Western Pacific tropical easterly jet significantly modulates tropical cyclone genesis over the western North Pacific

Understanding the processes that control the genesis frequency of tropical cyclones (TCs) is a critical component to knowing how active the TCs could be in a TC season. The western North Pacific (WNP) is a particularly difficult region to study TC genesis ingredients. This is because the region involves complex dynamic and thermodynamic interactions among the large-scale ocean and atmospheric circulations on various time scales. Previous studies show that the Tropical easterly jet (TEJ), an easterly jet stream in summer extending from the western North Pacific to the African continent in the upper troposphere, has had a significant impact on summer weather and climate throughout India and Africa. However, whether the TEJ also has a significant impact on summer weather and climate, such as the TC activity, over the WNP is yet to be explored.

On Jan. 21, a team of researchers published a new study in Advances in Atmospheric Sciences showcasing a new index that quantifies jet intensity in the entrance 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 TC genesis frequency in the WNP basin.

“TC genesis frequency over the WNP exhibits pronounced interannual variability.” said Ruifen Zhan, the paper's lead author and Professor with the Department of Atmospheric and Ocean Sciences at Fudan University. “For example, the WNP witnessed 34 named TCs in 1994 but only 14 in 2010. The new index introduced in our paper captured those extremes and showed the potential for predicting TC seasonal activity over the WNP."

Researchers concluded that the intensified WP_TEJ results in strong northerly winds perpendicular to the pressure gradient within the entrance region, inducing the large-scale divergence (convergence) in the upper (lower) troposphere and thus upward motion to the north over the main TC genesis region over the WNP. This favors deep convection, and eventually TCs formation over the WNP.

"We also found that factors controlling the interannual variability of the WP_TEJ intensity is different from that of the TEJ core…” adds co-author Yihui Ding, an academician of The Chinese Academy of Engineering and Professor of the National Climate Center, China.

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

"How future TC activity may change under global warming is a great concern to our society." said Yuqing Wang, co-author and Professor of Atmospheric Sciences at the University of Hawaii at Manoa in the United States. "Many studies have shown that the global warming tends to cause the weakening of the TEJ. If this is the case, our work would implicate a significant decrease in TC genesis frequency over the WNP in a warmer climate."