Novel dynamical indices for the variations of the South Asia high in a warming climate

The South Asian high (SAH) is an important circulation system that seriously affects the weather and climate over East Asia. Previous studies have proposed related indices based on geopotential height to monitor the intensity and movement of the SAH. However, the SAH indices defined by geopotential height have been questioned by some recent studies due to the overall enhancement of geopotential height in the context of climate warming. To objectively reflect the characteristics of SAH variability, we redefined the corresponding indices using the stream function R of horizontal circulation from the three-pattern decomposition of global atmospheric circulation (3P-DGAC) model and studied the interannual and interdecadal variations of SAH. The results indicate that from 1958 to 2023, the area index, intensity index, and eastward ridge point index exhibit significant interannual variability, while the interdecadal signals remain relatively constant. The ridge line index of SAH not only exhibits significant interannual variations but also shows a significant interdecadal movement. Before the late 1980s, the SAH ridge line shifted southward, while after the late 1980s, it shifted northward. Furthermore, the changing characteristics of the SAH defined by the stream function R more closely with those of relative vorticity and horizontal wind. The geopotential height presents a spurious interdecadal enhancement of SAH since 1990. The eddy geopotential height overestimates the interdecadal weakening of SAH around 1979. This shows that the stream function R provides a more objective and reliable representation of the interdecadal variations of the SAH compared to geopotential height and eddy geopotential height. Therefore, our study provides new research methods and index definitions for understanding the changing characteristics of SAH in the context of global warming, and it also gives important reference and methodological guidance for future relevant research.