The western North Pacific subtropical High (WNPSH) greatly influences the rainfall in East Asia, by modulating the moisture transport. Previous studies show that the interannual variability of the WNPSH is characterized by two distinct modes. These two modes are both characterized by anomalous anticyclones over the western North Pacific, but are associated with different SST anomalies. The first mode is associated with warm SST anomalies over the tropical Indian Ocean and the maritime continent, and cold SST anomalies over the equatorial central Pacific. The second mode is associated with cold SST anomalies over the WNP. There is still no consensus on the mechanism of the second mode. Some studies claimed that the second mode is forced by the local SST anomalies, while others argued that it is driven by atmospheric internal dynamics.

In order to clarify the relationship between these two modes and SST forcing, Dr. HE Chao and Prof. ZHOU Tianjun in IAP have evaluated the performance of the CMIP5-AMIP models on these two modes. The CMIP5-AMIP models, which are forced by the historical SST, perform well in simulating the first mode. The CMIP5-AMIP models also well capture the forcing mechanisms for the second mode, including the influence from tropical Indian Ocean through Kelvin wave, the influence from the maritime continent by local Hadley circulation anomaly, and the influence from the equatorial central Pacific through Rossby wave. These evidences suggest the first mode is an SST forced phenomenon.

In contrast, the second mode is only partially reproduced by the CMIP5-AMIP models. Among the five years associated with the second mode, the WNPSH anomalies are reproduced by the CMIP5-AMIP models in only two years. The local negative relationship between the anomalous anticyclone and the SST anomaly for the second mode is not reproduced by the CMIP5-AMIP models. These evidences suggest that the second mode cannot be explained as an SST forced phenomenon. The above results have improved our understanding on the mechanism for the interannual variability of the WNPSH.

The above results have been published in Climate Dynamics in 2014.

Reference:

He, C., and T. Zhou, 2014: The two interannual variability modes of the Western North Pacific Subtropical High simulated by 28 CMIP5–AMIP models. Clim Dynam, 43, 2455-2469.

Fig. 1. The spatial pattern of the vorticity and wind at 850 hPa for the two interannual variability modes of the WNPSH. (a) The first mode in the observation. (b) The first mode in the MME simulation. (c) The difference between MME simulation and the observation for the first mode. (d-f) Same as (a-c) but for the second mode.