| In the early21st century, especially since2008, cold winter has occurred frequently in Eurasia, the most significant place responsing to global warming. Contrast with the warming background since1980s, it has attracted special attention of scholars both at home and abroad. In this paper, using reanalysis data provided by NCEP/NCAR including monthly mean global surface temperature, height and wind, global SST reanalysis data provided by NOAA, and AO index data, aiming at the low temperature anomalies region in mid-high latitude in Eurasia, it could be divided into two winter temperature anomaly stages, before and after global warming (1980). We selected two different period of frequent winter low temperature, thus1960s-1970s and early21th century, and carried on a research on contrasting and analyzing their low temperature anomaly regions and the characteristics of large scale atmospheric circulation. We focused on the temporal variation rule and the characteristic of spatial distribution in detail of two important circulation systems for northern hemisphere winter, arctic oscillation (AO) and polar vortex. On this basis, we revealed the homogeny and complementarity of the effect on northern hemisphere climate anomaly of the arctic oscillation and the whole or different part of the polar vortex, especial the relationship between the both systems and the winter temperature anomaly in mid-high latitude of Eurasia. At last, we also conducted a further analysis on the cause and mechanism of frequent low temperature in winter since2008. We concluded that:1) In21th century especial since2008, low winter temperature in Eurasia ocurried frequently.The winters with low temperature has accounted for about2/3of the total winters since global warming, and it stays in a stage of low temperature, showing some similarities and differences of characteristic with that before1970s. The atmospheric circulations of the above two stages are similar. It is a relatively deep vertical structure from the lower to the upper troposphere in mid-hight latitude, and showing a significant annular structure in the northern hemisphere (AO). Northerly winds in Ural Mountains are strong and blowing towards east and west direction respectively. It is active of cold air in northern Eurasia.Before global warming, the height fields in mid-low latitudes of cold winters were consistently low, strong clod air in Polar Regions transfered southward to mid-high latitudes in mid-low troposphere, and E-P flux converged from low latitude and Polar Regions. However, since21th century, the height fields of cold winter represent an opposite phase distribution, thus, it is negative anomaly in lower troposphere, and it is positive anomaly in upper troposphere in mid-low latitudes. It is weak of northerly wind to blow southward, low temperature anomaly of stratosphere in mid-low latitude leans to northern Eurasia, and E-P flux only converged from tropopause in low latitude. Comprehensive analysis shows that it has a south-north difference between the two stages, which leading to the difference of the low temperature distribution in winter. Since21th century, low temperature anomaly area is only located in mid-high latitudes in Eurasia. However, before global warming, it covered the extensive regions from mid-low latitudes to northern Polar Regions. Such a planetary scale difference of low temperature in Eurasia could not be explained by regional circulation. So, it is necessary to conduct a research on horizontal and vertical structure characteristics of the arctic oscillation and the whole or different part of the polar vortex, and the relationship between the above with clod winter in Eurasia since21th century.2) Temporal variation rule and space distribution characteristics of AO and polar in winter.For AO, its vertical distribution presents a deep quasi-barotropic structure from sea surface to stratosphere. When it stays in a positive phase, height anomaly is negative in Polar Regions, and it is positive in mid and high latitudes. The significant region of teleconnection distribution is located in troposphere. While in latitudinal direction, it shows a markedly skewed distribution. When AO stays in a positive phase, the main characteristic is that the atmospheric pressure in North Pacific is lower than that in North Atlantic. When in a negative phase, it presents that the atmospheric pressure in North Atlantic is higher. For polar vortex in northern hemisphere, it is significant autocorrelation for polar vortex indexes both in200hPa and500hPa, which indicate that the polar vortex in the northern hemisphere was positive pressure state in middle and upper troposphere. Besides, in troposphere of Northern hemisphere the major positive pressure mode presents a negative oscillation between polar vortex and mid-high latitudes. And along with the height changes into stratosphere, it only presents an active polar vortex mode.In temporal variation rules, AO has significant decadal cycle. Especially in SLP and500hPa, the amplitude reached a peak in1970’s. And after2000’s, it changed into a new cycle. It also appears significant cycles in interannual scale and multi-interdecadal scales in the northern hemisphere and its four polar vortexes located in different regions, which showing a6-7waves distribution. And it also changed into a new long cycle stage in late2000s.3) It has some similarities and differences of anomalous relationship between sea-air system and AO or polar vortex (including its different partitions) in northern hemisphere. They also are complementary for each other.It is the annular distribution for the AO negative phase, showing the opposite anomaly between Polar Regions and mid-high latitudes in northern hemisphere. While, for polar vortex circulation, it is not only showing a annular distribution, but also charactering semipermanent atmospheric activity center in north Pacific better, and reflecting the skewness distribution characteristics of circulation anomalies in Eurasia. Sea surface temperature (SST) of west wind drift area in north Pacific and middle-east equator Pacific Ocean (ENSO) both have a relationship with the Pacific area index of polar vortex. While AO is related to SST in the north Atlantic drift area. After classifying anomalous phases of AO, we could find that positive AO anomaly corresponds with negative phase of PDO (La Nina), and negative AO anomaly corresponds with typical El Nino distribution. When AO stays in an extreme anomaly state, the corresponding relationship is opposite with the above. Both the effected area and strength of polar vortex in winter appear obvious distinction as the phases of ENSO or PDO change. It is significantly synchronous related for the phase anomalys of La Nina and PDO to AO and polar vortex, which has been proved by numerical simulation. Besides, AO is negatively/positively related to area/strength of polar vortex, and the significantly related regions are mainly located in North America and Atlantic nearby Europe. When the area of polar vortex in northern hemisphere is larger, it is relatively warmer in the polar region. At the same time, cold air concentrates at high latitudes in the land, embodying significantly AO negative phase distribution. On the other hand, the cold air is mainly to huddle in the polar region, has less effect on the high-latitude climate anomaly.4) It also has some conplementary for AO and polar vortex to the effect on both summer and winter climate in Eurasia.Polar vortex in the northern hemisphere, its different partition indexes, and AO index are negatively related to the winter temperature in polar area, high latitudes of Eurasia and China respectively. Among them, the temperatures of regions which are more significantly related to AO are located in polar area, the north Middle East, North Africa, and the north Atlantic. While polar vortex area index in the northern hemisphere is more significantly related to temperatures in the north Pacific west wind drift region and the Middle-East equator Pacific. When polar vortex area in the northern hemisphere is larger, it is colder from high latitudes of Eurasia to middle-east North America, and warmer in northern Africa and Greenland. However, the corresponding relationship is opposite for AO anomaly. Temperatures of middle and high latitudes in polar area are similar influenced by polar vortex area indexes in North America and the Atlantic nearby Europe. But the impact on anomaly scope, intensity of regional temperature is slightly different. Polar vortex area index in Asia is significantly related to temperatures not only in middle and high latitudes in Asia, but also in Eastern China and coastal areas and parts of South Asia like Pakistan and northwest Indian Ocean, which could precisely depict some characteristics that two main cold air burst out into low latitudes from the west and the east Asian continent. Overall, AO and polar vortex have similar thermal property to temperature of the northern hemisphere. While compared with the effect on the temperature field in the northern hemisphere winter, polar vortex in the northern hemisphere and its partition activity can better reflect regional characteristic on significantly related relationship between itself and temperature of each continent. The major mechanism, for polar vortex in Asia is that the area of the polar vortex could clearly reflect the activity characteristics of cold air, especially could depict the East Asian winter monsoon which is not only in East Asia, but also in Central Asia. Besides, AO and polar vortex area in winter are both well related to temperatures and precipitation in China during the same period and summer next year, showing that AO and polar vortex are the main factors leading to the temperature and precipitation changes in winter and next summer even though with great regional distinction. It fluctuates significantly of the areas of polar vortex and its partitions in the northern hemisphere, which has marked characteristics in winter. And the difference between the strong and weak earas of partition in north Pacific could also enforce the positive phase of NPO, which is an important early signal for low temperature of northeast China. The positive/negative phase of NPO conducives to the formation of constant super long wave trough/ridge, which will provide low/high value disturbance source for anomalous activities of cold vortex/blocking high in Northeast Asia under the function of non adiabatic heating by backward and slow wave.5) the diagnosis analysis of the effect of AO and polar vortex in northern hemisphere anomaly changes on winter low temperature in Eurasia and summer low temperature in northeast China since2008.After comprehensive analysis on individual cases, we could find that wide range and continuous low temperature, sleet and freezing disasters occurred in China from January to February2008are associated with AO positive phase distribution. Frequent low temperature events in winter in the northern hemisphere, especially the high latitude in Eurasia since2009, not only caused by the interseasonal AO negative phase (with the exception of the2011winter), and also are associated with the larger polar vortex area especially parts in Asia. Both low summer temperatures in northeast China and frequent cold vortex activity in2009are not only significantly related to AO positive phase in previous winter (January to February), but also have relationship with small polar vortex area in north Pacific (the negative phase of NPO). Comprehensive analysis shows that cold events occurried in Eurasia since2008are mainly affected by the anonamly changes of AO and polar vortex. AO and polar vortex are directly and main influence factors for frequent cold winter in mid-high latitudes of Eurasia since21th century. The anomaly phase of NPO which has relationship with polar vortex areas in Pacific is not only the important early signals for summer temperature anomaly in northeast China, but also the steady component in planetary scale which could be extracted in addition to chaotic components in synoptic-scale.6) It is complementary each other between AO and polar vortex (including its partitions) for the relationship with the major models of sea-air system, and especial for the mechanism of cold winter occurried in mid-high latitudes of Eurasia in early21th century. What we focus on in this paper is whether polar vortex and its four partitions in the northern hemisphere (in Asia, north Pacific, north America and Atlantic near Europe) have objectivity. |
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