El Niño and the Southern Oscillation (ENSO)
PAST, PRESENT & FUTURE
“This is the foundation of all: we are not to imagine or suppose, but to discover, what nature does or may be made to do”.
Francis Bacon (1561-1626)
For millennia, people have tried to understand, predict, forecast, and guess what might be the natural variations of local and regional climate on seasonal and on year-to-year time scales. Among the earliest recorded treatises on weather and climate was Aristotle's Meteorologica, written in the fourth century BC. Since then, seasonal and interannual variations have been a major concern to leaders and followers alike for many reasons, including food production, water resources management, shelter, and public safety. Other reasons for concern about climate and weather include curiosity, and an ever-present human desire to foresee the future.
As a general interest, meteorology introduced its semantic field in our everyday speech: “the president is under a cloud”, “the test was a breeze”, “the economy is in the doldrums”. Stockbrokers on Wall Street now mumble “El Niño” when the market is unpredictable; so does the commuters in Paris during exceptional traffic congestion. Philander (2004) pointed out that the phenomenon has so remarkable large effects on societies and economies of many counties by the severe floods and droughts it causes, that its name also become part of our vocabulary. But contrary to meteorogical metaphoric statement cited above, the meaning of El Niño and its physical characteristics through the history remains a mystery to most people, despite all the publicity he currently receives.
In general, the tropical Pacific Ocean is characterized by warm surface water (29-30°C) in the west but much cooler temperatures in the east (22-24°C). The body of warm water in the west is known as the Pacific “warm pool”, and it is associated with intense rainfall and the largest region of atmospheric heating. The warm pool is relatively deep, with the temperature decreasing slowly with depth to the thermocline, the region of rapidly changing temperature some 100-200 m below the surface, before dropping off more rapidly. The cooler sea-surface temperatures (SSTs) in the eastern Pacific are the result of cold water upwelled from below, forced by the trade winds converging into the warm pool. Collectively, the east-west SST gradient and the associated easterly trade winds constitute a state of quasi-equilibrium for the ocean-atmosphere system
Roughly every three to seven years, this state of quasi-equilibrium (driven by the so-called “Bjerknes feedback”) breaks down: ocean warming occurs across the entire basin, and it can last for a year or occasionally longer. This is an El Niño event. Typically, the warm pool is displaced to the east, causing a shift in the major precipitation regions of the tropics and the disruption of “normal” climate patterns at higher latitudes (Figure 1.b). During the past two decades there have been a number of major warm events. In 1982-83, the second-strongest El Niño of the century occurred. In 1986 a weaker warm event ensued and lasted through 1987. A far weaker one occurred in 1992, but was untypical because it appeared to continue for a further two years. In 1997-98, an unprecedented surface warming occurred over the tropical eastern Pacific Ocean (5°C higher than normal) bringing the so-called “strongest El Niño of the century”. Since then much weaker El Niño phenomenon occurred in 2002-03 and 2006-07, except for the current event which display a warming of 2°C over the tropical eastern Pacific. Usually, an opposite and cooler state of the tropical Pacific follows a year or so after an El Niño. This phase, known as La Niña, is marked by a distinct cooling in the eastern Pacific, and is also associated with perturbations of the global climate (Figure 1.c).
El Niño and La Niña depend jointly on oceanic and atmospheric processes, and are the result of the instability of the quasi-equilibrium state. Different accounts of the underlying dynamics are currently accepted. One of them underlines that the onset of an El Niño is often marked by a series of prolonged westerly wind bursts in the western Pacific, which persist for one to three weeks and replace the normally weak easterly winds over the warm pool. The most important impact of these wind bursts is to severely perturb the upper ocean (Ekman pumping) and excite the eastward propagation of large-scale Kelvin waves, which have wavelengths of thousands of kilometres, and have their maximum amplitude on the thermocline. Their main effect is not to advect warm water from the warm pool to the cooler eastern Pacific, but to suppress the upwelling of cold water in the central and eastern basin. The regular annual cycle of SSTs in the tropical Pacific would “normally” induce a cooling early in the northern spring; instead during an El Niño event the eastern ocean continues to warm. This observation has led to the speculation that the instability that produces El Niño results from the inability of the warm pool to export enough heat each year so that its heat content increases in time. In this sense, the warm pool is preconditioned for El Niño, and the westerly wind bursts may serve as triggers to release the stored energy. When the oceanic Kelvin waves hit the eastern Pacific boundary, they reflected as Rossby waves and reverse the anomalies. Therefore El Niño is not only a sporadic occurrence—an occasional departure from the “normal” conditions—but is part of a continual oscillation, the Southern Oscillation.
On top of that, the “coupling strength” between the ocean and the atmosphere play a major role in the alternation warmer/cooler period of the ENSO's cycle. This coupling is determined by a host of physical factors. Among the most important: the average wind, which influences the strength of wind stress from a certain wind anomaly; the amount of atmospheric heating generated by a given SST change, which will depend on mean atmospheric temperature and humidity; the sharpness and depth of the thermocline, which together determine how big a change in the temperature of upwelled water is realized from a given wind-driven change in the thermocline depth.
As we said before, this results in a period tending to stay within a 3-7 years band. This current irregularity of the cycle bring a broad disagreement, some attribute it to low-order chaotic dynamics, some to noise-weather systems and intraseasonal oscillations—shaking the essential linear and damped characteristics of the system that can be found looking back in the past.
Paleo-ENSO data (pollen data from Australia (Shulmeister, 1995), geoarchaeological evidence from Peru (Sandweiss, 1996), lake sediments in Ecuador (Rodbell, 1999), corals from Papua New Guinea (Tudhope, 2001) and organic molecular proxies (Makou et al., 2010)) display a dual control for ENSO consisting of an orbital precession component and a “glacial dampening” component. The evolution from weak ENSO in the early and middle Holocene to strong and variable ENSO today was related to precessional effects (Clement et al., 2000). Moderate ENSO strength around 38 to 42 ka, 85 ka, and 130 ka was the net result of the competing effects of glacial dampening and precessional enhancement (Tudhope et al., 2001).
Indeed, Clement (2000) show that the likely cause is the difference in the earth's orbital configuration at that time. Using the ENSO model of Zebiak and Cane (1987), they impose the perturbation heating due to orbital changes to an otherwise modern state. The model simulation has a weaker ENSO cycle during the early and middle Holocene. The average period between events is not greatly different, but strong events are rare. In both the model and real versions of the modern climate, ENSO events amplify through a “growing season” that runs through the boreal summer and into the autumn, after which growth ceases and anomalies begin to decay. (Thus El Niño and La Niña events peak around the end of the calendar year, when the rate of change is zero.) The growth is a consequence of the Bjerknes feedback (Figure.1); there is a positive feedback for only part of the year. In the model simulations of the early Holocene, the growth of anomalies ends around August, before the summer is over. This shorter growing season means that anomalies do not reach the peak values of today. In other words, the equatorial oceans received about the same annual solar radiation but its seasonal distribution is quite different. Northern Hemisphere insolation was stronger in the late summer and fall, so the Intertropical Convergence Zone, which tends to lie over the warmest water, was held in place in the higher tropical latitudes. One then should remind that a key link in the Bjerknes feedback is from SST to enhanced heating to changes in the winds, but the heating is associated with low level convergence, and if the convergence cannot be moved on to the Equator the link is broken and the ENSO anomalies do not grow. Thus, orbital changes alter the mean climate and this in turn changes ENSO behavior markedly.
If we look futher into the Zebiak-Cane model, this one considers only the tropical Pacific Ocean, and the orbitally forced model runs do not include any changes in atmospheric CO2 or changes in climate forced from outside the tropics. As a result, there is little change in the modeled mean temperature of the equatorial Pacific during glacial times, a scenario that now seems unlikely as showed by Tudhope (2001). Indeed, they suggest that ENSO was weakened by glacial conditions at times when the model, which looks only at orbital changes, maintains its strength. Therefore one may suggest that some aspect of the glacial climatic state may have had the effect of reducing the amplitude of ENSO. There are several possible candidates for this role. Significantly decreased equatorial Pacific temperature may have resulted in generally weaker coupled ocean-atmosphere interactions, thereby subduing the ENSO system. Alternatively, or in addition to this, the inferred increase in zonal SST gradient in the equatorial Pacific may have subdued ENSO through strengthening of the trade winds. Although these are perhaps the most probable controlling factors, changed meridional SST gradients, tropical/extra-tropical interactions, and changes in thermocline structure (possibly related to any or all of the above) may also be implicated.
This likely dual “orbital precession-glacial dampening” controlling mechanism of ENSO strength, leads to suggest that ENSO may be stronger now than at any other time over the past 150,000 years.
If we look now, further in the past, earlier than three million years ago, in a period of similar interglacial conditions, El Niño state was perennial (Kennett et al., 1985). In the light of this empirical information, the return of permanent El Niño conditions in response to the current rapid rise in the atmospheric concentration of carbon dioxide is also within the realm of possibilities (Philander & Federov, 2003).
Nevertheless conditions today appear to be both close to and far from those of three million years ago. We are close to the earlier “warm world” because, today, the distribution of the continents and the composition of the atmosphere are only slightly different from what they were then. We are far from that world because El Niño is intermittent today but was perennial then. The question of the factors that can favor an abrupt switch from the current conditions to those of the past may receive considerable attention. Of special interest are the different mechanisms that can cause an increase in the SST of the eastern equatorial Pacific. Today, the warming of that region during El Niño involves an east-west redistribution of the warm surface waters without the net addition of heat to the ocean (Figure.1.b). An alternative, less nimble way for warming the eastern Pacific is by adding heat to the ocean, thus increasing the volume of warm surface water and deepening the thermocline. Once the thermocline is so deep that the winds are unable to bring cold water to the surface, then El Niño conditions are permanent rather than intermittent (as described in paragraph 3). This unfamiliar phenomenon, which scientists are only beginning to explore, clearly involves changes in the oceanic heat budget.
In a state of equilibrium, the ocean has a balanced heat budget so that the gain of heat equals the loss of heat. Today the gain is mainly in upwelling zones of low latitudes, such as the eastern equatorial Pacific, where cold water rises to the surface. The loss of heat occurs in higher latitudes, especially where cold air off the Asian and North American continents flows over the warm Kuroshio Current and Gulf Stream. Oceanic currents transport heat from regions of gain to regions of loss, thus ensuring a balanced heat budget for the ocean. Earlier than three million years ago, before the appearance of cold surface water in upwelling zones of low latitudes, the constraint of a balanced heat budget was probably satisfied in a different manner—locally everywhere. Hence the oceanic circulation must have been different, and the thermal structure of the ocean, which the circulation maintains, must also have been significantly different.
Up to now, oceanographers investigating abrupt changes in oceanic heat transport to high latitudes have focused on the thermohaline circulation (or Overturning Meridional Circulation). Theoretical studies show that a freshening of the surface waters of the northern Atlantic could cause sudden changes in that component of the circulation, changes that could have a significant effect on the climate of the northwestern Europe. But this change in the salinity of the surface waters could also affect the other major component of the circulation, like for instance the wind-driven gyres. It appears that, as in the case of the thermohaline circulation, so the wind-driven circulation can be subject to abrupt changes, except that the changes manifest themselves differently. They become apparent, not in the northern Atlantic, but in the tropics, in the form of perennial El Niño conditions. These tentative results suggest that global warming, should it cause the melting of the polar ice caps and hence a freshening of the surface water in high latitudes, could result in a permanent El Niño. The events of three million years seem to be valuable tests for these theories.
Some three million year ago La Niña made an entrance, bringing to an end an era of permanent El Niño conditions. The associated climate changes—our planet become more temperate—favored the rapid rise of our species. Within a remarkably short time we acquired the ability to change the atmospheric composition. How ironic it would be if our activities result in the banishment of La Niña, and thus the termination of the very conditions that are allowing us to flourish.
But this is unlikely, Timmerman (1999) stipulate that this change in the mean state (toward an El Niño-state) will be superimposed by a stronger interannual variability, in other words the year-to-year variations may become more intense, with strong cold events (relative to the El Niño-warmer-mean-state) becoming more frequent under enhanced greenhouse conditions.
It has been suggested that “regional differences in the cloud-albedo feedback will lead to surface warming that is strongest in the equatorial east Pacific” (Meehl & Washington, 1996). The argument is that “the equatorial west Pacific is so warm that even modest additional warming would lead to a cloud shielding effect, with high cirrus clouds, reducing incoming solar radiation at the surface and inhibiting further warming” (Ramanathan & Collins, 1991). This “thermostat” would be less effective in the eastern equatorial Pacific, therefore it would warm more than the western Pacific. By the mean of the Bjerknes feedback it will lead to a slackening of the trade winds and result in overall El Niño-like conditions. Hence, these changes toward a warmer mean state could lead to a reduction of the ENSO variability as the surface zonal asymmetries across the equatorial Pacific would be reduced. Meanwhile, the air-sea interactions may be more energetic in a general warmer climate, increasing the interannual variability. Beside, changes in the vertical density structure of the water column may also alter the intensity of the interannual variability. The thermocline across the Equator becomes stronger in response to greenhouse induced warming (Timmerman, 1999): “temperatures near the surface rise, but those at deeper ocean levels fall. This cooling at subsurface levels can be attributed to a greater inflow of cold waters in response to the intensification of the atmospheric Hadley Circulation, especially in the Southern Hemisphere. If emissions of greenhouse gases continue to increase, the tropical Pacific climate system is thus predicted to undergo strong changes with more frequent El-Niño-like conditions and stronger cold events in the tropical Pacific Ocean”.
Nevertheless these likely predictions should be carried out with great caution. Indeed, recent research also suggests some important links with the so-called “Pacific Decadal Oscillation”(Verdon & Franks, 2006) and “North Atlantic Oscillation” which themselves are predicted to vary under the presumed climate change scenario.
As we saw, constant El Niño could soon become fickle. This is disturbing news because scientists are still having trouble coping with familiar past and current El Niño as this sentence from Joseph & Nigam (2006) underlines: “Predicting regional climate variability/change remains an onerous burden on models”. Will he grow more intense? Will his brief visits become prolonged? Could the sporadic visitor turn into a permanent resident? Answers have been addressed in the lines above but without definite certitude. However, we do know that a change in the properties of El Niño is inevitable should the atmospheric concentration of greenhouse gases continue to rise. We are rapidly changing the composition of the atmosphere, not by design, but as an unfortunate by-product of industrial and agricultural activities that bring enormous benefits—increasing standards of living for the rich and poor alike. For how long will these considerable benefits outweigh the possible adverse consequences of global climates changes that include an altered El Niño? We are gambling that, for the time being, the odds are in our favor. At what stage will the risks become unacceptable high? In a well-know American country song Kenny Rogers reminds us that gambler “got to know when to hold'em, know when to fold'em”. Do we?
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 This citation of Francis Bacon expresses the necessity of taking a long-term view of a complex phenomenon like El Niño to assess its present and future state. Hence, this extract justifies the chronological methodology used in this essay.
 The Intertropical Convergence Zone is the region where the northern and southern trades converge. The consequent upward motion makes this a zone of strong rainfall.