SUBTROPICAL MONSOON MOIST DYNAMICS AND ITS FUTURE CHANGE
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2022
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Different from classic mid-latitude dry baroclinic instability theory, atmospheric motions over the subtropical Meiyu front in boreal summer are dominated by synoptic-scale disturbances coupled with precipitation and moisture under a weaker background vertical shear. This moisture-precipitation-circulation interactive feature, along with a preferred zonal wavelength of about 3400 km and eastward phase propagation is explained by a moist baroclinic instability theoretical framework. The new framework is an extension of a traditional 2-level baroclinic model by considering a prognostic moisture equation, the moisture-precipitation-circulation feedback and an interactive planetary boundary layer. An eigenvalue analysis of the model shows that the most unstable mode has a preferred zonal wavelength of 3400 km, a westward tilted vertical structure, and a phase leading of maximum moisture and precipitation anomalies relative to a lower-tropospheric trough, all of which are in good agreement with observations. Both anomalous horizontal and vertical advection processes contribute to the moisture increase. Further sensitivity tests show that the instability and the zonal scale selection primarily arise from the moisture-convection-circulation feedback, while the vertical shear provides an additional energy source for the perturbation growth. This moist baroclinic instability theory explains well the observed characteristics of the development of synoptic-scale disturbances along the Meiyu front.
Dominant summertime disturbances along the subtropical Meiyu front are eastward-propagating synoptic-scale waves coupled with precipitation and moisture under a moderate background vertical shear. To what extent the intensity and structure of such synoptic disturbances would change under global warming is investigated by diagnosing 18 models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). The model diagnosis reveals that there is a robust increase in the intensity of synoptic-scale motions along the Meiyu front, while the wavelength and phase speed remain unchanged. The cause of such changes of the synoptic-scale variability in the future warmer climate is investigated through the analysis of a moist baroclinic instability model. It is found that the growth rate of the most unstable mode strengthens in the future warmer climate, while the zonal wavenumber and phase speed of the most unstable mode remain unchanged, which is consistent with the CMIP6 projections. The enhanced synoptic-scale variability is primarily attributed to the increase of background meridional and vertical moisture gradients under a warmer climate through strengthened positive moisture-convection-circulation feedback, while the changes of background vertical shear and convective adjustment times are insignificant.
The relative roles of the land and ocean surface warming in causing strengthened East Asian summer monsoon (EASM) circulation but weakened South Asian summer monsoon (SASM) circulation under global warming are investigated with an intermediate atmospheric model. By extending a 2.5-layer intermediate atmospheric model to cover both tropical ocean and land regions, we examine how the EASM and SASM circulations respond to idealized land and ocean surface warming patterns. The model is able to reproduce the gross features of the summer monsoon circulation in the present-day climate state, with pronounced southerlies over the EASM region and westerlies over the SASM region. Forced by the land and ocean surface warming patterns derived from Phase 5 of the Coupled Model Intercomparison Project (CMIP5) and CMIP6 model ensembles, the intermediate model simulates successfully a strengthened EASM circulation and a weakened SASM circulation, consistent with CMIP5 and CMIP6 projections. Idealized model experiments are further designed to reveal the relative roles of a greater land than ocean warming and an El Niño-like warming pattern in causing the distinctive circulation changes in South and East Asia. It is found that the strengthened EASM circulation is mainly attributed to the land-ocean warming contrast, whereas the weakened SASM circulation is primarily caused by an El Niño-like warming pattern in the tropical Pacific and an Indian Ocean Dipole-like warming pattern in the Indian Ocean.
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Atmospheric sciences
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101 pages
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