea one step further and suggested that many long-term variations in the world’s 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.

The quality of wine produced in the vineyards of Western Europe, some of the most acclaimed in the world, is often affected by a climate cycle (kin to El Niño) known as the North Atlantic Oscillation (NAO). Every few years high and low pressure systems over the North Atlantic Ocean flip-flop, causing changes in temperature and precipitation in Europe, Northern Africa, and North America. (Photograph copyright Corel Professional Photos)

The NAO strongly affects the winter weather in Western Europe. The top graph shows the difference in pressure anomaly between Portugal and Iceland for December–March. (Positive values indicate relatively high pressure in mid latitudes, and relatively low pressure over Iceland.) Temperatures in central England, shown in the lower graph, are closely linked to the pressure difference over the North Atlantic. (Graphs by Robert Simmon, based on NAO index data from the University Corporation for Atmospheric Research and temperature data from the UK Meteorological Office)

Over the past three decades since Bjerknes’s 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 don’t 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.

Brazil's seasonal rains are carried by the Atlantic trade winds. When these weather patterns change, driven by the movement of the Intertropical Convergence Zone, extended droughts can leave the tropical rainforest vulnerable to outbreaks of fire. (Photograph courtesy Dr. Compton Tucker, NASA Goddard Space Flight Center)

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.
 

Weather in the tropical Pacific varies with the size of the West Pacific Warm Pool, which fluctuates over a period of decades. (Photograph copyright Photodisc)

Now that scientists understand many of the Earth’s 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 Earth’s 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 isn’t 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.

References
(Ross, Tim and Neal Lott, 1999: Billion Dollar U.S. Weather Disasters 1980-1999, National Climactic Data Center, Ashville, North Carolina.)

(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.

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

The West Pacific Warm Pool is a large area of ocean centered in the seas around Indonesia. This region contains the warmest ocean water in the world, and slowly fluctuates in size. The extent of the West Pacific Warm Pool affects the size and frequency of El Niño. In the image at left, cold waters are blue, purple, red, and orange waters are warmer, and yellow indicates sea surface temperatures up to 35°C. (Image by Robert Simmon, based on sea surface temperature data from the Physical Oceanography DAAC at NASA’s Jet Propulsion Laboratory)