The Interannual And Interdecadal Characteristics And Mechaganisms Of Tibetan Plateau Vortex And The Future Projections

The Tibetan Plateau(TP)is surrounded by the Kunlun Mountains in the north,the Himalayas in the south,the Pamir in the west,and the Hengduan Mountains in the east.The average elevation of the TP is above 4000 m,covering an area of 2.9-million km².It is the largest and highest plateau in the world,known as the"Third Pole".It is the source of many major rivers in Asia,providing water for millions of people,and therefore is called the"Asian Water Tower".Since the major water supply for the"Asian Water Tower"is precipitation on the TP,it is important to study the spatial pattern and temporal variability of precipitation and related weather systems.The Tibetan Plateau Vortex(TPV)is a mesoscale weather system active in the near-surface layer,which is one of the major systems for the generation of precipitation in the Tibetan Plateau.However,no long-term observations of the TPVs are available due to the scarcity of observations in the TP.Thereby,long-term,high-accuracy reanalysis products are a reliable source for the analysis of characteristic TPV activities and possible mechanisms behind.The major results of this dissertation are as follows:(1)In the present study,an objective analysis method is implemented to obtain a complete dataset of TPV based on several reanalysis products,including the ERA-Interim,ERA40,JRA55,NCEP CFSR,and NASA MERRA2.The TPVs are detected and tracked by the minima in geopotential height at 500h Pa.Results indicate that the TPVs derived from multiple reanalysis products have quite similar spatial patterns and temporal variability.The characteristic parameters of the TPVs derived from a given reanalysis data are related to the resolution of this reanalysis product.Higher-resolution reanalysis products can yield more low-pressure systems,which explains why the TPVs derived from high-resolution reanalysis datasets generally have longer lifetime,stronger and larger vortex scale than those from low-resolution reanalysis products,although the total number of TPVs are similar.The annual average number of TPVs generated in the TP is 63.5,and these TPVs largely originate in the mountainous area of the central western TP around(34°N,78-95°E)and a belt zone(30°N,80-84°E)in the southern TP,where the elevation is around 5500 m.The TPVs often dissipate in low valleys and the lee side of mountains.They mainly occur in the warm season during May–September,most active in the summer and least active in the winter.More TPVs form in the daytime than in the nighttime.Those TPVs that move out of the TP account for less than 10-14%of all TPVs,and they usually follow three moving paths:eastward(6.9-7.8%),northeastward(2.3-3.4%)and southward(2.3-3.5%).(2)TPVs mainly occur in the warm season from May to September.In this paper,we investigate the inter-decadal change of TPVs in the warm seasons of 1979–2017by analyzing five widely used reanalysis datasets.A significant change of the TPVs'frequency appears around the mid-1990s,associated with less TPVs during1979–1996 and more TPVs during 1997–2017.The abrupt change is caused by a transition of the Atlantic Multi-decadal Oscillation(AMO)from a cold phase to a warm phase in the mid-1990s.The shift of AMO leads to a silk-road pattern wave train and a spatially asymmetric change of tropospheric temperature.It modifies the intensity of the subtropical westerly jet and the TP heating,leading to the inter-decadal change of TPV activities.(3)Using the Poisson regression,an empirical genesis index of TPVs G_TPV is constructed based on a long-term database of TPVs derived from the reanalysis datasets.The spatiotemporal characteristics of TPVs are well reproduced by G_TPV.Relative contribution of environmental factors shows that,the environmental factors reflected the sensible heat(elevation and the ground-air temperature difference)play a crucial role in the spatial distribution of TPVs,and the environmental factors represented the latent heat(the difference between upper-level and lower-level divergence,relative vorticity at 500h Pa,and the mid-tropospheric humidity)determine the interannual variations of TPVs.In the annual cycle of TPVs,both the latent heat and the sensible heat are of importance.The genesis climatology of TPVs via the Yearbook and G_TPV via other reanalysis datasets are also used to evaluate the performance of the genesis index.It implies the potential capability of G_TPV to understand the mechanism of the genesis and development of TPVs,and to analyze the TPVs from the GCMs with the coarse resolutions.(4)The characteristics of TPV-associated precipitation from 1979 to 2017 are quantitatively analyzed based on three daily precipitation datasets.The influence of TPVs on precipitation is more significant in the warm season(May–September)because the precipitation and TPV activities both peak in the warm season.The TPV-associated precipitation(VAP)dominates the total precipitation over most of the TP in the warm season.The VAP contributes more than 50%of the total precipitation in the warm season over 30%of the area over the TP,particularly in the central TP,while the VAP accounts for more than 25%of the total precipitation in the cold season over about 20%of the area over the TP.Therefore,TPVs are the major systems to produce precipitation on the TP in the warm season,and they are also important in the cold season in certain regions.The interannual variation of VAP is mainly determined by the number,rather than the precipitation intensity,of TPVs.Furthermore,the heating anomaly caused by the TP thermodynamic effect is an influential factor of VAP.Stronger(weaker)TP heating and resulting ascending motion strengthen(attenuate)the convergence near the TP surface and the divergence in the upper troposphere.The thermal adaption-associated circulation generates more(less)TPVs and VAP.(5)The climatology of TPVs has been investigated by many researches.However,the future of TPVs has not yet been projected under the global climate change.In this paper,the possible activities of TPVs under 1.5? and 2? warming scenarios above the pre-industrial level,which are the long-term warming targets of the 2015 Paris Agreement,are evaluated through the NCAR CESM(Community Earth System Model)Low-warming(CESM-LW)Experiments.The results show that the CESM-LW reproduces the spatio-temporal characteristics of TPVs in the historical run and the simulations under stabilized 1.5?(CESM-LW1.5)and 2?(CESM-LW2.0)warming.It suggests that TPVs under 2? warming will occur more frequently and related to a larger ratio of precipitation than that under 1.5? warming.Specifically,the annual mean number of TPVs during the warm season(from May to September)will increase by 11%with a value of 9.2 in CESM-LW2.0 compared with CESM-LW1.5.The changes of TPVs are closely related to the large-scale circulations.Under the stronger global warming,the convergence near the TP's surface and the divergence in the upper troposphere will be enhanced,resulting in the stronger ascending motion and cyclonic vorticity over the TP.Thus,more TPVs are generated.The changes of large-scale circulations are related to TP's heating effect.The assessment of future TPVs could provide a synoptic dynamic perspective on the climate change of precipitation and water resources,and deepen the understanding on the mechanisms of climate change over the TP.