Very strong warm episode (ENSO) conditions continued during October 1997, as sea surface temperatures (SSTs) greater than 28.0°C covered the tropical Pacific west of 125°W and values greater than 29°C extended from 170°E to 140°W (Fig. 1). SSTs in the east-central and eastern Pacific were the warmest for October in the historical record dating back to 1950. SST anomalies exceeded +2.0°C over the equatorial Pacific east of 170°W and +4.0°C east of 130°W, with the Niño 3.4, Niño 3, and Niño 1+2 average anomalies [2.8, 3.5, and 4.1, respectively (Fig. 2)] being the largest values recorded in October.

Tropical convection and rainfall during October were greatly enhanced across the central and eastern equatorial Pacific, and suppressed over Indonesia and the western Pacific (Fig. 3). Precipitation has been enhanced across most of the central and eastern equatorial Pacific since March These large-scale shifts in tropical convection have been associated with low-level westerly wind anomalies exceeding 10 m s-1 over the central tropical Pacific, and with a nearly complete collapse of the normal tropical low-level easterlies across the entire equatorial Pacific.

The equatorial oceanic thermocline during October continued to reflect strong ENSO conditions, increasing to more than twice the normal depth across the eastern Pacific and remaining shallower than normal in the western Pacific. Consistent with this structure, subsurface ocean temperatures were below normal in the western Pacific and much above normal in the east-central and eastern Pacific, with anomalies exceeding +9.0°C observed between 130°W and 105°W at thermocline depth.

The Southern Oscillation Index (SOI) was -1.8 during October, with small negative sea level pressure (SLP) anomalies observed at Tahiti (-0.3) and large positive anomalies again observed at Darwin (2.6). Large negative values of the SOI and large positive SLP anomalies at Darwin have been observed since March 1997. In fact, the five-month running mean of the Darwin SLP anomalies, has been surpassed only during the 1982/83 episode over the past 20 years.

Over the past few seasons the NCEP statistical (CCA) and coupled model predictions have consistently indicated the development and persistence of a strong warm episode. The latest NCEP coupled model forecasts indicate that strong warm episode conditions will continue through February-April 1998 (Fig. 4). In interpreting the SST anomalies from these forecasts (right hand panels), it is important to keep in mind that the smaller anomalies predicted during February-April 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 February-April 1998 is larger than during November - January 1997/98 (left hand panels). 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 May-July 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 continue over Indonesia, eastern Australia, Central America, the Caribbean, 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. During the northern winter 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

Climate Prediction Center - NOAA/NWS/NCEP World Weather Building - Washington, D.C. 20233 e-mail: wd52vk@hp31.wwb.noaa.gov

Back to Pacific ENSO Update - 4th Quarter 1997- Vol.3 No.4