ENSO variability and the eastern tropical Pacific: A review
Introduction
Our first knowledge of El Niño came from Peruvian geographers, who at the end of the 19th century were interested in the unusual climate aberrations that occurred along the Peru coast in the odd year (Eguiguren, 1894). They took note of what a knowledgeable ship captain said about the fishermen in northern Peru, who typically saw a switch from cold to tropical ocean conditions around Christmas of every year and attributed this to a southward warm “El Niño current”. This name was a reference to the annual celebration of the birth of the Christ child, who is much more prominent than Santa Claus/Saint Nicholas in Latin American traditions of the Christmas season. The geographers noted that in some years the onset of warm conditions was stronger than usual and was accompanied by unusual oceanic and climatic phenomena. Starting with the arrival of foreign-based scientific expeditions off Peru in the early 20th century, the concept gradually spread through the world’s scientific community that El Niño referred to the unusual events (Murphy, 1926, Lobell, 1942). The annual occurrence was forgotten.
It was separately noted by Sir Gilbert Walker in the 1920s and 1930s that notable climate anomalies occur around the world every few years, associated with what he called the Southern Oscillation (SO) (Walker, 1923, Walker, 1924, Walker, 1928, Walker and Bliss, 1932). The SO is a large interannual fluctuation in tropical sea level pressure (SLP) between the Western and Eastern Hemispheres (SO index is defined as SLP anomaly difference between Tahiti and Darwin). It was not until the 1960s that scientists came to realize that the warming off Peru is only part of an ocean-wide perturbation that extends westward along the equator out to the date line. Berlage, 1957, Berlage, 1966 recognized the linkage between the SO and episodic warmings of sea surface temperature (SST) along the coast of Southern Ecuador and Northern Peru, known locally as El Niño. El Niño became associated with unusually strong warmings that occur every two to seven years in concert with basin-scale tropical Pacific Ocean anomalies.
About the same time, the noted meteorologist Jacob Bjerknes proposed that El Niño was just the oceanic expression of a large-scale interaction between the ocean and the atmosphere. Using observed data in the context of earlier studies dating back to those of Walker, 1924, Bjerknes, 1966, Bjerknes, 1969 provided evidence that the long-term persistence of climate anomalies associated with the Walker’s SO (Walker and Bliss, 1932) is closely associated with slowly evolving SST anomalies in the equatorial eastern and central Pacific. Bjerknes recognized the importance of ocean–atmosphere interaction over the eastern tropical Pacific. He hypothesized that a positive ocean–atmosphere feedback involving the Walker circulation is responsible for the SST warming observed in the equatorial eastern and central Pacific. In his seminal paper, he stated (Bjerknes, 1969, p. 170):
This positive ocean–atmosphere feedback or coupled ocean–atmosphere instability leads the equatorial Pacific to a never-ending warm state. During that time, Bjerknes did not know what causes a turnabout from a warm phase to a cold phase (Bjerknes, 1969, p. 170):“A decrease of the equatorial easterlies weakens the equatorial upwelling, thereby the eastern equatorial Pacific becomes warmer and supplies heat also to the atmosphere above it. This lessens the east–west temperature contrast within the Walker Circulation and makes that circulation slow down.”
The positive ocean–atmosphere feedback of Bjerknes (1969) has influenced later studies. The essence of Bjerknes’ hypothesis still stands as the basis of present day work. Oceanographers and meteorologists began to combine their efforts to expand and refine the Bjerknes’ hypothesis by systematically studying the El Niño and the Southern Oscillation together in what we now call “El Niño-Southern Oscillation”, or ENSO.“There is thus ample reason for a never-ending succession of alternating trends by air–sea interaction in the equatorial belt, but just how the turnabout between trends takes place is not quite clear.”
After Bjerknes published his hypothesis, ENSO was not intensively studied until the 1980s. The intense warm episode of the 1982–1983 El Niño, which was not recognized until it was well developed, galvanized the tropical climate research community to understand ENSO and ultimately predict ENSO. The 1982–1983 El Niño was not consistent with the “buildup” of sea level in the western Pacific by stronger than normal trade winds prior to 1982, presumed to be a necessary precursor of El Niño (Wyrtki, 1975). Also, there was no warming off the west coast of South America in early 1982, considered to be part of the normal sequence of events characterized the evolution of El Niño (Rasmusson and Carpenter, 1982). This motivated the 10-year international TOGA (Tropical Ocean-Global Atmosphere) program (1985–1994) to study and predict ENSO. One outcome was to build the ENSO observing system that includes the TAO/TRITON array of moored buoys (Hayes et al., 1991, McPhaden et al., 1998), an island tide-gauge network, surface drifters, the volunteer ship program, and various satellite observations. TOGA also supported analytical and diagnostic studies of the ENSO phenomenon (Wallace et al., 1998), and the development of a sequence of coupled ocean–atmosphere models to study and predict ENSO (Philander, 1990, Neelin et al., 1998, Wang and Picaut, 2004). Since TOGA, our understanding of ENSO has been greatly advanced by focusing on interaction between the tropical Pacific Ocean and atmosphere. This paper will provide a brief review of ENSO observations and of our present understanding of ENSO, with a focus on patterns and processes in the eastern tropical Pacific Ocean.
ENSO variability in the eastern tropical Pacific is centered along the equator, but is closely related to variability of the tropical Western Hemisphere warm pool (WHWP), which has been defined as the region covered by water warmer than 28.5 °C (Wang and Enfield, 2001, Wang and Enfield, 2003). The WHWP is comprised of the eastern north Pacific west of Central America; the Intra-Americas Sea (IAS), i.e., the Gulf of Mexico and the Caribbean; and the western tropical North Atlantic. The WHWP is the second-largest tropical warm pool on Earth. Unlike the western Pacific warm pool in the Eastern Hemisphere, which straddles the equator, the WHWP is entirely north of the equator. The WHWP has a large seasonal cycle and the interannual fluctuations of its area are comparable to the annual variation, although it does not undergo large anomalous zonal excursions such as occur in the western Pacific. The WHWP is a critical component of the boreal summer climate of the Caribbean and surrounding land areas. From an oceanographic point of view, the WHWP can be separated into two parts by the Central American landmass: the eastern north Pacific warm pool and the Atlantic warm pool. To the atmosphere, the WHWP is a monolithic heat source that annually migrates and changes in size (Wang, 2002b), with little regard for the narrow landmass of Central America. We nevertheless recognize that WHWP development may involve oceanographic processes that are fundamentally different between the two oceans (Wang and Enfield, 2003). This paper will focus on review of seasonal and interannual variations of the eastern Pacific component of the WHWP, because of the eastern Pacific warm pool being part of the eastern tropical Pacific that is the subject of this review volume.
ENSO variability and the eastern Pacific warm pool are related to eastern tropical Pacific interdecadal variability reviewed by Mestas-Nuñez and Miller (2006), the ocean circulation of the eastern tropical Pacific by Kessler (2006), atmospheric forcing of the eastern tropical Pacific by Amador et al. (2006), and hydrography of the eastern tropical Pacific by Fiedler and Talley (2006). ENSO variability is associated with biological and ecological variability in the eastern tropical Pacific. We herein also briefly review biological and ecological effects of ENSO. The paper is organized as follows. Section 2 briefly describes major observed features of ENSO. Section 3 reviews our present understanding of ENSO. Section 4 presents seasonal and interannual variations of the eastern Pacific warm pool. Section 5 briefly reviews ENSO biological and ecological variability. Section 6 discusses changes in ENSO variability. Finally, Section 7 provides a summary.
Section snippets
Observations of ENSO
ENSO variability has been documented in the written record over hundreds of years (e.g., Quinn et al., 1987, Enfield, 1989). It is evident in paleoclimatic records, with slight changes in amplitude or frequency, over thousands of years (Diaz and Markgraf, 1992, Diaz and Markgraf, 2000). For example, Rodbell et al. (1999) showed that the frequency of ENSO variability increased progressively over the period from about 7000–5000 years ago, and archaeological evidence suggests that El Niño events
Mechanisms of ENSO
The eastern tropical Pacific is a region that can both involve local ocean–atmosphere interaction and be remotely affected by processes in the western Pacific, because the absence of a Coriolis effect causes the equatorial ocean to act as a waveguide (Gill, 1982). Bjerknes (1969) first hypothesized that interaction between the atmosphere and the equatorial eastern Pacific Ocean causes El Niño. In Bjerknes’ view an initial positive SST anomaly in the equatorial eastern Pacific reduces the
The eastern Pacific warm pool
As stated in Section 1, the Western Hemisphere warm pool (WHWP) is defined as the region covered by water warmer than 28.5 °C on both the Pacific and Atlantic sides of Central America (Wang and Enfield, 2001, Wang and Enfield, 2003). These are temperatures that have a significant impact on organized tropical convection (e.g., Graham and Barnett, 1987). The choice of 28.5 °C is based not only on limiting the WHWP to a closed region, but also on the fact that the depth of the 28.5 °C isotherm is
ENSO biological and ecological variability
ENSO-related changes in winds, insolation, hydrography and circulation in the eastern tropical Pacific are of sufficient magnitude and duration to affect organisms, populations and ecosystems. The species and communities of the region have evolved to persist through the quasi-regular disturbances imposed by ENSO events. Thus, typical or even exceptional events should not result in long-term, fundamental changes (Paine et al., 1998). Biological effects of recent El Niño events in the region have
Changes in ENSO variability
Changes in the characteristics, or modulation, of ENSO variability over the past one to two centuries have been described in instrumental and proxy records from the tropical Pacific. Mestas-Nuñez and Enfield (2001) found that the late 1970s climate shift that warmed the eastern equatorial Pacific (Niño3 region) by about 0.5 °C was also characterized by increased interannual variance through the 1980s and 1990s. An 1893–1994 coral record from Clipperton Atoll (within the eastern Pacific warm pool
Summary
ENSO shows an interannual variability in both the eastern and western tropical Pacific. During the warm phase of ENSO, warm SST and low SLP anomalies in the equatorial eastern Pacific and low OLR anomalies in the equatorial central Pacific are accompanied by cold SST and high SLP anomalies in the off-equatorial western Pacific and high OLR anomalies in the off-equatorial western Pacific. The off-equatorial anomalous anticyclones in the western Pacific initiate and produce equatorial easterly
Acknowledgements
CW thanks David Enfield for discussions of historical background of El Niño. We thank anonymous reviewers’ comments and Charles Miller’s editorial comments. Fig. 4 was provided by TAO project office. This work was supported by a grant from National Oceanic and Atmospheric Administration (NOAA) Office of Global Programs and by the base funding of NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML). The findings and conclusions in this report are those of the author(s) and do not
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