Modified from Kershaw, S. 2000. Oceanography: an Earth Science Perspective. Nelson Thornes.

1. El Niño

El Niño is important in climate change analysis because it embodies all that is not clear about global ocean circulation and its relation with the atmosphere. >>

Figure 1 shows a simplified version of the surface ocean system; notice that in the northern hemisphere there is a general clockwise flow in an approximate circular motion around the ocean basins.

Figure 1. World ocean surface circulation patterns. Note the Pacific ocean circulation, divided into the North Pacific Gyre and South Pacific Gyre.

These circular patterns are called gyres. Gyres occur because of wind patterns that flow across the ocean surface, shown for the southern hemisphere in Figure 2. The water is driven by winds to form currents. You can demonstrate this by blowing on your cup of tea; the tea flows along with your blowing because of friction between the moving air and the tea. If you blow off-centre, the tea then flows in a circle around the cup, because the walls of the cup force the water to be deflected. >>

This is similar to the ocean gyres on Earth: the land gets in the way of water flow, and deflects the water and the result is the formation of gyres. There are two gyres in the northern hemisphere: North Atlantic and North Pacific; note their clockwise flow. In the southern hemisphere there are three gyres; the South Pacific and South Atlantic gyres flow anticlockwise and meet the northern gyres around the equatorial zone. The reason for the gyre motion is that the air flow in the atmosphere is not uniform, as shown in Figure 2.

Figure 2. Wind circulation patterns, from a southern perspective; note the southeast and northeast trade winds, that drive ocean surface water to the west in the Pacific ocean.

The asymmetry of air flow (and therefore water flow) is due to the coriolis effect (erroneously called coriolis force; note that it is not a fundamental force. >>

Figure 3 demonstrates how the coriolis effect works. Thus the gyres develop because land interrupts the water flow, and forces it to turn.

Figure 3. How coriolis effect works; study the diagram carefully.

The Indian ocean has a gyre which switches direction because of the presence of the Himalayan mountains; the mountains influence the wind patterns blowing across the sea; in the summer, when the air is warm, air is drawn up the mountains and wind blows across the sea towards land, causing clockwise flow. In the winter, when the air is cold, air flows down from the mountains and forces the gyre to change direction. >>

As a result of the surface ocean water circulation patterns (Figure 1), water in the Pacific basin is usually piled up on its western side because of the rotation of Earth and the northeast & southeast trade winds (Figure 2) that developed because of coriolis forcing (Figure 3). In normal years, the south Pacific gyre causes water to flow northwards along the eastern margin (South American side). Also involved is a peculiar effect induced by coriolis forcing that plays a significant role in ocean water transport, called Ekman transport (no this is not a Scandinavian bus service), shown in a rather complex diagram in Figure 4.

Figure 4. Ekman transport system, that controls oceanic upwelling on continental margins and in the equatorial oceans. It is controlled by the coriolis effect. Ekman transport has a significant control on the flow of water in coastal regions, and has a great impact on nutrient supply to the surface oceans.

Ekman transport drives the overall water flow to the left (west), and leads to updrawing of water in the eastern side of the Pacific basin. This updrawing is normally referred to as upwelling and it brings cold nutrient-rich water from the deep Pacific, and a thriving Peruvian fish industry from the upwelled nutrients. Furthermore, because there is a piling of surface water on the west side, there is a thicker layer of warmer water in the western Pacific, so that the thermocline (boundary between warmer surface water and cooler deeper water) is deeper on the west, and slopes upwards to the east. >>

Each year around Christmas time (El Niño, the child), the pattern changes. The southeast trade winds weaken, and normality breaks down; warm water flows eastwards down the water gradient across the Pacific to interrupt the usual pattern off the west coast of South America. The thermocline flattens out as the warm ocean develops eastwards and there is a severe decrease in upwelling off South America, depriving surface water of nutrients and interrupting the fishing industry; a time for repairing nets and painting boats.

In certain years the change is more profound, leading to a set of events now commonly referred to as an El Niño event. In an El Niño (event) year, the whole process is intensified, resulting in catastrophic decrease in upwelling off South America and decimation of the Peruvian fishing economy.

It gets worse. In normal times, the southeast trades flowing across eastern Pacific waters draw cool, dry air from the subtropical high pressure zone. >>

In the southern hemisphere, the descending subtropical air is cool, dry and is deflected to the west by the coriolis effect; this air is not warm enough to rise to produce rainclouds until it gets across to the western Pacific region. Because the southeast trades weaken during El Niño, the rainclouds extend eastwards across the Pacific, causing considerable increase in rainfall in the central Pacific, and also in central South America. Shifting of the rain focus to central Pacific waters can leave western Pacific areas short of rainfall, giving droughts in Australia and Indonesia, and forest fires. Figures 5 and 6 show two different perspectives of the process.

Figure 5. The Pacific Ocean surface circulation in normal and El Niño years. Upper diagram shows the temperatue distribution and the piling of warm water along the western Pacific; the thermocline slopes up to the east, so that upwelling cold water on the eastern margin is able to reach the surface, stimulating ocean productivity in that area. The patterned arrow in the atmosphere is the compensating flow of warm moist air, with the whole system being called Walker Circulation. Lower diagram shows an El Niño year, when Walker Circulation diminishes, warm water flows east along the equatorial regions, suppressing eastern margin upwelling, and promoting dryness in the western Pacific. Temperature fields in the ocean are in oC.

Figure 6. same as Figure 5, but showing the diagram cut into two, and a cross section of the ocean water structure along the line of the cut.

Fluctuations of normal times and El Niño times led to the term El Niño-Southern Oscillation (ENSO); the ENSO system forms a complex Ocean-Atmosphere-Land interaction. >>

The oceanic part seems to involve mostly surface currents, but also the deep ocean (upwelling). The big question, of course, is why the southeast trade winds weaken to initiate the El Niño event.

2. La Niña

Less well known than El Niño is another phenomenon called La Niña (named to be opposite to El Niño, since La Niña means girl child). >>

La Niña is an intensification of the normal circulation shown in Figure 2, so that increased rainfall develops in the western Pacific, and may lead to intense flooding in eastern Australia (e.g. January 2011).

3. It is complex

Although the heat transport system of the atmosphere, plus Coriolis deflection, creates the trade winds, the controls are clearly more complex, and can lead to a general disruption of the climate, especially in the southern hemisphere. The La Niña event of Jan 2011, that caused severe flooding in western Australia, but also in Brazil, show how catastrophic these events can be, generally. >>

How much of the disturbance is related to heat transference between the atmosphere and oceans awaits further research, but more refined models indicate that, if future greenhouse warming predictions come true, then El Niño conditions will become more frequent and powerful. Because water has c.30x the heat capacity of air, it may not come as a surprise if it is eventually established that El Niño has an oceanic origin.

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