Forecast Forum - July 2007

          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 Forecast System Model (CFS03) are presented in Figs. F3 and F4a, F4b.  Predictions from the Markov model (Xue, et al. 2000: J. Climate, 13, 849‑871) are shown in Figs. F5 and F6.   Predictions from the latest version of the LDEO model (Chen 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 and Landsea 1997, Wea. Forecasting, 12, 633‑652) are shown in Fig. F12.  Niño 3.4 predictions are summarized in Fig. F13, 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

            ENSO-neutral conditions are expected to continue through August 2007, with a slightly greater than 50% chance of La Niña developing during the next couple of months.

Discussion     

ENSO-neutral conditions continued in the tropical Pacific during July 2007, with average to below-average sea surface temperatures (SSTs) extending from the date line to the west coast of South America (Fig. T18). The July SST departures were negative in the Niño 1+2 ("1.6ºC), Niño 3 ("0.8ºC), and Niño 3.4 ("0.3 ºC) regions, and positive in the Niño 4 (+0.2ºC) region (Table T2). Thus, while SSTs in the eastern equatorial Pacific have been cooler than average for the last six months, the departures continue to fall short of the threshold for La Niña (3-month running mean value of "0.5 ºC for the Niño 3.4 region: 5°N-5°S, 120-170°W).

Despite not meeting the SST threshold for La Niña, recent atmospheric circulation and tropical convection patterns are consistent with the evolution toward La Niña conditions. For example, the low-level easterly winds remained stronger than average in the west-central equatorial Pacific (Fig. T20), convection remained suppressed across most of the equatorial Pacific, and a weak area of enhanced convection covered parts of Indonesia and the far western equatorial Pacific (Fig. T25). Also, the upper-ocean heat content (average temperatures in the upper 300 m of the ocean) in the central and east-central equatorial Pacific remained below-average, but the magnitude of the departures continued to exhibit intraseasonal fluctuations. Collectively, the oceanic and atmospheric conditions reflect a continuation of ENSO-neutral conditions.

Nearly all of the ENSO models predict below-average SSTs in the Niño 3.4 region for the remainder of the year (Figs. F1, F2, F3, F4a, F4b, F5, F6, F7, F8, F9, F10, F11, F12 and F13).   The spread of the recent model forecasts range from ENSO-neutral to La Niña, with a majority of dynamical models indicating a more immediate transition to La Niña. However, over the last several months, the dynamical models have consistently predicted a stronger and more rapid cooling than has actually occurred. In contrast, a majority of the statistical models indicate a continuation of ENSO-neutral conditions, but some forecast weak La Niña conditions during the fall or winter. When considered collectively, recent atmospheric conditions and model forecasts suggest a slightly greater than 50% chance of La Niña developing during the next couple of months. Historically, the early fall season (August-September-October) has been a critical period for the onset of La Niña events.

            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/