Compounding factors causing the unusual absence of tropical cyclones in the western North Pacific during August 2014

The western North Pacific (WNP) is a region where tropical cyclones (TCs) occur most frequently. However, no TCs occurred in August, climatologically the month of most frequent TC genesis, in 2014. Such absence of TC activity in August was approximately 3 sigma below the climatological mean (5.1) and occurred for the first time during 1945-2014. This study investigates the large-scale factors responsible for the absence. While July-August 2014 was a period of weak El Nino condition, rainfall in the eastern North Pacific was unusually high, exceeding the 95th percentile for the period 1979-2014. We demonstrate that the unusually positive sea surface temperature anomaly in the eastern North Pacific (ENP) resulted in such high rainfall amount. Diabatic heating associated with the unusual rainfall resulted in a Walker circulation anomaly in the Pacific, which in turn led to anomalous downward motion in the West Pacific and suppressed TC genesis in the WNP. Moreover, an eastward propagating intraseasonal oscillation (ISO) was identified in the Indo-Pacific in August. The dry-phase ISO induced subsidence anomaly in the WNP. The combination of the dry ISO phase in the WNP and a west-east overturning circulation anomaly triggered by the warm sea surface temperature in the ENP led to extremely dry and warm conditions and the absence of TCs in the WNP. Both empirical diagnostics and numerical simulations confirm this result.