| The western Pacific subtropical high(WPSH)is a crucial member of the East Asian summer monsoon system,which has an important impact on weather and climate change over China and even the whole East Asia.Previous studies have often used the 500 hPa geopotential height to define area,intensity,western ridge point and ridgeline indices of the WPSH,but global warming caused a significant increase in geopotential height,particularly at the middle and lower latitudes,leading artificial results about the long-term trend and interdecadal evolution of the WPSH.Therefore,it is an important scientific problem to study the characteristic indices of the WPSH in a warming climate.The three-pattern decomposition of global atmospheric circulation(3P-DGAC)method proposed by Chou Jifan's team is an effective tool to describe the characteristics of large-scale circulations in the subtropical region under global warming,and it can describe the three-dimensional spatial structure characteristics of the WPSH comprehensively and accurately from the dynamical perspective.In this paper,the characteristic indices of the WPSH defined by the stream function R of horizontal circulation in the 3P-DGAC is used to study the climatic evolution characteristics and dynamical diagnosis of the WPSH in summer under global warming.The main conclusions are as follows:(1)Climatic evolution characteristics of the summer WPSH based on reanalysis datasetsIn the current warming climate,the 500 hPa geopotential height has a systematic increment at middle and lower latitudes in summer,which would bring about illusory information concerning the long-term trend and interdecadal evolution of the WPSH.However,the stream function R of horizontal circulation in 3P-DGAC removes the systematic deviation of the evolution of the WPSH,and it can well reflect the dynamical characteristics of the WPSH.We further analyze the characteristics of the WPSH by adopting the stream function R of horizontal circulation in 3P-DGAC.Results show that the circulation features of the WPSH can be revealed by the stream function R in 3P-DGAC in a warming climate.Besides,the rainbelt over East Asia is located at the northwest periphery of zero-value isoline of the stream function R.The local rainfall is connected to the zero-value isoline of the stream function R over the western Pacific Ocean under global warming.By analyzing the time series of the WPSH indices defined by adopting the stream function R of horizontal circulation in the 3P-DGAC,we find that the long-term trend of area,intensity,western ridge point and ridgeline indices display the modest contraction of area,weakening of intensity,eastward recession of the western ridge point and southward shift of the ridgeline from 1948 to 2018.Moreover,by analyzing interdecadal changes of the time series of the WPSH indices during1948–2018,we find that area and intensity indices decrease with time before the end of the 1970 s and increase slightly with time after the end of the 1970 s,the western ridge point index moves eastward with time before the end of the 1970 s and moves westward slightly with time after the end of the 1970 s,as well as there are no obvious interdecadal variations in the ridgeline index.We conclude that the climatological average WPSH is contracted and retreated eastward during 1979–2018 relative to1948–1978.In summary,the new indices defined in this study objectively describe the long-term trend and interdecadal changes of the summer WPSH,confirming that the stream function R in 3P-DGAC is an appropriate tool to describe the dynamical characteristics of the WPSH.The zero-value isoline of stream function R can be adopted as a metric to identify the position of and shape of the WPSH in a warmer climate.(2)Climatic evolution characteristics of the summer WPSH based on CMIP6modelsUnder future climate warming scenario,the 500 hPa geopotential height will increase greatly,and it will be a global phenomenon in the future.If we continue to use the geopotential height,it will not be able to accurately represent the western boundary of the WPSH.In addition,the relationship between local rainfall and geopotential height at 500 hPa over the western Pacific Ocean will change accordingly with climate warming.The local rainfall cannot be related to the constant contour of geopotential height,which will bring some difficulties to the precipitation prediction.However,under different climatic scenarios,the local rainfall is connected to the zero-value isoline of the stream function R over the western Pacific Ocean.Therefore,it has great rationality and reliability to associate the local rainfall with constant isoline of stream function R over the western Pacific Ocean under global warming.Furthermore,the long-term trend of area,intensity and western ridge point indices the WPSH display the contraction of area,weakening of intensity and eastward recession of the western ridge point from 2015 to 2100.Ridgeline index moves northward with time before the end of the 2060 s and moves southward with time after the end of the 2060 s.We conclude that the climatological average WPSH will be contracted and retreated eastward during ssp5-8.5 run relative to historical run.(3)Dynamical diagnostic analysis of the evolution of the WPSH in a warming climateBy analyzing the stream function R of horizontal circulation tendency equation in the 3P-DGAC,we find that the variations in advection transport of the absolute vorticity by the horizontal circulation A1,zonal-vertical transport the absolute vorticity by the zonal circulation A2,meridional-vertical transport of the absolute vorticity by the meridional circulation A3,the absolute vorticity divergence by the zonal circulation D1 and the absolute vorticity divergence by the meridional circulation D2 over the western Pacific Ocean in 500 hPa can reflect the intensity change and movement of the WPSH in a warmer climate.Compared with 1948–1978,the A1 increases in 1979–2018 and A1 is negatively correlated with the area and intensity indices of the WPSH.A3 increases and A3 is negatively correlated with the area and intensity indices.A3 is positively correlated with the western ridge point index.D2 decreases and D2 is positively correlated with the area and intensity indices.Therefore,the weakening and eastward retreat of WPSH are mainly caused by the changes of A1,A3 and D2 during 1979–2018 relative to 1948–1978.In addition,compared with the historical run,the A1 decreases in ssp5-8.5 run and A1 is positively correlated with the area and intensity indices of the WPSH.A1 is negatively correlated with the western ridge point index.A2 increases and A2 is negatively correlated with the area and intensity indices.A2 is positively correlated with the western ridge point index.D1 increases and D1 is negatively correlated with the area and intensity indices.D1 is positively correlated with the western ridge point index.Therefore,the weakening and eastward retreat of WPSH are mainly caused by the changes of A1,A2 and D1 during ssp5-8.5 run relative to historical run. |
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