Relative Controls of Asian–Pacific Summer Climate by Asian Land and Tropical–North Pacific Sea Surface Temperature

The dominant pattern of summertime tropical and North Pacific sea surface temperature (SST) is characterized by an out-of-phase relationship between the tropics and the extratropics. This pattern, defined as the tropical–North Pacific mode (TNPM) in this study, is closely correlated with the variability of climate over Asia and the Pacific Ocean. A high TNPM index, with positive (negative) SST anomalies over the extratropics (tropics) of the Pacific, is linked to deep negative anomalies of tropospheric temperature over the extratropical Pacific, with shallow positive anomalies in the lower troposphere, and is also linked to deep positive temperature over Asia. It is also found that these anomalies of tropospheric temperature and SST are significantly related to the Asian–Pacific Oscillation (APO), an extratropical zonal–vertical atmospheric pattern connecting Asia and the Pacific. Indeed, when the variability of APO is removed, the above-described climate anomalies weaken significantly. Although the above relationships observed between atmospheric circulation and SST can be captured by general circulation models, sensitivity experiments show that the variations of summertime Asian–Pacific atmospheric circulation may not be mainly forced by the Pacific SST. Instead, the Asian land elevated heating seems to play a more important role in generating the climate anomalies, as shown by model-sensitivity experiments in which changes in topographic height are included. Moreover, the relative importance of Asian land and Pacific SST for the variations of Asian–Pacific climate in summer and winter is compared in this study. In winter the most dominant mode of Pacific SST exerts a stronger impact on the Asian–Pacific climate.