Anomalous Tropical Cyclone Activity in the Western North Pacific in August 2014

2014 was the first year since 1949 that no TC formed in the western North Pacific (WNP) during August, which is climatologically a peak TC month. It is found that intraseasonal oscillation(ISO) played an important role in inhibiting TC genesis in August 2014 by inducing anomalous easterlies, which lead to decreased moisture and vorticity, and increased divergence. The strength of the modulation of TC frequency by the ISO over the WNP during August has been increasing since 1948 under three Pacific Decadal Oscillation (PDO) phases with the strongest during the recent negative PDO phase. However, the anomaly pattern as well as the interannual variations of 850hPa wind anomaly cannot be captured by the CMIP5 models, and therefore, we currently cannot determine whether the significant easterly trend as well as the observed TC activity is part of natural climate variability or climate change induced by anthropogenic forcing. In the future, application of downscaling techniques in atmospheric general circulation models may be needed to diminish the biases in the tropical areas.