Climatology Analysis of Blocking System in Northern Hemisphere on Iran

Document Type : Full length article


1 PhD Student in Climatology, University of Isfahan, Iran

2 Associate Professor of Hydro-climatology, University of Isfahan, Iran

3 Professor of Climatology, University of Isfahan, Iran

4 PhD of Climatology, University of Isfahan, Iran


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.


Main Subjects

درگاهیان، ف.؛ علیجانی، ب.؛ محمدی، ح. (1393). آشکارسازی و مطالعة اقلیم‏شناختی رخدادهای بلاکینگ مؤثر بر آب و هوای ایران (دورة آماری 1953ـ 2012) در فصل سرد، فصل‏نامة علمی‏- پژوهشی فضای جغرافیایی، 40(48): 237ـ256.
حبیبی، ف. (1385). تحلیل همدیدی و دینامیکی سامانه‏های بندالی، روش تشخیص سامانة بندالی و تأثیر آن بر روی منطقة ایران، مجلة فیزیک زمین و فضا، 32(3): 69ـ89.
عزیزی، ق. (1378). بلوکینگ، پژوهش‏های جغرافیایی، 36: 37ـ50.
عزیزی، ق. و خلیلی، م. (1390). نقش بلاکینگ در رخداد سرماهای فرین ایران، پژوهش‏های جغرافیای طبیعی، 77: 39ـ55.
کاویانی، م.؛ علیجانی، ب.(1386).مبانی آب و هواشناسی، انتشارات سمت، تهران.
Antokhina, O.Y; Antokhin, P.N; Martynova, Y.V; Mordvinov, V.I. (2016). The impact of atmospheric blocking on spatial distributions of summertime precipitation over Eurasia, IOP Conf. Series. Earth and Environmental Science 48, PP.1-5.
Austin, J.F. (1980). The blocking of middle latitude westerly winds by planetary waves, J. R. Met. Soc., 106: 327-350.
Athar, H. and Lupo, A.R. (2010). Scale analysis of blocking events from 2002 to 2004: A case study of an unusually persistent blocking event leading to a heat wave in the Gulf of Alaska during august 2004, Advances in Meteorology, PP. 1-15.
Athar, H.; Almazroui, M.; Islam, N.; Abid, A. and Ehsan, A. (2013). Effect of mid-latitude blocking anticyclones on the weather of the Arabian Peninsula, International Journal of climatology, 33: 585-598.
Azizi, Q. (199). Blocking, Geographical Research, 36: 37-50.
Azizi, Q. and Khalili, M. (2011). Blocking role in Iran's minimum temperature event, Natural Geography Research, 77: 39-55.
Barriopedro, D.; García-Herrera, R.; Lupo, A.R. and Hernández, E. (2006). A climatology of Northern Hemisphere blocking, Journal of climate, 19: 1042-1063.
Carrera, M.L; Higgins, R.W. and Kousky, V.E. (2004). Downstream Weather Impacts Associated With Atmospheric Blocking Over The Northeast Pacific, Journal of Climate, 17: 4823-4839.
 Charney, J.G.; Shukla, J. and Mo, K.C. (1981). Comparison of a barotropic blocking theory with observation, J. Atmos. Sci., 38: 762-779.
Cherenkovaa, E.A.; Semenovab, I.G.; Kononovaa, N.K. and Titkovaa, T.B. (2015). Droughts and Dynamics of Synoptic Processes in the South of the East European Plain at the Beginning of the Twenty_First Century, published in Aridnye Ekosistemy, 2(63): 45-56.
Cheung, H.N.; Zhou, W.; Mok, H.Y.; Wu, M.C. and Shao, Y. (2013). Revisiting the climatology of atmospheric blocking in the Northern Hemisphere, Advances In Atmospheric Sciences, 30: 397-410.
Darghahian, F.; Alijani, B. and Mohammadi, H. (2014). Climatic demonstration and study of blocking events affecting Iran's climate during cold season (Statistical period, 1953-2012), Journal of - Geographic Space Research, 40(48): 256-237.
Diao, Y.; Li, J. and Luo, D. (2006). A new blocking index and its application: Blocking action in the Northern Hemisphere, J. Climate, 19: 4819-4839.
Elliott, R.D. and Smith, T.B. (1949). A study of the effects of large blocking highs on the general circulation in the northern-hemisphere westerlies, Journal of Meteorology, 6(2): 68-85.
Glisan, M.J. (2007). Two extreme cases of atmospheric blocking over europe and north America, In Partial Fulfillment of the Requirements for the Degree Master of Science, Thesis Supervisor: Dr.A.R. Lupo, the Faculty of the Graduate School, University of Missouri, PP. 1- 108.
Habibi, F. (2006). Synoptic and Dynamic Analysis of Blocking System, Blocking System Detection and its Effect on Iran Region, Journal of Earth and Space Physics, 32(3): 69-89.
Hafez, Y.Y. and Almazroui, M. (2013). The Role Played by Blocking Systems over Europe in AbnormalWeather over Kingdom of Saudi Arabia in Summer 2010, Advances in Meteorology, PP.1-20.
Hartmann, D.L. and Ghan, S.J. (1980). A statistical stady of the dynamics of blocking, Monthly weather review, 108: 1144-1159.
Knox, J.L. and Hay, J.E. (1984). Blocking Signatures in the Northern Hemisphere: Rationale and Identification, Atmosphere-Ocean, 22(1): 36-47.
Lejenas, H. (1995). Long Term Variations of Atmospheric Blocking in the Northern Hemisphere, Journal of the Meteorological ociety of Japan, 73(1): 79-89.
Lejenas, H. and 0kland, H. (1983). Characteristics of northern hemisphere blocking as determined from a long time series of observational data, Tellus, 35A: 350-362.
Matsueda, M.; Kyouda, M.; Toth, Z.; Tanaka, H.L. and Tsuyuki, T. (2011). Predictability Of An Atmospheric Blocking Event That Occurred On 15 December 2005, Monthly Weather Review, 139: 2455-2470.
Namias, J. (1947). Characteristics of the general circulation over the Northern Hemisphere during the abnormal winter 1946–47, Monthly weather review, 75(8): 145-152.
Pelly, J.L. (2001). The predictability of atmospheric blocking, A thesis submitted for the degree of Doctor of Philosophy, The University of Reading, Department of Meteorology, PP. 198.
Rex, D.F. (1950a). Blocking action in the middle troposphere and its effect upon regional climate (I): An aerological study of blocking action, Tellus, 2: 196-211.
Rex, D.F. (1950b). Blocking action in the middle tropospheric westerlies and its effects on regional climate, II: A climatology of blocking action, Tellus, 2: 276-301.
Schalge, B.; Blender, K. and Fraedrich, K. (2011). Blocking detection based on synoptic filters, Advances in meteorology, PP. 2- 11.
Schwierz, C.; Croci-Maspoli, M. and Davies, H.C. (2004). Perspicacious Indicators of Atmospheric Blocking, Geophisical Research Letters, 31: 1-4.
Semenova, I.G. (2013). Regional atmospheric blocking in the drought periods in Ukraine, J. Earth Sci. Eng., 3(5): 341-348.
Sumner, E.J. (1954). A study of blocking in the Atlantic-European sector of the northern hemisphere. Q.J.R. Meteorol. Soc. Vol. 80: PP. 402 - 416.
Tibaldi, S.; Andrea, F.D.; Tosi, E. and Roeckner, E. (1997). Climatology of NorthernHemispher Blocking in the ECHAM model, Climate Dynamics, 13(9): 649-666.
Tibaldi, S. and Molteni, F. (1990). On the operational predictability of blocking, Tellus A,  42: 343-365.
Treidl, R.A.; Birch, E.C. and Sajecki, P. (1981). Blocking action in the northern hemisphere: A Climatological study, Atmosphere-Ocean, PP. 1-23.
Tyrlis, E.B; Hoskins, J. (2008). Aspects of a northern hemisphere atmospheric blocking climatology, Journal of The Atmospheric Sciences, Vol. 65, PP. 1638 – 1652.
Vivekanandan, N. (2007). Analysis of Trend in Rainfall Using Non Parametric statistical Methods; International symposium on rainfall rate and radio wave propagation, American institute of physics, PP. 101-113.
Whan, K.; Zwiers, F. and Sillmann, J. (2016). The Influence Of Atmospheric Blocking On Extreme Winter Minimum Temperatures In North America, Journal Of Climate, 29: 4361-4381.
Wiedenmann, J.M.; Anthon, R.L.; Mokhov, I.I. and Tikhonova, E.A. (2002). The Climatology of Blocking Anticyclones for the Northern and Southern Hemispheres: Block Intensity as a Diagnostic, Journal of climate, 15: 3459-3473.