ENSO events What does ENSO mean? Recall that ENSO is an acronym for El Niño-Southern Oscillation. Originally, El Niño referred to the advent of a warm current off Peru, often at Christmas time. More recently, it has come to mean the entire system that led to the observed warm currents the ENSO phenomenon refers to conditions affecting the entire Pacific Ocean, especially in the midlatitudes. It has recently been realized that the El Niños and their opposite, dubbed La Niña, are part of a naturally oscillating weather system. (31) The changing periodicity of ENSO indicates a role for chance causes in the system. The three suggested causes for the ENSO irregularity are deterministic chaos, weather noise, such as Madden-Julian oscillations, and the changing climate system. (32) The Southern Oscillation and the North Atlantic Oscillation/Arctic Oscillation There are several other oscillating long-distance weather patterns that operate over long terms that are connected with and affect the El Niño phenomena the Southern Oscillation, the North Atlantic Oscillation, the Pacific Decadal Oscillation (operating over several decades, as the name suggests), the Madden-Julian Oscillation, and the Quasi-Biennial Oscillation (operating over roughly two years, as the name indicates). The Southern Oscillation and the North Atlantic Oscillation/Arctic Oscillation are examples of correlated long-distance weather patterns. The Southern Oscillation index is a measure of the difference in pressure between Tahiti, French Polynesia and Darwin,
Energy, Ch. 16, extension 1 ENSO events 2 Australia. The strength of the Indian monsoon is connected to the Southern Oscillation, as mentioned in the Chapter. The North Atlantic Oscillation index is a measure of the difference in pressure between Reykjavik, Iceland and the Azores (or Lisbon), (33) originally identified by Wallace and Gutzler. The index is high when there is a low pressure system around Iceland and a lowpressure system around the Azores; winds blow from the west across the Atlantic into Europe bringing thermal energy and increased storm activity (and so increased rainfall). Temperatures are also higher in the eastern United States and lower in Greenland. The index is low when there is a high around Iceland and a low around the Azores; European continental temperatures are lower. (34) A clear picture emerges of decadal periodicity, though this is less regular than that of the Southern Oscillation. (35) A longer history has been identified from Greenland ice cores that extends the index back to 1650. (36) Written historical records mentioning weather can complement the data and give indications of the state of the Oscillation before reliable proxy readings are available. (34) In addition to affecting weather directly, the North Atlantic Oscillation plays a role in pelagic (marine) populations. (34,37) It has recently been recognized that the North Atlantic Oscillation is just a part of the Northern Hemisphere weather story. David Thompson and John Wallace have made a forceful case for the existence of an Arctic Oscillation, of which the North Atlantic Oscillation is a part. (38) They define an Arctic Oscillation index that is based on the pressure difference between the Arctic and the midlatitudes. The Arctic Oscillation accounts for about half the winter warming seen over Eurasia. (There is also an Antarctic Oscillation, but it affects few people and is therefore less important.)
Energy, Ch. 16, extension 1 ENSO events 3 The Arctic Oscillation flip-flops as should be expected, but since about 1970 has been mostly in what is called a positive phase. The connection with the North Atlantic Oscillation is clear. More recently, a connection between the Arctic Oscillation and the stratospheric vortex has been elucidated. Vortex condition changes antedate those of the Arctic Oscillation by several weeks. (39) This predictability in behavior may mean that the Arctic Oscillation is a bit less chaotic, and a bit more deterministic, than had been thought. This determinism may be connected to the ability of water to store a lot of thermal energy. (37) The Quasi-Biennial Oscillation is seen in the behavior of the Equatorial winds at high altitudes. Its cause is not known. a. b. Fig. E16.1.1 Sea surface temperature TOPEX/Poseidon satellite measurements. a. Warm phase (white is warm water). b. Cool phase (blue and purple are cool water). (NASA and CNES [Centre National d'etudes Spatiales, the French space agency]) The Pacific Decadal Oscillation (PDO) is an oscillation that endures 20 to 30 years. The Pacific Decadal Oscillation Index is defined as the leading principal component of
Energy, Ch. 16, extension 1 ENSO events 4 North Pacific monthly sea surface temperature variability (poleward of 20 N for the 1900-93 period). (41) Fig. E16.1.1 shows the satellite measurements of the sea surface temperatures characteristic of warm and cool PDO phases. Earth has just moved from a characteristically warm phase (which lasted from 1977 to 1999) into a characteristically cool phase. The color-coded temperatures in the pictures look almost to be inversions of one another. Previous PDO events have been identified: cool from 1890 to 1924, warm from 1925 to 1946, and cool from 1947 to 1976. (41) In the warm PDO phase, more rainfall is expected in the eastern Pacific (North American west coast). There are many similarities between ENSO events and the PDO, but the PDO lasts considerably longer and affects the Pacific northwest climate the most, while as we detail below, ENSOs affect mostly the tropics. The underlying causes of the PDO are still unknown. The Madden-Julian oscillation involves gusts of wind that travel across the western Pacific Ocean every 30 to 60 days. (42) It is believed that these gusts can strengthen or weaken El Niños formation, (43) and may have been the cause of the early demise of the 1997-98 El Niño. (44) The progress of an El Niño event In a normal year, the Pacific trade winds blow from east to west and push warm surface water across the Pacific Ocean from South America toward Australia. The winds exert a frictional drag on the surface of the ocean, making the sea surface 0.5 m higher off
Energy, Ch. 16, extension 1 ENSO events 5 Indonesia than off Peru. Rainfall is higher in the western Pacific, where the warm water evaporates. The water temperature in the west is about 8 C higher than in the east. Fig. E16.1.1 Normal Pacific conditions. Cold nutrient-rich water is within about 50 m of the surface off the coast of South America. (National Oceanic and Atmospheric Administration, TAO array of moored buoys) The wind circulation cell is driven by the high sea surface temperature in the western Pacific: (24) The winds pick up water vapor as they flow westward; they drop it in the western Pacific as rain, and the air rises, completing the circulation. When warm water builds up in the western Pacific Ocean, it pushes the cold layer downward there, which pushes the cold water upward at the other end of the swing (Fig. E16.4.1). Cold water full of nutrients rises to very near the surface just off the west coast of South America and the fish population increases.
Energy, Ch. 16, extension 1 ENSO events 6 Fig. E16.4.2 Condition of the Pacific during El Niño. (National Oceanic and Atmospheric Administration, TAO array of moored buoys) For the more common variety of El Niño, the Southern Oscillation index begins to fall late in the year before El Niño as the south Pacific high in Tahiti (measured of the Southern Oscillation) weakens. This causes the trade winds to weaken. (25) The sea surface temperature gets higher than normal off Peru, and the anomaly spreads to the west (Fig. E16.1.2). The winds that raise sea level along the South American coast send off a wave called a Rossby wave, which lowers sea level as it moves to the west. Moving about 25 to 85 km/d, it takes months for the wave to get to the west in the Indian Ocean. (45) As a result of the high sea surface temperature peaking in late spring off South America, about 2.5 C above normal, the cold water is pushed downward and fewer nutrients are transported to the surface, and so productivity may decline to 20% of what it was along longitude 95 W and to 5% along longitude 92 W. As the anomalies drift westward, the
Energy, Ch. 16, extension 1 ENSO events 7 easterlies die at midyear, and the ENSO decays over a period of months. (25) The Rossby waves that had gone from the South American coast to the Indian Ocean are reflected toward the equator there and then head back east at 250 km/d. The waves return forces the sea level to drop off South America, which can turn off the El Niño. Fig. E16.1.3 Sea surface temperature profile for the mid-latitude Pacific Ocean. a. December, 1998, La Niña. b. December, 1993, normal. c. December, 1997, El Niño. (National Oceanic and Atmospheric Administration) The sea surface is shown for three conditions in Fig. E16.1.3: for El Niño conditions (c), for normal conditions (b), and for La Niña conditions (a). As can be seen, red corresponds to high water temperatures, and blue to low water temperatures. The mean Equatorial water temperature is about 25 C. The La Niña is a condition roughly opposite to that of El Niño. Temperatures along the South American coast are lower than would be expected under normal conditions. The low temperatures stretch out far from the coast into the Pacific. Two types of ENSO The ENSO of 1982-1983 was of the rarer type (the last previous one was in 1940-1941). It began in the Pacific and spread east to the coast; it started in May 1982 (fall in the
Energy, Ch. 16, extension 1 ENSO events 8 Southern Hemisphere), not in the late spring around Christmas. In the western Pacific, the Indonesian low had moved north to the equator to meet the intertropical convergence zone as it moved south. (29) The intertropical convergence zone (ITCZ) is the boundary between the northeast and southwest trade winds (see Figure 14.2). As the convergence zone moved across the warm water, the low and ITCZ intensified each other. Warm wet air rose, supplying latent heat to the air, causing winds that brought in warm water from far away. (25,29) The wind change allowed the warm water that had been piled up in the western Pacific to head back along the equator toward Peru, where it covered the cool layer and pushed it down, causing the rise in sea surface temperature. (25) In the 1982-1983 episode, the sea surface temperature was 7 C higher along the coast than normal, and in the central Pacific it was 5 C higher than normal. (30) El Niño consequences Many fish perish or fail to reproduce as primary production typically falls (sometimes to as low as 5% of pre-enso values). (46,47) Peru fishermen notice that the anchovy catch crashes in an El Niño, and sometimes, as in 1982-1983 ENSO, fur seal pups die and the Christmas Island seabird reproduction fails. (29) In this episode, there was a drought in Indonesia, eastern Australia, and Melanesia; rainfall in Peru was the heaviest in 450 years, and the southwestern United States had one of the wettest winters on record. (25,46) Such results are typical of El Niños, but they are not usually so severe. Rainfall increases in southern Brazil, Uruguay, California (Fig. E16.1.4), the American southeast, and northern Kenya and Somalia. (48) The drought in the western Pacific sometimes leads to out-of-control brush fires in Australia and Indonesia. Droughts are also experienced in
Energy, Ch. 16, extension 1 ENSO events 9 the Brazilian northeast and in the African region around Tanzania and Mozambique and in Madagascar. It is unusually warm in the American and Canadian northeast, the Pacific Northwest, southern Alaska, and southeast China. (48) Fig. E16.1.4 Flooded area in Lakeport, California as a result of the 1998 El Niño event. (Federal Emergency Management Agency) Floods in the U.S. Southwest have clustered into times characterized by cool moist climate and frequent El Niños over the last 5,000 years. (49) According to FEMA, the 1982-1983 El Niño cost over $8 billion (over $2.2 billion of the total cost was in paid by American insurance companies and government aid, and about 160 Americans died in floods). (48) Crop losses in La Niñas are estimated at $10 billion. (48) In the 1997-1998 El Niño, 700 billion kilograms of carbon that would have been lost to the air were retained in the ocean. After the end of the El Niño, carbon dioxide emission jumped again and there was a huge bloom in the mid-pacific. The total difference
Energy, Ch. 16, extension 1 ENSO events 10 between high and low productivity regimes was 6 trillion kilograms of carbon (the difference between 117 trillion kilograms and 111 trillion kilograms). (50) A study found evidence that cholera prevalence in Bangladesh is correlated with ENSO events during the stronger phases of the El Niño record, as has been observed since 1976. (51) Over 70% in the variance of the prevalence of cholera is explained by the ENSO. This study raises the important question of whether other diseases are so correlated. Predicting El Niños Weather modelers are beginning to have some success in predicting the beginnings of ENSOs, but receive low marks for accuracy. (52,53) Ironically, Peruvian farmers might have found a better nontechnical method, which has to do with the apparent brightness of stars in the Pleiades constellation. (54) Given the costs of both El Niños and La Niñas, the prediction of the ENSO events could save huge amounts of money. The Quasi-Biennial Oscillation and Madden-Julian oscillation appears to be connected to the ENSO events (except after a particularly strong ENSO). (55) There are two sorts of El Niño events that occur with no obvious periodicity, varying between 4 to 7 years. (30,31) In both sorts of ENSO events, warm water is brought to the west coast of South America and can cover the cool, fertile water for extended periods (6 to 18 months). El Niño might possibly be a superposition of two frequencies, one corresponding to 2 years and one to 4-5 years. (28)
Energy, Ch. 16, extension 1 ENSO events 11 History of El Niño Obviously, there were no observers in the distant past who gauged the pressures determining the Southern Oscillation (see Chapter and above) and measured sea surface temperatures. In fact, we could not have learned all this about the ocean system in the pre-space age, so the distant past means past about 35 years ago!. However, there are alternative ways to find out about things that occurred in the past and measure conditions. These are called proxy measurements, because while they are not actual measurements, the desired measurements may be inferred from the proxies. Proxies that have been used for dating in order to learn about ENSOs of the past include tree rings, ice cores, beach ridges, ancient trash pits, and soils. (56) In addition, pollen content in Australian lake sediments, debris flows in an Ecuadorian lake, presence and absence of fauna that inhabit specific thermal layers off Peru s coast, and oxygen-18 anomalies in corals have also served to indicate what happened. (57,58,59,60) The pollen show that drought-adapted plants were missing before about 5,000 years ago, indicating a weak ENSO. ENSOs bring storms, which cause floods in Laguna Pallcacocha that leave inorganic layers; the layers were dated using accelerator mass spectrometry. (58) Measurements were also made with δ 18 O, the difference in proportions of two oxygen isotopes (see Extension 16.3, Proxy measurements). There was apparently a shift that took place between the pre-1976 and post-1976 El Niños. The change affected the thermal structure of the eastern Pacific. (61) Guilderson and Schrag suggest that this change is associated with the differences in frequency and intensity of El Niños since then. (61)
Energy, Ch. 16, extension 1 ENSO events 12 The result of all these records is agreement that the El Niño was less prominent and less variable before the current era, and that the El Niños of the last few centuries were much more varied. (62) The El Niño is also linked (through the Southern Oscillation) to the Asian monsoon. There are indications that the monsoon is changing as well. (31,62,63) Where there was once a connection between weak summer monsoons and warm water over the Pacific (El Niño), this has recently changed, possibly to underlying climate change (see Ch. 15). (63) The changes in ENSO events since the 1970s apparently are occurring because of the slowing of transfers of warm tropical surface waters from the equator into the northern Pacific Ocean, and the return flow of cooler water back toward the equator. (64) The average temperature of the sea surface increased by about 0.8 C during the same period. This changes the net primary productivity of the region. (65) As a result of the change in temperature of the sea surface, outgassing of CO 2 from the Pacific has decreased since around 1975. (64) This change in water transport is concurrent with unprecedented warming, and might well be correlated. Cobb et al. examine 1,100 years of fossil-coral oxygen isotope records from Palmyra Island in the Pacific. Warmer rainy conditions apply at Palmyra during an El Niño, and this decreases the relative abundance of 18 O in the corals. During La Niñas, the abundance of 18 O increases. The skeletons of the coral sit layer upon layer, and so the traces of abundance of 18 O form a record over the period the extrusion of coral grows. Periodically, pieces of the reef are washed away, and Cobb et al. gathered older pieces from the top of the living coral reef and analyzed those. Individual windows of coral data are spliced together to produce the majority of the entire 1,100 year record. Some gaps do remain in the record. (66)
Energy, Ch. 16, extension 1 ENSO events 13 Cobb et al. first assure themselves that twentieth-century corals correlate with records of known ENSO events. They can see evidence of the mean climate over the 1,100 year period and its variation on the scale of decades and centuries. They also can detect ENSO events and find these to be poorly correlated with the global mean climate record from the northern hemisphere (they do not see evidence of the Little Ice Age or the Medieval Warm Period). (66) (There is other evidence from the Pacific indicative of the Little Ice Age. Sea surface salinity decreased in step with the end of that period along the Great Barrier Reef. (67) ) Cobb et al. find that variations in the historical record are greater than those exhibited by the corals of the twentieth century, implying stronger ENSO events in the past. There is no apparent driving force for these large fluctuations, so they assume they arise from internal dynamics and point out that similarly large, abrupt changes in the character of tropical Pacific climate variability may occur in the future. (66) The twelve year cycling observed by Cobb et al. is intriguingly close to the 11 year solar cycle. Indeed, as has been noted in the literature, some have argued that solar activity drives ENSOs. (64) At present, this connection remains highly speculative. Warming in the late twentieth century stands out as different from the temperature fluctuations of earlier times. (66) The perspective of an 1,100 year time record allows this judgment to be made. Study of ENSOs over longer periods farther back in time continues. Obviously the uncertainties multiply as the time horizon gets pushed back. Lake sediments from lakes close to the ocean can detect changes. This approach led to a 12,000 year record that
Energy, Ch. 16, extension 1 ENSO events 14 suggests that ENSO events decreased in intensity and frequency in modern times compared to the more distant past. (68) Evidence of this past in the ocean is in the form of reconstructions of sea surface temperature and salinity from measurements of the magnesium-calcium ratio in seawater, which is reflected in the elemental composition of foraminifera and from δ 18 O as found in the shells of foraminifera. (69,70,71,72) In one investigation, it is observed that during the last glacial maximum, sea surface temperature differences decreased; it is speculated that this is due to a weakening of trade winds such as is observed in modern ENSO events. (71) In another, the existence of super ENSOs is suggested, seen in the Pacific salinity record that is correlated with Dansgaard-Oeschger events (see Ch. 17). (72) Evidence from Australian grassland peat over 45 kyr find similar correlations of dry periods with Dansgaard-Oeschger events. (73) Processea involved in El Niños Rosenfeld and Broccoli suggest that there is a need for new proxies because the interrelations characteristic of modern times might not have held in the distant past. They recommend that paleoclimate experts cooperate with those having expertise in climate dynamics to develop synergies. The richness of the climate models could help paleoclimate in gathering their data. Conversely, experts in climate dynamics, including modelers, can sharpen the questions they investigate through a better awareness of the paleoclimate record. (70) This could lead to a sharper focus on the salient questions about changes in ENSO variability and mean climate state in the field. (70) Research has suggested that El Niños are correlated with volcanic eruptions as well as the see-sawing of the ocean and atmosphere. Adams et al. claim that volcanic forcing
Energy, Ch. 16, extension 1 ENSO events 15 roughly doubles the probability that an El Niño event will occur. (74) Since we saw above that El Niños come roughly every 5 years, this would mean a normal probability of 20%, so after an eruption the probability of an El Niño event is approximately 40%. Adams et al. imply that increased fifteenth century volcanism partially compensated for the contemporaneous reduction in the solar constant (see the chapter for a discussion of the Maunder Minimum). It has been possible to use relatively current El Niños to train a model, which, when applied to the historical record, postdicted actual El Niños very well. (75) The researchers claim that they are able to predict El Niños at long lead times. (75) They claim that over the last 148 years their model produces few false positives. The validity of their model calculations rests on precise past sea surface temperature data that was generated from proxy measurements (see Extension 16.3, Proxy measurements) during the past decade. (75) The El Niño record generated by the consonance of the proxy records goes back considerably father than 150 years, albeit with more uncertainty as we go back farther in time, as we have seen above.