Abstract
The evolution of plateau monsoons is essential to synoptic climatology processes over the Qinghai-Xizang Plateau. Based on ERA-Interim Reanalysis data covering 1979–2014 from the European Centre for Medium-Range Weather Forecasts (ECMWF), we propose a new plateau monsoon index (ZPMI) that can effectively reflect the evolution of monsoons and compare this new index with the existing Plateau Monsoon Indices (PMI), i.e., the Traditional Plateau Monsoon Index (TPMI), the Dynamic Plateau Monsoon Index (DPMI), and the PMI proposed by Qi et al. (QPMI). The results show that the onset and retreat of plateau monsoons determined by the TPMI are approximately 1–2 months earlier than those of the ZPMI and DPMI and that the ZPMI can better reflect seasonal and inter-annual variations in precipitation over the plateau. The plateau summer and winter monsoons have similar inter-annual and inter-decadal variation characteristics and show a rising trend, but the increasing trend of the summer monsoon is more significant. The ZPMI is also capable of effectively reflecting meteorological elements. In stronger plateau summer monsoon years, more (less) precipitation and a higher (lower) air temperature appear over the eastern and central (western) plateau. The ZPMI and soil moisture in April and May are used to explore the influence of soil moisture on plateau monsoons, and a significant correlation is found between the plateau soil moisture in the spring (April–May) and plateau summer monsoons. It is found that when the soil moisture over the central and eastern plateau is higher (lower) than normal (while the soil moisture over the western plateau is lower (higher)), the plateau summer monsoon may be stronger (weaker).
Highlights
Research on the Qinghai-Xizang Plateau monsoons, as well as their influence on climate, have existed for more than half a century
The shifts of cold and heat sources in the winter and summer over the plateau can cause the seasonal transformation of the cold high and the warm low, which can lead to a monsoon phenomenon over the plateau, similar to that caused by land-sea thermal differences [2]
Contrast Analysis and Evolution Characteristics of Plateau Monsoon Indices rotation, and the airflow over the plateau converges toward the warm low center
Summary
Research on the Qinghai-Xizang Plateau (hereafter referred to as “plateau”) monsoons, as well as their influence on climate, have existed for more than half a century. In the mid-1950s, Ye et al [1] first proposed that the surface winds around the plateau in the summer are basically due to cyclonic rotation and that the convergence of the airflow causes an ascending motion over the plateau. They suggested that this phenomenon is a direct result of the thermal effect over the plateau. The shifts of cold and heat sources in the winter and summer over the plateau can cause the seasonal transformation of the cold high and the warm low, which can lead to a monsoon phenomenon over the plateau, similar to that caused by land-sea thermal differences [2]. Remote Sens. 2016, 8, 493; doi:10.3390/rs8060493 www.mdpi.com/journal/remotesensing
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