| Based on the daily minimum temperature data of 903 stations north of 35°N in China from 1980 to 2019 and NCEP/NCAR daily reanalysis data,the regional strong cooling event(RSCE)in winter in the north has been defined.After finding significant cycles of RSCEs,this paper studies their atmospheric oscillation characteristics,extracts extended-period forecast signals,and then further compares the differences in event characteristics in the ENSO cold and warm phase backgrounds.The main results are as follows:(1)In the winter of 40 years,there were 30 RSCEs,with the majority of days lasting 3 to 5days and the longest(shortest)being 9 days(2 days).The decline was greatest in Inner Mongolia,the border between Shaanxi and Shanxi,and the border between Liaoning and Jilin.The regional minimum temperature mainly has 10–20 d and 30–60 d oscillations,and the RSCEs as a whole have the characteristics of 10–20 d quasi-biweekly oscillations.10–20 d B components correspond well with the original temperature distance level series.(2)The Ural Mountain blocking high at the 500 h Pa low-frequency height field moves east-south and weakens,and the trough between Lake Balkhash and Lake Baikal strengthens;the Siberian high pressure on the sea level pressure field moves and expands with the blocking high;the 850 h Pa strong cold center and cold advection continue to strengthen;and low-frequency rapids at the 300 h Pa latitudinal wind field are weaker in the north and stronger in the south.The 2PVU line in vertical isentropic surfaces is decreasing,and the low-frequency high potential vorticity(PV)reservoir at the tropopause shifts southward in the315 K isentropic surface.The high PV cold air column in the troposphere extends vertically,and the cold air is driven by the rapid blockage and meridional circulation downward along the 300–315K isentropic surfaces to the lower latitudes,which makes the low-frequency positive vorticity enhance and the low trough deepen.Also,the convergence at mid-high level,low-level dispersion,and strong downward motion behind the trough all maintain the southward pressure of Siberian high pressure.The above factors together cause the occurrence of RSCEs.The positive(negative)anomaly in the low-frequency 500 h Pa potential height field in the Barents Sea(Western Siberian Plain)at 10 d is a predictable signal for the pre-RSCEs.(3)The RSCEs are classified into El Ni?o types,La Ni?a types and normal types according to the ENSO positive and negative phases.In El Ni?o years,cold air follows a southward path to the northeast due to a strong Ural Mountain high ridge,the strong West Siberian high pressure,and a weak trough.Meanwhile,the high PV has a strong influence in the first and middle stages of the cooling process.In La Ni?a years,the high ridge is weak and westward,the trough is strong,and the cold air path is northwestward.The 2PVU line on the315 K isentropic surface is shifted deeper southward.The high PV reservoir is strong,and the PV column is large in the meridional direction.The whole layer of positive vorticity in the troposphere is strong.The low trough is very deep.In normal years,both large trough ridges and Siberian high pressure are weak,and the cold air path is westward and southward.Apart from this,the high PV column is the weakest,but the height of it is low,which favors the development of low-level anomalies.The positive height anomaly located in the Barents Sea at-10 day and the high PV anomaly located in Novaya Zemlya at-17 day in El Ni?o years;the negative(positive)anomaly located in northern Xinjiang(West Siberian Plain)of China at-8 day(-4 day)and the high PV anomaly located in the border area between Sweden and Finland at-17 day in La Ni?a years;the negative(positive)anomaly located in northern Xinjiang(northeast of the Aral Sea)at-8 day(-4 day)and the high eddy anomaly located in the Baltic Sea at-18 day in normal years all can be used as extended range forecast signals in each SST background. |
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