Causes of the persistent merging of the western North Pacific subtropical high and the Iran high during late July 2022

During late July 2022, the western North Pacific subtropical high (WNPSH) and the Iran high merge persistently, leading to concurrent extreme climate events over multiple regions. Such extraordinary circulation results from two extreme anticyclonic anomalies centered over the WNP-South Asia and Central Asia, respectively. The WNP-South Asia anticyclonic anomaly is related to La Nina SSTA pattern and the enhanced convection over the tropical eastern Indian Ocean (TEIO). SSTA serves as a seasonal-mean background, with abnormally cold tropical central Pacific (TCP) favoring the anticyclonic anomaly through Rossby wave response. The TEIO convection is extremely strong during late July and induces the anticyclonic anomaly via Kelvin wave response and a local Hadley circulation. The TEIO convection anomaly during late July results from different time scales, with the seasonal mean accounting for 30% and the 25-90-day intraseasonal oscillation (ISO) for 50%. The seasonal mean TEIO convection anomaly is related to La Nina, triggered by not only the local positive SSTA but also the negative TCP SSTA through the Walker circulation. The 25-90-day ISO features a northward-propagating convection dipole over the Indian Ocean. An enhanced ISO convection center develops over the TEIO during late July, which moves northward and is gradually replaced by suppressed convection. On the other hand, the Central Asia anticyclonic anomaly is associated with the Silk Road pattern propagating eastward along the westerly jet. During late July, a remarkable anticyclonic node moves across Central Asia, cooperating with the enhanced TEIO convection to facilitate the merging of the WNPSH and the Iran high.