West Pacific tropical eastern jet signifi

image: Tropical Storm Higos approaches Zhuhai, Guangdong, China on August 18, 2020. Higos formed from a tropical disturbance north of Luzon, Philippines on August 16 and made landfall in Zhuhai with a maximum wind speed of 35 m s-1 (126 km/h) on August 19, 2020
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Understanding the processes that control the frequency of tropical cyclone (TC) genesis is critical to knowing how active TCs might be during a TC season. The western North Pacific (WNP) is a particularly challenging region to study the ingredients of TC genesis. Indeed, the region involves complex dynamical and thermodynamic interactions between large-scale oceanic and atmospheric circulations at different time scales. Previous studies show that the Tropical East Jet (TEJ), a summer easterly jet stream extending from the western North Pacific to the Indian subcontinent in the upper troposphere, has had a significant impact on summer weather and climate all over India and Africa. However, it remains to be determined whether the TEJ also has a significant impact on summer weather and climate, such as TC activity, on the WNP.

On January 21, a team of researchers published a new study in Advances in atmospheric science presenting a new index that quantifies the intensity of jets in the entry region of the TEJ throughout the tropical Western Pacific (WP_TEJ). With this new index, they revealed that the TEJ has a significant impact on the frequency of TC genesis in the WNP basin.

“The frequency of TC genesis on the WNP exhibits pronounced interannual variability.” said Ruifen Zhan, lead author of the paper and a professor in the Department of Atmospheric and Oceanic Sciences at Fudan University. “For example, the WNP had 34 named TCs in 1994, but only 14 in 2010. The new index introduced in our paper captured these extremes and showed the potential for predicting seasonal activity of TCs on the WNP.”

The researchers concluded that the intensified WP_TEJ results in strong northerly winds perpendicular to the pressure gradient in the entry region, inducing the large-scale divergence (convergence) in the upper (lower) troposphere and thus upward movement towards the north over the main TC genesis region on the WNP. This promotes deep convection and eventually the formation of TC on the WNP.

“We also found that the factors controlling the interannual variability of WP_TEJ intensity are different from those of the TEJ core…” adds co-author Yihui Ding, Academician of the Chinese Academy of Engineering and Professor at the National Climate Center , China.

Professor Ding is also the first scientist to propose that the TEJ has two discernible branches over the tropical Western Pacific. One branch diverges to the north and the other to the south. Professor Ding recommends that future research focus on the processes leading to the year-to-year changes in WP_TEJ relative to the variation in the core of the TEJ.

“How the future activity of CTs might change as a result of global warming is of great concern to our society.” said Yuqing Wang, co-author and professor of atmospheric science at the University of Hawaii at Manoa in the United States. “Many studies have shown that global warming tends to cause the TEJ to weaken. If so, our work would imply a significant decrease in the frequency of genesis of TC compared to WNP in a warmer climate.”

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