Variable correspondence between western North Pacific tropical cyclone frequency and East Asian subtropical jet stream during boreal summer: A tropical Pacific SST perspective

Fluctuations in the summer relationship between western North Pacific tropical cyclone (TC) frequency (TCF) and East Asian subtropical jet stream (EAJS) are investigated using observations and composite analysis. Here TCF is divided into four categories to assess how different tropical Pacific sea surface temperature (SST) anomalies modulate TCF-EAJS associations. Results show that high TCF would weaken (strengthen) the south (north) part of EAJS, but with different locally poleward shifts during La Nina- and El Nino-like events. In contrast, during low TCF, a clearly southward shifted, strengthened EAJS occurs only when the western Pacific SST cools, but no significant change is observed when the western Pacific SST warms up. Further analysis shows that the possible reason why TCF has diverse linkages to EAJS is that the TC genesis locations and TC tracks are quite different under different tropical Pacific SST anomalies, even for quasi-constant TCF. Potential mechanisms responsible for the changing TCF-EAJS associations can be physically explained by the exceptional configurations of tropospheric meridional temperature gradient anomaly and the deformed-and-displaced Pacific-Japan-like wavetrain, which is mainly due to non-negligible differences in large-scale oceanic and atmospheric environments that modulate the TC genesis locations and TC tracks. This study highlights the importance of a tropical SST perspective when analysing TCF-EAJS associations.