Forecast Forum - August 2004

The canonical correlation analysis (CCA) forecast of SST in the central Pacific (Barnett et al. 1988, Science, 241, 192-196; Barnston and Ropelewski 1992, J. Climate, 5, 1316-1345), is shown in Figs. F1 and F2. This forecast is produced routinely by the Prediction Branch of the Climate Prediction Center. The predictions from the National Centers for Environmental Prediction (NCEP) coupled ocean/atmosphere model (Ji et al. 1998, Mon. Wea. Rev, 126, 1022-1034) are presented in Figs. F3 and F4a, F4b.  Predictions from the Markov model (Xue, Y. et al. 2000: ENSO prediction with Markov model: The impact of sea level. J. Climate, 13, 849-871) are shown in Figs. F5 and F6.   Predictions from the latest version of the LDEO model (Chen, D. et al. 2000, Geophys. Res. Let., 27, 2585-2587) are shown in Figs. F7 and F8. Predictions using linear inverse modeling (Penland and Magorian 1993, J. Climate, 6, 1067-1076) are shown in Figs. F9 and F10. Predictions from the Scripps / Max Planck Institute (MPI) hybrid coupled model (Barnett et al. 1993, J. Climate, 6, 1545-1566) are shown in Fig. F11.   Predictions from the ENSO-CLIPER statistical model (Knaff, J. A. and C. W. Landsea 1997, Wea. Forecasting, 12, 633-652) are shown in Fig. F12.  Niño 3.4 predictions are summarized in F13, which is provided by the Forecasting and Prediction Research Group of the IRI.

The CPC and the contributors to the Forecast Forum caution potential users of this predictive information that they can expect only modest skill.

Outlook

Warm-episode (El Niño) conditions are expected to continue through early 2005.

Discussion

Positive sea surface temperature (SST) anomalies persisted in the central and western equatorial Pacific during August 2004 (Table T2). Positive SST anomalies greater than +1°C were found between 170°E and 130°W, while negative SST anomalies less than -0.5°C were found between 100°W and the South American coast (Fig. T18).  The recent increase and eastward expansion of the area of anomalous warmth in the central equatorial Pacific during July-August  (Fig. T9) indicate the early stages of a warm (El Niño) episode.  Through the end of August conditions were not yet indicative of a basin-wide El Niño, particularly due to the presence of below normal sea-surface temperatures in the far eastern equatorial Pacific (Fig. T18). In spite of the anomalous warmth in the central equatorial Pacific during August, there appears to be little or no reflection of that warmth in the pattern of deep convection (precipitation) (Figs. T25, E3) or in the pattern of low-level winds (Fig. T20) over the region.

Considerable intraseasonal variability (MJO activity) in recent months has resulted in week-to-week and month-to-month variability in many atmospheric and oceanic indices (Table T1).  Most recently, a strong oceanic Kelvin wave, initiated by weaker-than-average easterly winds in June (Fig. T13), propagated eastward resulting in a substantial deepening of the oceanic thermocline (Figs. T15 and T16) and an increase in the subsurface temperature anomalies in the central and east-central equatorial Pacific (Fig. T17).  This Kelvin wave reached the South American coast during late August, resulting in an increase in SSTs along the coasts of Ecuador and northern Peru.  Another substantial weakening of the equatorial easterlies occurred during late August (Fig. T13), which appears to have initiated another eastward-propagating Kelvin wave in the central equatorial Pacific.

The NOAA operational definition for El Niño [Oceanic Niño Index (ONI), a three-month running mean of the Niño 3.4 index, greater than or equal to +0.5°C] was satisfied for the period June-August 2004 with an ONI value of +0.7°C.  A majority of the statistical and coupled model forecasts indicate that El Niño conditions will continue for the next 3-6 months with the remaining forecasts indicating near neutral conditions in the tropical Pacific through the end of 2004 (Figs. F1, F2, F3, F4a, , F4b, F5, F6, F7, F8, F9, F10, F11, F12, F13) Based on the recent evolution of oceanic and atmospheric conditions and on the statistical and coupled model forecasts, it seems most likely that SST anomalies in the Niño 3.4 region will remain positive, at or above +0.5°C, through early 2005. At this time it is not clear what, if any, impacts this event will have on ocean temperatures in the classical El Niño region (Niño 1+2) along the west coast of South America. CPC will continue to monitor the situation in the tropical Pacific and will provide more detailed information on possible regional impacts due to this event in coming months. 

Weekly updates of SST, 850-hPa wind, OLR and features of the equatorial subsurface thermal structure are available on the Climate Prediction Center homepage at: http://www.cpc.ncep.noaa.gov.