A simple coupled ocean/atmosphere model has recently been developed at the Bureau of Meteorology Research Center (BMRC) (Kleeman 1993) in order to explore the physical basis of ENSO predictability. In particular, a variety of very simple ocean models with varying thermodynamical equations governing SST have been coupled to a simple atmospheric model which performs well when forced by a full range of ENSO SST anomalies (Kleeman 1991).

The coupled models are somewhat similar to that of Cane and Zebiak (Cane and Zebiak 1987), but differ in aspects of the coupling, atmospheric convection and heating, and ocean thermodynamics.

The hindcast skill of these coupled models was tested using the ocean models initialized at regular 3 month intervals between January 1972 and July 1986 using FSU winds, and it was determined that optimal skill was obtained when the ocean model SST was determined purely by equatorial thermocline perturba-tions.

The initialization of the coupled model was improved (Kleeman et al. 1995) by using a space-time variational (adjoint) technique to assimilate sub-surface thermal data, as well as the usual wind data, into the ocean model. This resulted in an increase in the model skill. Figure 1 shows the skill of the improved system and persistence for forty forecasts with start dates from 1982 to 1991, inclusively.

Displayed in Fig. 2 is the most recent forecast of Niño 3, which uses the FSU winds up to July 1997 and sub-surface thermal data up to June 1997 to initialize the model. The forecast shows that strongly warm conditions are expected to persist through to early 1998. Subsequent to this the warm event is expected to repidly subside.

Kleeman, R., 1991: A simple model of the atmospheric response to ENSO sea surface temperature anomalies. J. Atmos. Sci., 48, 3-18.

Kleeman, R., 1993: On the dependence of hindcast skill on ocean thermodynamics in a coupled ocean-atmosphere model. J. Climate, 6, 2012-2033.

Kleeman, R., A.M. Moore and N.R. Smith, 1995: Assimilation of sub-surface thermal data into an intermediate tropical coupled ocean-atmosphere model. Mon. Wea. Rev, 123, 3103-3113.

Fig. 1. Hindcast skill in predicting Niño 3 SST as a function of lead time for the low order coupled ocean-atmosphere model used at Australia's Bureau of Meteorology Research Center (BMRC) in Melbourne. Skill is shown as a temporal anomaly correlation and as root-mean-square error, and is compared with persistence skill.

Fig. 2. Current forecast for Niño 3 SST using the BMRC low order coupled model. FSU winds up to July 1997 and sub-surface thermal data up to June 1997 are used for intialization.