26 Feb 2021

26 Feb 2021

Review status: this preprint is currently under review for the journal WCD.

The interaction of tropical and extratropical air masses controlling East Asian summer monsoon progression

Ambrogio Volonté1,2, Andrew G. Turner1,2, Reinhard Schiemann1,2, Pier Luigi Vidale1,2, and Nicholas P. Klingaman1,2 Ambrogio Volonté et al.
  • 1Department of Meteorology, University of Reading, Reading, RG6 6ET, UK
  • 2National Centre for Atmospheric Science, University of Reading, Reading, RG6 6ET, UK

Abstract. The East Asian summer monsoon (EASM) is a complex phenomenon, influenced by both tropical and mid-latitude dynamics and by the presence of the Tibetan Plateau. The EASM front neatly separates tropical and extratropical air masses as the monsoon marches northwards. Although the different factors behind EASM progression are illustrated in a number of studies, their interactions, in particular between tropical and extratropical air masses, still need to be clarified. In this study we apply Eulerian and Lagrangian methods to the ERA5 reanalysis dataset to provide a comprehensive study of the seasonal evolution and variability of the EASM, and we highlight the dynamics of the air masses converging at its front.

A frontal detection algorithm is used to perform a front-centred analysis of EASM evolution. The analysis highlights the primary role of the sub-tropical westerly jet (STWJ) in controlling the strength and the poleward progression of the EASM front, in particular during Mei Yu, one of the stages of EASM progression. The upper-level mid-latitude forcing acts in conjunction with the southerly advection of low-level moist tropical air, modulated by the seasonal cycle of the South Asian monsoon and by the location of the Western North Pacific subtropical high. The Mei Yu stage is distinguished by an especially clear interaction between tropical and mid-latitude air masses converging at the EASM front. The analysis of composites based on the latitude of the EASM front during Mei Yu reveals the influence of the STWJ on the strength of the mid-latitude flow impacting on the northern side of the EASM front. In turn, this affects the extent of the warm moist advection on its southern side and the distribution and intensity of resultant rainfall over China.

This study shows the validity of an analysis of EASM evolution focused on its front and on the related low-level airstreams, at least in the Mei Yu stage. The framework highlighted shows how the upper-level flow drives the low-level airstreams that converge at the EASM front, thus controlling the shape of EASM progression. This framework provides a basis for studies of climate variability and extreme events and for model evaluation.

Ambrogio Volonté et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2021-12', Anonymous Referee #1, 31 Mar 2021
  • RC2: 'Comment on wcd-2021-12', Anonymous Referee #2, 02 Apr 2021
  • CC1: 'Comment on wcd-2021-12', David Schultz, 06 Apr 2021

Ambrogio Volonté et al.

Ambrogio Volonté et al.


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Short summary
In this study we analyse the complex seasonal evolution of the East Asian summer monsoon. Using reanalysis data, we show the importance of the interaction between tropical and extratropical air masses converging at the monsoon front, particularly during its northward progression. The upper-level flow pattern (e.g. the westerly jet) controls the balance between the airstreams, and thus the associated rainfall. This framework provides a basis for studies of extreme events and climate variability.