Surface Winds Across Most of the Tropical Pacific Normally Move from East to West. The

El Nino

Surface winds across most of the tropical Pacific normally move from east to west. The trade winds blow from the normally high-pressure area over the eastern Pacific (near Central and South America) to the normally stable low-pressure area over the western Pacific (North of Australia). However, for reasons that are unclear, these pressure areas change places at irregular intervals of roughly three to eight years: High pressure builds in the western Pacific, and low pressure dominates the eastern Pacific. Winds across the tropical Pacific then reverse direction and blow from west to east: The trade winds weaken or reverse. This change in atmospheric pressure and thus in wind direction is called the Southern Oscillation. Ten of these oscillations have occurred since 1950.

The Southern Oscillation is the atmospheric counterpart of El Niño. It is an oscillation in air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia. El Niño episodes are associated with negative values of the SOI, meaning that the pressure at Tahiti is relatively low compared to Darwin.

The trade winds normally drag huge quantities of water westward along the ocean’s surface on each side of the equator, but as the winds weaken these equatorial currents crawl to a stop. Warm water that has accumulated at the western side of the Pacific – the warmest water in the world ocean – can then return east along the equator toward the coast of Central and South America. The eastward-flowing warm water usually arrives near the South American coast around Christmas time. In the 1890s it was reported that Peruvian fishermen were using the expression Corriente del Nino (current of the Christ child) to describe the flow; hence the curent’s name El Nino. The phenomena of the Southern Oscillation and El Nino are coupled; so the terms are often combined to form the acronym ENSO, for El Nino/Southern Oscillation. An ENSO event typically lasts about a year, but some have persisted for more than three years. The effects are felt not only in the Pacific; all ocean areas at trade wind latitudes in both hemispheres can be affected.

Normally, a current of cold water, rich in upwelled nutrients, flows north and west away from the South American continent. When the propelling trade winds falter during an ENSO event, warm equatorial water that would normally flow westward in the equatorial Pacific backs up to flow eastward. Much of the flow is in the nature of a greatly strengthened equatorial countercurrent, but oceanographers have recently discovered that the Pacific Equatorial Undercurrent also increases greatly in volume during these times. The normal northward flow of the cold Peru Current is interrupted or overridden by the warm water. Upwelling within the nutrient – laden Peru Current is responsible for the great biological productivity of the ocean off the coasts of Peru and Chile. Although upwelling may continue during and ENSO event, the source of the upwelled water is nutrient-depleted water in the thickened surface layer approaching from the west. When the Peru Current slows and its upwelled water lacks nutrients, fish and seabirds dependent on the abundant life it contains die or migrate elsewhere. Peruvian fishermen are never cheered by this particular Christmas gift!

During major ENSO events, sea level rises in the eastern Pacific, sometimes by as much as 30 centimeters (12 inches) in the Galapagos . Water temperature also increases by up to 7°C (13°F). The warmer water causes more evaporation, and the area of low atmospheric pressure over the eastern Pacific intensifies. Humid air rising in this zone, centered some 2,000 kilometers (1,200 miles) west of Peru, causes high precipitation in normally dry areas. The increased evaporation intensifies coastal storms, and rainfall inland may be much higher than normal. Marine and terrestrial habitats and organisms can be affected by these changes.

The two most severe ENSO events of this century occurred in 1982-83 and 1997-98. In both cases effects associated with El Nino were spectacular over much of the Pacific and some parts of the Atlantic and Indian Oceans. In February of 1998, 40 people were killed and 10,000 buildings damaged, by a “wall” of tornadoes advancing over the American Southeast. This record-breaking tornado event was spawned by the collision of warm, moist air that had lingered over the warm Pacific and a polar front that dropped from the north. In the eastern Pacific, heavy rains through the 1997-98 winter left at least 250,000 people homeless in Peru, destroyed 16,000 dwellings, and closed every port in the country for at least one month. Hawaii, however, was left with record drought, and some parts of southwestern Africa and Papua New Guinea received so little rain that drops failed completely and whole villages were abandoned to avoid starvation. Most of the United Stated and Canada escaped serious consequences – indeed, the Midwestern states, Pacific Northwest, and Eastern Seaboard enjoyed a relatively mild fall, winter, and spring. But California’s trials were widely reported – rainfall in most of the state exceeded twice normal amounts, and landslides, avalanches, and other weather – related disasters crowded the evening news. Conditions did not return to near normal until the late spring of 1998. Estimates of worldwide 1997-98 ENSO – related damage exceeded $25 billion.

Normal circulation sometimes returns with surprising vigor, producing strong currents, powerful upwelling, and chilly and stormy conditions along the South American coast. These contrasted colder - than – normal events are known as La Nina. As conditions to the east cool off, the ocean to the west (north of Australia) warms rapidly. The renewed thrust of the trade winds piles this water upon itself, depressing the upper curve of the thermocline to more than 100 meters (328 feet) deep. In contrast, the thermocline at the same time in the eastern equatorial Pacific rests at about 25 meters (82 feet). Studies of the ocean and atmosphere in 1982-83 and 1997-98 has given researchers new insight in to the behavior and effects of the Southern Oscillation. Some researchers believe that the 1982-83 event was triggered by the violent 1982 eruption of El Chichon, a Mexican volcano which injected huge quantities of obscuring dust and sulfur-rich gases in to the atmosphere. No similar trigger occurred before the 1997-98 ENSO, however, and researchers has recently suggested that the southern oscillation may be triggered by pulses of heat entering the Pacific at crustal spreading zones near the equator. Some of this heat is moved by currents to the ocean surface. The warming of surface water may cause air above the ocean to warm, atmospheric pressure to fall, and the east-to-west trade winds to slacken. Though the exact cause or causes of the Southern Oscillation are not yet understood, subtle changes in the atmosphere permit meteorologists to predict a severe El Nino more than a year in advance of its most serious effects.

Introduction to El Nino

El Niño is a warm, nutrient poor, ocean current that flows southward along the coast of northern Peru and creates sometimes catastrophic effects on local and sometimes global fishing and biogeochemical systems. This event, named El Niño (the {Christ} Child) because it occurs around Christmas time, was once thought of as happening only abnormally. It is known now, however, that El Niño is actually the result of a Pacific Ocean oriented cycle that lasts from three to five years. The event itself lasts about 12 to 18 months.

The El Niño or, more precisely, the El Niño Southern Oscillation (ENSO), for which it and its related phenomena are called, is produced primarily by the interaction between the winds in the atmosphere and the sea surface in the Pacific Ocean. This interaction is tracked by observing, through remote satellites, the changing patterns of sea surface temperatures in the Pacific, sea level changes, and pressure oscillations.

In 1904, Sir Gilbert Walker, Director General of the Observatory in India, began studying the sea level pressure swing between the East Pacific and the West Pacific. He correlated the swing with data from around the globe, including sea surface temperature, and called it the Southern Oscillation (the change in pressure is called the Southern Oscillation Index -SOI- today). Walker failed to make the connection between this oscillation and El Niño however.

Normally, when there is no El Niño (often referred to as La Niña, the trade winds (winds that flow toward the equator) blow from east to west across the coastal waters of the eastern Pacific. In doing so, they drag the warm surface waters of the equatorial Pacific with them. This causes an upwelling of cooler, nutrient-rich waters which the fish population in the region depends on. This 'air/sea interaction' is accompanied by the circulation of large internal waves (waves that have their peaks under the ocean surface) across the Pacific Ocean mainly in the equatorial region. These internal waves are referred to as Kelvin and Rossby waves. Kelvin waves travel eastward along the equator and are not subjected to the Coriolis force because of this. Rossby waves, unlike Kelvin waves, travel westward and are a result of the changes that the Kelvin waves introduced to the area. Rossby waves, since they move away, but parallel, to the equator are not relieved of the Coriolis force; thus they travel up to about three times slower.

During an El Niño, the east-to-west trade winds weaken causing the upwelling of deep water to cease. The prevailing reason for this is that cumulus clouds, produced by the warm surface waters of the Pacific, move eastward along the Pacific altering the surface winds and weakening the prevailing east-to-west trade winds. As the trade winds weaken and the ocean surface is warmed, the El Niño is strengthened and a positive feedback loop forms.

The ENSO cycle can also be explained through the movement of waves in the Pacific as mentioned above. The cycle starts with warm water traveling from the western Pacific to the eastern Pacific in the form of Kelvin waves. After roughly three to four months of traveling across the Pacific along the equator, the Kelvin waves reach the western coast of South America where they mix with the cool Peru Current system; therefore raising sea levels and sea level temperatures in the region. Upon reaching the coast, the water forces to the north and south and causes El Niño conditions to the south. Because of the changes in sea-level and sea-temperature due to the Kelvin waves, an infinite amount of Rossby waves are formed and move back over the Pacific. The Rossby waves, as mentioned before, are much slower than the Kelvin waves and can take anywhere from nine months to four years to cross the Pacific. Waves move slower when the distance from the equator is increased. (This wave delay is key to the ENSO cycle.) When the Rossby waves arrive at the western Pacific they bounce off the coast and become Kelvin waves and again travel back across the Pacific towards South America. This time, however, the waves decrease the sea-level and sea surface temperature returning the area to normal or La Nina conditions.

This sea surface temperature (SST) oscillation is accompanied by an oscillation in pressure (called the Southern Oscillation). During an El Niño, sea level pressure in the east becomes real low while pressure in the rest rises. The situation reverses itself during La Niña years. .

This 3 to 5 year cycle is then repeated and each El Niño phase is different, some being mild and some being very strong. A complete ENSO cycle requires that the Pacific be crossed by two pairs of Kelvin and Rossby waves. Because of El Niño's devastation to local and sometimes global economies and agriculture, the benefits of being able to predict an El Niño even a short while in advance are great.

Indian Ocean Dipole

The Indian Ocean Dipole (IOD) is a coupled ocean and atmosphere phenomenon in the equatorial Indian Ocean and characterized by anomalous cooling of SST in the south eastern equatorial Indian Ocean that affects the climate of Australia and other countries that surround the Indian Ocean basin. It is normally anomalous warming of SST in the western equatorial Indian Ocean. Associated with these changes the normal convection situated over the eastern Indian Ocean warm pool shifts to the west and brings heavy rainfall over the east Africa and severe droughts/forest fires over the Indonesian region. The name " Indian Ocean Dipole (IOD) " was coined by Prof. Yamagata and other researchers of the Climate Variations Research Program (CVRP) of Frontier Research Center for Global Change (FRCGC) to represent the zonal dipole structure of the various coupled ocean-atmosphere parameters such as SST, OLR (outgoing longwave radiation) and Sea Surface Height anomalies. Generally, this configuration is also called positive IOD.

The IOD is commonly measured by an index that is the difference between sea surface temperature (SST) in the western (50°E to 70°E and 10°S to 10°N) and eastern (90°E to 110°E and 10°S to 0°S) equatorial Indian Ocean. The index is called the Dipole Mode Index (DMI). The map below shows the east and west poles of the IOD for November 1997; a positive IOD year.