Influences of boreal summer intraseasonal oscillation on tropical cyclone genesis and intensification over the Western North Pacific at the interannual timescale

The climatological connection between tropical cyclones (TCs) and boreal summer intraseasonal oscillation (BSISO) over the western North Pacific (WNP) is known, but here we additionally find that the influences of May-October BSISO on TC genesis and intensification at the interannual timescale reveal distinctive regional features. The strengthened BSISO variability over the subtropical WNP (7.5 degrees-27.5 degrees N, 145 degrees E-165 degrees W) contributes positively to the basin-total TC genesis count, while the BSISO amplitude over the central-western equatorial Pacific (5 degrees S-5 degrees N, 140 degrees E-160 degrees W) is significantly correlated with the average intensity that TCs can reach. The physical processes causing the differences in TC behavior associated with interannual changes in BSISO are quantified from the viewpoint of eddy energetics. During summers with enhanced BSISO over the subtropical WNP, more TCs form there. Composite diagnostic analysis results for TC genesis days show that local synoptic eddies (or TC embryos) gain more kinetic energy to grow into TCs from both the seasonal-mean and BSISO anomalies via barotropic energy conversion. Eddy self-augmentation processes related to baroclinic conversion and eddy geopotential flux also aid the generation of TCs. During summers with enhanced central-western equatorial BSISO, TCs stay longer over the open ocean before they reach maximum intensity. Further diagnosis along the TC tracks shows that these eddies may grow faster (with a larger kinetic energy tendency) and reach a higher intensity with the help of enhanced barotropic and baroclinic energy conversion processes.