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Natural Climate Variability on Decade-to-Century Time Scales
Griffin, 1992), Bermuda (Pätzold and Wefer, 1992), and Puerto Rico (A. Winter, pers. commun., 1992). Records from sites spanning the tropics are in progress at many institutions (Figure 1; see also Dunbar and Cole, 1993).
CORAL MONITORS OF ENSO VARIABILITY
The ENSO phenomenon consists of the large-scale oscillation of the tropical Pacific ocean-atmosphere system between two extreme states, characterized by alternately warm and cool SST anomalies in the eastern Pacific (Philander, 1990; Deser and Wallace, 1990). Both warm- and cool-phase ENSO conditions exhibit coherent patterns of coupled oceanic and atmospheric anomalies that leave distinct signatures in the isotopic and trace-metal content of coral skeletons (Figure 2). The influence of ENSO dominates interannual climate variability throughout the equatorial Pacific and the global tropics, and ENSO-related climate anomalies propagate to higher latitudes via mechanisms that include the displacement of upper atmospheric pressure patterns and the generation of troughs that penetrate into southwestern North America (van Loon and Rogers, 1981; Rasmusson and Wallace, 1983; Horel and Wallace, 1981).
The cool phase of ENSO is characterized by strong easterly trade winds at the surface, a convective maximum (the Indonesian Low) over Indonesia and northern Australia, and return flow aloft that brings dry subsiding air over most of the eastern and central Pacific. This zonal atmospheric pattern that spans the tropical Pacific is known as the Walker circulation. The strong trade winds during this phase of
Schematic cross-section of equatorial Pacific showing major features of the warm and cool phases of ENSO in relation to selected sites where coral paleoclimatic reconstructions of ENSO are under way: Bali, Sulawesi, Tarawa, Kanton, and the Galapagos (after Cole, 1992).
ENSO intensify the upwelling of cool, nutrient-rich waters in the eastern Pacific and transport surface waters westward, generating a strong zonal SST gradient and an eastward slope in sea level.
Transition to the warm phase of ENSO may occur when the trade winds relax or reverse west of the date line, depending on surface ocean conditions. As the warm phase of ENSO begins, the western Pacific warm pool spreads eastward (Lukas et al., 1984; McPhaden and Picaut, 1990), and the Indonesian Low convective system migrates to the region near the date line and the equator. In the eastern Pacific, surface waters become warm and oligotrophic, as the thermocline deepens and warm waters move in from the west. Trade winds remain weak and variable as a result of the diminished zonal SST gradient. Dramatic shifts in precipitation patterns occur across the entire tropical Pacific, from South America to southeast Asia, in response to Indonesian Low migration and the development of convection over newly warmed ocean regions.
Frequency-domain analyses of climate data that span the past four decades indicate that ENSO operates on three fundamental time scales. The annual cycle sets the phasing of ENSO anomalies; the evolution of ENSO extremes appears phase-locked to the annual cycle (Rasmusson and Carpenter, 1982). Several aspects of ENSO variability, including SST, winds, rainfall, and sea level pressure (SLP), also possess a quasi-biennial pulse that varies in intensity throughout the instrumental record (Trenberth, 1980; Rasmusson et al., 1990; Barnett, 1991; Ropelewski et al., 1992). Finally, ENSO warm extremes recur approximately every 3-7 years, lending a ''low-frequency" beat to the spectrum of ENSO variability (Rasmusson and Carpenter, 1982; Rasmusson et al., 1990; Barnett, 1991; Ropelewski et al., 1992). Recent analyses of multi-decadal records of Pacific climate suggest that ENSO variability may also experience significant decadal-scale shifts (Elliott and Angell, 1988; Cooper et al., 1989; Trenberth, 1990). Such a shift is evident over the interval between 1977 and 1988, which exhibited no strong cool anomalies (Trenberth, 1990; Kerr, 1992).
The impacts of the Southern Oscillation both within and beyond the tropical Pacific fluctuate on a multi-decadal time scale (Trenberth and Shea, 1987; Elliott and Angell, 1988; Cole et al., 1993; Michaelsen and Thompson, 1992; Rasmusson et al., 1995, in this volume). During the first 20 years of this century, for example, the interannual pulse of the Southern Oscillation was strong and highly correlated with climatic records both within the tropical Pacific and in sensitive teleconnected sites. The subsequent three decades, however, witnessed a general decline in the correlations among Pacific climate variables and between Pacific climate and teleconnected sites. Elliott and Angell (1988) propose that these fluctuations may result from the migration of the centers of strongest coherent variability in the Southern Oscillation over the course of the present century.