Revisiting the Extreme 1959 Western North Pacific Tropical Cyclone Season: The Effects of Environmental Conditions and Sea Surface Temperatures

The mechanisms responsible for the extreme 1959 tropical cyclone (TC) season over the western North Pacific (WNP) are investigated. Observational analysis shows that the enhanced basin-mean lifetime maximum intensity (LMI) is jointly contributed by the shift of TC genesis toward a high-LMI region and the more frequent rapid intensification (RI). These TC features are linked to large-scale circulation anomalies, including an enhanced monsoon trough, a weakened subtropical high, and an eastward-shifted tropical upper-tropospheric trough, which together create favorable environmental conditions. Genesis potential indices verify that increased vorticity, moisture, upward motion, and reduced vertical wind shear over the open basin favor TC formation in the high LMI region. Composite analysis of TC nearby environments reveals that the enhanced southwesterly flow and moisture to the south of TCs, which are associated with the monsoon trough, might play a role in increasing RI frequency. To assess the role of the sea surface temperature (SST) pattern, we conducted ensemble simulations with the 28-km Nonhydrostatic Icosahedral Atmospheric Model (NICAM). Sensitivity experiments reproduce the intensified TCs, shifted genesis locations, and anomalous circulations, with the proportion of extreme TCs doubling compared to the climatological run. Similar responses in the realistic 1959 and tropical North Pacific warming experiments suggest that tropical North Pacific SST anomalies contributed to circulation changes broadly consistent with a Matsuno–Gill-type atmospheric response, thereby modulating TC intensity. These results highlight the importance of a basin-wide SST pattern in shaping extreme TC seasons.