SST predictions with an intermediate coupled model of the tropical Pacific.

contributed by Boris Dewitte¹, Dasha Gushchina ², Yves duPenhoat ³ and Rodrigo Abarca del Rio⁴

¹IRD/LEGOS B.P. A5 98848 Nouméa Cedex, New Caledonia
²Meteorological Department, Geographical Faculty, Moscow State University, Vorobyevy gory, Moscow, 119899, Russia
³ LEGOS/CNES/IRD 14 Av. E. Belin, 31401 Toulouse Cedex 4, France
⁴ DEFAO, University of Concepcion, Casilla 160-C, Concepcion, Chile

Forecasts of the tropical Pacific SST are presented here using two intermediate coupled models. The oceanic component consists in a three baroclinic mode ocean and the mixed layer model differs from the Cane and Zebiak (1987) model in the parameterization of the vertical advection terms and the basic state (cf. Dewitte, 2000). The atmospheric component is either a statistical atmospheric model based on the singular value decomposition (SVD) of observed sea surface temperature (SST) and wind stress anomalies (see Périgaud et al., 2000) or a Gill(1980)'s dynamical tropical atmosphere. Initial conditions for the prediction runs are produced in a coupled mode by nudging the observed winds (FSU winds for the period 1961-1992, ERS1-2 winds from may 1992 until september 2000, QuikSCAT winds from october 2000) to the simulated winds as in Chen et al. (1995). The skill of the models for the NINO3 SST index over various periods and for the 1997-1998 El Niño is presented in Dewitte et al. (2002).

Figure 1 shows model predicted SST and wind stress anomalies in the tropical Pacific for the next four seasons in the case of the statistical atmosphere. These are ensemble averages of 12 forecasts started from Sept-Oct-Nov conditions. For producing the initial conditions of the individual forecasts, random noise was added to the system with an approach similar to Kirtman and Schopf (1998).

The model is predicting persistent warm conditions throughout the year 2003. The peak phase of the 2002/2003 El Niño is for the Dec-Jan-Feb 2002/2003 season with large zonal scale anomalies of ~2°C. Note that the warming is of the same order of magnitude for both versions of the model (i.e. with the statistical atmosphere (figure 1) or the dynamical atmosphere (figure 2))

Caveat: The forecasts shown above are experimental in nature. The reader is forewarned that the methods/forecasts are new and subject to future change and improvement.

References:

Chen, D., S. E. Zebiak, A. J. Busalacchi, and M. A. Cane, 1995: An improved procedure for El Niño forecasting: Implication for predictability. Science, 269, 1699-1702.

Dewitte B., 2000: Sensitivity of an intermediate coupled ocean-atmosphere model of the tropical Pacific to its oceanic vertical structure. J. Climate, 13, 2363-2388.

Dewitte B., D. Gushchina, Y. duPenhoat and S. Lakeev, 2002: On the importance of subsurface variability for ENSO simulation and prediction with intermediate coupled models of the Tropical Pacific: A case study for the 1997-1998 El Niño. Geoph. Res. Lett., in press.

Gill, A., Some simple solutions heat-induced tropical circulation. Q. J. R. Meteorol. Soc., 106, 447-462, 1980.

Kirtman, B. P. and P. S. Schopf, 1998: Decadal variability in ENSO predictability an prediction, J. Climate, 11, 2804-2822

Perigaud C. C. Cassou, B. Dewitte, L.-L. Fu and D. Neelin, 2000: Using data and intermediate coupled models for seasonal-to-interannual forecasts. Mon. Wea. Rev., 128, 3025-3049.

Zebiak, S. E. and M. A. Cane, 1987: A model El Nino-Southern Oscillation. Mon. Wea. Rev., 115, 2262-2278.

Figure Captions:

Fig. 1:  model  forecast of tropical Pacific SST (°C) and wind stress (Dyn/cm²) anomalies for DJF 2002/2003, MAM 2003 and JJA 2003 and SON 2003. Each forecast is an ensemble average of 12 sets of prediction runs initialized from wind forced model outputs perturbated by random noise. Observed data through 14 November 2002 was used to produce the forecast. Contour interval 0.5°C. Regions with SSTA amplitude larger than 0.5°C are shaded and dashed contour line are used for value below zero. The longuest wind stress arrow on each map corresponds to the value indicated on the right hand side. Only wind stress anomalies for which wind modulus is larger than 0.1 Dyn/cm2 are plotted.

Fig.2: same as Fig. 1 but for the coupled model using the dynamical atmosphere.