The two leading modes of winter clear-sky days over China and their formation mechanisms

Abstract

Using gauge data during 1979–2013, the spatial and temporal characteristics of winter clear-sky days (CSDs) in China are investigated. The frequency of winter CSDs in China has two independent and significant modes, which account for 41% and 10% of the total variance, respectively. Based on evidence from observational analysis and numerical experiments, the formation mechanisms of these two leading modes are unraveled. The first mode is a homogenous mode with coherent year-to-year variation of CSDs over most parts of China, and is closely related to a large-scale cyclonic/anticyclonic anomaly over the Asian continent. Anomalous Indian Ocean SST and Arctic sea ice are responsible for the associated large-scale circulation anomalies via inducing a mid-to-high latitude Rossby wave train. The Indian Ocean SST warming can induce wave energy propagation upwards to the stratosphere and then northwards to the polar region, where it moves down to the troposphere and then travels southeastwards from the polar region to East Asia. Meanwhile, the decreased Arctic sea ice can directly induce a Rossby wave train emanating from the polar region that travels southeastwards into East Asia. These two factors jointly lead to a dominant equivalent barotropic cyclonic/anticyclonic anomaly over China, and in turn the homogenous pattern of winter CSDs in the region. The second mode is a seesaw mode with out-of-phase variations of CSDs between northern and southern China. This mode is closely related to a meridional dipole circulation anomaly pattern over the East Asian sector, which is generated by the so-called ‘Mega-ENSO’ SST anomaly over the Pacific Ocean. This Mega-ENSO SSTA pattern can induce convective heating/cooling over the tropical western Pacific, which further leads to the meridional dipole circulation anomaly pattern over the East Asian sector, resulting in the south-to-north seesaw mode of winter CSDs in China.