EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC ADVISORY

Issued by the Climate Prediction Center, NCEP

December 11, 1997

Very strong warm episode (ENSO) conditions continue, as sea surface temperatures (SSTs) and anomalies increased in the central and eastern equatorial Pacific during November. SSTs in the east-central and eastern tropical Pacific were the warmest for November in the historical record dating back to 1950. SST anomalies exceeded +2.0°C over the equatorial Pacific east of 170°W and were greater than +5.0°C from 115°W to 90°W, with the Niño 3.4, Niño 3, and Niño 1+2 average anomalies (2.9, 3.8, and 4.2°C, respectively) being the largest November values recorded in the last 50 years. The November value for Niño 3 is the largest anomaly observed during any month. Individual monthly SST index records have been set in these Niño regions since August 1997. The abnormally warm SSTs along the South American coast have resulted in well above-normal temperatures in coastal Peru (November temperatures 5-6°C above normal), with record monthly air temperatures recorded in Talara, Peru since May.

Tropical convection and rainfall during November were again greatly enhanced across the central and eastern equatorial Pacific, and suppressed over Indonesia and the western Pacific. This pattern of anomalous convection, which has persisted for the last several months, has been associated with low-level westerly and upper-level easterly wind anomalies over the central equatorial Pacific. The upper-level equatorial easterly anomalies have been accompanied by an anomalous anticyclonic couplet straddling the equator near 140°W and enhanced westerlies in the lower midlatitudes of both the Northern and Southern Hemispheres. These anomalous anticyclonic circulations are typical of strong, mature ENSO conditions.

The dramatic shift of heavy tropical convection away from Indonesia and the western Pacific and the resulting large-scale pattern of descending motion across Indonesia and the eastern Indian Ocean was associated with low-level easterly wind anomalies of 6-12 m/s across the central equatorial Indian Ocean during the month. The enhanced easterly flow in this region has contributed to excessive precipitation throughout equatorial eastern Africa during the last two months. Observed rainfall totals have been 400-600 mm above normal in sections of Kenya, and have resulted in extensive flooding in the region.

The oceanic thermocline during November continued to reflect strong ENSO conditions, remaining more than twice the normal depth across the eastern Pacific during the month, while remaining much shallower than normal in the western Pacific. Consistent with this structure, subsurface ocean temperatures were 2-3°C below normal in the western Pacific and much above normal in the east-central and eastern Pacific, with anomalies exceeding +10.0°C between 135°W and 125°W at thermocline depth.

The Southern Oscillation Index (SOI) was -1.4 during November. In recent months, the SOI has reflected large positive sea level pressure (SLP) anomalies at Darwin as anomalies at Tahiti have been near zero. Large negative values of the SOI and large positive SLP anomalies at Darwin have been observed since March 1997. Another (and possibly better) measure of the pressure gradient across the equatorial Pacific can be calculated from gridded SLP data from the CDAS/Reanalysis archive. This index (EQSOI) shows that the current warm episode is at least as strong as the 1982/83 episode, with large positive anomalies over the western Pacific and large negative anomalies over the eastern Pacific. There is a nearly equal contribution from both regions to the equatorial SOI.

Collectively, the oceanic and atmospheric anomaly patterns indicate strong mature-phase ENSO conditions. Given the magnitude of the anomalies and the absence of any signs of weakening, it appears likely that strong ENSO conditions will continue at least into the boreal spring of 1998. The latest NCEP coupled model and statistical (CCA) forecasts agree with this assessment. The NCEP coupled model forecasts indicate that strong warm episode conditions will continue through March-May 1998. In interpreting the SST anomalies from these forecasts, it is important to keep in mind that the smaller anomalies predicted during March-May 1998 do not represent a weakening of the warm episode, but rather are a function of the SST annual cycle which reaches its peak in the tropical eastern Pacific at that time of the year. In particular, note that the areal extent of SSTs greater than 28°C (the threshold for deep convection) during March-May 1998 is larger than during December - February 1997-98. Thereafter, the NCEP coupled model indicates a weakening of the warm episode. The NCEP CCA forecasts a similar evolution, with an accelerated decrease in SST anomalies beginning during June-August 1998.

Based on current conditions in the tropical Pacific, on the NCEP SST predictions, and on results from historical studies on the effects of ENSO, we expect drier-than-normal conditions to occur over Indonesia, eastern Australia, northern South America and southern Africa during the next few months. Wetter-than-normal conditions should continue over the central and eastern equatorial Pacific and southeastern South America through early 1998. Also, we expect wetter-than-normal conditions over California and the southern tier of the United States, and warmer-than-normal conditions along the northern tier of states.

Weekly updates for SST, 850-hPa wind, and OLR are available on the Climate Prediction Center homepage at: http://nic.fb4.noaa.gov (ENSO Update).

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