[Previous Article]
[Next Article]
Forecasts of Nino 3 Tropical Pacific SST
Using a Low Order Coupled OceanAtmosphere
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 NiZo
3, which uses the FSU winds up to April 1996 and sub-surface thermal data
up to March 1996 to initialize the model. The forecast shows that the recent
mildly cool conditions are expected to end in June and a mild warm event
is expected to peak in early 1997. Compared to the previous forecast in
the March issue of the Bulletin, the warming is now a little weaker and
is expected to dissipate earlier (towards the end of 1997).
Kleeman, R., 1991: A simple model of the atmospheric response
to ENSO sea surface temperature anomalies. J. Atmos. Sci., 48,
318.
Kleeman, R., 1993: On the dependence of hindcast skill
on ocean thermodynamics in a coupled oceanatmosphere model. J. Climate,
6, 20122033.
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 oceanatmosphere 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 Nino 3 SST using
the BMRC low order coupled model. FSU winds up to April and sub-surface
thermal data up to March 1996 are used for intialization.