Different Mechanisms for Interannual Variability of the Northwest Pacific Subtropical Anticyclone in June and July

The interannual variability of the summertime Northwest Pacific (NWP) subtropical anticyclone (NWPSA) has been widely investigated for its important role in the variability and predictability of the East Asian summer monsoon. A preceding El Nino-Southern Oscillation (ENSO) event influences the NWPSA by anchoring ocean memory in the North Indian Ocean, NWP region, etc. This study reveals a discontinuity of the impact of ENSO on the interannual variability of the summertime NWPSA, which is small in June but significant in July. The different mechanisms for the interannual variability of the NWPSA in June and July are further examined using observations, reanalysis data, and an atmospheric general circulation model. The interannual variability of the NWPSA in June and July arises from different physical processes. The latter is closely related to a preceding ENSO, while the former is attributed to the sea surface temperature (SST) anomalies over the southern tropical Atlantic (STA) in the preceding spring that can persist into June, independent of a preceding El Nino event. The STA SST warming could cause an anomalous descent in the central Pacific via modulating the Walker circulation, leading to low-level anomalous divergence and easterlies in the western equatorial Pacific and thereby an anomalous NWPSA. The different mechanisms underlying the NWPSA variability in June and July may be rooted in the annual cycle of the SST and Intertropical Convergence Zone in the Atlantic region. This study has important implications for the potential predictability of the NWPSA.