Spatiotemporal Variations of Spring Indices in China and Their Physical Mechanisms

Abstract

AbstractSpring indices refer to the onset date of Spring, which quantify the phenological events in Spring. The average spring index (Avg‐SI) is defined as the average of the first leaf index (the date of early spring) and the first bloom index (the date of late spring) in this study. This study calculated the Avg‐SI in China for 2,780 weather stations during 1951–2014. The long‐term mean Avg‐SI suggested that the spring onset gradually delayed with the increase of latitudes and altitudes. The long‐term trend indicated earlier onset of spring in nearly all sub‐regions in China. The spatiotemporal variations of Avg‐SI were analyzed using the empirical orthogonal functions (EOF) method. The first EOF mode suggested a homogenous pattern across China. This mode is closely linked to the Atlantic Multidecadal Oscillation (AMO), which impacts the atmospheric circulation and temperature across Eurasia. Warm phase of AMO is associated with positive geopotential height (HGT) anomalies and warmer spring in China. The second EOF mode suggested a north‐south zonal pattern, which is linked to the Arctic Oscillation (AO) and the East Asian Trough Strength. Positive phases of AO lead to positive HGT anomalies over the northern China but negative HGT anomalies over the southern China. Together they lead to a weaker East Asian Trough and less cold air intrusions to the northern China, which in turn cause earlier onset of spring in this region.