Season-dependent forecast skill of the dominant atmospheric circulation patterns over the North Pacific and North American region

【摘要】：Multi-model ensemble (MME) seasonal forecasts are analyzed to evaluate numerical models' performance in predicting the leading forced atmospheric circulation pattern over the extratropical Northern Hemisphere. Results show that the time evolution of the leading tropical Pacific sea surface temperature (SST)-coupled atmospheric pattern (MCA1), which is obtained by applying a maximum covariance analysis (MCA) between 500-hPa geopotential height (Z500) in the extratropical Northern Hemisphere and SST in the tropical Pacific Ocean, can be predicted with a significant skill in MAM, JJA and DJF one-month ahead. However, most models perform poorly in capturing the time variation of MCA1 in SON with August 1 st initial condition. Two possible reasons for the models' low skill in SON are identified. Firstly, the models have the most pronounced errors in the mean state of SST and precipitation along the central equatorial Pacific. Due to the link between the divergent circulation forced by tropical heating and the midlatitude atmospheric circulation, errors in the mean state of tropical SST and precipitation may lead to a degradation of midlatitude forecast skill. Secondly, examination of the potential predictability of the atmosphere, estimated by the ratio of the total variance to the variance of the model forecasts due to internal dynamics, shows that the atmospheric potential predictability over the North Pacific-North American (NPNA) region is the lowest in SON comparing to the other three seasons. The low ratio in SON is due to a low variance associated with external forcing and a high variance related to atmospheric internal processes over this area.