Forecast of NINO3 using a low order coupled ocean-atmosphere model

contributed by R. Kleeman

Bureau of Meteorology Research Centre, Melbourne, AUSTRALIA

A simple coupled ocean/atmosphere model has recently been developed at 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 entry 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.

Later (Kleeman et al., 1995), the initialization of the coupled model was 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. The forecasts made here are with an unaltered version of this 1995 model.

The model skill was assessed in two different ways: First, predictions made 1 month apart from Jan 1981 to Dec 1994 were compared with the observations. Figure 1 shows a scatter plot of predicted and observed six and twelve month forecasts. This diagram should allow users to obtain some idea of both the skill and uncertainties of the real time prediction below. Second, historical forecast skill was compared with that obtained by other prediction schemes using the recent intercomparison of Kirtman and Shukla (1998). In this study, skill was computed for a common subset of forecasts for the years 1982-84, 1986-89 and 1991. This was repeated for the system here and anomaly correlations of .83, .82 and .80 were obtained for NINO3 forecasts of duration 6, 9 and 12 months respectively. These levels of skill compare well with those obtained from the other systems in the intercomparison.

Displayed in Figure 2 is the most recent forecast of NINO3 which uses the FSU winds up to October 1998 and sub-surface thermal data up to September 1998 to initialize the model. The forecast shows that the present weak cold conditions are expected to persist for two seasons after which further cooling is anticipated.

References

Kirtman B.P. and J. Shukla, 1998, Current Status of ENSO Forecast Skill. A report to the Climate Variability and Predictability (CLIVAR) Numerical Experimentation Group (NEG).

Kleeman R., 1991, A simple model of the atmospheric response to ENSO sea surface temperature anomalies, J. Atmos. Sci. , 48 , pp3-18.

Kleeman R., 1993, On the dependence of hindcast skill on ocean thermodynamics in a coupled ocean-atmosphere model, J. Clim. , 6, pp2012-2033.

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. Weath. Rev. 123, pp3103-3113.