The Interdecadal SST Variations In The Subtropical Ocean And Its Impacts On East Asian Summer Monsoon

Using the observed datasets and the four models results of IPCC AR4 20C3M, the paper studied the interdecadal variations in the subtropical Pacific. It was found that the interdecadal SST variations over eastern subtropical North Pacific are more significant than other regions of North Pacific, and are significant only during boreal summer and autumn. Besides interdecadal SSTs variations, the interdecadal atmospheric variations over the Subtropical North Pacific are significant. The study also examined the possible causes leading to interdecadal SST variations in the Subtropical North Pacific. Applied with SVD analysis, the momentum flux plays more important roles than freshwater flux, latent heat, and sensible heat flux in maintaining the interdecadal SST variations. Furthermore, the analyses suggested that the local atmospheric forcing, other than tropical teleconnection forcing from the tropics, results in the SST interdecadal variations in the Subtropical North Pacific. The paper firstly pointed out the roles of interdecadal SST variations in the subtropical North Pacific on PDO evolution. The subtropical SST anomalies could propagate tropical Pacific. The further results showed that the SST anomaly in the Subtropical North Pacific region would reach tropical Pacific, and be taken to midlatitude Pacific by Kuroshio Current, and finally changes original SST anomaly signal in the midlatitude Pacific, thus results in the change of PDO phase. With the analysis of the four models results of IPCC AR4 20C3M, the significant interdecadal SSTA variations in the Subtropical North Pacific and its characters have been confirmed. However, the four coupled models are hardly simulating the interdecadal variation of atmosphere in the Subtropcial North Pacific.The paper also found that the SSTs and sea level pressure in the Subtropical South Pacific have the significant interdecadal variations, and the interdecadal SSTA mode is significant only during boreal winter (DJF). The interdecadal variation of SSTA firstly occurring in the subtropical South Pacific propagates to the western boundaries of the South Pacific, then moves northeast to cross the equator, and finally reaches the central tropic Pacific. It takes about 8 years to propagate from southeast subtropical Pacific to the north hemisphere. Moreover, the paper showed that the interdecadal SSTA variability originated in the subtropical southeast Pacific is related closely with interannual variability in the tropic Pacific.The paper found that the subtropical north Pacific SSTA is significant correlated with Mei-yu rainfall on the interdecadal timescales, which influences the Mei-yu rainfall variations through changing the SST and wind in the tropical Pacific. Both the observed and model results showed the impacts of SSTA dipoles in the South Indian Ocean on Mei-yu rainfall. When the SSTA dipoles are in positive phase, Mei-yu rainfall is more and vice verse.The asymmetry impacts of ENSO on Mei-yu rainfall were studied in the paper, especially the interdecadal impacts of La Ni?a on Mei-yu rainfall in next year. After La Ni?a occurs, the Mei-yu rainfall is more/less than normal level before/after 1976. Our results showed that the such changing relationship between La Ni?a and Mei-yu rainfall is due to the equatorial Pacific mean SSTs changes in 1976/77. Finally, the paper examined the relationships between summer Indian rainfall and SSTA in different areas, including the Arabian Sea, the equatorial Indian Ocean, the southern Indian Ocean and the equatorial eastern Pacific, and indicated the interdecadal variations of influences of SSTA on Indian rainfall.