Morphological variation of the western Pacific subtropical high and its possible thermodynamic causes under the background of climate warming

Based on the NCEP/NCAR reanalysis data, the variation of spatial pattern of the western Pacific subtropical high (WPSH) and the thermodynamic mechanisms leading to the variation, under the background of global warming, are investigated through correlation and composite analysis and calculation of the atmospheric heat sources. It shows that the pattern of the WPSH on all levels varies significantly following the global warming. The scope of the WPSH is found to expand westward and southward above 850-700 hPa, particularly at 500 hPa. The patterns of the WPSH are consistent with the distributions of surrounding heat sources and voticitity both in the cold and warm epochs. Along with the global warming, the heat sources strengthen on the western and southern sides of the WPSH above 850-700 hPa, particularly at 500 hPa, while the heat sources at the southern side move southward. Meanwhile, the anticyclonic vorticity tends to increase on the western and southern sides of the WPSH through the troposphere. Specifically, the most obvious increasing tendency is concentrated in the layer between 850 hPa and 700 hPa on the western side and at 500 hPa on the southern side, while the heat sources as a whole move southward. In summary, the anticyclonic vorticity increases as the heat sources strengthen along with the global warming at the western and southern sides of the WPSH. Consequently, the WPSH expands westward and southward and moves southward as a whole, since it tends to expand following the increasing anticyclonic vorticity.