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An intermediate dynamical ocean-empirical atmosphere coupled model is currently being used in the Atmospheric,
Oceanic and Planetary Physics Department at the University of Oxford to predict sea surface temperature (SST)
anomalies in the tropical Pacific. The model, whose detailed description and performance are documented in Balmaseda
et al. (1994), consists of a tropical Pacific ocean model with two active layers coupled to a statistical model that relates
SST anomalies, heat content (HC) anomalies, and surface wind stress anomalies. The anomalies are relative to monthly
climatology. The ocean model is first forced by observed wind stress (based on data from Florida State University
[FSU]; Goldenberg and O'Brien 1981) during the period 1961-91. The output of this simulation run is used to build the
statistical atmospheric model, which assumes that the wind stress anomalies are a linear function of the first 6 principal
components (PCS) of the model SST and HC anomalies, with seasonal variation.
The hindcast skill of the model was tested using a set of 252 hindcast experiments, each of 24 months duration,
with initial conditions taken from the simulation run at one month intervals during the period 1970-91. The correlation
skill (sk) and root mean square error (RMSE) for hindcasts of SST anomalies versus observed values have been
calculated using ensemble means of hindcasts from three consecutive months.
The model shows good reliability in regions of the central equatorial Pacific--in particular, in the regions Niño 3
and Eq2 (130-170oW, 5oN-5oS). Figures 7-1a and 7-1b in the December, 1994 issue of this Bulletin show the correlation
and RMSE skills, respectively, for model hindcasts of the SST anomalies in the Niño 3 and Eq2 regions. Correlation
skill at 6 months lead is about 0.59 for Niño 3 and about 0.62 for Eq2, while at 12 months lead they are about 0.50 and
0.55, respectively. The error in the initial conditions is quite high, because only wind information is used to obtain the
model initial state.
Figure 1 shows current forecasts of SST anomalies for 6 and 9 month lead times. The vertical bars are two RMSE
in length, based on the 1970-91 period. The forecasts for Niño 3 (left column) at both lead times and Eq2 (right column)
at 9 months lead show considerable recovery of the SSTs relative to the strongest negative anomalies. The Eq2 forecast
at 6 months lead indicates new cooling this winter. Although the observations have recovered from their early winter
1995-96 lows, those of Eq2 show some recent decline. During 1996 the forecast anomalies have tended to be more
negative than the observed anomalies. However, at both lead times the forecast anomalies have had the correct sign. In
the present forecast the initial conditions represented by the FSU wind stress continue to favor cool conditions.
The last four individual forecasts, with initial conditions from July to October 1996, are shown in Fig. 2 for Niño
3 and Eq2. The thick solid lines are the observed SST anomalies, and the thin solid lines show the control run from
which the initial conditions are taken. The trajectories of the individual forecasts are generally in good agreement for
both Niño 3 and Eq2, with later initial conditions producing cooler forecasts. Gradual warming is indicated throughout
1997 following a cold beginning.
Balmaseda, M.A., Anderson, D.L.T. and M.K. Davey, 1994: ENSO prediction using a dynamical ocean model
coupled to statistical atmospheres. Tellus, 46A, 497-511.
Goldenberg, S.D. and J.J. O'Brien, 1981: Time and space variability of tropical Pacific wind stress. Mon Wea.
Rev., 109, 1190-1207.
Fig. 1. Oxford coupled model forecasts of the SST anomalies in the Niño 3 (left column) and Eq2 (130-170oW;
right column) regions for 6 and 9 month leads. The latest forecast was initialized from October 1996 data. The vertical
bars represent the RMSE-based 95% confidence intervals for the relevant lead time, based on predictions for 1970-91.
Each prediction is the average of forecasts from three consecutive months. Correlation skill (sk) and RMSE values
(fractions of observed standard deviation, or std) are indicated in each panel. Thick lines indicate observed SST
anomalies.
Fig. 2. Individual initial condition predictions of SST anomalies in the Niño 3 and Eq2 regions made with the
Oxford coupled model. The initial conditions are for July (dotted line), August (dashed line), September (dash-dot), and
October (dash-dot-dot-dot) 1996. Each prediction has a duration of 12 months. The ocean initial conditions (thin solid
line) are produced by forcing the ocean model with observed FSU winds and also by sub-surface assimilated data. The
thick solid line shows the observed SST anomalies.