Rapid Weakening of Tropical Cyclones in Monsoon Gyres Over the Western North Pacific: A Revisit

Kexin Song,Li Tao,Jianyun Gao

doi:10.3389/feart.2021.688613

Kexin Song, Li Tao + Show 1 more

Open Access

https://doi.org/10.3389/feart.2021.688613

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Abstract

The low-level monsoon trough over the western North Pacific (WNP) can evolve into a large cyclonic circulation, which is often termed a monsoon gyre (MG). Previous studies have revealed that tropical cyclones (TCs) embedded in MGs can experience rapid weakening (RW) and such RW might be attributed to the convective activity in the southeastern quadrant of the MG, which could induce asymmetries in a TC’s inner core structure, while the environmental factors, including the sea surface temperature (SST) and vertical wind shear (VWS), were not primary contributors to RW events. In this study, the possible role of large-scale environmental factors in association with the RW of TCs in MGs over the WNP is revisited based on the best-track TC and global reanalysis data during 2000–2018. Results indicate that TCs tend to weaken rapidly when they are embedded in the eastern semicircle of a MG, with the extreme RW events often occurring in the southeastern quadrant of a MG. However, different from previous studies, results from this study demonstrated that lower SST and strong large-scale VWS in the eastern semicircle of a MG are two major environmental factors contributing to the RW of TCs in MGs over the WNP. The different findings in this study from those in previous studies could be partly due to the different methods used to obtain the MG circulations and partly due to the environmental factors being analyzed in different quadrants of MG in this study.

Highlights

The low-level monsoon trough over the western North Pacific (WNP) and South China Sea, characterized by the cyclonic shear and confluence zone between the southwesterly on the equatorial side and the northeasterly on the poleward side, is the most common climate feature of the summer monsoon circulation over the WNP (Holland, 1995)
This study has revisited the effect of monsoon gyre (MG) on rapid weakening (RW) of tropical cyclones (TCs) during May–October over the WNP based on best-track TC data and global reanalysis during 2000–2018
Results from statistical analyses indicate that TCs are more likely to weaken rapidly during the MG period than during the non-MG period with most of the RW events occurring in the eastern semicircle of MGs and with the extreme RW events often occurring in the southeastern quadrant of a MG

Summary

INTRODUCTION

The low-level monsoon trough over the western North Pacific (WNP) and South China Sea, characterized by the cyclonic shear and confluence zone between the southwesterly on the equatorial side and the northeasterly on the poleward side, is the most common climate feature of the summer monsoon circulation over the WNP (Holland, 1995). Based on statistical analysis of TC RW events during 2000–2014, Liang et al (2018a) found that more than 40% of TC RW events over the WNP were associated with MGs, and the RW might be related to TC structural changes induced by the interaction between the TC and the MG They indicated that the large-scale environmental factors, including changes of sea surface temperature (SST), VWS, and mid-level environmental humidity, were not primary contributors to the RW of TCs, though the VWS increased slightly when the RW occurred during the MG period. The shallow VWS shows the similar distribution with positive anomalies in the southeastern and northeastern quadrants of the composite MG (Figure 10D) This suggests that VWS may play important roles in causing TCs to weaken rapidly in the eastern semicircle of a MG. The region with strong convective activities is farther upstream of the regions with RW of TCs in the MG, suggesting that strong convection and high relative humidity in the southeastern quadrant of the MG might contribute marginally to RW of TCs in a MG

CONCLUSION AND DISCUSSION

Findings

DATA AVAILABILITY STATEMENT