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INTERNATIONAL JOURNAL OF CURRENT LIFE SCIENCES
RESEARCH ARTICLE
ISSN: 2249- 1465
International Journal of Current Life Sciences - Vol.5, Issue, 4, pp. 577-581, April, 2015
IDENTIFICATION AND ANALYSIS OF THE NORTH ATLANTIC BLOCKINGS
Ghasem Azizi*, Hossain Mohammadi, Mostafa Karimi, Ali Akbar Shamsipour and Iman Rousta
Faculty of Geography, Department of Climatology, University of Tehran, Tehran – Iran
AR TIC L E
I NF O
Article History
th
Received 4 , December, 2014
Received in revised form 20th, February, 2015
Accepted 15th, April, 2015
Published online 28th, April, 2015
Key words:
North Atlantic, Blocking, Frequency, Duration,
Geographical Position.
ABS TR AC T
Atmospheric blocking is one of the most striking features of extratropical lowfrequency Variability. This paper identification North Atlantic Blockings (in the
range 0 to 90 degrees North and Longitude 40°W to 70°E) according to the
Barriopedro et al. method. Daily data 500 geopotential heights in the period
1948-2013 from network data NCEP / NCAR was extracted. After identifying
North Atlantic Blockings, they were analyzed. 4299 days with blocking
conditions identified that this days involve 506 periods of Blockings which was
held from 5 to 33 days. The most blocking events occurred in April and May,
and least at September and August were occurred. The years 1975, have most
number and 1978, 1998 and 2009, least number of blocking periods. The longest
periods of blocking happened on 1968, 1956 and 1953 years. Among geographic
latitude, the latitudes of 55°N with 2408 days and, geographical longitudes
17.5°W to 2.5°E, with 1309 days with blocking conditions, have the maximum
number of days with blocking events.
© Copy Right, IJCLS, 2015, Academic Journals. All rights reserved.
INTRODUCTION
During the last several decades some studies have applied
subjective blocking criteria based on surface and
midtroposphere observations of typical blocking flow
configurations. Atmospheric blocking potentially triggers
various climate extremes over the extratropics, which can
be
extended
toward
the
subtropical
region.
Climatologically,the westerly jet circulates around the
extratropical region; this is called zonal-type circulation.
Occasionally, the jet is split and its speed slows down
such that the normal atmospheric flow appears to be
“blocked” (Cheng et. al 2013). Typical blocking is
composedof a barotropic high pressure center and a
surface frontal zone (Treidl et al., 1981). While warmer
air masses comprise its upstream portions, colder air
masses constitute its downstream portions. Long periods
of such a quasi-stationary state persistently maintain the
meridional flow driving north–south air mass and energy
exchange. This may trigger extreme climate events, such
as heat waves, cold waves, drought, and flooding. Two
opposite extremes may occur simultaneously at its
different components. For example, wildfires in the Ural
Mountains, heat waves in Russia, and severe flooding in
Pakistan during summer 2010 were the consequences of a
long duration of blocking in the vicinity of the Ural
Mountains (Dole et al., 2011;Matsueda, 2011; Lau and
Kim, 2012).Following the traditional Rex(1950a)
criterion, a blocking event can be identified through a
split-flow regime in the middle troposphere as a double jet
detectable over more than 45° in longitude and persisting
for more than 10 days. Sincethen, there have been
modifications to the original Rex definition, demanding
lower durations or extensions (Treidl et al. 1981;Azizi
1996; Azizi et al. 2009; Azizi et al. 2011; Azizi et al.
2011; KhoshAkhlagh et al. 2011, Rousta et al. 2014) as
well as new restrictions in latitude location (White and
Clark 1975) to exclude semi permanent subtropical
anticyclones. Recently, numerous criteria have been
proposed in order to identify objectively atmospheric
blocked flows. Most of them were based on zonal flow
indices computed from meridional height gradients at the
middle troposphere (Lejenäs and Øakland 1983,here after
LO83; Tibaldi and Molteni 1990, hereafter TM90;Tibaldi
et al. 1997; Trigo et al. 2004, Azad 2006). Other
methodologies detected blocking events as positive height
anomalies at the midtroposheric flow persisting for
several days (Charney et al. 1981; Dole and Gordon
1983) or from normalized indices based on daily height
projections over mean blocking patterns (Liu 1994;
Renwick and Wallace 1996). The most recent
methodologies combine traditional subjective and
objective criteria, as those used by Lupo and Smith (1995;
hereafter LS95) or Wiedenmann et al. (2002; here after
WI02), or use quantities derived from dynamical
properties related to blocking patterns, such as the
meridional potential temperature (θ) gradient on a
potential vorticity(PV) surface representative of the
tropopause (Pelly and Hoskins 2003), or negative
anomalies of vertically integrated potential vorticity
within the 500–150-hPalayer (Schwierz et al. 2004).As a
result, several long-term studies focused on North
*Corresponding author: Ghasem Azizi*
Faculty of Geography, Department of Climatology, University of Tehran, Tehran – Iran
This article extracted from the Iman Rousta’s PhD thesis
International Journal of Current Life Sciences - Vol.5, Issue, 4, pp. 577-581, April, 2015
Hemisphere blocking events have been previously
published (Rex 1950a,b; LO83; Dole and Gordon1983;
TM90; Tibaldi et al. 1994; LS95; WI02).However, some
of them were confined to certain regions or limited to
single seasons. Moreover, the behaviour of the North
Hemisphere blocking has been traditionally described in
terms of frequency, duration, and favoured occurrence
regions, not considering other characteristics such as
genesis location. On the other hand, there are not many
studies addressing long-term blocking variability,
especially at inter decadal scales (e.g., Chen and Yoon
2002). Some authors have reported that blocking
occurrence may be affected by North Hemisphere largescale patterns such as the North Atlantic Oscillation
(NAO) (e.g., Shabbaret al. 2001).The relationship
between blocking and theEl Niño–Southern Oscillation
(ENSO), however, has been widely discussed (Renwick
and Wallace 1996;Watson and Colucci 2002; Mokhov
and Tikhonova 2000; WI02).Nevertheless, this linkage
has been derived for certain regions or single seasons and
the ENSO-related variability, if any, has not been clearly
established. This paper tries to identify North Atlantic
Blocking systems as one of the most important
phenomenon affecting weather in the northern hemisphere
in the 1948-2013 period and frequency variations These
systems will be studied.
Fig 1 the Study Region
Data and detection algorithm design
The dataset used in this study was The 66-yr record
(1948–2013) of mean daily 500-hPa height geo potential
from the National Canters for Environmental Prediction–
National Center for Atmospheric Research (NCEP–
NCAR) gridded reanalysis, for the latitude 0 to 90 north
degrees and longitude -40 to 70 degree in Northern
Hemisphere which are described in more detail by Kalnay
et al. (1996) and Kistler et al. (2001). These analyses are
archived at NCAR and are available from the NCAR
mass-store facilities in Boulder, Colorado. The 0000 and
1200 UTC NCEP– NCAR reanalysis used here for the
calculations are the 500-hPa gridded (2.5 degree lat and
2.5 degree lon) heights available at 6-h interval.
a-Blocking index
Since blocking patterns are characterized by an
appreciable mass difference between high and middle
latitudes (Namias and Clapp 1951; Austin 1980; Treidl et
al. 1981) and anomalous easterly winds, the blocking
index used here is an adapted version of the TM90 index,
which is based on the original criterion proposed by
LO83, that recently updated by Barriopedro et al. (2006;
hereafter BA06). According to the LO83 criterion, a
blocking event can be identified when the averaged zonal
index computed as the 500-hPa height difference between
40° and 60°N, is negative over 30° in longitude and
during five or more days. However, TM90 noted that cut
off lows displaced pole ward could also yield negative LO
values. To exclude these, TM90 demanded an additional
negative height gradient northward of 60°N. According to
BA06 index, a blocking event was detected when at least
five (12.5°) or more contiguous longitudes appeared as
blocked during at least five days. Following this
methodology, two 500-hPa height geo potential gradients
(GHGN and GHGS) have been simultaneously computed
for each longitude and for each day of study over the
region of study in agreement with expression (1):
=
( ,
=
( ,
( ,
)−
)
( ,
)
≥0
)− ( ,
−
( ( ,
)
/ deg
≤ −10
)) > 0
Equation. (1)
/
= 80.0 ° +△
0 = 60.0 ° +△
= 40.0 ° +△
△= −5.0 °, −2.5 °, 0 °, 2.5 °, 5.0 °
Where Z (λ, ϕ) is the 500-hPa height geopotential at
latitude ϕ and longitude λ. GHGS is proportional to the
zonal geo-strophic wind component and provides a
measure of the zonal flow intensity for each longitude,
while the GHGN gradient is imposed in order to exclude
non-blocked flows. Thus, an arbitrary longitude is
considered blocked when both GHGN and GHGS verify
the condition expressed by Eq. (1) for at least one of the
five values and simultaneously the ϕο height anomaly is
positive. This requirement minimizes the problem of
identifying cutoff lows as blocked flows. Also, the
procedure incorporates better spatial resolution and more
blocking opportunities by allowing five △ values.
The temporal algorithm can be summarized in three steps
1. For each day shall be a minimum of 12.5 degrees in
longitude.
2. The minimum period of three conditions, is 5 days.
3. The above two features must be maintained for the
entire period of blocking, and discontinuity is not
allowed to feature 1 and 2.
Findings
Recent research shows that in the 55-yr period of
study, a total of 1514 blocking events were detected
over the Northern Hemisphere, giving an annual
average of about 27 events. As a consequence, about
half of the number of days in any given year was
blocked on average over any region of the Northern
Hemisphere (BA06). Although fewer events were
obtained by earlier studies (e.g., Treidl et al. 1981;
LO83), this result is close to those reported in the later
works, such as WI02, who, using NCEP–NCAR datasets,
found an annual frequency of 25 events. In this paper
and this study area, based on the criteria used
(Equation 1 and the three conditions) also a total of
506 blocking events were detected over the study
region, giving an annual average of about 7.7 events.
As a consequence, about 65.2 numbers of days in any
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International Journal of Current Life Sciences - Vol.5, Issue, 4, pp. 577-581, April, 2015
given year were blocked on average over any region of
study area. General trend in the number of days per
year and the number of periods of blocking per year
shows a weak decreasing trend in all of the 66-yr
period of study (Figure 3).
1948
1952
1956
1960
1964
1968
1972
1976
1980
1984
1988
1992
1996
2000
2004
2008
2012
12.0
11.0
10.0
9.0
8.0
7.0
6.0
5.0
Fig 4 Annual average of blocking events duration, in the periods
(1948-2013)
Latitude
75
1
72.5
2
70
31
days with blocking
67.5
89
65
210
62.5
345
60
485
57.5
728
2408
55
Fig 5 Number of days, with blocking conditions divided north
latitude, in the periods (1948-2013)
170
120
70
20
-40
-32.5
-25
-17.5
-10
-2.5
5
12.5
20
27.5
35
42.5
50
57.5
65
Days With Blocking
220
Longitude
Fig 6 Number of days, with blocking conditions divided
longitude, in the periods (1948-2013).
FIG 2 Schematic of the automated blocking detection method and
their analysis
15
10
5
1948
1952
1956
1960
1964
1968
1972
1976
1980
1984
1988
1992
1996
2000
2004
2008
2012
0
1948
1953
1958
1963
1968
1973
1978
1983
1988
1993
1998
2003
2008
2013
120
100
80
60
40
20
0
Fig 3 Number of days (right) and number of periods of
blocking (left) in statistical periods (1948-2013).
The year of 1975 with 13 periods of blocking, was the
greatest and 1978, 1998 and 2009, each with 3 periods of
blockingwas the least. Also the year of 1993with 11.6
days, has the greatest and 1964, with 5.9days, has the
least of average of blocking durations (Table 1). The
longest period of blocking in 1968, 1956 and 1953,
respectively 33, 28 and 27 days have occurred. Overall,
the average of blocking duration periods during 19482013 periods shows a weak increase (Figure 4).
The results showed that during the study period (24106
days), 4299 Days with blocking events was detected. That
these days include 506 of period which are included have
continuity between 5 to 33 days.April and May each with
57 blocking period, have the largest number of events and
September and August respectively with 23 and 24
blocking period events, had the lowest periods. Most days
with blocking conditions was during April and February,
respectively, with 508 and 502 days and minimum days
with blocking conditionswas in September and August,
respectively, with 148 and 172 days during the period
(Table 2).
Greater frequencies of blocking events werenear 10°E
(BA06). But in the results of this paper,greater
frequencies of blockings events were located in 17.5°W to
2.5°E.the latitude 55°N with 2408 blocking days (56% of
all detected blocking days) had the highest number of
days with blocking conditions.With increasing latitude
number of days with blocking conditionsis decrease and
in above 75°N latitude that have only one day blocking
during a period of 66 years, there is no blocking event
(Fig. 5).Therefor 55 to 75°N latitude is the center of the
formation, development and eventual disappearance
blocking phenomenon in the study area. That latitude
55°N plays a far more important role. 17.5°W to 2.5°E
longitude with 1309 days with blocking conditions
(30.5% of all blocking days detected) has the greatest
number of blocking days and 35°W and 67.5°Elongitude
respectively with 27 and 28 days with blocking
Conditions, has minimum days (Figure 6).
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International Journal of Current Life Sciences - Vol.5, Issue, 4, pp. 577-581, April, 2015
Table 1 Number of days, periods and average of blocking events duration, in the periods (1948-2013).
Year
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
Days with Blocking Periods
74
8
44
6
43
5
63
7
68
10
92
10
60
6
84
10
83
9
82
9
57
6
56
7
34
5
53
8
77
9
108
12
47
8
48
6
44
6
89
11
64
10
84
8
58
7
82
9
112
12
64
8
80
8
104
13
104
12
64
7
20
3
57
6
75
7
Mean Duration
9.3
7.3
8.6
9.0
6.8
9.2
9.4
8.7
9.2
9.1
8.5
8.0
6.8
6.6
8.6
9.0
5.9
8.0
7.3
8.1
8.5
7.3
9.0
8.4
10.6
6.9
10.0
8.0
8.5
9.5
6.7
9.5
10.7
Year
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
Days with Blocking
47
43
45
86
71
53
67
49
64
30
98
86
87
26
64
101
69
21
52
70
39
42
71
33
73
74
91
75
30
105
58
61
44
Periods
5
6
6
11
8
5
7
7
9
4
9
8
8
4
8
11
9
3
7
8
4
6
8
5
8
9
11
11
3
10
7
8
5
Mean Duration
9.4
8.1
6.4
7.8
8.9
10.6
9.6
7.0
7.1
7.5
10.9
10.2
11.6
6.5
8.0
9.4
7.1
7.0
7.4
8.8
9.8
7.0
8.9
6.6
9.1
8.2
8.3
6.6
10.0
10.0
8.8
7.6
8.8
Table 2 Monthly number of days and periods of blocking events in the periods(1948-2013)
Month
Periods
Days
Jan
40
478
Feb
50
502
Mar
55
468
Apr
57
497
May
57
508
CONCLUSIONS
Blocking events the atmospheric phenomenon is not too
much repetition but has high life and has many effects on
the climate of mid latitude and can lead to the creation of
conditions, including droughts, floods, at extremely cold
temperatures. This paper attempts to identify North
Atlantic Blockings in period 1948-2013, according to the
BA06 method. The results showed that during the study
period, 4299 Days with blocking events was detected.
That these days include 506 of period which are included
have continuity between 5 to 33 days. Total of 506
blocking events were detected over the study region,
giving an annual average of about 7.7 events. As a
consequence, about 65.2 numbers of days in any given
year were blocked on average over any region of study
area. General trend in the number of days and periods per
year shows a weak decreasing. April and May have the
Maximum and September and August, had the Minimum
blocking period events in period study. Most days with
blocking conditions were during April and February, and
minimum days with blocking conditions were in
September and August. The latitude 55°N had the highest
number of days with blocking conditions. With increasing
latitude, number of days with blocking conditions is
decrease and in above 75°N latitude during a period of
66-yr, there is no blocking event. So 55 to 75°N latitude is
the center of the formation, development and eventual
disappearance blocking phenomenon in the study area,
Jun
46
314
Jul
28
213
Aug
24
172
Sep
23
148
Oct
36
263
Nov
34
298
Dec
56
438
that latitude 55°N plays a far more important role.Greater
frequencies of blockings events were located in 17.5°W to
2.5°E.And 35°W and 67.5°E longitude has minimum days
in period study. Overall, in 1948-2013 periods, the
average of blocking duration periods shows a weak
increase but number of days and periods of blocking
events had a weak decrease.
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