Role of the intraseasonal IPCO in the absence of typhoons in July 2020

Abstract

The influence of the intraseasonal Indo-western Pacific convection oscillation (IPCO) on the absence of typhoons in July 2020 over the western North Pacific (WNP) was explored. While observation analysis shows that necessary conditions such as sea surface temperature (SST) and vertical wind shear in July 2020 meet the basic requirement of or even are conducive to the formation of typhoon, the unprecedented absence of typhoon over the WNP occurred in July 2020, and it is the first time that no typhoon in July since 1951. Additionally, significant differences were found in the number of typhoons in July between the different phases of the intraseasonal IPCO, and the number in the positive phase of the intraseasonal IPCO was significantly higher than that in the negative phase of the intraseasonal IPCO. In July 2020, the intraseasonal IPCO was in a strong negative phase, with the third lowest index in history and had the strongest inhibition effect on convection over the WNP on record, leading to large-scale circulation anomalies. The strongest descending movement on record inhibited the upward transport of water vapor and the development of cumulus convection, thereby reducing the release of latent heat of condensation and making it difficult to form a typhoon warm-core structure. In addition, the geopotential height increased over the WNP, and the western Pacific subtropical high moved southerly, which inhibited typhoon formation. Simultaneously, the South China Sea monsoon trough weakened significantly, with increased negative vorticity anomaly in the response scale, which hindered disturbance generation. The lowest genesis potential index confirmed that the large-scale circulation anomaly caused by the intraseasonal IPCO had an unprecedented restraining effect on typhoon generation, leading to the absence of typhoons over the WNP in July 2020.