Formation Mechanism of the ENSO-Independent Summer Western North Pacific Anomalous Anticyclone

The western North Pacific anomalous anticyclone (WNPAC) is the key circulation modulating the East Asian summer climate. In this study, the formation mechanism of the summer WNPAC that is independent of El Nin similar to o- Southern Oscillation (ENSO) is investigated. Although ENSO has a significant relationship with WNPAC, except for the super El Nin similar to o years, the WNPAC index remains almost unchanged after removing ENSO's impact, suggesting the possibil-ity of other origins of the WNPAC apart from ENSO. An Atlantic-to-Pacific two-step mechanism is proposed for the formation of ENSO-independent summer WNPAC. In boreal spring, diabatic heating induced by the positive sea surface temperature anomalies (SSTAs) over the tropical Atlantic could stimulate a stationary equivalent barotropic Rossby wave train that travels across the Eurasian continent and ends in the tropical North Pacific. At the end of the Rossby wave train, the lower-level anomalous anticyclone advects negative moist enthalpy into the equator, which suppresses the local convection over the tropical North Pacific and equatorial central Pacific, and thus triggers the lower-level equatorial east-erly anomaly to its west. During boreal summer, the lower-level easterly anomaly leads to the zonal dipole SSTA pattern with a negative center in the tropical central Pacific and a positive one in the Maritime Continent. Then, this dipole SSTA pattern over the Pacific exerts a relaying effect that further reinforces and westward shifts the dipole convection anomaly pattern, generating the WNPAC as a Gill-type response. This study underpins the independent role of Atlantic oceanic forcing through the extratropical route in the formation of ENSO-independent summer WNPAC.

Automated summary

This study investigates the formation mechanism of the summer Western North Pacific Anomalous Anticyclone (WNPAC) that is independent of El Niño-Southern Oscillation (ENSO). It is found that besides the significant relationship with ENSO, the WNPAC index remains almost unchanged after removing the impact of ENSO, suggesting the possibility of other origins of the WNPAC. A two-step mechanism from the Atlantic to the Pacific is proposed for the formation of ENSO-independent summer WNPAC.