Abstract
In the summer of 2020, tropical cyclone (TC) activities experienced a contrastive transition over the western North Pacific (WNP), from silence in July to unusually active in August. Furthermore, the generation location of TCs was further northwestward in August 2020, resulting in more typhoons landing, and three TCs successively moving northward, which is rare in history. Based on diagnoses with the total genesis potential index (GPI) in July and August 2020, it is suggested that the variation of mid-tropospheric relative humidity and upward convective motion is the major factor for the transition of TC genesis in summer 2020, while the changes of SST, low-level vorticity, and vertical wind shear anomalies played a secondary role. The exceptional variation of the Madden–Julian Oscillation (MJO) activity from July to August 2020 contributed to the transition of the environmental conditions over WNP. In July 2020, MJO was restricted in the Indian Ocean, thus generating an anomalous low-level anticyclone over WNP that intensified the WNP subtropical high. While in early August, MJO propagated eastward to enhance convective activities over the South China Sea and the Philippine Sea, favorable to TC genesis. Thus, MJO activity is a potential predictability source for intraseasonal variation of TC genesis anomalies in WNP.
Highlights
As one of the most destructive natural disasters in the world, tropical cyclones (TCs) are frequently generated over the western North Pacific (WNP), with the highest fraction of the global annual mean number of TCs (Chan, 2000; Chan, 2005; Ching et al, 2010)
If we further examine the Madden–Julian Oscillation (MJO)’s modulation effect on WNP TC genesis during the typical TC season (June to November) through the phasedependent MJO–TC relationships (Supplementary Figure S8 in Liu et al, 2021), it is shown that the whole TGR is favorable for TC genesis when MJO is in phases 6–7, while only the western part of TGR is favorable when MJO is in phases 4–5
In the summer of 2020, TC activities experienced a contrastive transition over WNP, from silence in July to unusually active in August
Summary
As one of the most destructive natural disasters in the world, tropical cyclones (TCs) are frequently generated over the western North Pacific (WNP), with the highest fraction of the global annual mean number of TCs (Chan, 2000; Chan, 2005; Ching et al, 2010). In addition to the large-scale atmospheric circulation, previous studies have noted that El Niño and Southern Oscillation (ENSO) and Indian Ocean sea surface temperature (SST) anomalies have a tremendous impact on the intraseasonal–interannual variation of TC frequency (Sobel and Maloney, 2000; Camargo et al, 2007; Chen and Tam, 2010; Wang and Chan, 2002; Du et al, 2011; Zhan et al, 2011; Huangfu et al, 2018) These anomalous modes affect TC activity through their impacts on the thermal and dynamic conditions over WNP. The maximum potential intensity is determined by SST and the vertical structure of temperature and moisture
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