International Journal of Geosciences, 2012, 3, 992-999
http://dx.doi.org/10.4236/ijg.2012.325099 Published Online October 2012 (http://www.SciRP.org/journal/ijg)
Impact of North-South Shift of Azores High on Summer
Precipitation over North West Europe
Shahnaz Ali Rashid1,2, Muhammad Jawed Iqbal2,3, Muhammad Arif Hussain4
1Department of Sciences & Humanities, National University of Computer & Emerging Sciences (FAST), Karachi, Pakistan
2Department of Mathematics, University of Karachi, Karachi, Pakistan
3Institute of Space & Planetary Astrophysics, University of Karachi, Karachi, Pakistan
4Institute of Business and Technology, Karachi, Pakistan
Received August 21, 2012; revised September 23, 2012; accepted October 19, 2012
ABSTRACT
Several Studies demonstrate that North Atlantic Oscillation (NAO) influences variability of climate over Europe. As
NAO is has significant influence on climate of Europe during boreal cold season (November to April), we use the cen-
ters of action approach for the study of summer precipitation (June to August) variability over Europe, taking into ac-
count variations in the components of the NAO North Atlantic Oscillation (NAO), the Azores High and the Icelandic
Low pressure systems. This study shows that north-south shifts of the Azores High has significant impact on interan-
nual variations of summer precipitation over North West Europe, there being more precipitation when the Azores High
shifts southward versus when it is northward. Thus this article demonstrate that when the Azores High system is to the
south there is flux of moist and warm air from the Atlantic into NW Europe. We present a regression model for summer
precipitation over North-west in which the Azores High latitude and the Icelandic low longitude are independent vari-
ables and it explains 53 percent of the variance of precipitation during 1952-2002, a significant enhancement over the
NAO value of R2 = 0.10.
Keywords: NAO; COA; Azores High Pressure; Icelandic Low Pressure
1. Introduction
North Atlantic Oscillation (NAO) is the most prominent
and persistent pattern of atmospheric variability over
middle and high latitudes of the Northern Hemisphere. It
refers to swings in the atmospheric sea level pressure
difference between Arctic and subtropical Atlantic that
are most perceptible during boreal cold seasons (No-
vember to April) and linked with changes in the mean
wind speed and direction [1-4]. Such changes alter the
seasonal mean heat and moisture transport between the
Arctic and the neighboring countries, as well as the in-
tensity and number of storms, their path and their weath-
er. Therefore, NAO has been identified as one of the five
principal research areas of the DecCen component of
CLIVAR [5].
However, NAO is a useful concept to explain the va-
riability of precipitation during winter season; it is less
useful during summer, as the Icelandic Low ceases to be
important [6]. Therefore, several studies even employ
winter NAO for describing the summer precipitation va-
riability [7,8]. This article investigates possible role of
Azores High on precipitation over Europe during the
summer season. In this paper, centers of action ap-
proach [9-11] is employed for the mentioned purpose.
The centers of action (COA) which are the large semi-
permanent pressure systems seen in the global distribu-
tion of sea level pressure [12], such as the Azores High
or the Icelandic Low, dominating the atmospheric circu-
lation over a large region. Each COA is characterized by
three indices: the longitudinal and latitudinal positions of
its center of gravity and its central pressure. Hence, in
contrast to the NAO, the COA approach makes use of the
information on position as well as pressure. It therefore
provides additional degrees of freedom so as to more
directly explain the interannual variations of summer
precipitation over Europe. In fact, this paper examines
how the intensity and position of Azores High as well as
Icelandic Low have impact on precipitation during sum-
mer seasonan.
2. Data
We use monthly precipitation data obtained from Climate
Research Unit, University of East Anglia:
http://www.cru.uea.ac.uk/cru/data/hrg/cru_ts_2.10 for the
period from 1952 to 2002 for the Iberian Peninsula.
Monthly averaged gridded SLP data are also used for
C
opyright © 2012 SciRes. IJG

S. A. RASHID ET AL. 993
calculating objective COA indices for the monthly aver-
aged pressure, latitude and longitude of the Icelandic Low
pressure, the Azoress High pressure and Siberian High
pressure systems as described by [13]. NAO, AO and SOI
monthly indices are available at the Climate Data Center,
National Center for Environmental Prediction.
3. Method
As we know that the NAO index is defined by measure-
ment of the pressure difference between the Icelandic
Low and Azores High at two fixed locations, Lisbon,
Portugal and Stykkisholmur, Iceland. However, it is
known that these two pressure systems have extended
structures that migrate considerably and their motions are
not entirely coupled. As such, a better estimate of influ-
ence of atmospheric pressure fluctuations on the climate
variability in this region can be attained through a more
quantitative assessment of the fluctuations in the pressure
and locations of Icelandic Low and Azores High. Each of
these COA exhibits a characteristic seasonal cycle. Dur-
ing winter the Icelandic Low and the Azores High are
most pronounced. In summer the low-pressure center
weakens, and the Azores High dominates. In addition to
a weakening and strengthening of the pressure centers,
there is also displacement; for example, in summer the
Azores High moves to the northeast compared to the
summer season. The position and the strength of each
COA are captured by three indices representing its lon-
gitude, latitude, and pressure. Recently, this approach has
successfully been used to explain the role of the move-
ment and intensity of individual atmospheric COAs on
such varied biogeochemical systems as copepod abun-
dance [14], the variation in the location of the Gulf
Stream [15] the variability of the transport of African
dust [16], impact of Indian Ocean High pressure on pre-
cipitation over Western Australia [17] and influence of
Azores High on Middle Eastern Rainfal [18].
The pressure index Ip is defined as an area-weighted
pressure departure from a threshold value over the do-
main (I, J):


,
11
,
11
cos
cos
IJ
ij tt
ij
pt IJ
ij
ij
PP
I







,
,
1M
ijij t
ij t








where Pij,t is the SLP value at grid point (i, j)average
over a time interval t, in this case monthly SLP values
are taken from NCAR, Pt is the threshold SLP value (Pt =
1014 mb for Azores High and Icelandic Low),

ij is the
latitude of the grid point (i, j), M = 0 for the Azores High
and for the Icelandic Low.
 = 1 if (−1)M(Pij,t − Pt) > 0
and
 = 1 if (−1)M (Pij,t − Pt) < 0, this ensures that the
pressure difference is due to an Azores High or Icelandic
Low system. The intensity is thus a measure of the ano-
maly of the atmospheric mass over the section (I, J).
Similarly, the latitudinal index is defined as:
,,
11
,
,,
11
cos 1
cos 1
IJ M
ij ttijijij t
ij
tIJ M
ij ttijij t
ij
PP
IPP

 












and the longitudinal index I
 ,t.
We first calculate seasonal average of summer pre-
cipitation for the months from June to August over the
European region bounded by 30˚N - 70˚N and 10˚W -
30˚E at each grid point. We next compute correlation of
summer precipitation at each grid point over Europe with
NAO and COA indices (Azores High pressure, Azores
High latitude, Azores High longitude, Icelandic Low
pressure, Icelandic Low latitude, Icelandic Low longi-
tude). The purpose is to identify regions where a signifi-
cant amount of variation can be possibly explained by
further study.
Next we identify the indices with large contributions
that could be significant. Correlations are calculated
among the Centers of Action (CAO) indices. Only mutu-
ally independent indices are considered for further inves-
tigation. Thus, significant independent COA indices are
identified for the construction of a linear regression
model of summer precipitation of north Europe.
4. Results
We construct the correlation maps for examining the
influence of NAO as well as COA variables on summer-
time (JJA) precipitation with p = 0.05. NAO has signifi-
cant relation with summer precipitation in some places in
England and Ireland (see, Figure 1). However, Figure 2
also shows that summer precipitation over a large region
of north-west Europe is strongly correlated with Azores
High Latitude (see, Figure 2). Therefore, we select the
region (10˚W - 8˚E, 44˚ - 58˚N) in north-west Europe for
the further analysis of summer precipitation
Then, we next compute correlations between six indi-
ces of COA (Icelandic Low latitude, Icelandic Low lon-
gitude, Icelandic Low pressure, Azores High pressure,
Azores High latitude, Azores High longitude), AO, Ni-
no34 and NOA with the mean of summer precipitation
for the region of north-west Europe.
The average summer precipitation over Northwest
Europe (10˚W - 8˚E, 44˚ - 58˚N) is strongly correlated
with NAO, AO, Nino 3.4, Icelandic Low pressure, Ice-
landic Low longitude, Icelandic Low latitude, Azores High
longitude and Azores High latitude (see Table 1).. From
the correlation coefficients, we find that summer Icelandic
Low latitude and Azores High latitude have highest cor-
relation coefficients −0.70 and −0.69 respectively while

S. A. RASHID ET AL. 994
Figure 1. Correlation summer precipitation over Europe
and NAO.
Figure 2. Correlation summer precipitation over Europe
and Azores High latitude.
Table 1. Correlation coefficients of summer precipitation
over north-west Europe with NAO, AO and the center of
action (COA) variables.
# Variables Correlation Coefficients
1 NAO −0.29
2 AO −0.52
3 Nino34 0.29
4 ILPS 0.49
5 AZPS −0.11
6 ILLT −0.70
7 ILLN 0.31
8 AZLT −0.69
9 AZLN −0.44
10 ILLNA & ILLT 0.74
(55)
11 ILLN & AZLT 0.725
(53)
summer NAO, AO and Nino34 have −0.29, −0.5 and
0.29 respectively with the precipitation of NW Europe.
To examine the variable which has dominant influence
on summer precipitation over NW Europe, we compute
the partial correlation coefficients for NAO index, AO
index, Nino34, Azores High Longitude and summer
precipitation. The partial correlation coefficients report
the contribution from a given index on the winter pre-
cipitation while holding all other independent variable
fixed. As shown in Table 2, Azores High latitude and
winter precipitation are correlated 95% percent confi-
dence level, when these indices are held fixed. Thus the
partial correlation, r = −0.57, which shows that the posi-
tion of Azores latitude has a direct and significant effect
on the summer precipitation over NW Europe. Similarly,
the partial correlation between AO index and the pre-
cipitation is −0.32, keeping other variables as fixed.
However, the partial correlations between NAO index
the precipitation and between Nino34 and precipitation
are not significant. Thus it shows that Azores High lati-
tude has dominant influence on summer precipitation
over NW Europe.
As Icelandic Low longitude and Icelandic Low latitude
has not significantly correlated (correlation between
them is −0.11), we construct a linear model of summer
precipitation over NW Europe using these two variables.
Thus, we have a linear model of precipitation:
NWER = 1210.452 − 14.922 (ILLat) + 1.613 (ILLon).
R2 for the region is 0.55, a significant enhancement
over the NAO value of R2 = 0.10.
Similarly, we also construct a linear model using two
independent variables i.e. Azores High latitude and Ice-
landic Low longitude. Thus, we have another linear
model of precipitation:
NWER = 1655.677 − 40.216 (AHLat) +1.613 (ILLon).
R2 for the region is 0.53, a significant enhancement
over the NAO value of R2 = 0.10.
By separating the NAO index into COA components,
we have been able to isolate the main variable related to
the precipitation and thus improve our regression model
by 43% over the fit to the NAO data alone. The regres-
Table 2. Partial correlation matrix for winter precipitation
of Iberia with respect to NAO index, AO, Nino 34 and,
Azores High latitude.
#Variables Partial Cor relation Coefficients
1NAO index and Rain −0.01
2AO index and Rain −0.33
3Nino34 and Rain −0.07
4AHLT and Rain −0.57

S. A. RASHID ET AL.
IJG
995
air from the Atlantic into NW Europe. sion with Azores High latitude and Icelandic Low lati-
tude (including Icelandic Low Longitude) both sepa-
rately capture the major patterns of wintertime observed
precipitation variations from 1952 to 2002 over NW
Europe (Figures 3-4).
The pressure composite for the years of extreme south
minus extreme north of Azores High at 850 mb is shown
in Figures 7. This figure shows that there is anomalous
low pressure band extends over North Atlantic and west-
ern Europe. Figure 8 shows the vector wind differences
at 850 mb between the winters in which the when Azores
High latitude was located to extreme south. There is
anomalous cyclonic flow Over Northern Atlantic and
NW Europe. We know that the seasonal mean cyclonic
anomalies represent greater frequency of synoptic scale
storm in the region. As shown in Figure 9, when Azores
High latitude was located to extreme north, vector wind
anomaly shows that there is anomalous anticyclonic flow
over Northern Atlantic and NW Europe. We know that
the seasonal mean anticyclonic anomalies represent
greater frequency of blocking episode in the region. Thus
our analysis confirms that regional circulations of the
atmosphere and the ocean are consistent with the em-
pirically determined relationship between summer pre-
cipitation and Azores High latitude.
5. Mechanisms for the R e la t io n sh ip s b e tween
Azores High Latitude and the
Precipitation Variability
In this section we present evidences that regional circula-
tions of the atmosphere and the ocean are consistent with
the empirically determined relationships. We construct
different composites with the phase of Azores High lati-
tude using NCEP/NCAR Reanalysis monthly averaged
fields of winds, pressure and humidity. Figure 5 shows
composite mean of 850 mb vector wind for 10 years
when Azores High latitude was located to extreme north
(dry over north-west Europe). The strong jet is located at
50˚N. While, Figure 6 shows composite mean of 850 mb
vector wind for 10 years when Azores High latitude was
located to extreme south (more rain over north-west Eu-
rope). The jet is displaced southward by 5˚N. There is
direct westerly flow over north-west Europe. When the
Azores High system is to the south there is flux of moist
6. Conclusions and Discussion
The North Atlantic Oscillation has been considered the
0
50
100
150
200
250
300
350
1952 1956 1960 1964 1968 1972 1976 1980 19841988 1992 1996 2000
Years
Precipitation (mm)
Observed
Regression
Figure 3. A comparison of the summer precipitation of NW Europe and our modeled values based on linear regre ssion model
for the years 1952-2002. The independent variable in our regression model is June, July and August (JJA) averaged Icelandic
Low Latitude (ILLat) and Icelandic Low Longitude. The variance in the winter precipitation explained by our regression
model is R2 = 0.55.

S. A. RASHID ET AL. 996
0
50
100
150
200
250
300
350
1952 1956 1960 1964 1968 19721976 1980 1984 1988 1992
Years
Precipitation ( mm)
1996 2000
Observed
Regression
Figure 4. A comparison of the summer precipitation of NW Europe and our modeled values based on linear regre ssion model
for the years 1952-2002. The independent variables in our regression model are June, July and August (JJA) averaged
Azores High Latitude (AHLat) and Icelandic Low Longitude. The variance in the winter precipitation explained by our re-
gression model is R2 = 0.53.
Figure 5. 850 mm vector wind speed shows that when Azores High displaced northwards, jet is located around 50˚N.

S. A. RASHID ET AL. 997
Figure 6. 850 mm vector wind speed shows that when Azores High displaced southwards, jet displaced southward by 5 de-
grees.
Figure 7. Difference in surface pressure between summers in which Azores High is southward and northward shows that
anomalous low pressure band extends over North Atlantic and Western Europe.

S. A. RASHID ET AL. 998
Figure 8. Composite anomaly of vector wind at 850 mb shows that the anomalous cyclonic flow over Northern Atlantic and
NW Europe when Azores High displaced southward.
Figure 9. Composite anomaly of vector wind at 850 mb shows that the anomalous anticyclonic flow over Northern Atlantic
and NW Europe when Azores High displace d northward.
dominant modulator of regional variability over Europe.
The results presented in this work show that north-south
shifting of Azores High latitude and Icelandic Low lati-
tude is the primary dominant dynamical mode for ex-
plaining variations in summer precipitation over NW
Europe. This mode explains significantly greater vari-
ance in summer precipitation variability than the North
Atlantic Oscillation or the Arctic Oscillation. When the
Azores High system is to the south there is flux of moist
and warm air from the Atlantic into NW Europe.
There has been a discussion in the literature on weath-
er the NAO or the AO represent the best paradigm for
representing atmospheric variability in the north Atlantic
region [19]. Wallace (2000) has suggested criteria for
choosing between the two by considering their impact on
regional climate: “If its impacts prove to be largely at-
tributable to (1) anomalous temperature advection in-
volving the strong thermal contrasts between the North
Atlantic and the upstream and downstream con tinents , (2)
changes in the latitude or intensity of the North Atlantic
storm track and its downstream extension into Europe, (3)
anomalies in the stationary wave configuration by dia-
batic heating and/or storm track dynamics over the
North Atlantic, or (4) changes in the frequency of block-
ing in the North Atlantic sector, it would argue in favor
of the NAO paradigm. On the other hand, if the impacts

S. A. RASHID ET AL. 999
can be shown to be more pervasive and extensive than
can be accounted for processes operating in or remotely
forced from the Atlantic sector, it would argue in favor of
annular mode paradigm”. In the results presented in this
paper we see that the four criteria listed by Wallace are
satisfied and that would favor the NAO as the better pa-
radigm. However, the correlation of precipitation over
NW Europe with the NAO is smaller than with the AO.
The results presented in this paper show that this is be-
cause the NAO definition, as traditionally defined, does
not include the interannual variations in the sea level
pressure distribution in the north Atlantic. The north At-
lantic is the dominant influence on of precipitation over
NW Europe, but the relevant dynamics cannot be repre-
sented by time variations of affixed pattern such as an
EOF, a formalism that can be used to define the NAO
and AO.
This article suggests that the north-south fluctuations
of the Azores High is a dynamical mode of tropospheric
variability which is not adequately represented by either
the AO or the NAO paradigms. This is because both the
AO and the NAO are defined as EOFs (Wallace, 2000).
However, a major limitation of the EOF method is that it
seeks to find a time independent pattern in the data field,
and gives the principal component as the time variation
of the fixed pattern. We suggest that the COA is a better
paradigm in comparison with NAO or NAM for under-
standing regional variations.
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