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 wind stress and sea level analysis (Xue et al. 1999). The model is trained for 1980-95 and verified for 1964-79.

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 wind stress is the FSU wind stress product (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 Markov model is built with three multivariate EOFs in which the anomalous fields of SST, wind stress and sea level are equally weighted. The model contains 12 monthly transition matrices. The cross-validated skill for 1980-95 and hindcast skill for 1964-79 have been published in the issue of September 1998 of the Experimental Long-Lead Forecast Bulletin.

Fig. 1 shows the time evolution of NINO3.4 forecasts up to 12 month leads by the Markov model initiated monthly from October 1996 to July 1999. The model predicted the 1997/98 El Niño and 1998/99 La Niña events well. The latest predictions suggest that the current cold SST anomaly will increase slightly in the fall and peak at the end of 1999. This cold SST anomaly ( -1^OC) will probably dissipate quickly in the spring of 2000. The seasonal mean SST anomalies forecast by the model initiated from July 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., A. Leetmaa and M. Ji, 1999: ENSO predictions with Markov models: the impact of sea level. J. Climate, in press.