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 July 1996 and sub-surface thermal data up to June
1996 to initialize the model. The forecast shows that the recent mildly
cool conditions are expected to switch to weakly warm conditions for the
first half of 1997. Compared to the previous forecast 3 months ago in this
Bulletin, a warming is now significantly reduced.
References
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.
Figures
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 July and sub-surface thermal data
up to June 1996 are used for intialization.