Neural-dynamical hybrid coupled model forecasts of the tropical Pacific sea surface temperatures

Youmin Tang and William Hsieh


A neural-dynamical hybrid coupled model has been developed for giving seasonal predictions of the tropical Pacific sea surface temperatures. A 6-layer dynamical ocean model of the tropical Pacific is driven by the FSU wind stress, then during the forecasting period (Tang and Hsieh, 2001a), the ocean model is coupled to a nonlinear neural network atmospheric model, which estimates the surface wind stress anomalies from the upper ocean heat content anomalies (Tang et. al 2001).

For better forecast skills, different types of data-- surface wind stress, upper ocean heat content anomaly (HCA) (White, 1995), sea surface temperature (SST) and sea surface height anomaly-- have been assimilated into the coupled model using a 3D-Var data assimilation scheme (Derber and Rosati 1989). The results show that assimilating HCA (kindly provided by Warren White and Ted Walker at SIO) yields the greatest improvement in the forecast correlation skills (Tang and Hsieh 2001b). Fig. 1 shows the correlation skills of the predicted SST anomalies (SSTA) in the NINO3 region in the equatorial eastern Pacific during 1980-1989 (green line) and 1990-1999 (red line) using our model with HCA assimilation. The predictions were made at three months intervals (starting on 1 January, 1 April, 1 July and 1 October) and continued until a lead time of 15 months.

Fig.1 Correlation skills of predicted NINO3 SSTA



Fig. 2 shows the prediction of the NINO3 SSTA starting from April 1994 to April 2001 for lead times of 3 months, 6 months, 9 months and 12 months.

Fig.2 Observed and predicted NINO3 SSTA at lead times of 3, 6, 9 and 12 months


Fig 3 shows our latest forecasts (initialized using data till the end of April, 2001), indicating that the moderate anomalies (cool in the western equatorial Pacific and warm in the eastern equatorial Pacific) present during late summer, 2001, will fade away towards near-normal to slightly cool conditions by winter, 2001-2002. By spring, 2002, cooling develops in the eastern equatorial Pacific, turning into a La Nina by summer, 2002.

Fig.3 Predicted SSTA of the tropical Pacific


Contour intervals are 0.5 degree Celsius, with positive anomalies indicated by solid contours, negative anomalies by dashed contours, and zero by thick contours. Positive anomalies above 1 degree are shaded in red, negative anomalies below -1 degree are in blue, and the zero contour is in purple.


References:

  • Derber, J. and A. Rosati, 1989: A global oceanic data assimilation system. J. Phys. Oceanogr., 19, 1333-1347.


  • Tang, Y., and W. W. Hsieh, B. Tang and K. Haines, 2001: A neural network atmospheric model for hybrid coupled modelling. Clim. Dynam., 17, 445-455.


  • Tang, Y. and W.W. Hsieh, 2001a: Hybrid coupled models of the tropical Pacific -- ENSO prediction. Clim. Dynam. (submitted)[ http://www.ocgy.ubc.ca/~tym/papers.html]


  • Tang, Y. and W. W. Hsieh, 2001b: Impact of data assimilation on ENSO simulations and predictions. J. of Climate (submitted). [ http://www.ocgy.ubc.ca/~tym/papers.html]


  • White, W.B., 1995: Design of a global observing system for gyre-scale upper ocean temperature variability. Prog. in Oceanogr., 36, 169-217.


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