14.2 Variable Equatorial Circulation: El Niño/La Niña
The trades are remarkably steady, but they do vary from month to month and year to year, especially in the western Pacific. One important source of variability are Madden-Julian waves in the atmosphere (McPhaden, 1999). If the trades in the west weaken or even reverse, the air-sea system in the equatorial regions can be thrown into another state called El Niño. This disruption of the equatorial system in the Pacific disrupts weather around the globe.
Although the modern meaning of the term El Niño denotes a disruption of the entire equatorial system in the Pacific, the term has been used in the past to describe several very different processes. This causes a lot of confusion. To reduce the confusion, let's learn a little history.
A Little History
The Peruvians noticed that in some years the El Niño current was stronger than normal, it penetrated further south, and it is associated with heavy rains in Peru. This occurred in 1891 when (again quoting from Philander's book)
The El Niño of 1957 was even more exceptional. So much so that it attracted the attention of meteorologists and oceanographers throughout the Pacific basin.
Just months after the event, in 1958, a distinguished group of oceanographers and meteorologists assembled in Rancho Santa Fe, California to try to understand the Changing Pacific Ocean in 1957 and 1958. There, for perhaps the first time, they began the synthesis of meteorological events with oceanographic observations leading to our present understanding of El Niño.
While oceanographers had been mostly concerned with the eastern equatorial Pacific and El Niño, meteorologists had been mostly concerned with the western tropical Pacific, the tropical Indian Ocean, and what they called the Southern Oscillation. Hildebrandsson, the Lockyers, and Sir Gilbert Walker noticed in the early decades of the 20th century that pressure fluctuations throughout that region are highly correlated with pressure fluctuations in many other regions of the world (Figure 14.6). Because variations in pressure are associated with winds and rainfall, they were wanted to find out if pressure in one region could be used to forecast weather in other regions using the correlations.
The early studies found that the two strongest centers of the variability are near Darwin, Australia and Tahiti. The fluctuations at Darwin are opposite those at Tahiti, and resemble an oscillation. Furthermore, the two centers had strong correlations with pressure in areas far from the Pacific. Walker named the fluctuations the Southern Oscillation.
The Southern Oscillation Index is sea-level pressure at Tahiti minus sea-level pressure at Darwin (Figure 14.7) normalized by the standard deviation of the difference. The index is related to the trade-winds. When the index is high, the pressure gradient between east and west in the tropical Pacific is large, and the trade-winds are strong. When the index is negative trades, are weak.
The connection between the Southern Oscillation and El Niño was made soon after the Rancho Santa Fe meeting. Ichiye and Petersen (1963) and Bjerknes (1966) noticed the relationship between equatorial temperatures in the Pacific during the 1957 El Niño and fluctuations in the trade-winds associated with the Southern Oscillation. The theory was further developed by Wyrtki (1975).
Because El Niño and the Southern Oscillation are so closely related, the phenomenon is often referred to as the El Niña Southern Oscillation or ENSO. More recently, the oscillation is referred to as El Niño/La Niña, where La Niña refers to the positive phase of the oscillation when trade-winds are strong, and water temperature in the eastern equatorial region is very cold.
Definition of El Niño
Trenberth (1997), based on discussions within the Climate Variability and Predictability program, recommends that those disruptions of the equatorial system in the Pacific shall be called an El Niño only when the 5-month running mean of sea-surface temperature anomalies in the region 5°N - 5°S, 120°W - 170°W exceeds 0.4°C for six months or longer.
So El Niño, which started life as a change in currents off Peru each Christmas, has grown into a giant. It now means a disruption of the ocean-atmosphere system over the whole equatorial Pacific.
Theory of El Niño
Sometimes the trades in the western Pacific not only weaken, they actually reverse direction for a few weeks to a month, producing westerly wind bursts that quickly deepen the thermocline there. The deepening of the thermocline launches an eastward propagating Kelvin wave and a westward propagating Rossby wave. (If you are asking, What are Kelvin and Rossby waves? I will answer that in a minute. So please be patient.)
The Kelvin wave deepens the thermocline as it moves eastward, and it carries warm water eastward. Both processes cause a deepening of the mixed layer in the eastern equatorial Pacific a few months after the wave is launched in the western Pacific. The deeper thermocline in the east shuts o the upwelling of cold water, and the surface temperatures offshore of Ecuador and Peru warms by 2 - 4°. The warm water reduces the temperature contrast between east and west, further reducing the trades and hastening the development of El Niño.
With time, the warm pool spreads east, eventually extending as far as 140°W (Figure 14.8). Plus, water warms in the east along the equator due to reduced upwelling, and to reduced advection of cold water from the east due to weaker trade-winds.
The warm waters along the equator in the east cause the areas of heavy rain to move eastward from Melanesia and Fiji to the central Pacific. Essentially, a major source of heat for the atmospheric circulation moves from the west to the central Pacific, and the whole atmosphere responds to the change. Bjerknes (1972), describing the interaction between the ocean and the atmosphere over the eastern equatorial Pacific concluded:
It is these far reaching events that make El Niño so important. Few people care about warm water off Peru around Christmas, many care about global changes the weather. El Niño is important because of its influence on the atmosphere.
After the Kelvin wave reaches the coast of Ecuador, part is reflected as an westward propagating Rossby wave, and part propagates north and south as a coastal trapped Kelvin wave carrying warm water to higher latitudes. For example, during the 1957 El Niño, the northward propagating Kelvin wave produced unusually warm water offshore of California, and it eventually reached Alaska. This warming of the west coast of North America further influences climate in North America, especially in California.
As the Kelvin wave moves along the coast, it forces other Rossby waves which move west across the Pacific with a velocity that depends on the latitude (14.4). The velocity is very slow at mid to high latitudes and fastest on the equator. There the reflected wave moves back as a deepening of the thermocline, reaching the central Pacific a year later. In a similar way, the westward propagating Rossby wave launched at the start of the El Niño in the west, reflects off Asia and returns to the central Pacific as a Kelvin wave, again about a year later.
El Niño ends when the Rossby waves reflected from Asia and Ecuador meet in the central Pacific about a year after the onset of El Niño (Picaut, Masia, and du Penhoat, 1997). The waves push the warm pool at the surface toward the west. At the same time, the Rossby wave reflected from the western boundary causes the thermocline in the central Pacific to become shallower when the waves reaches the central Pacific. Then any strengthening of the trades causes upwelling of cold water in the east, which increases the east-west temperature gradient, which increases the trades, which increases the upwelling (Takayabu et al. 1999). The system is then thrown into the La Niña state with strong trades, and a very cold tongue along the equator in the east.
La Niña tends to last longer than El Niño, and the full cycle from La Niña to El Niño and back takes around three years. The cycle is not exact and El Niño comes back at intervals from 2-7 years, with an average near four years (Figure 14.7).
Equatorial Kelvin and Rossby Waves
Two types of waves are especially important for El Niño: internal Kelvin waves and Rossby waves. Both waves can have modes that are con ned to a narrow, north-south region centered on the equator. These are equatorially trapped waves. Both exist in slightly different forms at higher latitudes.
Kelvin and Rossby wave theory is beyond the scope of this book, so I will just tell you what they are without deriving the properties of the waves. If you are curious, you can nd the details in Philander (1990): Chapter 3; Pedlosky (1987): Chapter 3; and Apel (1987): §6.10 - 6.12. If you know little about waves, their wavelength, frequency, group and phase velocities, skip to Chapter 16 and read §16.1.
The theory for equatorial waves is based on a simple, two-layer model of the ocean (Figure 14.9). Because the tropical oceans have a thin, warm, surface layer above a sharp thermocline, such a model is a good approximation for those regions.
Equatorial-trapped Kelvin waves are non-dispersive, with group velocity:
g' is reduced gravity, ρ1, ρ2 are the densities above and below the thermocline, and g is gravity. Trapped Kelvin waves propagate only to the east. Note, that c is the phase and group velocity of a shallow-water, internal, gravity wave. It is the maximum velocity at which disturbances can travel along the thermocline.
Typical values of the quantities in (14.2) are:
At the equator, Kelvin waves propagate eastward at speeds of up to 3 m/s, and they cross the Pacific in a few months. Currents associated with the wave are everywhere eastward with north-south component (Figure 14.10).
Kelvin waves can also propagate poleward as a trapped wave along an east coast of an ocean basin. Their group velocity is also given by (14.3), and they are confined to a coastal zone with width x = c / ( b y ).
The important Rossby waves on the equator have frequencies much less than the Coriolis frequency. They can travel only to the west. The group velocity is:
The fastest wave travels westward at a velocity near 0.8 m/s. The currents associated with the wave are almost in geostrophic balance in two counterrotating eddies centered on the equator (Figure 14.10).
Away from the equator, low-frequency, long-wavelength Rossby waves also travel only to the west, and the currents associated with the waves are again almost in geostrophic balance. Group velocity depends strongly on latitude:
The wave dynamics in the equatorial regions differ markedly from wave dynamics at mid-latitudes. The baroclinic waves are much faster, and the response of the ocean to changes in wind forcing is also much faster than at mid-latitudes. For the planetary waves waves con ned to the equator, we can speak of an equatorial wave guide.
Now, let s return to El Niño and its " far-reaching large-scale effects."
|Department of Oceanography, Texas A&M University
Robert H. Stewart, firstname.lastname@example.org
All contents copyright © 2005 Robert H. Stewart,
All rights reserved
Updated on November 7, 2007