Boreal Summer Negative Correlation Relationship Between Interannual SST and Precipitation Anomalies in the Tropical and Subtropical Western North Pacific

Western North Pacific (WNP) is a key region for ENSO development and its effect on East Asian summer monsoon. Different from most other tropical oceanic areas, sea surface temperature (SST) and precipitation anomalies over the WNP are negatively correlated in boreal summer, indicating the ocean is mainly driven by the atmosphere rather than the opposite over there. Two interannual variability modes of such negative correlation relationship are revealed with SVD method, called tropical and subtropical WNP air-sea coupled modes (WNP-TCM and WNP-STCM) in terms of their locations accordingly. Their core regions are both consistent with the near-zero line of low-level zonal wind, corresponding to the Northwest Pacific monsoon trough and the west ridge of WNP subtropical high, respectively. In WNP-TCM, precipitation anomaly is caused by local vorticity anomaly, which is the result of Matsuno-Gill response to SST anomalies with the reversed signs in central equatorial Pacific and eastern equatorial Indian Ocean. In WNP-STCM, it is produced by local divergence anomaly, which is remotely forced by SST anomalies with the same signs in equatorial Pacific and Indian Ocean through anomalous Hadley circulation. SST anomalies in the two modes are both controlled by ocean thermal process, especially the solar radiation anomaly. The key factors determining such unique WNP air-sea interaction are the weak latent heat flux and horizontal SST advection anomalies, which are both closely related to the near-zero low-level zonal wind. In addition, WNP-TCM often happens in ENSO developing summer whereas WNP-STCM usually appears in ENSO decaying summer.