How Does Coldwave Frequency in China Respond to a Warming Climate?

Under the background of a warming climate, regional climate responses may be different from place to place. How cold extreme events in China respond is still an open question. This study investigates responses of coldwave frequency (CWF) in China from observation and modeling perspectives. Observational evidences show that CWF significantly reduces across China during the warm period (1978-2009) in comparison with that during the cold period (1957-1977), concurrent with extreme value centers located in northern China during 1957-1977 and southern China during 1978-2009. The empirical orthogonal function (EOF) leading mode of CWF in the cold period is also dominant by an extreme value center prevailing over northern China, while the center exhibits a southward shift in the warm period. A seven-member multi-model ensemble (MME) from coupled model intercomparison project#3 (CMIP3) shows that southern China tends to experience more coldwaves than northern China in the twenty first century (2045-2064 and 2080-2099) under the global warming A1B forcing (with atmospheric CO2 concentration of 720 ppm). This feature can also be seen in the leading EOF mode of MME. These results indicate that the primary response of CWF to a warming climate may be the southward shift of the maximum loading center. The response of CWF may be associated with the weakened Siberian High (SH) and the enhanced western Pacific Subtropical High (WPSH) in the warm period. Cold and dry air is transported from the north via a "northwest pathway" to southern China during 1978-2009. Meanwhile the enhanced WPSH and weakened SH may result in anomalous southerlies which bring warm and wet air to southern China. Under the joint action of these two air masses, coldwaves may easily generate in southern China as observed in recent extreme cold events in this region. The variation of the CWF might also be intimately associated with the echanced and southward shift of the storm track from the cold period through the warm period. Negative correlations are found between coldwaves and the North Pacific storm track. The stronger storm activities in the warm period indicate less coldwaves. The significance change of the baroclinic growth rates around 40°N tends to be accompanied by a consistent change of the synoptic eddies in the troposphere, which may shift the preferred latitude for the growth of eddies. As a result, the storm track tends to move southward, suggesting the possibility of the increased storminess in southern China due to the increased baroclinic instability in the troposphere.