Simulation of the northward propagation of the 30-60 day oscillation by ECHAM T42 and T106

The onset of Asian Summer Monsoon is characterized by the sudden enhancement of the heavy precipitations and the abrupt change of large-scale circulation. Intraseasonal oscillation (ISO) has been suggested to be an important climate factor to the onset of Asian Summer Monsoon. The purpose of this study is to estimate the forecast skill of the 30-60 day oscillation of several ensemble methods of T42 and T106. In general, the pattern correlation of the individual simulations significantly decline from April and reach minimum value in May. The occurrence of this poor performance of model simulation of ISO in May is coincided with the sudden northward development of the 30-60 day OLR in May. Ensemble forecasts may improve the performance of the T42 individual simulation in Asian monsoon region by enhancing the individual model response of the SST boundary forcing. It is found that the performance of T42 ensemble forecast in May is positively correlated to the SST anomalous over the equatorial Eastern Pacific Ocean and Indian Ocean, but negatively correlated to the SST anomalous over the off-equatorial Western Pacific Ocean. The T106 simulation is better than the traditional mean ensemble of T42 in May. The performance of T106 forecast in May is also positively correlated to the SST anomalous over the equatorial Eastern Pacific Ocean and Indian Ocean, while insignificantly correlated to the SST anomalous over the off-equatorial Western Pacific Ocean. The atmospheric internal dynamics may play an important role on the northward propagation of ISO over the off-equatorial Western Pacific Ocean in spring.