Impacts of the Low-Frequency Oscillation over the Extra-tropics of the Northern Hemisphere on the Extreme Low Temperature Event in Northeast China in the Winter of 2012/2013

Using daily data of NCEP/DOE reanalysis Ⅱ and the temperature data collected at conventional observation stations of National Meteorological Information Center, the characteristics of the low-frequency oscillation (LFO) of the regionally averaged temperature during the extreme low temperature event in Northeast China in the winter of 2012/2013 is analyzed. Changes in the spatial configuration of low-frequency circulation systems in the mid- and lower troposphere in the extratropics of the Northern Hemisphere and their linkage with cold air activities are analyzed. Furthermore, the propagation character of the low-frequency circulation systems and their impacts on temperature changes are also investigated. Results show that: (1) the regionally averaged temperature in Northeast China in the winter of 2012/2013 exhibits a strong 10-30-day oscillation feature, which is accompanied by a significant 30-60-day oscillation and closely related to actual temperature drops. (2) for the 10-30-day oscillation, the phase 7 (phase 3) corresponds to maximum decrease (increase) in temperature over Northeast China; Northeast China is under control of the strong northerly (southerly) air current between the low-frequency high (low) pressure system around Lake Baikal and low-frequency low (high) pressure system nearby the Sea of Japan at 500 hPa; meanwhile, East China from the north to the South China Sea is mainly controlled by low-frequency northerly (southerly) wind at 850 hPa, which strengthens (suppresses) the circulation system of East Asian winter monsoon. As a result, Northeast China undergoes a cooling (warming) process. With such a spatial configuration and evolution of low-frequency circulation systems in the mid- and lower troposphere, Northeast China is affected by strong (or stronger) cold airmass that follows an easterly and/or northerly path and extremely low temperature event happens. (3) A low-frequency wave train at 500 hPa propagates along the path of Urals-Baikal Lake-Northeast China-western Pacific. It is the circulation system that actuates the LFO of East Asian winter monsoon and the extreme low temperature event in Northeast China.