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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 (see the Cane and Zebiak forecast in this
issue; 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 perturbations.
Recently (Kleeman et al. 1995) the initialization of the coupled model has been improved 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 has resulted in a significant increase in the skill of the model as seen in Fig. 1 which shows the skill of the
old system, the new 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 October 1996 and sub-surface thermal data up to September 1996 to initialize the model. The forecast shows that the recent cool conditions
are expected to switch to mildly warm conditions during early 1997. Compared to the previous forecast 3 months ago
in this Bulletin, the onset of the warming is delayed by a couple of months but reaches a slightly higher peak in mid-1997.
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 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 for the new SST data assimilation
version of the model is compared with that for the previous wind forced SST initialization system, and both of these
are compared with persistence skill.
Fig. 2. Current forecast for Niño 3 SST using the BMRC low order coupled model. FSU winds up to October and
sub-surface thermal data up to September 1996 are used for intialization.