ATMOSPHERIC PREDICTABILITY OF SEASONAL, ANNUAL, AND DECADAL CLIMATE MEANS AND THE ROLE OF THE ENSO CYCLE - A MODEL STUDY

The characteristics of extratropical low-frequency variability are exa mined using a comprehensive atmospheric general circulation model. A l arge experiment consisting of 13 45-yr-long integrations forced by pre scribed sea surface temperature (SST) variations is analyzed. The pred ictability of timescales of seasonal to decadal averages is evaluated. The variability of a climate mean contains not only climate signal ar ising from external boundary forcing but also climate noise due to the internal dynamics of the climate system, resulting in various levels of predictability that are dependent on the forcing boundary condition s and averaging timescales. The focus of this study deviates from the classic predictability study of Lorenz, which is essentially initial c ondition sensitive. This study can be considered to be a model counter part of Madden's ''potential'' predictability study. The tropical SST anomalies impact more on the predictability over the Pacific/North Ame rica sector than the Atlantic/Eurasia sector. In the former sector, mo re significant and positive impacts are found during El Nino and La Ni na phases of the ENSO cycle than during the ENSO inactive period of ti me, Furthermore, the predictability is significantly higher during El Nino than La Nina phases of the ENSO cycle. The predictability of seas onal means exhibits large seasonality for both warm and cold phases of the ENSO cycle. During the warm phases, a high level of predictabilit y is observed from December to April. During the cool phases, the pred ictability rapidly drops to below normal from November to March, The s pring barrier in the atmospheric predictability is therefore a distinc t phenomenon for the cold phase, not the warm phase, of the ENSO cycle . The cause of the barrier can be traced to the smaller climate signal and larger climate noise generated during cold events, which in turn can be traced back to the rapidly weakening negative SST anomalies in the tropical Pacific east of the date line. Due to the fact that the s ignal to noise ratio of this model climate system is very small, an up per bound in atmospheric predictability is present, even when a perfec t model atmosphere is considered and large ensemble mean predictions a re exploited. The outstanding issues of the dynamical short-term clima te prediction employing an atmospheric general circulation model are e xamined, the current model deficiencies identified, and continuing eff orts in model development addressed.