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Role of the South Asian High in the Onset Process of the Asian Summer Monsoon during Spring-to-Summer Transition

1,2,3,*, 1,3, 1,3 and 4
1
School of Atmospheric Sciences, and Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Zhuhai 519082, China
2
Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing 210044, China
3
Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519000, China
4
Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong 00852, China;
*
Author to whom correspondence should be addressed.
Atmosphere 2019, 10(5), 239; https://doi.org/10.3390/atmos10050239
Received: 16 April 2019 / Revised: 28 April 2019 / Accepted: 30 April 2019 / Published: 1 May 2019
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Abstract

The evolution of the South Asian high (SAH) and its role in the onset process of the Asian summer monsoon (ASM) during the spring-to-summer transition are investigated by using the NCEP-DOE reanalysis II dataset, with a focus on climatology and interannual time scales. Our results show four sudden changes of the SAH in its Northwestward evolution from the Western Pacific to the South China Sea (SCS), the Indochina Peninsula and the South Asian plateaus, coincide with the ASM onset over the Bay of Bengal, the SCS, and the Indian summer monsoon region. The physical process for the mutual promotion between the SAH and ASM rainfall is revealed. Accompanying the SAH evolution, the upper-level Easterly wind along the Southern flank and the upper-level divergence associated with the SAH shift Northwestward accordingly. The upper-level Easterly wind coordinates with the lower-level Southwesterly wind, and forms the summer circulation structure in the ASM regions gradually. Besides, the upper-level divergence associated with the SAH enhances ascending motion in ASM regions and increases the monsoon rainfall accordingly. Subsequently, the latent heat associated with the monsoon rainfall in the monsoon onset region excites an anticyclone to its Northwest in the upper level, which keeps strengthening the SAH and moving it Northwestward. This mutual promotion between the SAH and ASM rainfall can be affected by the sea surface temperatures (SSTs) in the Western Pacific and tropical Indian Ocean in the previous month. Colder (warmer) SSTs over the Western Pacific and inactive (active) convection over the Southern Philippines suppress (favor) the Northwestward development of the SAH in late April. In addition, the warmer (colder) SSTs in the tropical Indian Ocean excites anomalous anticyclone (cyclone) in the upper level near the equator, which keeps the SAH in the lower latitudes (promotes the SAH to the North), and delays (advances) the mutual promotion between the SAH and ASM rainfall. As a result, the entire ASM onset process is later (earlier) than normal. View Full-Text
Keywords: South Asian high; spring-to-summer transition; Asian summer monsoon; monsoon onset South Asian high; spring-to-summer transition; Asian summer monsoon; monsoon onset
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Wei, W.; Wu, Y.; Yang, S.; Zhou, W. Role of the South Asian High in the Onset Process of the Asian Summer Monsoon during Spring-to-Summer Transition. Atmosphere 2019, 10, 239.

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