Interdecadal changes in the interannual variability of the summer temperature over Northeast Asia

This study reveals the interdecadal changes in the interannual variability of the summer temperature over Northeast Asia (NEA), which presents an enhancement around the early 1990s and a reduction after the mid-2000s. The stronger NEA temperature variability after the early 1990s is favored by the enhanced influence of the Pacific–Japan (PJ) teleconnection, which is remotely modulated by the southeastern tropical Indian Ocean (SETIO). After the early 1990s, the mean state over the SETIO presents relatively warmer SST and ascending motion, favoring a good relationship between the local SST and convection. Therefore, the SETIO SST could prominently influence the local convection and subsequently modulate the convection over the western North Pacific (WNP) via a cross-equatorial overturning circulation. The abnormal convection over the WNP further triggers the PJ teleconnection to influence NEA. However, these ocean–atmosphere processes disappear before the early 1990s. In this period, the mean state over the SETIO features relatively colder SST and subsiding motion, accompanied by a poor relationship between the local SST and convection. Therefore, the variability of convection over the SETIO is weak, thus the atmospheric variability over the WNP is also weakened and the PJ teleconnection presents a different distribution that could not influence NEA. The reduced variability of NEA temperature after the mid-2000s is related to the feeble influence of the PJ teleconnection and the reduced variability of the SETIO SST, which is modulated by the SST over the tropical central–eastern Pacific during the preceding winter to spring.

Possible Cause of Seasonal Inhomogeneity in Interdecadal Changes of Tropical Cyclone Genesis Frequency over the Western North Pacific

An abrupt decrease in annual tropical cyclone genesis frequency (TCGF), which is statistically significant only from October to December (OND), has been noticed over the western North Pacific Ocean. However, the seasonal inhomogeneity of interdecadal changes in TCGF between OND and the other seasons (from January to September) and the associated mechanisms are not clearly documented. This study examines and compares the different interdecadal changes in OND and in January–September from 1979 to 2018. According to our analysis, the TCGF decrease in OND (2.2) accounts for 79% of the total decrease (2.8) in annual TCGF after 1998, whereas the TCGF in January to September remains unchanged. The key differences in large-scale environment are found from the extension of equatorial easterly wind anomalies and attendant anticyclone anomalies in the subtropics. Under similar sea surface temperature (SST) warming pattern in the tropical Indo-Pacific region (i.e., the La Niña–like SST warming), tropical precipitation is significantly enhanced over the area where its seasonal peak occurs: the tropical Indian Ocean in OND and the tropical western Pacific in January–September. Thus, the equatorial easterly wind anomalies extend westward to 110°E in OND and to 145°E in January–September. Different extension of easterly wind anomalies results in different expansion of attendant large-scale anticyclone anomaly over the subtropical western Pacific, which dominates the entire main development region in OND but not in January–September. To summarize, the different extensions of easterly wind anomalies under similar La Niña–like SST warming are responsible for the seasonal inhomogeneity of interdecadal changes in TCGF.