A229-0015
Seasonal Progression of the East Asian Summer Monsoon: Air-Mass Analysis and Tropical-Extratropical Drivers in ERA5

Wednesday, 16 December 2020
Poster
Ambrogio Volonté, National Centre for Atmospheric Science, University of Reading, Department of Meteorology, Reading, United Kingdom, Andrew G Turner, National Centre for Atmospheric Science, Department of Meteorology, University of Reading, Reading, United Kingdom, Reinhard Schiemann, National Center for Atmospheric Science, Reading, United Kingdom, Nicholas P Klingaman, Univ of Reading, Reading, United Kingdom and Pier Luigi Vidale, University of Reading, Department of Meteorology and National Centre for Atmospheric Science (NCAS), Reading, United Kingdom
Abstract:
China receives most of its rainfall during the East Asian summer monsoon (EASM). The EASM is a complex phenomenon, influenced by both tropical and mid-latitude large-scale dynamics and by the presence of major orography, such as the Tibetan Plateau. The inherently multiscale nature of the EASM is exemplified by the characteristics of its front, a tight nearly zonal band with a steep gradient in equivalent potential temperature, neatly separating tropical and extratropical air masses, particularly as the monsoon marches northwards.

A comprehensive study of the seasonal evolution of the EASM and of its front is presented here. This study consists in the use of Eulerian and Lagrangian methods on the ERA5 reanalysis dataset, with particular focus on tropical and extratropical drivers of front progression and on the dynamics of the air masses involved.

A new algorithm for the detection of the EASM front, adapted from literature, has been applied to the reanalysis data. The results from this algorithm, together with the analysis of rainfall and thermodynamics variables, allow us to clearly identify and depict the four main stages of evolution of the EASM, in agreement with recent studies.

These results emphasise the role of the subtropical westerly jet, not only over the Tibetan Plateau but also further east, as a control for the strength and position of the EASM front during its poleward progression (mei-yu stage). This upper-level mid-latitude forcing acts in conjunction with the low-level advection of moisture from the tropics, modulated by the seasonal cycle of the South Asian Monsoon and by several factors at shorter time scales (e.g. tropical cyclones).

Lagrangian trajectories are used to isolate air masses playing a role in EASM front evolution, detailing their paths and revealing the processes (e.g. moist diabatic) shaping their dynamics. Preliminary results confirm the interaction between moist tropical and dry continental air masses at the EASM front, particularly during the mei-yu stage, highlighting the importance of remote moisture sources for the advection of moist tropical air.

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