A Common Base Mode of Asian Summer Monsoon Variability across Timescales
AbstractThe summer intraseasonal oscillation (ISO) is characterized by a northward-moving rain band in the Indo-western Pacific warm pool region. Physical origin of the ISO is not fully understood, masked by strong interaction of convection and circulation. This study examines intraseasonal to interannual variability during June to August over the Indo-western Pacific warm pool region. The results show that the tropical Northwest Pacific anomalous anticyclone (NWP-AAC) is a fundamental mode on both intraseasonal and interannual timescales, destabilized by the monsoon mean state, specifically through barotropic energy conversion and convective feedback in the low-level confluence between the monsoon westerlies and easterly trade winds. On the interannual timescale, the NWP-AAC shows a biennial tendency, reversing phase from the summer of El Niño to the summer that follows; the AAC in post-El Niño summer is excited indirectly through sea surface temperature anomalies in the Indo-NWP. On the intraseasonal timescale, the column-integrated moisture advection causes the NWP-AAC-related convection propagates northward. Our results provide a unifying view of multi-scale Asian summer monsoon variability, with important implications for subseasonal to seasonal prediction.