Before we can examine the atmospheric manifestation of El Nino, we should look at some aspects of the General Circulation of the Atmosphere (GCA).
The GCA describes the large scale motions of the atmosphere with the smaller scale motions being smoothed. In addition to the rotation of the earth, the major underlying cause of the GCA is the differential heating which occurs at the earth's surface. Remember, that when considering the radiation balance, the earth's equatorial regions receive a net surplus of energy, while the regions nearer to the pole, experience a net deficit. To prevent the equatorial regions from getting warmer and warmer, and the polar regions from continually cooling, there must be a transport of heat from equatorial regions to polar regions. This transport of heat is accomplished by the atmosphere and ocean circulations. The transport in the atmosphere can take place as tboth sensible and latent heat. Some smaller scale manifestations of this heat transport can be seen in hurricanes and frontal systems. Hurricanes, for example, may evaporate water in the tropics and later release the latent heat they acquired when condensation occurs in mid-latitudes.
To help explain the GCA, various models have been developed. One, which is quite helpful is explaining the basics of the GCA is termed the Three Cell Model. It is a simple model, but goes a long way to explaining much of what we observe. The assumptions made in constructing this model are: the earth rotates; the sun shines directly overhead at the equator; and the earth is everywhere covered by the same substance which we assume to be water. When the earth rotates under these conditions, we see that longitudinal (meridional) circulations develop in both the northern and southern hemispheres. In each hemisphere, three of these meridional circulations form. One which is caused by surface heating in the vicinity of the equator results in rising, poleward moving air at the equator. As this air moves poleward, it cools radiatively, and sinks near 300 latitude, where it then returns at low levels toward the equator. This meridional circulation cell is called the Equatorial Hadley Cell. It is with this circulation cell, that we are concerned in our discussion of El Nino. Now, over the oceans, because of surface uniformity, the large scale meridional flow most closely follows what the Three Cell Model predict. As the flow around the tropical cell descends near 300 and moves equatorward, it is turned to the right in the northern hemisphere (to the left in the southern) resulting in what we call the east or northeast tradewinds in the northern hemisphere, and the east or southheast tradewinds in the southern hemisphere.
As the easterly trades move at low levels across the Pacific Ocean, the air rises in the Western Pacific and then descends again in the Eastern Pacific. This large scale zonal circulation, results in rising air over the Western Pacific and descending air over the Eastern Pacific and is called a Walker Cell. The zonal Walker Cell can be thought of as being super-imposed upon the meridional flowing Hadley Cell. The air flowing westward across the oceans acquires eastward angular momentum from the earth's surface. As it then rises in the Western Pacific and is turned to the east by the Coriolis Force, and moves poleward, it acquires an increasing westerly wind component, resulting in the subtropical jet stream located at the poleward boundary of the tropical Hadley Cell.
The atmosphere is said to be stable if an air parcel resists displacement in the vertical. In practice, we are usually concerned with parcels near the earth's surface. So in a stable atmosphere, if a parcel is displaced upward, it will return to the surface. In an unstable atmosphere, it will continue to rise after being displaced vertically.
Concepts which are important to the understanding of stability include adiabatic expansion and cooling, the ability of air to "hold" moisture, and density. When an air parcel rises, it cools. If the relative humidity of the parcel is < 100%, it cools at the dry adiabatic lapse rate (10 C/1000 m or 5.5 F/1000 ft). As the air cools, the relative humidity increases until saturation is reached and cloud droplets begin to form. At this point, because latent heat is being released, the rate at which the ascending air parcel cools decreases. Now, it must be remembered, that at identical pressures cold air is more dense than warm. So, if an air parcel is made to rise, it cools dry adiabatically. It this process results in it being colder (and therefore more dense) than its surroundings, in the absence of a forcing mechanism, it will return to the surface. On the other hand, if an air parcel is warmer (less dense) than its surroundings, if made to rise, it will continue to rise, as long as it is less dense than its surroundings.
When the concept of stability is applied to the air over the subtropical oceans, we see that on the east side of the subtropical highs, we generally have descending air on the east side of the Walker circulations. We also have relatively cold upwelled water. This water chills the low level air by conduction, so that it is much colder than the air higher up. These two factors combine to essentially stop positive vertical motions in those areas, resulting in a very stable atmosphere. As a result the coasts of Baja California, Peru and Northern Chile experience very low annual precipitation totals. The immediate coastal areas also experience a great deal of fog and low clouds resulting from the cooling of the air just above the water to its dewpoint.
On the west side of the oceans, we have a different situation. The water is warm (27 C to 31 C), and the Walker circulation predicts large scale rising of air. As a result, the subtropics lying on the west side of the subtropical high, experience a great deal of instability and ample precipitation, particularly during the summer months.
Around the southern periphery of the northern hemisphere's subtropical high, we have the northeasterly trade winds. Analogously, around the north side of the southern hemisphere's subtropical high, exist the southeasterly trades. Where these wind fields come together is called the ITCZ. It is generally an area of convection with rain showers and thunderstorms. The mean position of the ITCZ varies from just south of the equator in January to about 100 north in July with a mean position over the Pacific Ocean of about 50 north.
The ITCZ is a band of low pressure which forms over the regions of the warmest waters and land masses in the tropics. The ITCZ is identified on the satellite image as the band of bright clouds located just north of the equator.
The ITCZ is not a stationary band but tends to migrate to the warmest surface areas throughout the year. In the early part of the calendar year, the high sun occurs in the Southern Hemisphere causing a southward displacement of the ITCZ. As the high sun period travels from the Southern Hemisphere to the Northern Hemisphere, the ITCZ follows by migrating northward attaining its maximum northward displacement during the month of June.
While El Nino conditions prevail, the normal migration of the ITCZ is disrupted due to the unusually warm sea surface temperatures which occur in the tropical Pacific.