Journal of the Meteorological Society of Japan. Ser. II
Online ISSN : 2186-9057
Print ISSN : 0026-1165
ISSN-L : 0026-1165
Article
Sensitivity of Western North Pacific Summertime Tropical Synoptic-Scale Disturbances to Extratropical Forcing – A Regional Climate Model Study
Ying Lung LIUChi-Yung TAM Andie Yee Man AU-YEUNG
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2022 Volume 100 Issue 1 Pages 167-180

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Abstract

The role of extratropical forcing on the summertime tropical synoptic-scale disturbances (TSDs) in the western North Pacific has been investigated by conducting parallel integrations of the Regional Climate Model. The suite of experiments consists of a control run (CTRL) with European Centre for Medium-Range Forecasts Reanalysis data as boundary conditions, and an experimental run (EXPT) with the same setting, except that signals with zonal wavenumber > 6 were suppressed at the northern boundary (located at 42°N) of the model domain. Comparison between CTRL and EXPT showed that without extratropical forcing, there is weaker TSD activity in the June-to-August season, with reduced precipitation over the TSD pathway. Associated with suppressed TSD, southeastward-directed wave activity is also reduced, leading to less active mixed Rossby gravity waves in the equatorial western Pacific area. Further analysis revealed that extratropical forcing and associated circulation changes can modulate the TSD wavetrain and its coherence structure, in relation to low-level vorticity in the far western North Pacific. In CTRL, west of about 140°E, TSD-related circulation tends to be stronger; in EXPT, vorticity signals and vertical motions are found to be slightly more coherent in the more eastern portion of the TSD wavetrain. The latter enhanced coherency of TSD east of 140°E, from the EXPT simulations, might be due to changes in wave activity transport channeled by modulated upper-level mid-latitude westerlies in EXPT. Our results serve to quantify how extratropical forcing and related general circulation features influence western North Pacific summertime TSD activities. Implications on the understanding of the initiation of TSD are discussed.

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© The Author(s) 2022. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution 4.0 International (CC BY 4.0) license.
https://creativecommons.org/licenses/by/4.0/
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