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Forecasts of Niño 3 Tropical Pacific SST
Using a Low Order Coupled Ocean-Atmosphere Dynamical Model
contributed by Richard Kleeman
Bureau of Meteorology Research Center, Melbourne, Australia
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 January
1997 and sub-surface thermal data up to December 1996 to initialize the model. The forecast
shows that the recent mildly cool conditions are expected to switch to weakly warm conditions in
the northern spring of 1997. The forecast has changed little in timing or amplitude since the
previous forecast in this Bulletin.
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 January 1997 and sub-surface thermal data up to December 1996 are used for intialization.
