LONG-TERM FLUCTUATIONS OF NORTHERN HEMISPHERE ATMOSPHERIC CIRCULATION ACCORDING TO DZERDZEEVSKII’S CLASSIFICATION

The long-term series of fluctuation of monthly and annual Northern Hemisphere atmospheric circulation in non-tropical latitudes from 1899 to 2008 according to Dzerdzeevskii classification have been discussed. The differences in atmospheric circulation between circulation epochs have been identified. The circulation and climatic characteristics of extreme decades of circulation epochs in the Northern Hemisphere and its six sectors—Atlantic, European, Siberian, Far East, Pacific, and American—have been given. The recent, the 1981–2007 period, is characterized by the increase in frequency (number of cases) and duration (number of days) of the southern meridional circulation group.

At the present time, the analysis of long-term fluctuations of atmospheric circulation is lacking proper attention. However, without studying these fluctuations it is impossible to explain alternation of the periods of increase and decrease in air temperatures and total precipitation in isolated regions and the entire Northern Hemisphere; it is also impossible to explain global and regional distinctions in warming of the 1920s-1940s and the last warming.

METHODS AND DATA
Analyses of synoptic daily maps allow one to isolate 41 elementary circulation mechanisms (ECM). They differ in direction and quantity of blocking and of southern cyclone outlets. The important feature of ECM is that they are seasonal in nature. Each ECM has a unique cyclone and anticyclone trajectory scheme and description [Dzerdzeevskii, 1968;Dzerdzeevskii, Kurganskaya, Vitvitskaya, 1946], maps of see level pressure and temperature, height of AT 500 and temperatures at AT 500 for 1970-1978 [Savina, Khmelevskaya, 1984], annual series and long-term series of fluctuation for 1899-2008 .
ECM have been grouped in 13 types, and 4 groups (Table 1, Figure 1). The first group is the zonal (types 1 and 2: anticyclone on the North Pole, 2-4 of southern cyclone

FLUCTUATION OF ATMOSPHERIC CIRCULATION
The basic purpose of the classification of the atmospheric circulation over the Northern Hemisphere is the analysis of long-term climatic fluctuations and forecast. To address this goal, ECM were grouped in different categories using the History of the alternation of ECM and the ECM duration was calculated on a monthly, circulation season, and yearly basis for the entire period beginning in 1899 (Table 1). These data are also placed on the website www.atmospheric-circulation.ru.
First results of research efforts to study multi-year atmospheric circulation over the Northern Hemisphere based on the History of alternation of ECM, have been published in 1956 [Dzerdzeevskii, 1956]. At that time, the first representations of generalized (composite) circulation groups (i.e., the zonal (the zonal itself and disturbances of the zonal) and the meridional (the northern and southern)) and circulation epochs (i.e., periods with positive or negative deviations of the zonal circulation from its long-term average duration value) were made.   . During the modern, the meridional epoch, the duration of the meridional ECM is greater than during the first epoch, as it has been previously suggested by B.L. Dzerdzeevskii [Dzerdzeevskii,. 1968]. The maximal total duration of all meridional ECM was 319 days per year on average in 1997-   Table 2).
Differences between the first and modern meridional epochs are also associated with the duration of northern and southern ECM ( Figure 3).
There were almost no meridional southern processes in the beginning of XX century (Fig. 3).
Their duration started to increase in 1920s; only in 1963, it reached the mean value for the 1899-2008. Beginning in 1980s, it grew fast and now, the duration of the meridional southern circulation is over one-third of the year with weather patterns determined by alternations in meridional northern and southern processes. group. The meridional northern circulation group prevails practically during the entire period (the average duration of 193 days per year). The years 1932The years , 1938The years , 1939The years , 1940The years , 1945The years , 1951The years and 1975 represent the exception. During these years, the zonal circulation was longer in duration than the meridional northern. All years, except for 1975, can be referred to the zonal epoch. In 1989In , 1991In , 1992In , 1994 и 1997, the meridional southern circulation was the longest.
During the meridional epoch, the duration of northern processes was the longest (246 days per year on average, with a 274 days maximum in 1915 ( Table 2)). During the zonal epoch and especially in the period from 1932 to 1951, years with the prevalence of the meridional northern circulation alternated with years when the zonal circulation prevailed. A new increase in the duration of the meridional northern circulation that started in 1957 led to its absolute dominance in 1969 (268 days per year with a further decrease to a 91 days per year minimum in 1992 ( Figure 4). By that time, the duration of meridional southern processes increased significantly while the duration of the zonal processes reached even greater minimum than in the beginning of XX century. As a result, during the period from 1965 to 1977, the durations of the zonal and meridional southern processes were almost similar. In 1977-1985, at an absolute dominance of the meridional northern circulation, the duration of zonal processes increased and took over the meridional southern processes. From 1986 to 1977, the durations of the meridional northern and southern circulation groups was about the same and exceeded substantially the duration of the zonal group. The duration of the southern group more than by a factor of three exceeded the mean value. This situation determined an extremely instability of atmospheric circulation that has not been seen during the entire previous period. A sharp alternation in atmospheric processes became a reason for a fast growth in re-occurrence of meteorological extremes and hazardous natural events resulting from these meteorological conditions [Kononova, 2007]. In 1997, the duration of the meridional northern processes was 147 days per year compared to 196 days on average for 1899-2008; the duration of the meridional southern processes was 179 days per year compared to 45 days on average for the entire period, i.e., it was four times greater than the longterm average value. From 1998, the duration of meridional southern processes started to decrease, while the duration of the northern processes grew (Figure 4).
From 1998 to the present time, there is a decrease in the duration of the meridional southern circulation with the dominance and a new increase in the duration of the meridional northern circulation and minimal duration of the zonal group. In 2007, the Thus, while the meridional northern and zonal epochs were relatively uniform, the meridional southern epoch can be subdivided into 4 periods with different combinations of the circulation group durations (Table 3). Table 2 presents the durations of circulation groups in extreme decades of circulation epochs and specific periods of the third epoch, including the last decade. During the period of decrease in zonal and increase in northern meridional ECM during modern epoch (1957-1985, Fig. 4), there was some decrease in temperature over the Northern Hemisphere (Fig. 5). The year 1976 was the coldest for this period with the deviations of -0,294°C. The last negative deviation (-0,134°С) was noted in 1985.

FLUCTUATIONS OF CLIMATE OF THE NORTHERN HEMISPHERE
The period from 1986 to 1997 when there was maximal duration of the meridional southern ECM, was marked with a warming (Fig. 5). A sharp increase in the duration of southern meridional processes correlated to climatic changes in the system "oceanatmosphere" [Byshev, Kononova, Neiman,   [Chaplygina, 1961], and there were 11 times more reverse transitions from type 13 to type 12a. No such alternations happened in 1899-1923. Alternation in ECM types 12 and 13 provides for the best conditions for deepening of atmospheric fronts that leads to increase in re-occurrence of meteorological and ecological extremes registered in recent years.

CIRCULATION EPOCHS IN SECTORS OF THE NORTHERN HEMISPHERE
The elementary circulation mechanism that acts as a uniform and complete mechanism of macro-circulation exchange manifests itself in different ways depending on the location in the Northern Hemisphere: while there may be blocking in one area, a southern cyclone outlet or zonal circulation are possible in another. To study these processes, the Northern Hemisphere was subdivided into six sectors [Dzerdzeevskii, 1968;Dzerdzeevskii, 1970] with their boundaries drawn considering positions of the continents and oceans that determine seasonal character of development of atmospheric circulation. These boundaries are as follows: Atlantic -60°W-0°; European Circulation epochs in different sectors during the same Hemisphere epoch appear to also differ due to features discussed previously.
Relative prevalence of the duration of the zonal or meridional processes in a sector serves as a criterion for defining the circulation epochs in the sector and for the Hemisphere in general. However in this case, the terms "zonal" and "meridional" are applied to circulation processes for the entire Hemisphere, while, in individual sectors, the terms "latitudinal" and "longitudinal" are used.
The principles of isolation of latitudinal and longitudinal circulation in individual sector have been developed by B.L. Dzerdzeevskii [Dzerdzeevskii, 1968;Dzerdzeevskii, 1970]. The direction of the air stream in the atmosphere over the sector was used as a criterion to describe a character of circulation. Trajectories of cyclones and anticyclones from weather maps and the direction of the air stream from maps AT 500 were used as input data. Breaking ECM into groups with similar circulation patterns in each of the six sectors of the Northern Hemisphere is presented in [Dzerdzeevskii, 1968;Dzerdzeevskii, 1970]. Additional groups, compared to the ECM grouping in the Northern Hemisphere as a whole, were isolated to characterize, for example, such a poison when Arctic intrusion is displaced to one of the boundaries of the sector. In this case, there may be a penetration of a southern cyclone far north within the rest of the sector, or the latitudinal circulation may be maintained. For such cases, combined definitions of circulation processes over the sector, i.e., "longitudinal northern and longitudinal southern" or "longitudinal northern and latitudinal western" were applied. Similar to these instances, there can be various descriptions of a direction of the air stream in the northern and southern parts of the sector. For example, when there is an intrusion of the air from the north into a stationary anticyclone located in the middle latitudes, the group "longitudinal northern and stationary position" is defined. When there is interfluence of a southern cyclone with cyclones formed on the Arctic front, the term "latitudinal western and longitudinal southern" is used. The maximal number of groups in one sector is 9 (American) and the minimal is 5 (Far East).
To solve some problems (e.g., establishment of boundaries of circulation epochs in different sectors and defining their seasonal character) it is necessary to use, as in the case of the Northern Hemisphere, only two generalized groups: latitudinal which includes all ECM when latitudinal trajectories of cyclones and anticyclones prevail over the sector, and longitudinal. The generalized latitudinal group includes 3 groups: (1) latitudinal western, (2) latitudinal western and longitudinal southern and (3) latitudinal western and stationary position. All others groups form the generalized longitudinal group. The long-term variations in the duration of the generalized latitudinal groups in each of the six sectors are presented in Fig. 6.
Comparison of these graphs with the graphs for the zonal circulation for the entire Northern Hemisphere showed a shift in the boundaries of the circulation epochs in each sector relative to their boundaries within the Hemisphere.
The boundaries of the circulation epochs in the sectors are presented in Table 5. Analysis of this table indicates that the changes in relative prevalence of zonal over meridional processes and vice versa, occur in oceanic sectors earlier, than in continental.
Besides, the circulation epochs in different sectors during the same Hemisphere epoch, are different in character: they may be zonal in one sector and meridional in another. We will address a character of the circulation epochs of the Northern Hemisphere in each of its six sectors in more details.
There are a different number of circulation epochs in different sectors for the period  Table 5 and Fig 6, there are 2 circulation epochs in the European, Far-East, and Pacific sectors: (1) zonal and (2) meridional; as it was noticed above, these sectors, since the second half of XX century to the present time, represent the generators of the southern cyclones.
In the Siberian sector, there are 3 circulating epochs: (1) meridional, (2) one without a clear prevalence of either latitudinal or longitudinal processes, and (3) zonal, with a substantial role of southern cyclones, i.e., the circulation group "latitudinal western in combination with longitudinal southern. " In the Atlantic and American sectors, there are 4 opposite in sign circulation epochs: during a zonal epoch in the Atlantic sector there is a meridional in the American.
Thus, the first, meridional, epoch in the Northern Hemisphere characterized by the development of the blocking processes in the Siberian and American sectors, has manifested itself in the meridional epochs specifically in these sectors. The zonal epochs in the very beginning of XX century occurred in the oceanic (the Atlantic and Pacific) and their dependant European and Far-East sectors. The epochs that coincide in time with the third, i.e., the meridional southern, Hemisphere epoch, reflect periods in its development presented in Table 3. At the present time, the Pacific, Far-East, and European sectors are experiencing the bulk of re-occurring southern cyclones [Byshev, Kononova, Neiman, Romanov, 2004].
Characteristic periods of fluctuations of the latitudinal circulation in all sectors may be identified from data presented in Fig. 7. Thus, from 1899 to 1934-1943, the duration of the latitudinal circulation differed significantly between individual sectors. From 1934From -1943From to 1969From -1978, it varied about the mean in all sectors, with the exception of the Far-East and Pacific sectors where it was above the average till 1954-1963, with a decrease in the future years. Beginning in 1970-1979 till the present time, the duration of the latitudinal circulation between individual sectors differed even to a greater degree then in the beginning of the century. The greatest differences in the duration of the latitudinal circulation are between individual sectors in 1989-1998. Therefore, during the periods of maximal development of the meridional Hemisphere circulation (both northern and southern) there are the maximal differences in the durations of latitudinal circulation between individual sectors. B.L. Dzerdzeevskii [Dzerdzeevskii, 1969] thought it was necessary to have a set of climatic data for each circulation epoch because just a mean value for the entire period alone can not be used to characterize modern climate. This thesis is especially important now when many regions are experiencing extreme weather evens with a low probability of occurrence of one in 20, 50, and even 100 years.

REGIONAL FEATURES OF ECM
Within each sector, individual ECM are characterized by different circulation modes in different parts of the sector. Often, within latitudinal ECM, in northern areas there is movement of cyclones, while anticyclones move or stationary within southern sectors. In longitudinal ECM, blockings in western sectors are accompanied by outlets of southern cyclones in their eastern parts. In order to describe circulation and climatic properties of individual territories within the sectors, each sector has to be broken into areas based on cyclonic or anticyclonic nature of ECM. This procedure has been applied to the territory of Russia [Kononova, 2005].
Using the Black Sea coast of Krasnodar region and the western steppe region of the Altai region as examples, we will demonstrate the relationships between circulation and climatic characteristics. As shown in Fig. 8-11, fluctuations of annual precipitation sums in these regions are consistent with variations in the duration of their cyclonic circulation. This is important for the analysis of hazardous natural processes that occur due to meteorological conditions (mudflows, landslides, avalanches, etc.), for which monitoring is conducted from time to time. According to known data on hazardous natural processes for particular regions, there is a connection with certain ECM. Data on fluctuations of the duration of these ECM for 1899-2008 may be used to establish a degree of hazard from increased activity of analyzed processes in the present and near future.
The average annual air temperature over the Northern Hemisphere during this period was lower than the average temperature for the period 1961-1990 accepted by WMO as the standard period.
The second circulation epoch  was marked by long deviations of the total annual duration of the zonal circulation from the 1899-2008 average. The average annual air temperature over the Northern Hemisphere during this period was above the average for 1961-1990. Especially significant In the extreme decade of the zonal circulation epoch (1930)(1931)(1932)(1933)(1934)(1935)(1936)(1937)(1938)(1939), there was the longest total annual duration of the generalized zonal group of circulations: 230 days/year. The greatest positive deviation of the average annual air temperature over the Northern Hemisphere during this decade was 0,141°C (1938). The year 1944 was the warmest with the deviation of 0,163°C.
In 1960-1969, the total annual duration of the meridional northern processes was comparable to the level for 1906-1915 (268 and 274 days/year, respectively). The duration of the meridional southern processes in 1963 reached its average longterm value for 1899-2008 and continued to grow. The average annual air temperature over the Northern Hemisphere decreased with the deviations of -0,222°C in 1964 and -0,294°C in 1976. The average annual total precipitation over the northern Hemisphere increased as a result of the development of atmospheric fronts.
In 1988-1997, the maximal total annual duration of the meridional southern processes was noted, with the maximum of 201 days in 1989. The maximal positive deviation of air temperatures in XX century occurred immediately after this decade (in 1998, 0,608°C).
In 1998-2008, the total annual duration of the meridional southern processed decreased with the increase in the duration of the meridional northern processes; as a result, the duration of the meridional processes on average for the decade was 319 days and has reached its maximal value in 2000 (i.e., 346 days/year). That year was marked with decrease in the average annual temperature of the Northern Hemisphere with the deviation of 0,357°C. This decade had the maximal (for the entire observation period) positive deviation of 0,625°C in 2005. Similar to the period 1960-1969, there was an increase in the annual total precipitation on average for the Northern Hemisphere.
The fluctuations of the duration of the atmospheric circulation processes of the Northern Hemisphere cause fluctuations of the air temperatures, precipitation, and, as a result, the increase in reoccurrence of the hazardous natural events in different regions of the northern Hemisphere.
It is important to consider the character of circulation epochs in modeling efforts that target circulation of atmosphere and climate because alternations of circulation epochs impact fluctuations of air temperatures and precipitation over the Northern Hemisphere.