The seasonal dependence of atmospheric short-term climate (i.e. seasonal to interannual) predictability is studied. This is accomplished by analyzing the output from ensemble integrations of the European Center for Medium Range Weather Forecast Model. The integrations use the observed evolution of sea surface temperature (SST) as prescribed boundary forcing. Forced by the interannual variation of SST, the short-term climate predictability of the atmospheric circulation is geographically and seasonally dependent. The predictability is larger in tropics than the extratropics and is greater in the Pacific-Atlantic Ocean sector compared to the Indian Ocean-Asian Monsoon region. Predictability is also higher in the winter hemisphere than in the summer hemisphere. In average, the weakest predictability in the Northern Hemisphere occurs during the boreal autumn. However, it is noted that the 1982/83 strong El Nino event produced stronger atmospheric predictability than the 1988/89 strong La Nina event during the boreal spring; and, the predictability pattern is reversed during the boreal autumn.

Predictability is further partitioned into its internal and external components. The external component is defined as the interannual variation of ensemble average and the internal component is the sample-to-sample variance. The temporal and spatial structure in the external variability resembles most of the structure in the SST forced atmospheric predictability. However, there are regions in the tropics such as over the monsoon region where the external and internal variabilities show roughly the same magnitude. Overall, internal variability is largest in the extratropics. Specifically, it is larger in the northern extratropics during the boreal autumn and larger in the southern extratropics during the boreal spring. An opposite asymmetry between the two equinox seasons is seen in the external variability (e.g. smaller in the northern extratropics during the boreal autumn).

The study also shows that major features of the SST forced atmospheric predictability is determined by the external variability in the tropics. And, in the extratropics, the predictability is determined by seasonal variations in both internal and external variabilities. The weakest predictability that occurs in the northern extratropics during the boreal autumn is the result of a conjunction of local increase in internal variability and decrease in external variability at the same time.

Furthermore, the external variability is controlled by seasonality in the forcing over the tropical Pacific Ocean. The seasonality in the forcing over the tropical Pacific Ocean is largely determined by the following two mechanisms: (1) Annual-cycle/ENSO interaction over the tropical Pacific Ocean; (2) Non-linear effects of hydrological processes associated with the annual-cycle/ENSO interaction.