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Historical Background |
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The effects of the ENSO mechanism on agriculture played an important role in motivating early research. The critical dependence of Indian food production on monsoonal rainfall and the failure of the Indian monsoon in 1877 inspired early forecasting attempts by the India Meteorological Department (Allan et al., 1996). Related research involved efforts to summarize large scale atmospheric conditions associated with the Indian monsoon, which lead to the identification of a near-global scale oscillation in surface pressure over the Indian and Pacific Oceans that Sir Gilbert Walker referred to as the Southern Oscillation. The term El Niño has been in use along the Peruvian coast since at least 1895 (Philander, 1990) and refers to a southward moving counter-current of warm water appearing around Christmas of each year which fishermen dubbed El Niño ("the boy child") in honor of Christ. Year-to-year variability in the state of the Southern Oscillation and in the strength and persistence of the El Niño current were considered as occurring independently until the 1960's, when Bjerknes (1969) proposed a dynamic synthesis linking the large scale variation in atmospheric surface pressure and eastern Pacific sea-surface temperature. The current conception of ENSO (Philander, 1990; Trenberth, 1991; Allan et al., 1996) is that of a coupled ocean-atmosphere phenomenon primarily active over the tropics and subtropics of the Pacific and Indian Oceans. The connection between atmospheric and oceanic components are expressed in a mutual interplay between the organization of large scale vertical circulations in the atmosphere and the warm and cold sea-surface temperature anomalies (SSTA) that both drive and are driven by these circulations. The state of ENSO is measurable through anomalies in surface pressure and sea-surface temperature which vary in amplitude and sign over multiple-year time scales. The related oscillatory nature of the underlying mechanism has led to its description in terms of two distinct phases: a La Niña or cold phase state, and an El Niño or warm phase state. |
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La Niña Conditions |
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La Niña ("the girl child") conditions are marked by strong large scale atmospheric rising motion over the warm sea-surface temperatures of the western equatorial Pacific, and sinking motion over the equatorial and subtropical regions of the eastern Pacific. These patterns of rising and sinking motion are the vertical components of an east-west oriented Walker circulation, which is also distinguished by extensive surface trade wind regimes linking the sinking motion in the eastern Pacific to the rising motion in the west. The Walker circulation's systematic redistribution of atmospheric mass causes surface pressure to drop below normal in the western tropical Pacific, and to rise above normal over the subtropics of the eastern Pacific. This surface pressure variability is measurable through a Southern Oscillation Index (SOI)(Ropelewski and Jones, 1987), which is proportional to the difference between the normalized surface pressure anomaly at Tahiti (T') minus the normalized surface pressure anomaly at Darwin (D'). La Niña conditions cause surface pressure to rise above normal at Tahiti, and to sink below normal at Darwin, resulting in positive (T'-D' >0) SOI values. The oceanic surface stresses associated with the Walker circulation's trade winds force the upwelling of cold sub-surface water along the equator in the eastern Pacific and the western coast of South America, thus the designation cold phase. The ability of the ENSO mechanism to affect North American climate is closely dependent on its affects on atmospheric circulation over the North Pacific. Trenberth and Shea's (1987) composite correlation of Darwin vs. global surface pressure suggests that cold phase conditions are associated with above normal annual mean surface pressure over the northeastern Pacific. Barnston et al.'s (1991) correlation of SOI vs. 700 mb geopotential heights suggests a tendency to above normal heights over the northeastern Pacific during winter periods consistent with La Niña conditions. Together, such positive anomalies of surface pressure and geopotential height suggest that winter La Niña conditions are accompanied by strengthened ridging over the northeastern Pacific. Evidence of anomalous ridging during northern winter periods is also apparent in the cold phase surface pressure and 500 mb height composites of Hoerling et al. (1997), although that ridging is somewhat westward of that implied by Barnston et al.(1991). |
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El Niño Conditions |
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El Niño conditions are marked in the atmosphere by a disruption of the east-west organization of the Pacific Walker circulation. A related weakening of the easterly trade wind flow in the central and eastern Pacific diminishes the influence that atmospheric surface stresses exert on SST and sea level in the western equatorial Pacific (Wyrtki, 1985). This shift in ocean-atmosphere dynamics leads to the eastward propagation of equatorial warm water masses in the western Pacific, producing warm water anomalies in the central and eastern equatorial Pacific (hence, "warm phase"). These positive sea-surface temperature anomalies in turn support the development of anomalous atmospheric deep convection and rising motion in the vicinity of the dateline. The weakening of the Walker Circulation and associated shift in atmospheric mass redistribution causes surface pressure to sink below normal over Tahiti (T' < 0) and to rise above normal at Darwin (D'>0), leading to negative SOI values(T'-D'<0). The correlations of Trenberth and Shea (1987) and Barnston et al.(1991) suggest that warm phase conditions are accompanied by below normal surface pressure and geopotential height in the northeast Pacific, which is consistent with a more active and extended storm track over the North Pacific. The composites of Hoerling et al. (1997) show negative anomalies of both surface pressure and 500 mb height in the northeast Pacific during northern winter El Niño periods, which suggests anomalous storm track activity off the western coast of the United States. |
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