EL NINO/SOUTHERN OSCILLATION (ENSO)

DIAGNOSTIC ADVISORY 95/5

issued by

CLIMATE ANALYSIS CENTER / NMC

October 18, 1995

Most atmospheric anomaly patterns indicate that conditions in the tropical Pacific are near normal, while oceanic patterns indicate cooler-than-normal conditions in the equatorial Pacific east of the date line. Sea surface temperature (SST) anomalies for the period from mid-September through mid-October were negative throughout the equatorial Pacific east of the date line (Fig. 1), as the equatorial thermocline remained shallower than normal in that region. However, the low-level equatorial easterlies were near normal in the central Pacific and also the Southern Oscillation (SOI) index was near zero (Fig. 2).

Since the beginning of this year, SST anomalies have decreased steadily in the equatorial Pacific (Fig. 3) with negative anomalies gradually appearing in many areas of the equatorial eastern Pacific. The magnitude of the negative SST anomalies increased markedly during August and September, as the depth of the oceanic thermocline decreased along the equator east of the date line. These oceanic patterns are similar to those observed during the early stages of cold episodes. Since the mid-1980s there have been two periods characterized by abnormally cold SSTs in the equatorial central Pacific - 1984/85 and 1988/89. As both of these cold episodes developed, stronger-than-normal equatorial easterlies persisted over the central Pacific (120-180W) at low-levels for several months (Fig. 4), a feature that has not been observed as yet this year.

None of the statistical and numerical models indicate any further development towards a cold episode. Instead, a majority of these models indicates a gradual decrease in the magnitude of the negative SST anomalies with near-normal or slightly cooler-than-normal SSTs prevailing during the next three to six months.

Given the recent trend in oceanic temperature anomalies east of the date line, it seems likely that at least weak cold episode conditions should prevail during the next three to six months. Critical to any further intensification of these conditions is the strength of the low-level easterlies over the central equatorial Pacific. An increase to stronger-than-normal easterlies would accentuate equatorial upwelling and produce a further increase in the magnitude of the negative SST anomalies in that region.

If a full-fledged cold episode does develop during the next few months, one can expect anomalous atmospheric circulation, temperatures and precipitation patterns that are nearly opposite to those observed during warm (ENSO) episodes - wetter than normal over Indonesia, Northeast Brazil and southeastern Africa, drier than normal over the central Pacific, United States Gulf Coast, west coast of South America and central South America (Uruguay, southern Brazil and northeastern Argentina), colder than normal over western Canada and the northern contiguous United States, and warmer than normal over portions of the Gulf Coast and southeast United States.

This situation will be closely monitored by the Climate Prediction Center- with updated discussions available in future ENSO Advisories and in the monthly Climate Diagnostics Bulletin, which are available on the World Wide Web via Mosaic at http://nic.fb4.noaa.gov.

Climate Prediction Center

National Centers for Environmental Prediction

NOAA/National Weather Service

World Weather Building - Washington, D.C. 20233

e-mail: wd52vk@hp3l.wwb.noaa.gov

FIGURE 1. Sea surface temperature anomalies for 17 September - 14 October 1995. Contour interval is 0.5 deg-C. Anomalies are departures from the adjusted OI climatology (Reynolds and Smith, submitted to J. Climate).

FIGURE 2. Five-Month running mean of the Southern Oscillation Index (SOI) (top) and teh standardized 850-hPa zonal wind anomalies for 175-140W (bottom). Individual Monthly values are indicated by "x"s. The x-axis labels are centered on july. For the wind index, Positive (negative) values indicate easterly (westerly) anomalies.

FIGURE 3. Time-longitude section of anomalous sea surface temperature (5N-5S). Contour interval is 0.5 deg-C. Anomalies are departures from the adjusted OI climatology (Reynolds and Smith, submitted to J. Climate).

FIGURE 4. Time-longitude section of three-month averaged 850-hPa zonal wind anomalies (meters /sec). Anomalies are departures from the 1979-1988 base period means.
[the label at the top of the figure has incorrect years for the base period - ed.] .