Modulation of western North Pacific tropical cyclone formation by central Pacific El Nino-Southern Oscillation on decadal and interannual timescales

This study investigates the impact of central Pacific (CP) El Nino-Southern Oscillation (ENSO) on tropical cyclone (TC) frequency over the western North Pacific (WNP) on decadal and interannual timescales. The CP ENSO-TC frequency relationship is strong and significant only on decadal timescales, with enhanced TC formation over most of the WNP associated with warm CP ENSO phases. TC formation changes, driven by CP ENSO, exhibit a southeast-northwest dipole pattern on interannual timescales, similar to the typical pattern induced by eastern Pacific ENSO. Associated with warm CP ENSO phases on decadal timescales are increases in TC formation east of 125 degrees E, while increases in TC formation east of 140 degrees E are observed with warm CP ENSO phases on interannual timescales. These increases in TC activity are primarily caused by favourable dynamic conditions, including increased 850-hPa relative vorticity and 200-hPa divergence and decreased 850-200-hPa vertical wind shear. The main TC formation difference on decadal and interannual timescales is concentrated over the longitudinal band of 125 degrees-140 degrees E, where reduced TC formation is observed on interannual timescales during CP El Nino years, predominately due to suppressed thermodynamic factors, for example, reduced maximum potential intensity and 700-500-hPa relative humidity. We find insignificant (significant) changes in thermodynamic conditions between 125 degrees E and 140 degrees E due to weak (strong) descending motion in the western cell of the anomalous Walker circulation on decadal (interannual) timescales. This can be explained by different patterns of CP ENSO-induced sea surface temperature anomalies on different timescales.