Spatiotemporal variations of Spring Indices in China and their mechanismin recent decades

Abstract:

Spring index (SI) refers to the onset date of Spring, which quantifies the phenological events in Spring in a given region. In this study, the SI in China is defined as the averaged of the first leaf index (the date of early spring) and the first bloom index (the date of late spring). The long-term mean SI suggested that the spring onset gradually delayed with the increase of latitudes and altitudes. The long-term trend suggested earlier onset of spring in nearly all sub-regions in China. The spatiotemporal variations of 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). 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 southern China.