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