Forecast of Tropical Pacific SST Using a Markov Model

contributed by Yan Xue and Ming Ji

Environmental Modeling Center, National Centers for Environmental Prediction, NOAA,

Camp Springs, Maryland

Forecasts of the tropical Pacific SST anomaly are presented here using a linear statistical model (Markov model). The Markov model is constructed in a reduced multivariate EOF space of observed sea surface temperature (SST), surface winds and sea level analysis (Xue and Leetmaa 1997). The SST from 1964 to 1981 is the reconstruction of historical SST by Smith et al. (1996) and the SST from 1982 to present is the SST analysis by Reynolds and Smith (1994); the surface winds from 1964 to present is the FSU pseudo windstress (Goldenberg and O'Brien 1981); the sea level from 1964 to 1979 is from a model simulation which uses the GFDL MOM1 model forced by the FSU winds and the sea level from 1980 to present is from the ocean analysis at NCEP (Behringer et al. 1998). All the data are monthly values and cover the tropical Pacific region within 20^O of the equator.

The training period is from 1980 to 1995 and the annual cycle for this period is removed from the data. The model is built in a combined EOF space with 3 retained EOFs where the anomalous fields of SST, winds and sea level are equally weighted. The model contains 12 monthly transition matrices as that of Xue et al. 1994. The skill of the model is tested for the training period with a cross-validation method (one year is held out at a time) and for an independent period from 1964 to 1979. The hindcast skills for the two periods have been shown in the issue of the Experimental Long-Lead Forecast Bulletin in September 1998.

Fig. 1 shows the time evolution of NINO3.4 forecasts up to 12 month lead times by the Markov model initiated monthly from October 1996 to January 1999. It is seen that the model has predicted the 1997/98 warm event well. The predictions initiated prior to the early spring of 1997 underestimated the strength of the warm event, but the predictions made afterwards simulated the event reasonably well. The model predicted the decay phase of the warm event well, and predicted a cold event for the winter of 1998/99. The latest predictions suggest that the current cold anomaly will remain through the end of 1999. The seasonal mean SST anomalies forecast by the model initiated from January 1999 are shown in Fig. 2.

References:

Behringer, D. W., M. Ji and A. Leetmaa, 1998: An improved coupled model for ENSO prediction and implications for ocean initialization. Part I: The ocean data assimilation system. Mon. Wea. Rev., 126, 1013-1021.

Goldenberg, S. B. and O'Brien, J. J., 1981: Time and space variability of tropical Pacific wind stress. Mon. Wea. Rev., 109, 1190-1207.

Reynolds, R. W., and T. M. Smith, 1994: Improved global sea surface temperature analyses using optimum interpolation. J. Climate, 7, 929-948.

Smith, T. M., R. W. Reynolds, R. E. Livezey, and D. C. Stokes, 1996: Reconstruction of historical sea surface temperatures using empirical orthogonal functions. J. Climate, 9, 1403-1420.

Xue, Y., M. A. Cane, S. E. Zebiak and M. B. Blumenthal, 1994: On the prediction of ENSO: a study with a low-order Markov model. Tellus, 46A, 512-528.

Xue, Y. and A. Leetmaa, 1997: Predictability of ENSO: a study with Markov models. Proceedings of the twenty-second annual climate diagnostics and prediction workshop, Berkeley, Ca, Oct. 6-10, 1997.