Weekend's Special: El Nino and La Nina, the Dancing Ocean and Atmosphere

The Christ Child and Little Girl

By now most people have heard of El Niño, if only to know the name refers to some kinds of abnormal weather. The definition of "abnormal" varies widely with geography, though. For people who live in Indonesia, Australia, or southeastern Africa, El Niño can mean severe droughts and deadly forest fires. Ecuadorians, Peruvians, or Californians, on the other hand, associate it with lashing rainstorms that can trigger devastating floods and mudslides. Severe El Niño events have resulted in a few thousand deaths worldwide, left thousands of people homeless, and caused billions of dollars in damage. Yet residents on the northeastern seaboard of the United States can credit El Niño with milder-than-normal winters (and lower heating bills) and relatively benign hurricane seasons.

Originally, the name El Niño (Spanish for "the Christ child") was coined in the late 1800s by fishermen along the coast of Peru to refer to a seasonal invasion of warm southward ocean current that displaced the north-flowing cold current in which they normally fished; typically this would happen around Christmas. Usually, trade winds blow west across the Pacific, causing warmer ocean waters to "pile up" in the west: near Indonesia, the sea's surface is actually about a foot and a half higher than it is near Ecuador. However, on a cyclical basis, the trade winds die down, disrupting this normal pattern. The warmer waters stay east, raising the water temperature, rather than migrating west. This changes atmospheric circulation and weather patterns around the globe -- what we call El Niño.

Today, the term no longer refers to the local seasonal current shift but to part of a phenomenon known as El Niño-Southern Oscillation (ENSO), a continual but irregular cycle of shifts in ocean and atmospheric conditions that affect the globe. El Niño has come to refer to the more pronounced weather effects associated with anomalously warm sea surface temperatures interacting with the air above it in the eastern and central Pacific Ocean.

Its counterpart--effects associated with colder-than-usual sea surface temperatures in the region--was labeled "La Niña" (or "little girl") as recently as 1985. La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific, as compared to El Niño, which is characterized by unusually warm ocean temperatures in the Equatorial Pacific. La Niña tends to bring nearly opposite effects of El Niño to the United States — wetter than normal conditions across the Pacific Northwest and dryer and warmer than normal conditions across much of the southern tier. The impacts of El Niño and La Niña at these latitudes are most clearly seen in wintertime. In the continental U.S., during El Niño years, temperatures in the winter are warmer than normal in the North Central States, and cooler than normal in the Southeast and the Southwest. During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.

The shift from El Niño conditions to La Niña and back again takes about four years. Understanding this irregular oscillation and its consequences for global climate has become possible only in recent decades as scientists began to unravel the intricate relationship between ocean and atmosphere. Although meteorologists have long been forecasting daily weather based on atmospheric measurements taken around the world, they had relatively little information about conditions in many parts of the world's oceans until the advent of arrays of fixed unmanned midocean buoys in the Pacific Ocean and orbiting satellites.

But technological advances were not the only key. As the following article recounts, atmospheric and oceanographic researchers, after years of independent inquiry into the basic workings of air and sea, at last joined forces. An elegant synthesis of these two fields of research now enables climatologists and oceanographers to construct theoretical models to simulate and predict the broad climate changes associated with ENSO. For example, scientists can now warn vulnerable populations of an impending El Niño event several months in advance, providing precious time in which to take steps to mitigate its worst effects. Invaluable as this prediction of El Niño is, it is just the first step toward the much longer-term goal of providing the climatic counterpart to the daily weather prediction that we have come to take for granted.

The Impact and Cycles

El Nino and La Nina can have profound effects on global weather and ocean conditions. Under normal conditions, trade winds on the equatorial Pacific normally blow from east to west (Peru towards Asia). This causes the warm waters of the equatorial Pacific to migrate toward Asia, and cool water to upwell off Peru. Generally, high pressure builds over the cool Pacific waters off Peru producing stable atmospheric conditions while low pressure and tropical precipitation develops off Asia.

Under El Nino conditions, the trade winds reverse direction, blowing from west to east (Asia towards Peru). The warm waters off Asia start migrating across the tropical Pacific towards Peru, and the waters cool off Asia. Typically the cycle starts in early summer and the warmest waters reach Peru right around Christmas. Drought sets in over Asia as high pressure builds over the cooler ocean waters, and flooding occurs over the east Pacific as low pressure moves in. Precipitation follows the warmer waters across the Pacific.

Kelvin waves often signal the start of an El Nino cycle. Eastward propagating Kelvin waves are driven by the Madden-Julian Oscillation (MJO), a cyclical weather pattern with duration of 30-60 days per cycle. The MJO is characterized by a weak low pressure center that develops in the east Indian Ocean and tracks eastward along the equator, against the normal flow of tropical systems. It can propagate into the North Pacific as far east as the dateline. The MJO has been implicated in the development of the wet winter weather pattern known as the 'Pineapple Express', a conditions which can inundate the Western US and Hawaii with rain. The MJO reaches it's peak during the transition from winter-to-summer and summer-to-winter. The southern quadrant of a strong northern hemisphere MJO pulse that pushes into the the North Pacific produces easterly wind, which in-turn pushes warm surface water of the West Pacific eastward. If it is strong enough it can potentially producing something known as a Kelvin Wave. Kelvin waves are not waves in the classical sense, but more like a pool of warmer-than-normal water that travels east under the oceans surface at a depth averaging about 150 meters. A Kelvin wave can be evidenced at the surface by a slight rise in sea surface height (8 cm) and slight temperature increase that covers hundreds of square miles of ocean surface. Once a Kelvin wave hits the South American coast (normally near Ecuador), the warm water it carries erupts upward, creating a large warm water pool at the surface. That warm water also starts to migrate south along the coast of Peru and north up towards Central American and Mexico, often reaching well into Northern California. A Kelvin wave is tracked primarily using an array of 70 buoys anchored along the entire width of equatorial Pacific, from Papua New Guinea to just off the Ecuador coast. Temperature sensors located a various depths along the buoys anchor-line record subsurface water temperature. The sensors relayed their data real-time via satellite to a central processing facility. At the processing facility the real-time temperature measurements are compared to historical and seasonally adjusted average water temperatures for each buoy location. A variety of reports are generated daily and posted on the Internet. Some reports highlight deviations from the 'normal' expected temperatures. Such deviations are called 'anomalies' and can register as either warmer than normal (El Nino) or cooler than normal (La Nina).

But there are other signals monitored that indicate a change in the El Nino/Southern Oscillation (ENSO) cycle. Wind speed and direction along the equator can be monitored by the buoy array and by the QuikSCAT satellite. Deviations in the normal pattern are easily highlighted. A persistent slackening or even change toward easterly winds in the West Pacific is a telltale sign of a developing El Nino. Conversely, strong westerly winds in the East Pacific are signs of a developing La Nina.

Early signs of a developing El Nino pattern can appear as early as late fall a year before a mature El Nino pattern sets in. But just because these signals appear, it does not always mean an El Nino will follow. The period from March to April during the year El Nino is forecast can make or break the forecast. It is not fully understood what physical processes take place during the seasonal transition, also known as the "Spring Barrier". But during this time, early signals of El Nino can disappear, to be replaced by a neutral to slightly La Nina like pattern. But if the signs persist through the Spring Barrier, the odds of a full scale El Nino developing go up significantly.

From a surf perspective during El Nino conditions, the north Pacific subtropical jetstream is enhanced, and a strong semi-permanent low develops just east of the dateline and south of the Aleutians. This situation enhances the potential for development of large winter storms and surf, but can also produce locally stormy conditions along the US west coast. During pronounced episodes, the jetstream can drive storm energy straight from Siberia clear across the Pacific and directly into the US, often taking a track right into the drier regions of Central and Southern California. This can bring significant rainand snowfall to dry regions of the southwest US in winter.

Conversely, La Nina has the opposite effect. Colder than normal water starts to develop in the eastern equatorial Pacific in the mid summer as the trades start raging from east to west (Peru to Asia). Strong high pressure builds over the eastern equatorial Pacific while low pressure follows the warm waters towards Asia. By Christmastime the North Pacific jetstream is displaced well north, driving up towards the Aleutians into Alaska and northern Canada, and high pressure dominates the NE Pacific pattern.

For the Atlantic Ocean, El Nino and La Nina have equally dramatic effects, but in the opposite way as experienced over the Atlantic. During El Nino, the strong jetstream winds and storms blowing into the western US also blow across the equatorial Atlantic, shearing the tops of developing hurricanes, reducing water temperatures off the eastern US and the amount of energy and moisture available to feed tropical storms that emerge off the African coast. Conversely, during La Nina, a consistent flow off Africa at all levels of the atmosphere creates conditions ripe for tropical storm development.