The north movement of Intertropical Convergence Zone on synoptic-scale causes heavy rainfall in the northern South China Sea in autumn

Near the equator, the Intertropical Convergence Zone (ITCZ) is a boundary where the northern and southern trade winds converge. On a synoptic scale, the ITCZ tends to shift north during autumn in La Niña years. According to new research by Prof. Lifang Sheng and a team of scientists at the College of Oceanic and Atmospheric Sciences, Ocean University of China, this pattern alteration causes heavy autumn rainfall in the northern South China Sea (SCS) region.

Alongside Prof. Sheng and her group, additional researchers from the Key Laboratory of South China Sea Meteorological Disaster Prevention and Mitigation of Hainan Province collaborated to find the mechanisms behind this rainfall anomaly, using the La Niña event of autumn 2010 as a case study. They just published their work in Advances in Atmospheric Sciences.

“The ITCZ is the main tropical system causing rainfall over the SCS.” said Dr. Sheng. “However, the ITCZ is typically held south of 10°N by prevailing northeasterly and weakening southwesterly winds in autumn.”

Despite the normal predominant autumnal winds, in a La Niña season, the ITCZ may still move north into a favorable position that triggers heavy rainfall in the northern SCS. This process can last between 2–14 days. This is possible due to the air-sea drag effect from nearby typhoons.

“However, the ITCZ can still move north on a synoptic scale without the help of typhoons, resulting in heavy rainfall across the northern SCS. In this case, typhoon drag mechanisms remained notably absent.” said Prof. Sheng.

The study further revealed that the ITCZ and continent high will establish the easterly jet over the northern SCS in autumn. And the decrease of jet area can make room for the northward movement of the ITCZ.

“We can attribute strengthening of the equatorial easterlies to the enhanced high pressure on the east side of Australia, as well as the Walker circulation.” explained Dr. Sheng. “Therefore, the ITCZ is more likely to move north due to the abnormal equatorial easterlies in La Niña years.”

For future extensions of this study, Prof. Sheng suggests that scientists explore the impact of thermal and dynamic fields on the northward movement of the ITCZ while still considering the abnormal equatorial easterlies.

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