Influence of Negative Indian Ocean Dipole on Strengthening of North Pacific Subtropical High Leading to Heat Wave in Japan around Late June 2022

The Japan Meteorological Agency reported that a record strengthening of the upper-level high and the North Pacific subtropical high (NPSH) at the surface in late June 2022 was the primary cause of the unprecedented heat wave in Japan from late June to early July 2022. Using observational data and a linear baroclinic model (LBM), this study explains the mechanism behind the NPSH strengthening associated with the negative Indian Ocean Dipole (IOD) occurring at that time. A lower-level anticyclonic circulation anomaly to the south of Japan did not appear in the LBM experiment for June with a negative-IOD-related heat source over the eastern Indian Ocean. However, it appeared in another LBM experiment where the heat source was located over the northern Indian Ocean. From mid to late June 2022, enhanced convection over the southeastern Indian Ocean gradually expanded toward the northern Indian Ocean, accompanied by the synchronous development of a low-level anticyclonic circulation anomaly south of Japan. Similar phenomena occurred in the same period in previous years, indicating that the negative IOD played a key role of the strengthening the NPSH around late June 2022.