In recent years,a wide range of abnormal weather events have been occurring frequently in China,imposing considerable impacts on people's daily lives.For example,the severe winter in South China in 2008 and the strong cold air over North China in 2009.These abnormal events have generated great interest among experts and scholars.A large body of research shows that the evolution of the atmospheric low-frequency oscillation in the weather and climate plays an important role.Building upon this previous research,the present paper attempts to discuss the characteristics and regularity of continuous abnormal events in North China,as well as the relationship with atmospheric low-frequency oscillations.
Based on 1959-2009 NCEP/NCAR daily reanalysis data and daily observed temperature data in China,we begin by defining persistent abnormal low temperature.When the average air temperature is less than or equal to -10 ℃ for five to seven consecutive days or more,with only 1 day of discontinuity permitted,we argue that the region is subject to a persistent abnormal low temperature process.At the same time,the first day of the continuous low temperature process is defined as the start date,and similarly,the end day is the last day of the average temperature being less than -10 ℃.On the basis of this definition,the characteristics of winter persistent abnormal low temperature events in North China are studied.Then,the main relationships between winter persistent abnormal low temperature in North China and atmospheric low-frequency oscillation activities are discussed.
A regular pattern is found insofar as,during 1959-2008,persistent abnormal low temperature processes occurred 26 times in North China,featuring obvious interdecadal variation characteristics.During the 1960s to 1990s,persistent abnormal low temperature events were more frequent than at other times.However,there was a rising trend after 2000.Importantly,persistent abnormal low temperature events were focused in two periods,late December to early January and late January to early February,which could be key for extended-range forecasting.
Previous research has pointed out that atmospheric low-frequency oscillations bear a close relationship with abnormal events,playing an especially important role in periodic,continuous abnormal extreme events and high impact events.It provides the background field for such abnormal events,whilst at the same time,the occurrence of persistent abnormal low temperature events usually takes place through adjustment of the circulation field.By analyzing the processing of persistent abnormal low temperature events,we found that the structure from the upper troposphere to the lower troposphere is consistent,corresponding to the establishment of a ridge of high pressure in the Ural Mountains,followed by its development and demise,and the maintenance,deepening and weakening of the East Asian trough.
Meanwhile,persistent abnormal low temperature events in North China are closely related to the change in the atmospheric low-frequency circulation field.Though a series analysis,we find that both 10-20-day and 30-60-day low-frequency activities influence persistent abnormal low temperature events greatly.When persistent abnormal events happened,a 10-20-day low-frequency cyclone occurs in Northeast and North China,and an interim one in the central Siberian Plateau.Meanwhile,an anticyclone near the western Siberian Plateau and a cyclone near the Sea of Japan are the important 30-60-day low-frequency systems caused by persistent abnormal low temperature events.