Good boy, bad boy
A Review by Richard Shelton

Meteorology and its close relative physical oceanography are arguably the most important of the environmental sciences. Without its atmosphere and oceans, with their great capacity to contain and redistribute solar energy, our planet would be a place of climatic extremes with an average global temperature of 18 degrees celcius. We can also be certain that, in such a dry and inhospitable place, the carbon atom would not have had the opportunity to exhibit the remarkable versatility the most complex expression of which we call sentient life. Air and water are fluids with very different properties, the one a mixture of gases at all natural temperatures on earth, the other occurring in all three phases and with the strange trick of possessing a lower density when solid than when flowing freely as a liquid.

For all the differences in the physical characteristics of air and water, their turbulent embrace is so intimate that to understand the ocean it is necessary to understand the atmosphere and vice versa. Two-way traffic between atmospheric gases, now dissolved in sea water, now released from it, helps to maintain the life-giving properties of both. Water molecules evaporating from the surface of the sea and later condensing to form clouds effect enormous transfers of energy while at the same time acting as cooling reflectors of sunlight. Differences in air pressure cause winds to blow, and surface water masses to move in ponderous response above a deeper "thermo-haline" circulation driven by the temperature differences between the poles and the tropics and, like the winds, modified in direction by the rotation of the earth. It is all rather a lot to take in and even more to explain, especially without the extensive use of diagrams and equations. S. George Philander, Distinguished Professor of Geosciences at Princeton University, has done this in a cleverly indirect way by using El Nino as his vehicle.

El Nino literally means "little boy"; expressed in capitals, it refers to the Christ child. That the natives of Peru and Ecuador applied so august a name to the Christmastide warming of the seas off their coasts is an indication that the arrival of the warm water was once seen as a great blessing. At that time, around the seventeenth century, the most valuable local fisheries were for relatively large predatory species like Spanish mackerel, skipjack and yellowfin tuna, which came inshore with the warm water and thus within the reach of the local fishermen.

Back in those days, boats were small and fishing gear was primitive. The high levels of exploitation made possible by acoustic fish detection in combination with purse seine and pelagic trawl nets lay far in the future. As a result, the abundance of the resource was much greater and the feeling that a blessing had been conferred by the same almighty hand that had miraculously warmed the waters was all the more obvious.

Increased efficiency in detecting and catching fish has not been the only change in the structure of the Eastern Pacific fisheries. There, as elsewhere in the world, the twentieth century saw the rapid growth of so-called industrial fishing in which the objective is not to provide fish for direct human consumption but the production of fish meal and oil for use in the manufacture of animal feed. The market for these products in the developed world is enormous and has been made all the larger in recent years by the increased demand for protein and oil for use in the intensive culture of carnivorous fishes like Atlantic salmon.

It so happens that one of the largest of the world's industrial fishery resources is the anchoveta, a small herring-like fish which occurs off the coasts of Peru and Ecuador. The anchoveta is numerous because, unusually among sea fishes, it lives mainly on diatoms, the microscopic mid-water plants that form the broad base of the marine food pyramid. Diatoms have two main requirements, bright sunlight to power their photosynthesis, and a source of nutrients like nitrate phosphate and silicate to sustain their growth and reproduction. There is no shortage of bright sunlight in tropical seas but, over large areas, the supply of nutrients in surface waters is severely depleted. By contrast, deep water, where the intensity of the light is too low to sustain photosynthesis, is often rich in nutrients. Where such water rises to the surface by the process that oceanographers refer to as "upwelling", bright sun and a continuous source of nutrients come together, the production of diatoms explodes and with it the organisms, like the anchoveta, that prey upon them.

The seas off Peru and Ecuador are host to one of the largest upwelling zones anywhere in the world. The snag is that, every so often, the El Nino that brings the warm water, and the highly prized but relatively scarce human consumption fishes with it, also smothers and cuts off the supply of upwelling cold water, which is rich in nutrient salts, and sustains the much more valuable industrial fishery for anchoveta. Further afield, the changes in air pressure which drive extreme El Nino episodes may be associated with drought and floods in South America, droughts in Australia, India and southern Africa and cyclones among Pacific islands.

To the world of science, El Nino is neither good nor evil. It is part of the Southern Oscillation, a long-established and highly complex series of interactions between the atmosphere and the sea in which the alternation of high and low air pressure between the Pacific and Indian Oceans is accompanied at a slower pace by wind-driven changes in the distribution of surface water. During the El Nino phase, a weakening of the trade winds allows warm surface water to extend across the full width of the tropical Pacific, a symmetrical distribution in which the warmest water corresponds with the most intense sunlight. It is a paradox that this phase of the Southern Oscillation, the one picked out for special mention by man the economist, is in truth more normal, isothermically than the asymmetries of sea surface temperature that mark La Nina -- the name relatively recently given to the opposite phase in which cold, nutrient rich water is exposed to sunlight off the South American coast to the great benefit of the anchoveta fishery. How aptly Philander quotes Confucius' observation that "a common man marvels at uncommon things; a wise man marvels at the commonplace". Why, for instance, is the layer of warm surface water so shallow, even in the tropics, that it can readily be swept aside by air movements? The shallowness of the warm water above the "thermocline", the name used by oceanographers to mark the sharp discontinuity between the balmy heat of the surface and the shuddering cold of the deep, certainly surprised Captain Harry Ellis who made the discovery in 1751. Being something of a bon vivant, he put his discovery to immediate use by drawing water from below the thermocline to cool his wine.

Why, though, should there be such a thing as a thermocline? Surely the effect of sunlight shining with tropical intensity, day after day, century after century, would heat the sea uniformly from top to bottom? Perhaps it would, but for the unique properties of water, and for the temperature differences between the poles and the equator which power the so-called "thermo-haline" circulation of the world's oceans. Thus, in polar latitudes, extreme cold, evaporation and, during ice formation, the concentration of salts in the liquid phase, make the sea water so dense that it sinks to the bottom of the ocean, pushing the water already there towards the Equator. Once again the commonplace surprises because only in the southern hemisphere is the input of cold water symmetrical about the pole. Even at its northernmost limits, the Indian Ocean is too warm for the sea to be dense enough to sink. The sea in the northern Pacific also has a low density because of its relatively low salinity. Only in the far North Atlantic is the sea cold and saline enough for the surface waters to sink off Greenland, ultimately to cross the Equator and join the Antarctic Circumpolar Current. This eastward current connects all three of the great oceans and supplies the Indian and Pacific with the cold deep water that once cooled Captain Ellis's wine and, until blanketed by El Nino, fattens the anchoveta and thereby, as we have seen, rather a lot of the world's intensively reared livestock.

There is much that we still do not know about the global conveyor belts that link the world's oceans and, by redistributing heat, share with the atmosphere control of the earth's climate. We are especially ignorant about the routes by which the water that courses slowly through the deep sea eventually finds its way back to the surface. In the same way, despite modern success in forecasting the short term atmospheric changes we call weather, we still have some way to go in explaining the overall effect of cloud formation and distribution on climate. It is Philander's eloquently expressed thesis that, despite the gaps in our knowledge, we know enough about "the two concentric spherical shells of water and air that envelop our planet" to make sense of El Nino.

In summary, to explain El Nino requires an understanding of the Southern Oscillation. To understand the Southern Oscillation requires an understanding of the ways in which the atmosphere, the oceans and the continental land masses interact to create the current climate and weather systems within which El Nino and La Nina swing their wayward southerly pendulum. George Philander provides this understanding simply and authoritatively. He does so, not by losing the reader in elaborate descriptions of data acquisition and mathematical modelling, but by the apt use of analogies drawn from the viewpoints of the poet, musician and painter.

The final step, the construction of a mathematical model that would enable us to predict the likely future behaviour of the Southern Oscillation with real precision, may never be possible. However, on present evidence and, in the knowledge that the climate of our vulnerable little spaceship is getting, at least for the time being, both warmer and less stable, we may find the El Ninos of the immediate future cleansers of temples more often than they are healers of lepers.

Richard Shelton is Research Director of the Atlantic Salmon Trust. His memoir, The Longshoreman, was published in paperback in January.