El Niño and Sea Level Anomalies: A Global Perspective - Prof. Aulinas, Ejercicios de Geología

¡Descarga El Niño and Sea Level Anomalies: A Global Perspective - Prof. Aulinas y más Ejercicios en PDF de Geología solo en Docsity! 215© Blackwell Publishing Ltd, Geology Today, Vol. 23, No. 6, November–December 2007 FEATURE The anomalous weather patterns observed world-wide associated with El Niño are often accompanied by changes in local sea levels. Sea level anomalies are transmitted through various oceanic and atmospheric pathways and data suggest that this effect is currently on the order of tens of centimetres in some areas both within and outside of the equatorial Pacific. Positive sea level anomalies either generated or enhanced by El Niño have been linked to powerful storm waves and coastal flooding. famous for his arguments in favour of uniformitarianism, believed that the British landscape had been carved by seawater during a major marine transgression and that the action of rivers and glaciers had been minimal by comparison. Although Earth scientists now emphasize fluvial, and in some landscapes, glacial processes, the great impact of dramatic, short-term rises in local sea level, known as ‘extreme events’, cannot be denied. The earthquake that occurred off the west coast of Sumatra on 26 December 2004 produced a series of tsunami with wave heights that reached up to 30 metres, severely altering coastal landscapes in the area and, unfortunately, claiming a reported total of 275 000 human lives in the process. While the extreme weather events associated with El Niño are widely recognized, less well-known and studied are the sometimes dramatic local sea level anomalies that are generated not only in the equatorial Pacific, but also throughout the Pacific basin, in the Indian Ocean, and possibly even in the Atlantic Ocean. Whereas predictions for rates of eustatic (global) sea level rise range from less than a millimetre to a few millimetres per year, local changes in sea level that arise during El Niño can be tens of centimetres and can fluctuate over time periods of days to months. In order to understand this phenomenon, we first consider what constitutes an El Niño event and examine more closely both ‘typical’ and ‘El Niño’ conditions in the equatorial Pacific. El Niño and the Southern Oscillation The term El Niño was originally used by fishermen in Peru and Ecuador to refer to the unusually warm Abbie H. Tingstad1 & David E. Smith2 1Department of Geography, University of California, Los Angeles, 1255 Bunche Hall Box 951524, Los Angeles, California, 90095, USA. abbie1@ucla.edu; 2School of Geography, Oxford University Centre for the Environment, Oxford, UK Feature El Niño and sea level anomalies: a global perspective The first indication that something out of the ordinary was afoot came at the beginning of July 2006: sea surface temperatures (SSTs) in the eastern equatorial Pacific had begun to rise, and as they hung at a few tenths of a degree (Celsius) above mean values, scientists world-wide waited to see if SSTs would return to normal as the summer progressed. They didn’t. Through late summer and autumn, SSTs in the eastern equatorial Pacific continued to deviate at increasing rates from seasonal norms, and by December and January, surface wind patterns were changing, the thermocline (boundary gradient between warm surface waters and cool water below) was getting deeper in the east, sea level had begun to rise in the east and fall in the west, and SSTs continued to rise sharply in the east, now three degrees Celsius above the seasonal mean in some places. A dramatic event was once again being played out in the theatre of the equatorial Pacific: El Niño, ‘the Child’, was here again. Sea level rise and ‘extreme events’ Beginning roughly around the Renaissance period, Western scientists started to take an interest in the mechanical power of the sea (particularly when it rose dramatically) and incorporated this into a number of theories regarding the formation and subsequent dissection of the Earth’s surface. In Italy, Nicolaus Steno (1638–86) published on the geological history of the Tuscan landscape, suggesting that the present topography he observed was formed, in part, through continental collapse and subsequent submersion of valleys by seawater during The (Biblical) Flood. Charles Lyell (1797–1875), most © Blackwell Publishing Ltd, Geology Today, Vol. 23, No. 6, November–December 2007216 FEATURE ocean water that occasionally appeared around Christmas-time (hence the reference to ‘the [Christ] child’). During the early- and mid-twentieth century, scientific interest in the origin of this warm coastal water grew, and research began to confirm the presence of this phenomenon. Particularly influential was the work of Klaus Wyrtki, professor at the University of Hawaii, who used tide-gauge data to demonstrate that El Niño was related to changes in basin-wide circulation. On the other end of the Pacific, the catastrophic failures of the Southeast Asian monsoon in 1877 and 1899 had contributed to widespread famine in India on both occasions. Following this, the problem of predicting these monsoon failures was actively pursued and in 1904, Sir Gilbert Walker became the Director-General of Observatories in India and took over this task. Although he did not completely solve the monsoon puzzle, he did notice that pressure over the equatorial Pacific and Indian Oceans fluctuated on an interannual timescale, and that the changes in pressure over the eastern Pacific and the Southeast Asian region were opposite in sign. Walker called this pressure fluctuation the Southern Oscillation and he published a series of important papers on it between 1923 and 1937. Others followed suit but, like Walker, did not fully appreciate that a connection existed between changes in atmospheric pressure and the appearance of warm surface water in the eastern equatorial Pacific. Research on El Niño and the Southern Oscillation continued independently of each other until the 1960s, when oceanographers began to connect the two phenomena. Jacob Bjerknes, professor at the University of California, Los Angeles, wrote the first papers fully linking the two, and proposed that the primary coupling mechanism involved thermally- driven, closed-loop circulation over the equatorial Pacific which he termed ‘Walker Circulation’, after the man who first described the Southern Oscillation. As a result of Bjerknes’ insight, we now refer to the quasi-periodic coupled ocean-atmosphere interaction in the equatorial Pacific as the El Niño– Southern Oscillation, or simply ENSO. The ENSO phenomenon has two phases, El Niño and La Niña. Whereas SSTs (sea surface temperatures) rise in the eastern equatorial Pacific, pressure gradients relax, trade winds weaken or fail, and the sea level slope across the Pacific slackens during El Niño, these conditions are reversed during La Niña: differences in SSTs, pressure, winds, and sea level across the Pacific are reinforced. There tend to be two SST and sea level anomaly peaks in the eastern equatorial Pacific during an El Niño; the first occurs around December and January and is thought to be related to a period of strong westerly surface wind anomalies, and the second occurs around April or May and is connected to the final stages of trade wind collapse. ENSO events last between 18 and 24 months and tend to fall within a 2–8 year recurrence interval, but are highly variable in terms of periodicity, duration, magnitude and spatial extent. Two major instrumental indices are currently used to document ENSO events and variability, trends for which can be extended back around 150 years. The Southern Oscillation Index (SOI) is calculated as the anomaly in the standardized sea level pressure difference between Papeete, Tahiti and Darwin, Australia. This index represents the degree to which the pressure ‘see-saw’ across the Pacific has departed from the mean state. When the SOI is very negative, the pressure difference is very low and the El Niño phase is taking place. When the SOI is very positive, the pressure difference is larger than the mean state and the La Niña phase is taking place. The other primary index is NINO3, which represents the SST anomaly in the eastern equatorial Pacific from longitudes 90ºW to 150ºW and latitudes 5ºS to 5ºN (larger or smaller regions are sometimes used and these are also called ‘NINO’ indices). When the NINO3 index is either strongly positive or negative, SSTs have departed significantly from the mean state; in the former case, an El Niño is taking place and in the latter case, a La Niña is taking place. Figure 1 shows annual SOI and NINO3 indices for 1900 through 2005 with arrows indicating the recent strong El Niño events of 1982–83 and 1997– 98. ‘Typical’ conditions in the equatorial Pacific Although it can be argued that no ‘typical’ conditions exist in the equatorial Pacific, due to the short recurrence interval of ENSO events, we attempt here to broadly describe the ocean-atmosphere interactions in the equatorial Pacific as they are when the system is not in an ENSO phase. The Humboldt Current delivers cold Antarctic and subtropical waters up the coast of South America. Upwelling of cold water along the South American coast and in the eastern equatorial Pacific reinforces cool temperatures. This water is then transported westward across the Pacific in the South Equatorial Current that is driven by southeasterly trade winds. The ocean circulation loop closes with water transport south along the eastern coast of New Guinea and then in an easterly direction, eventually Fig. 1. NINO3 and SOI indices for 1900–2005. Data for the NINO3 index was obtained from the IRI/LDEO Climate Data Library (associated with Columbia University, New York, USA). Data for the SOI index was obtained from the National Center for Atmospheric Research (Colorado, USA).