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 previous 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.

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 can be seen in Fig. 1 which shows the skill of the old system, the new system and persistence for forty forecasts with start dates between 1982 and 1991 inclusively.

Displayed in Fig. 2 is the most recent forecast of NINO3 which uses the FSU winds up to February 1998 and sub-surface thermal data up to January 1998 to initialize the model. The forecast shows that warm conditions are expected to persist until late 1998 after which a moderate cold event is expected. Compared to the previous forecast the cooling occurs now occurs somewhat later. 

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. Clim., 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 in the NINO3 region as a function of lead time for the low order coupled ocean-atmosphere model.

Fig. 2: Current forecast for NINO3 SSTA using FSU winds up to Feb. 1998 and sub-surface thermal data up to Jan. 1998.