U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
The Pacific ocean has several well known modes of seasonal to interannual period variability. In particular, there are times during which tropical South American coastal sea surface temperatures (SSTs) are anomalously warm, others during which central equatorial Pacific SSTs are anomalously high, and others during which there are significant swings in the sea level pressure (SLP) difference between Tahiti and Darwin. While names have been associated with periods of sustained warm coastal SSTs (El Niņo) and the Tahiti-minus-Darwin SLP difference (Southern Oscillation), there is no generally accepted language for the periods of warm central equatorial Pacific SSTs. Sometimes periods of central Pacific SST anomaly in excess of 1°C for a season or more are termed equatorial warm events, but this usage is not generally accepted. It is common to speak of El Niņo-Southern Oscillation (ENSO) events (e.g., Rasmusson and Wallace, 1983; Philander, 1990), as if this is a single phenomenon that encompasses all three of the above aspects. This awareness of significant connections between the three phenomena came largely from the work of Bjerknes (1966, 1969). Much remains unclear, however, about the extent to which these phenomena arise from a single coupled ocean-atmosphere mode, or from separate physical processes that can occur in different sequences. Deser and Wallace (1987) and others have shown that they are not inevitably linked and do sometimes occur separately. A great deal of work is still underway to examine the evolution, dynamics and predictability of these phenomena, under the programmatic rubric of "TOGA" (tropical ocean-global atmosphere) research.
Both terrestrial and ocean surface data sets have been used to explore the historical characteristics of these TOGA phenomena. Rasmusson and Carpenter (1982, hereafter RC), carried out the seminal modern work using ocean surface data and created a composite of El Niņo events between 1946-1976. They provided the first comprehensible basin scale view of SST and wind evolution during these events. Terrestrial time series of rain, wind and SLP have been explored, for example, by Taylor (1973), Gutzler and Harrison (1987), Kiladis and van Loon (1988), and Harrison and Luther (1990) among others. Trenberth (1976), Wright (1984), Barnett (1985), van Loon and Shea (1985, 1987), Trenberth and Shea (1987), Deser and Wallace (1987, 1990), and Wright et al. (1988) have also done important work on the basin scale variability using, variously, wind, SST, SLP and out-going long wave radiation (OLR) data. The preparation of the COADS surface data set (Woodruff et al., 1987) has greatly facilitated basin scale work.
The data collected since 1976 have raised many questions about the utility of the RC composite as subsequent ENSOs have departed from the RC composite in conspicuous ways. The RC composite has motivated so many other studies that advanced our knowledge of TOGA phenomena, however, that we have undertaken to update and extend the composite picture of the ocean surface behavior before, during, and after an El Niņo.
We composite over events between 1946 and 1993, and over as much of the ocean surface as can usefully be examined with the COADS data set. We also examine the month to month variability in the regions of substantial signal in order to discuss the degree to which the main features of the composite are representative of the individual events. We find and present here large scale signals in the near-global SLP data. We present our SST and surface wind analyses, and discuss their relationships to SLP variability in a companion paper (Harrison and Larkin, 1996).
The following questions provide the framework for this study:
1) Are the COADS SLP data useful for studies of behavior during El Niņo periods? In particular, can we reproduce the low frequency behavior of the Darwin and Tahiti based Troup Southern Oscillation Index using the COADS data, and are there plausible large scale patterns of SLP anomalies? How much smoothing in time and space is needed to bring out the large scale patterns?
2) What is the structure of the composite SLP anomaly (SLPA) during El Niņo periods? What statistical significance can we attach to these signals?
3) How representative of the SLPA behavior during individual El Niņo events is the composite? Which features are most robust? Least robust?
4) What do our results say about the utility of the composite approach to characterizing SLPA variability during El Niņo times? What phenomena do the results identify as deserving of additional study?
Section 2 details the data used and describes the smoothing filters applied in this study. Section 3 compares the traditional Tahiti/Darwin SLP time series and Troup Southern Oscillation Index (SOI) with a Troup-SOI surrogate index using COADS data, to help support the use of COADS for the rest of our study. Section 4 presents the 1946-1993 composite ENSO event. In Section 5 monthly time series are used to examine event-to-event variations. A new ENSO index is proposed. Finally, in Section 6, we review the major results and discuss their implications for ENSO forecasting, diagnosis, and further observation. Due to the many issues of connections between ENSO and the seasonal march, an appendix discussing the average SLP field and climatological seasonal march is provided.
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