عنوان مقاله [English]
Blocking is one of the most protruding flow patterns and has received attention during the last decades due to its effect on seasonal weather characteristics. The first qualitative conditions for the blocked atmospheric flow was introduced by Rex in 1950 telling the atmospheric jet becomes divided into two separated parts with a considerable connected mass transport, the flow to be meridional at the splitting point and downstream, and the state to continue for at least ten days with a zonal width of at least 45 degrees. The new ideas formulated by Rex became the basis for several blocking indices. Most of the indices detect typical atmospheric patterns. For shorter time series, particular criteria have been used, although, the majority of the analyses are in the 500 hPa geopotential height.
In the face of the differences between the index classifications, they draw similar conclusions regarding the patterns of the Northern Hemisphere blocking climatology. However, the blocking frequency is the percentage of time steps an assured longitude. It can be as low as 5% or higher than 20%. The main aims for this wide range of frequencies in automatic recognition are modifications of the Rex criteria and the use of different parameters.
Due to its simplicity, the index of Tibaldi and Molteni (TM-Index) has become a standard in automated blocking detection analyses. An important problem of the TM-Index is that it cannot effectively distinguish between blocking and cut-off low patterns, since both fulfill the TM-Index criteria. The adapted TM-Index MTM has a higher rejection rate for cut-off lows. Limits in the Rex conditions can be measured as implementations of independent filters which show synergetic effects if they are used in run.
Materials and methods
We have used the 500 hPa geopotential height of the NCEP/NCAR reanalysis dataset (1951–2012) with a spectral truncation of (2.5*2.5on a Gaussian grid).
We have defined blocking in north hemisphere by Tibaldi and Molteni index. The blocking can be defined at a longitude as GHGS > 0, GHGN < −10 m lat, with the geopotential height gradients in a northern and southern region.
In the present study, the TM-Index is modified by introducing different ranges for the gradients (this will be denoted as the basic setup in the following):
And for Iran:
where Z is the geopotential height and φ indicates latitudes. So, the 500 hPa geopotential height field is analyzed for a pattern with a positive geopotential height gradient in the southern region and a strongly negative gradient in the northern region. The explanation of a range for Δ leads to a larger number of detected blocking events.
According to this definition, it is possible to find blocking structures with a maximum between the center and southern region. For example, If only the southernmost latitude in the mid-latitude region has higher geopotential height than that of the more northern latitudes of the southern region, GHGS > 0 and blocking will be detected. Without this modification, such a combination is not possible and blocking is not identified, because the high geopotential is located too far south and the lower geopotential too far north, associated with a relatively small meridional extent of the block. The increase of the blocking frequency by the variable gradient is ranged in the basic setup amounts to 20% in regions with high blocking frequency and to 50% in regions with low blocking activity compared with the original TM-Index.
Results and discussion
Location of blocking system
Most of the blocking systems in the northern hemisphere are developed in all seasons and extended over the Atlantic and Pacific. The analysis of the frequency of blocking systems in (2012 -1951) show that most of the blocking systems occurred in the Atlas region 11.2%, the 4.8% pacific, Europe 3.9%, the United States 3.7%, and Asia 0 .75%. The highest frequency of blocking systems was observed in the central part of the Atlantic on the W ° 30, then the Pacific in the western and central part on W ° 140, in the west of the United States on W ° 125, west Europe on the W ° 10 and in Asia in the region E ° 85 - E60 (Figure1 and 2).
Fig. 1. The location of the blocking systems in the northern hemisphere.
Duration of blocking
Between 1951-2012, the Atlantic was 20.8% in 1954 and 19.5% in 1998, with the highest annual rates in 1968 and 4.6% in 1987, and 5.5% in 1987, with the lowest incidence of 5.5% (Figure 3) and Pacific Ocean in 1985 with 12.9% and 1998 in 10.7% were the highest in 1957 with 3.6% and with 3 .8% in 1968 and 1997 as the lowest incidence (Fig. 4).
Fig (3) Blocking System Frequency at 1954
Fig (4) Blocking System Frequency at 1985
Blocking as one of the most protruding flow patterns can greatly affect seasonal weather characteristics. The findings of this research show that most of the blocking systems in the northern hemisphere can occur in all seasons over the Atlantic and Pacific. The analysis of the frequency of blocking systems in (2012 -1951) shows that most of the blocking systems in Iran are very scarce. In 1975, the highest frequency of blocking systems has been observed in the east of the country. The seasonal distribution of the systems showed that the highest frequency of occurrence of blockade systems is in summer, autumn, winter and spring.