ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus GmbHGöttingen, Germany10.5194/angeo-33-207-2015The influence of solar activity on action centres of atmospheric circulation in North AtlanticSfîcăL.sfical@yahoo.comVoiculescuM.HuthR.Faculty of Geography and Geology, Alexandru Ioan Cuza University, Iaşi, RomaniaFaculty of Science and Environment, Dunărea de Jos University, Galaţi, RomaniaFaculty of Science, Charles University, Prague, Czech RepublicInstitute of Atmospheric Physics, Academy of Sciences of the Czech Republic, Prague, Czech RepublicL. Sfîcă (sfical@yahoo.com)26February201533220721517June201421January201526January2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/33/207/2015/angeo-33-207-2015.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/33/207/2015/angeo-33-207-2015.pdf
We analyse the response of sea level pressure and mid-tropospheric (500 hPa)
geopotential heights to variations in solar activity. We concentrate on the
Northern Hemisphere and North Atlantic in the period 1948–2012. Composite and
correlation analyses point to a strengthening of the North Atlantic
Oscillation and weakening (i.e. becoming more zonal) of the Pacific/North
American pattern. The locations of points with lowest and highest sea level
pressure in the North Atlantic change their positions between low and high
solar activity.
Meteorology and atmospheric dynamics (Climatology)Introduction
There is a lot of interest in establishing the influence of solar activity
upon atmospheric circulation as accurately as possible, since this
contributes to quantifying effects of solar variability on terrestrial
climate. This also offers the possibility of increased accuracy for decadal
predictions (Smith et al., 2013). The horizontal pattern of sea level
pressure (SLP) field is the main driver of the atmospheric circulation.
Strong correlations between wintertime temperature and pressure disturbances
in Europe, on the one hand, and solar activity, on the other hand, are
observed at long-term timescale (decennial–centennial) (LeMouel et al.,
2009; Woollings et al., 2010). Variations in phase with solar cycles were
observed in North Atlantic SLP (Kelly, 1977). The early work of Rumney (1968)
suggested that the strongest solar signal over the North Atlantic is
associated with regions where the polar fronts are mainly located. These are
in principal regions of high cyclonic activity (Gleisner and Thejll, 2003).
Correlations between various solar proxies and sea level pressure in the
Arctic region of Canada and Greenland (Mansurov et al., 1974; Page, 1989)
and a clear relation between solar activity and the position and intensity
of Aleutian Low and Californian High (Christoforou and Hameed, 1997) were
found using the mean composite difference method. Recent studies show that
there is an apparent clear response to the solar cycle in the lower
stratospheric geopotential structure (Labitzke and van Loon, 1995, 1988; van Loon
and Labitzke, 1993; Bochníček et al., 2012).
Several studies indicate that atmospheric circulation in the northern
mid-latitudes, and in particular in the Euro-Atlantic sector, becomes more
zonal during high solar activity (Bochníček and Hejda, 2002; Huth et
al., 2006, 2008; Barripedro et al., 2008). This may be related to a
strengthening of the North Atlantic Oscillation (NAO), which is an
atmospheric oscillation (“seesaw”) between the Icelandic low and Azores high
with a direct impact on the atmospheric circulation in the Northern
Hemisphere and on climate conditions in most of Europe (Hurrell et al.,
2001). A correlation between sunspot numbers and the NAO index was
identified on long timescales (Boberg and Lundstedt, 2002), with high solar
activity related to positive NAO phase. Other studies found a negative
correlation between the same parameters, however (Kirov and Georgieva,
2002). Studies on Maunder Minimum have indicated a direct relationship
between the low solar activity during this period and the negative NAO phase
(Wanner et al., 2008; Slonosky et al., 2001; Luterbacher et al., 2001;
Xoplaki et al., 2001; Shindell et al., 2001; Langematz et al., 2005). A
reduction in SLP at 20–40∘ N in the Pacific sector during high
solar years was mentioned by van Loon and Meehl (2008) in a study applying
the composite mean difference method. These results were confirmed by Roy
and Haigh (2010) by means of multiple linear regression applied to time
series of 155 years. They found a region of positive anomaly of about
5 hPa
in the North Pacific corresponding to a weakening of the Aleutian low during
high solar years. Generally, an expansion of the zonal mean Hadley cell and
a poleward shift of the Ferrel cell was observed during solar maxima
(Brönniman et al., 2006; Haigh, 2003, 1996; Larkin et
al., 2000; Matthes et al., 2006).
Due to a weak signal of solar activity in SLP, some authors have tried in
the last period to apply a multiple linear regression technique to estimate
the SLP response to solar forcing during northern winter as a function of
phase lag. Recent studies of Gray et al. (2013) and Hood et al. (2013)
investigated the SLP response to solar signal as a function of phase lag
during northern winter, using a long data set (more than 130 years). Hood et
al. (2013) found that the NAO index progress from a mainly negative phase
prior to solar maximum to a mainly positive phase at and following solar
maximum while Gray et al. (2013) found that the NAO index is significantly
positively enhanced several years after solar maximum.
Data and methodology
We use monthly averages of SLP and 500 hPa geopotential height (GPH) from
the NCEP/NCAR reanalysis (Kalnay et al., 1996), for a 65-year period
1948–2012, which constitutes 6 full solar cycles, for a region between
20∘ N and the North Pole. SLP data are available on a grid with
the zonal and meridional step of 5∘. Similar to the majority of
other studies, the cold season (October to March) and winter (December to
February) are only analysed here. NCEP/NCAR data were selected because of
the possibility of using both SLP and 500 hPa GPH data in our attempt to
search for a relation throughout the lower and middle troposphere between the
atmospheric circulation and solar activity, and also because they are
regularly updated, and so available until recently.
Data were divided into low and high solar activity months according to the
solar activity level, using the sunspot number as a proxy. Solar activity is
considered to be high (low) if the associated sunspot number is in the upper
(lower) third of the entire data set, similarly to Barriopedro et al. (2008).
Differences of the composite GPH and SLP between the high and low solar
activity were then calculated. The statistical significance of the
differences between high and low solar months was tested using the
Student test for the difference of means. Direct and 1-month lagged Pearson
correlations between solar activity and SLP (GPH) were calculated. This
method is used for simplicity and in the future we plan to use a multiple
regression method which is attested in recent works (Gray et al., 2013) as
a method that can yield information about other possible sources of
pressure variations, such as El Niño–Southern Oscillation. On the other hand, the composite approach that
we use can be considered more robust and general than linear regression
because the latter assumes a linearity of the atmospheric response, which
may not be realistic.
The positions of the major centres of action of atmospheric circulation were
localized as the lowest (highest) value of SLP in the Euro-Atlantic domain,
extending between 20 and 90∘ N and 70∘ W and
30∘ E. Separate frequency maps, showing where these centres are
located, were created for high and low solar months. The lowest SLP in the
domain can be identified with the Icelandic low, while the highest SLP
coincides either with the Azores high or with the high-pressure centre over
Greenland. Differences in the latitudinal and longitudinal distribution of
both pressure formations between high and low solar activity were assessed
using the two-sample Kolmogorov–Smirnov test. In brief, the two-sample KS-test
determines whether two data sets were drawn from the same distribution or
generating process and its main statistic (D) looks for the largest
difference between the empirical cumulative distribution functions of the
two samples (Wilks, 2006).
Mean difference between months with high and low solar
activity for the 500 hPa GPH (in metres) (left) and SLP (in hPa) (right), for
winter. Regions with statistical significance of differences are delimited
by continuous (10 % significance level) and dotted (5 % level) lines,
indicating positive (black) and negative (white) correlation.
Results and discussionsComposite and correlation analysis
Figure 1 shows the mean difference between high and low solar activity for
500 hPa GPH (in metres, left) and for SLP (in hPa, right) in winter. Regions
where the difference is statistically significant are shown with black or
white lines. The differences between high and low solar activity reach for
500 hPa GPH as much as 50 m (when the whole cold season is considered; not
shown) and 30 m during winter. The SLP variation reaches ±3hPa
during winter.
The relation of tropospheric circulation to variations in solar activity is
strongest in the North Atlantic. The statistical significance of differences
is mostly rather marginal, only small regions exhibiting significant
differences. An increase in GPH and SLP during high solar conditions can be
seen over the Iberian Peninsula and western Mediterranean; similar effects
were obtained on a longer period by Brugnara et al. (2013) and Gray et
al. (2013). On the other hand, decreases of GPH and SLP concentrate in the
vicinity of Iceland and over the North Sea. This can be interpreted as a
strengthening of both permanent pressure formations, and possibly also their
slight shift eastwards. Taken together, this implies a strengthening of the
NAO and an intensification of the westerly flow from the North Atlantic into
Europe. This also supports Woollings et al. (2010) and van Oldenborgh et al. (2013)
who found the same pattern of relationship between solar activity and
500 hPa GPH for a longer period in the North Atlantic–European sector and is
in a very good agreement with e.g. Bochníček and Hejda (2002), Huth
et al. (2006, 2008) and Barriopedro et al. (2008) who report an
intensification of westerlies or NAO under high solar conditions, using
different analysis tools. On the other hand, our results disagree with Kirov
and Georgieva (2002), who however analysed autumn, and an older study by
Girs and Kondratovich (1978) cited by Kirov and Georgieva (2002); both of
these studies found a shallower Icelandic low and weaker Azores High to be
related to high solar activity. However, this contradiction might be only
apparent, since the correlation between solar activity and NAO is not
constant on various timescales depending on the secular phase of solar
cycle (Georgieva et al., 2012), the level of geomagnetic activity (Li et
al., 2011), and possibly also the phase of the quasi-biennial oscillation
(van Loon and Labitzke, 1988; Huth et al., 2009).
Pearson correlation between solar activity and 500 hPa GPH: direct
(left) and 1 month lagged (right). White means no significant correlation,
light red (blue) stands for significant positive (negative) correlations at
10 %; dark red (blue) shows regions of positive (negative) correlation
(5 % significance).
In the North Pacific/North American sector, the solar signal is more
significant in the SLP field. The association of the Aleutian low with solar
activity is opposite to the Icelandic low: its intensity tends to decrease
(i.e. its GPH and SLP tend to increase) with increasing solar activity.
This is in line with several previous studies (Christoforou and Hameed,
1997; van Loon and Meehl, 2008; Roy and Haigh, 2010; Gray et al., 2013; Hood
et al., 2013; Scaife et al., 2013); however, we note that the amplitude and
significance of the difference between high and low solar activity is
smaller than in some of these studies. The disagreement in the amplitudes is
probably due to the fact that the period of our analysis is much shorter (65
against at least 130 years). A similar response, i.e. higher GPH and SLP in
high solar activity, occurs over the eastern US, while the opposite,
although insignificant and only in GPH, is observed over the western shore
of North America where high solar activity is accompanied with lower GPH
values. The spatial pattern of the differences is reminiscent of the
mid-latitude part of the Pacific/North American (PNA) teleconnection
pattern, the four centres of which are located over Hawaii, the Aleutian
Islands, western US/Canada, and southeastern US; for more details on
the PNA pattern and its relevance for the North American climate, see e.g. Leathers et al. (1991). The response of the PNA pattern to solar activity
can be interpreted as its weakening, i.e. shift to lower values,
accompanied with a more zonal flow over North America, in solar maxima.
Changes in the position of the action centres of the PNA pattern, although
not in its amplitude, in response to solar activity were reported by Huth et
al. (2006). Atmospheric circulation, and more so SLP, seems to be sensitive
to solar effects also in Central Asia.
The composite analysis is accompanied by directly calculating Pearson
correlations between solar activity and 500 hPa GPH (Fig. 2a). The areas of
significance are of similar size and strength as for the composites in
Fig. 1a. The similarity between the patterns of significant differences and
significant correlations suggests that the response to variations in solar
activity are mostly linear because Pearson correlations are a measure of the
linearity of association while the composite analysis reflects a
monotonicity of the association rather than its linearity. Since the
response of tropospheric circulation to solar activity is unlikely to be
immediate, we calculated also 1 month lagged Pearson correlations between
GPH and solar activity (Fig. 2b). They show only marginal differences
relative to simultaneous correlations, which confirms the appropriateness of
analysing circulation data with zero lag relative to solar activity.
(a) Upper panel: relative frequency (%) of the lowest mean SLP
occurring during high (left) and low (right) solar months for the cold
season (October–March).
(b) Lower panel: relative frequency (%) of the lowest mean SLP occurring
during high (left) and low (right) solar months for winter
(December–February).
(a) Longitudinal relative frequency of the minimum SLP occurrence
during cold season (left) and winter (right) for different solar activity
conditions.
(b) Latitudinal relative frequency of the minimum SLP occurrence during cold
season (left) and winter (right) for different solar activity conditions.
(a) Longitudinal relative frequency of maximum mean SLP during cold
season (left) and winter (right).
(b) Latitudinal relative frequency of maximum SLP during cold season (left)
and winter (right).
Cumulative latitudinal position frequency of minimum SLP (a) and maximum
SLP (b) for different solar activity conditions (horizontal line, mean
position; red box – frequency between first and third quartile; blue box –
frequency between first and last decile; whiskers – appearance).
Cumulative longitudinal position frequency of the minimum
SLP (a)
and maximum SLP (b) for different solar activity conditions (horizontal
line, mean position; red box – frequency between first and third quartile;
blue box – frequency between first and last decile; whiskers – appearance).
Solar activity versus SLP centres
A small but significant shift in the position of the grid point with lowest
SLP in the Euro–Atlantic sector, which approximately represents the position
of the Icelandic low, is observed when the solar activity varies from high
to low. Figure 3 shows the frequency of occurrence of the lowest SLP value.
During high solar activity the position of the cyclone is more
geographically confined and its most frequent position is south of
Greenland, in the area bounded by 25–55∘ W and
55–65∘ N. There is also a second preferred location of the lowest SLP near
and north of the Norwegian coast, between 5 and 15∘ E, which may
correspond to the eastward shift of the NAO pattern during its positive
phase presented by Peterson et al. (2003). One should note that the former
region (around 40∘ W and 60∘ N) is also known as the
main domain of Icelandic cyclogenesis (Schneidereit et al., 2006) at the
polar front; thus suggesting a possible connection between solar activity
and cyclogenesis, mentioned e.g. by Veretenenko et al. (2006). The overall
shape of the area where Icelandic cyclones occur, mainly its spatial
consistency over the North Sea in high solar activity, points to the fact
that during high solar conditions, the northeasterly track of cyclones over
this area is more favoured, supporting results of Huth et al. (2006) and
Barriopedro et al. (2008) who observed a tendency toward a stronger zonal
circulation.
A comparison of the latitudinal and longitudinal positions of the lowest and
highest SLP between the high and low solar activity is provided in Figs. 4
and 5, respectively. First we examine the Icelandic low (Fig. 4). Its
longitudinal distribution is clearly bimodal, with maxima around
30 to 40∘ W and 10∘ E. In high solar
activity, the western maximum is more geographically confined; in particular
the frequencies west of 45∘ W are much lower than in low solar
activity. The other feature of interest is higher frequencies between the
two longitudinal maxima, i.e. between 0 and 20∘ W in
high solar activity. This again points to a more zonal-like circulation in
high solar activity. The differences are stronger for winter than for the
entire cold season, suggesting that the solar effects are indeed most
pronounced in the winter months in the North Atlantic–European region.
Figure 5 shows the variation of the maximum SLP location in the
Euro–Atlantic region with solar activity. The location of the SLP maximum
appears to be less dependent on solar activity compared to the SLP minimum.
Two action centres manifest as the positions of the highest SLP: the Azores
anticyclone and the high over Greenland. They can be easily distinguished by
their latitude: while the former is located mostly between 30 and
50∘ N, the latter occurs mainly between 70 and
80∘ N. Both act as the SLP maximum with approximately the same
share of 50 %. Worth mentioning is the tendency of the Azores anticyclone
to be located more southward in solar maxima and of the Greenland high to be
more southward in solar minima; both effects are more pronounced in winter
than in the longer cold season. A more northward position of the maximum SLP
in low solar activity reflects a stronger and/or more frequent Greenland
high, which is in line with the finding by Barriopedro et al. (2008) that
blocking situations over the North Atlantic are more frequent and last
longer in low solar activity. On the other hand, the Azores high plays a
stronger role in high solar activity, reflecting, among others, its
considerably larger geographical extent (Kodera, 2003; Huth et al., 2006).
Figures 6 and 7 synthesize information from Figs. 4 and 5 using boxplots.
They demonstrate a slight shift of the Icelandic low towards north and west
in high solar activity and a southward shift of the high SLP centre. We may
look at the longitudinal and latitudinal distributions of the positions of
the lowest and highest SLP as statistical distributions. The two-sample
Kolmogorov–Smirnov test comparing the high and low solar months suggests
that the null hypothesis that both the high and low solar data were drawn
from the same distribution cannot be rejected in any of the four cases
(highest and lowest pressure; latitudinal and longitudinal distribution).
Conclusions
The composite and correlation analyses suggest that the response of lower
tropospheric circulation, represented by SLP and 500 hPa GPH here, to solar
activity is strongest in the North Atlantic where both the Icelandic low and
Azores high are strengthened, and so is the NAO. The signal related to solar
activity is somewhat weaker in the Pacific and North American domain where a
tendency towards a negative phase of the PNA pattern in high solar activity
is observed.
Solar maximum conditions favour a higher occurrence of the minimum of SLP
around the main region of Icelandic development of low-pressure systems,
along the northeasterly track of the cyclones. During solar minimum
conditions, the spatial distribution of the minimum SLP is more diffuse,
extending to subtropical latitudes on a wider area. The occurrence of lows
close to the Norwegian coast seems to be favoured also by low solar activity.
Differences of ±50m (cold season) and ±30m (winter) were
observed for 500 hPa, while a high to low solar difference of ±3hPa
sea level pressure could lead to a significant strengthening of the westerly
flow in mid-latitudes.
Generally the direct connection between solar activity and the location of
pressure centres of action in the North Atlantic is rather small, but this
could be a signal which can propagate in the lower troposphere by other
positive feedback mechanisms. Maximum solar months are related to a
stronger pressure gradient between the Azores high and the Icelandic low,
i.e. to positive NAO phases. The lowest and highest pressure in
the North Atlantic shift their position, as can be seen from the histograms
of their longitude and latitude, but these shifts do not reach statistical
significance.
The solar effect on atmospheric circulation in the North Atlantic can be
described as a tripole mechanism. During solar maximum conditions the
differences between the Icelandic Low and Azores High increase, while the
Greenland High decreases. Solar minimum conditions reinforce the high
pressure above Greenland together with a weakening of the other two North
Atlantic pressure centres.
Acknowledgements
L. Sfîcă was supported by the strategic grant POSDRU/159/1.5/S/133391 – Project “Doctoral and
Post-doctoral programs of excellence for highly qualified human resources
training for research in the field of Life sciences, Environment and Earth
Science” cofinanced by the European Social Fund within the Sectorial
Operational Program Human Resources Development 2007–2013”. M. Voiculescu
acknowledges project PN-II-ID-PCE-2011-3-0709, SOLACE of the Romanian NPRDI-II, UEFISCDI. Support from the
Ministry of Education, Youth, and Sports of the Czech Republic under project
LD12053, is acknowledged by R. Huth. Site http://www.esrl.noaa.gov/ is gratefully
acknowledged for free use of data. The study benefited from networking within
the COST through ES1005 Action TOSCA: “Towards a more complete
assessment of the impact of solar variability on the Earth's climate”. We
thank the referees for their useful comments and observations and Pavel
Ichim (UAIC) for support in mapping design. Topical Editor C. Jacobi thanks three anonymous referees for their help in
evaluating this paper.
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