By Edwin Schiele
Over the past millennia, the climate has remained remarkably stable. Yet even the most stable climates contain variability. Regional conditions in the ocean and atmosphere shift back and forth as natural variations in winds, currents, and ocean temperatures temporarily alter regional weather patterns. If these deviations become extreme enough, they can ripple across the globe. Such disruptions, especially if they are not foreseen, can devastate communities that rely on predictable weather patterns for their livelihoods.
The most infamous of these disruptions is heralded by the arrival of warm currents into the chilly waters off the Peruvian coast. In the 19th century, Peruvian fishermen named this phenomenon El Niño, Spanish for “the Christ child,” because the warm water typically arrived around Christmas. Normally this incursion of warm water is a short-term seasonal event. But every two to seven years, these warm waters stick around for up to 12 to 18 months, signaling a temporary shift in the interaction between the ocean and atmosphere over the tropical Pacific Ocean.
Today El Niño refers to this long-term incursion of warm water and its climatic consequences. Torrential rains that normally fall over the western tropical Pacific shift eastward, flooding the normally arid Peruvian and Ecuadorian coasts and leaving Indonesia and eastern Australia high and dry. These rainfall shifts may in turn disrupt the ocean and atmospheric circulation well beyond the tropical Pacific. During the severe El Niño events of 1982–83 and 1997–98, droughts and floods struck some of the most vulnerable areas in the world, including parts of Africa, Southeast Asia, and Central and South America. In the United States, unusually warm water made its way up the west coast, triggering torrential rains in California. Changes in the jet stream increased the frequency of floods and tornadoes in the southern states. On a brighter note, the northeast states enjoyed warmer winters and the number of Atlantic hurricanes decreased.
All told, floods, mud slides, crop failures, forest fires, and the spread of diseases attributed to El Niño have contributed to thousands of deaths around the world, displaced hundreds of thousands of people, and cost countries tens of billions of dollars.
With so much at stake, physical oceanographers and meteorologists have joined forces to learn what triggers El Niño and why it lasts only 12 to 18 months, although as discussed below, El Niño is often followed by a second climate anomaly called La Niña.
British mathematician Sir Gilbert Walker was the first researcher to study this phenomenon. In the early 20th century, he examined 40 years of meteorological data and observed that high pressure over the eastern South Pacific was always coupled with low pressure over the western Pacific and Indian Ocean. In this situation, heavy rains from monsoons fall over India and Indonesia while the eastern tropical Pacific remains dry. Walker deduced that the gradient between high pressure in the east and low pressure in west generates the east to west trade winds along the equator. Air then rises over the area of low pressure, loops back east through the upper layers of the atmosphere, then sinks back into the area of high pressure. This atmospheric loop is now called the Walker Circulation.
Walker also observed that these high and low pressure areas sometimes switched positions. He named this flip-flopping of high and low pressures the Southern Oscillation. Today scientist recognize that the Southern Oscillation and El Niño are tied together and call the phenomenon ENSO (El Niño Southern Oscillation.)
In the 1960s and 1970s, Jacob Bjerknes at UCLA then Klaus Wyrtki at the University of Hawaii linked Walker’s Southern Oscillation to oceanic processes. Scientists have since sketched out the following basic picture.
During normal conditions with high pressure over the east and low pressure over the west, the trade winds push currents north along the South American coast then west along the equator. Water piles up in the west, raising the sea level. But more importantly, the currents cause the thermocline to deepen in the west. The thermocline is the boundary between the sun-drenched top ocean layer and the colder deeper layer. In the eastern tropical Pacific, the thermocline is shallow (15–50 meters). But in the west, the currents push the thermocline down to 150–200 meters.
The depth of the thermocline can strongly influences the temperatures at the surface. Due to the Earth’s rotation, the northbound currents curve west away from the South American coast, and the equatorial currents driven by the Trade Winds diverge slightly towards the poles. Along the South American coast and in the east where the thermocline is shallow, cold water from the deep layer wells up to fill the void. This upwelling creates a cold tongue of water that extends along the equator from Peru to the International Dateline (180° longitude east). But in the west where the thermocline deepens, the cold deep water never reaches the surface. The top layer therefore remains warm.
These sea surface temperature differences in turn influence the positions of the high and lower pressure systems. Air and moisture rise up over the warm western waters, creating massive convective storms. In the east, the chilly water prevents these storms from forming and high pressure predominates. The resulting pressure gradient as described by the Walker Circulation generates the trade winds that keep the cycle going.
Every two to seven years, this cycle breaks down. First westerly wind bursts appear over the warm pool in the west, undermining the trade winds. These westerly wind bursts sometimes generate long waves called Kelvin waves, which travel east along the equator at about 2.5 meters per second. Scientists have observed these equatorial waves using buoys and satellite altimetry. If conditions are right, these Kelvin waves, push down the thermocline in the eastern tropical Pacific, preventing cold water from welling up to the surface. At the same time, new west-to-east currents associated with the Kelvin waves move warm water from the warm pool east towards South America, further warming the surface waters. As the eastern waters warm, the low pressure and its convective storms move east. The Walker Circulation and its associated trade winds weaken further. Persistent westerly winds generate more Kelvin waves and push more warm water east. Eventually a full scale El Niño may develop. Upwelling along the Peruvian coast ceases and no longer carries up nutrients from the ocean bottom that fuel the fisheries. Without these nutrients this productive ecosystem starves.
After 12 to 18 months, the El Niño perturbation breaks. Scientists have identified several mechanisms that may be involved. One of the most intriguing relates back to the westerly wind bursts and the equatorial waves they generate. The same westerly wind bursts that create Kelvin waves also create second types of waves called Rossby waves. These equatorial waves travel in the opposite direction of Kelvin waves at less than one meter per second. They then reflect off of Indonesia and return east as Kelvin waves. But unlike the original of Kelvin waves, these newer Kelvin waves pull the thermocline up in the east. Now cold water can once again well up and the water cools. Meanwhile, the original Kelvin waves reflect off of the South American coast and return west as Rossby waves. These new Rossby waves begin to draw cooler water from the east towards the west and further cool the surface waters.
As the ocean surface cools in the east, the normal high and low pressure systems and the trade winds reestablish themselves. Yet sometimes the system overshoots the normal conditions. The trade winds and upwelling become unusually strong and the ocean surface cools far below normal. This phenomenon, called La Niña, causes widespread climate disruptions that often have the opposite effect of those caused by El Niño. For example, areas that suffer from drought during El Niño may experience heavier rains than usual, and the number of Atlantic hurricanes may increase. Eventually the entire system settles back to normal.
Models that can reliably predict the onset of El Niño are vital to regions that need to prepare for the extreme weather. But despite some success predicting some of the most recent El Niño events, scientists still have much to learn. The fact that El Niño occurs so irregularly and that no two events behave the same makes the task of developing accurate models more daunting. At the local level, scientists need to better understand small scale turbulence and mixing as it relates to upwelling. They also must account for variability even within normal conditions. The ocean and atmosphere are never static.
Yet ultimately El Niño is tied into the entire global circulation of the ocean and atmosphere. To fully understand El Niño, scientists must develop global ocean circulation models that accurately replicate surface currents. Worldwide networks of buoys along with programs such as OSCAR (Ocean Surface Currents Analysis Real-time), which calculates surface currents based on satellite measurements, provide scientists with data on which to build and test their models. Such models should also help scientists determine how the frequency and severity of El Niño events might change as global warming alters Earth’s stable climate.