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Maintenance of the Basin-Dependent Quasi-Biweekly Mode in the Indian Ocean during Summer

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  • 1 aFirst Institute of Oceanography, and Key Laboratory of Marine Science and Numerical Modeling, Ministry of Natural Resources, Qingdao, China
  • | 2 bLaboratory for Regional Oceanography and Numerical Modeling, Pilot National Laboratory for Marine Science and Technology, Qingdao, China
  • | 3 cShandong Key Laboratory of Marine Science and Numerical Modeling, Qingdao, China
  • | 4 dSchool of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai Campus, Zhuhai, China
  • | 5 eSouthern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China
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Abstract

The atmospheric 10–20-day quasi-biweekly mode (QBWM) significantly modulates the active–break spells of the South Asian monsoon. Current knowledge, however, is limited concerning the diversity of the QBWM in the Indian Ocean (IO). Based on extended empirical orthogonal function analysis, two dominant summer modes are constructed in the IO. The first mode (QBWM1) generally depicts IO basin-dependent variability, while the second mode (QBWM2) exhibits a close relationship with the northwestern Pacific. QBWM1 initiates in the equatorial western IO and propagates toward the eastern IO along the equator. Two Rossby wave cells evolve in the off-equatorial eastern IO when convection encounters the Maritime Continent, and subsequently the northern cell develops and moves westward in the South Asian monsoon region. In contrast, QBWM2 originates in the northwestern Pacific and passes westward across the South Asian monsoon region in the form of convectively coupled Rossby waves. The maintenance mechanism of the peculiar IO basin-dependent QBWM1 is understood in terms of moisture dynamics. Significant moisture anomalies are found to precondition convection initiation in the western IO and subsequent eastward movement along the equator. Afterward, two off-equatorial moisture centers are generated in the double Rossby wave cells along with convection dissipation in the eastern IO, and the moisture anomalies are delivered from the southern cell toward the convection initiation area in the western IO via a moisture conveyor belt without coupling with convection. Moisture budget analysis indicates that the horizontal moisture advection associated with QBWM1 is regulated by the mean clockwise circulation in the tropical IO.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Zhi Li, lizhi@fio.org.cn

Abstract

The atmospheric 10–20-day quasi-biweekly mode (QBWM) significantly modulates the active–break spells of the South Asian monsoon. Current knowledge, however, is limited concerning the diversity of the QBWM in the Indian Ocean (IO). Based on extended empirical orthogonal function analysis, two dominant summer modes are constructed in the IO. The first mode (QBWM1) generally depicts IO basin-dependent variability, while the second mode (QBWM2) exhibits a close relationship with the northwestern Pacific. QBWM1 initiates in the equatorial western IO and propagates toward the eastern IO along the equator. Two Rossby wave cells evolve in the off-equatorial eastern IO when convection encounters the Maritime Continent, and subsequently the northern cell develops and moves westward in the South Asian monsoon region. In contrast, QBWM2 originates in the northwestern Pacific and passes westward across the South Asian monsoon region in the form of convectively coupled Rossby waves. The maintenance mechanism of the peculiar IO basin-dependent QBWM1 is understood in terms of moisture dynamics. Significant moisture anomalies are found to precondition convection initiation in the western IO and subsequent eastward movement along the equator. Afterward, two off-equatorial moisture centers are generated in the double Rossby wave cells along with convection dissipation in the eastern IO, and the moisture anomalies are delivered from the southern cell toward the convection initiation area in the western IO via a moisture conveyor belt without coupling with convection. Moisture budget analysis indicates that the horizontal moisture advection associated with QBWM1 is regulated by the mean clockwise circulation in the tropical IO.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Zhi Li, lizhi@fio.org.cn
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