Revision and application of the western pacific subtropical high indices

The Western Pacific Subtropical High (WPSH) is one of the major circulation systems that cause climate anomalies in the East Asian monsoon region. The WPSH is mainly described by five WPSH indices in the operational climate monitoring of the CMA National Climate Centre, including area (GM), intensity (GQ), ridgeline (GX), northern boundary (GB), and westernmost point (GD) indices. Under the background of global warming, the current operational WPSH indices can no longer accurately depict the WPSH spatial morphology due to the enhancement of the subtropical geopotential height in the Northern Hemisphere, and the corresponding relationship between the WPSH and regional extreme weather processes such as flood or summer heat also weakens. Thus, this work proposes several revisions necessary to improve the WPSH indices, including: (1) first determining the GD and GB indices, and then calculating the GM and GQ indices; (2) considering the western extension of the WPSH in the future under the warming background, the GM index is specified as the area enclosed by the 5880 gpm isoline from the WPSH westernmost point to 150°E; (3) adding the western and eastern GX indices, and the western and eastern GB indices, which can well characterize spatial variation of the WPSH and its impact on precipitation and high temperature anomalies in China. The comparison with the operational indices shows that the revised WPSH indices improve the inductiveness of the WPSH to the evolution of climate anomalies in China. Under a warming background, the WPSH in all months would significantly enlarge and strengthen and extend northwestward without a trend in the ridgeline location. Specifically, the WPSH would expand northward and affect northern China in mid-summer, while its westward extension would affect western China in other seasons. The revised indices have a better correspondence with both summer precipitation and high temperature in China than the operational indices, especially for the GD and western GX indices. Their spatial correlation distributions with summer precipitation and high temperature reflect that the high correlation area between precipitation and high temperature also gradually moves northward following the northward advance of the East Asian summer monsoon.