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Nina Bachmann, HS 2007.
The North Atlantic Oscillation (NAO)
Research, mechanisms and future outlook
Contents
Abstract
1
1. Introduction
2
2. History and trends in NAO research
3
3. Temporal structure of NAO
4
4. Spatial Structure of NAO
5
5. Dynamics and possible mechanisms of NAO
8
6. Influences of NAO on Europe
10
7. Simulations of future NAO development
13
8. Conclusions and Outlook
14
References
16
Abstract
The North Atlantic Oscillation (NAO) phenomenon describes the varying strength of two atmospheric pressure
systems lying over the subpolar and the subtropical region of the North Atlantic. The variability in the strengh
of this pressure gradient can be expressed by an Index, which gets positive if the pressure systems are well
established and negative when the pressure gradient between them is weaker. Especially during northern hemisphere winter (December to March), the NAO has a strong influence on weather conditions in Europe: A positive phase of NAO causes high precipitation and mild temperatures in Northern Europe which indirectly influences for example the European biosphere or the amount of snow in the Alps. Therefore, predictability of
NAO is of high interest for meteorologists. Unfortunatly, it is still unclear which mechanisms are driving
NAO. Due to this, short-term predictability has been impossible so far. In view at climate change, long-term
modeling of future NAO development became the focus of recent NAO research. Some models indicate an
upward trend of NAO Index towards higher positive values, which would have severe effects on European
weather conditions. Here, actual knowledge and future possible trends in NAO research are discussed, with a
focus on the consequences for the European continent.
Nina Bachmann, D-UWIS, [email protected]
1.
2
Introduction
The analysis of pressure fields shows that their variability always exhibit certain patterns, which are
called variability patterns or modi (Brönnimann, 2005). The North Atlantic Oscillation (NAO) is one
of the most prominent patterns of atmospheric circulation variability and is the only atmospheric
mode, which can be observed during the whole year in the northern hemisphere (Hurrell et al.,
2003). The NAO consists of opposing variations of barometric pressure over the subpolar and the
subtropical region of the North Atlantic (IPCC, 2007). In general, there is a high pressure system
over the subtropical region near the Azores, and a low pressure situation over the subpolar region
near Iceland (Wanner et al., 2001). The strength of the pressure gradient between the two so-called
centres of action varies with time within stronger and weaker phases. The measure for this variation,
the NAO Index, gets positive in a stronger phase with a high pressure gradient and negative in phases with a weaker pressure gradient (Brönnimann, 2005).
The pressure difference between the Azores and Iceland is associated with westerly winds across the
Atlantic into Europe, and as the pressure gradient fluctuates, the strength of the westerly winds also
varies. This has a high importance for weather and climate in the whole North Atlantic region: With
western winds, mild and humid air is transported from the Atlantic into Europe and determines temperature and precipitation, especially during northern hemisphere wintertime from December to
March (Hurrell and Dickson, 2004). Thompson et al. (2000) showed, that for the winter months
from 1968 to 1997, NAO (or NAM, see below) accounts for 1.6°C of the 3.0°C warming in Eurasian
surface temperatures (IPCC, 2007). There were also significant effects on ocean heat content, sea
ice, ocean currents and ocean heat transport (IPCC, 2007). That means, understanding the processes
that govern NAO variability is important for weather and climate predictability and is therefore of
high priority, especially in the context of global climate change (Hurrell et al., 2003).
The definition of an Index for NAO to relate the variability of two centres of action is only one possibility to explain a variability modus. Another is a more statistical approach where time series and
spatial patterns are analysed through main component analysis. Through this approach, another variability pattern, the Arctic Oscillation (AO) or Northern Hemispheric Annular Mode (NAM) is defined. Despite of the different definition approaches, the NAO and the AO/NAM show a similar spatial pattern, but AO/NAM has a third center of action in the pacific and the surface land pressure in
the polar region is more important than for NAO, where the polar low is equally weighted like the
subtropical high (Brönnimann, 2005). It is still an open question, whether NAO and AO/NAM are
the same phenomena or whether NAO and AO/NAM are not only different definitions but also different concepts. The suggestion that they are different concepts would also mean, that the underlying
processes for both wouldn‘t be the same (Hurrell et al., 2003). Here, only the NAO is adressed, for
further information the reader is referred to Wallace (2000).
In the following section, there is a short overview about history of NAO research, followed by a more detailled description of the temporal and spatial structure of the phenomena. In chapter 5, different mechanisms which might be forcing NAO are discussed. Chapter 6 gives informations about the
consequences of NAO for European climate and weather conditions, whereas Chapters 7 and 8 discuss future NAO development and trends in NAO research.
Nina Bachmann, D-UWIS, [email protected]
2.
3
History and trends in NAO research
The NAO phenomenon was first discovered through the observation of different weather and climate conditions during winter. The occurrence of periods with mild winters and others with severe winters lead to descriptions of NAO-related weather observations already several hundert years ago. In
the nineteenth century, more data became available and researchers began to investigate temperature
series in the northern hemisphere. The focus of most meteorologists and climatologists at that time
was mainly on forecasting weather and therefore more on data observation of the phenomenon than
on understanding its physical processes (Wanner et al., 2001).
One of the most cited researchers from the early 20th century in todays NAO related studies is G.T.
Walker. With his statistical research of pressure data he found three modes which dominate world
weather. In his paper from 1924 he defined the terms North Atlantic Oscillation, North Pacific Oscillation and Southern Oscillation (Walker, 1924). Walkers concept of NAO became very popular, and
a few years later Walker and Bliss (1932) constructed the first quantitative measure for the strength
of NAO, a NAO Index. This first Index involves several temperature and pressure data from Europe
and North America and can be calculated through a rather complex, iterative statistical procedure.
Until today, Walkers and Bliss‘ NAO Index was adapted several times, but it‘s principle remained
the same (see chapter 3).
In the 1960s, new, more dynamical and less descriptive theoretical concepts of climate variability in
the Atlantic-European region were developed. Bjerknes (1964) reviewed ocean-atmosphere interactions in relation to North Atlantic climate variability. He mentioned that heat exchange in the atmosphere and sea surface temperatures (SSTs) must play an important role for the NAO. In addition,
the interaction dynamics of the atmosphere circulation and pressure centres over land were described
by leading meteorologists of that time (for example Lorenz, 1967).
In the 1970s and 1980s some important studies were made which lead to more detailled knowledge
about the NAO concept. Van Loon and Rogers (1978) for example found, based on Bjerkes‘ research, significant correlations between atmosphere circulation and sea surface temperature and
Barnston and Livezey (1987) showed, that the NAO is the only circulation pattern in the northern
hemisphere which is found in every month in the year, although it‘s quite weak in European summer
time.
In the 1990s, researchers focused on climate modelling with respect on the NAO anomalies. With a
publication by Hurrell (1995), the interest in NAO and its influence on climate increased rapidly.
Hurrell related the NAO phenomenon with temperature and pressure variability over Europe in a
interannual to decadal timescale. He also defined a new Index for NAO, which is todays most used
NAO Index in climate research (see chapter 3 for more details). More research has been done in the
following years on reconstructing NAO Indexes on larger timescales, in order to explain low frequency or short-term atmospheric variability over the Atlantic-European area. Most recently, the role
of NAO in climate change was discussed, an overview about this is given in chapter 7 and 8.
Nina Bachmann, D-UWIS, [email protected]
3.
4
Temporal structure of NAO
A measure for the state of the NAO is an Index called the North Atlantic Oscillation Index (NAO
Index). There are several ways to calculate this Index, depending on the statistical analysis of meterological parameters used (Wanner et al., 2001). The NAO Index which was defined by Hurrell
(1995) for example, uses weather data from Stykkisholmur (Iceland) and Lisbon (Portugal): Sea
level pressure data are normalized to „normal“ pressure, which is defined through mean sea level
pressure over several years. The normalization gives the anomaly of the actual pressure compared to
normal pressure. The normalized pressures of Iceland is then subtracted from the normalized pressure of Portugal. The NAO Index gets positive for example, if there is a lower than normal pressure
over Iceland, respectively a higher than normal pressure over Portugal, or if both anomalies occur
(Hurrell et al., 2003).
The „Hurrell-Index“ has the advantage, that it can be calculated back to the XIX century, because
weather data from more than 100 years are available from the two weather stations which allows the
construction of time series of NAO Index (Wanner et al., 2001). The actualized time series of NAO
Index for the winter months calculated by Hurrell (1995) and updated until 2003 is shown figure 1.
6.00
NAO Index 1864 – 2007
4.00
2.00
0.00
-2.00
-4.00
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64
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69
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74
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79
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84
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94
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99
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04
19
09
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64
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74
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79
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84
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89
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94
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99
20
04
-6.00
year
Figure 1: Updated NAO wintertime Index from 1864 to 2007 with value of NAO Index on y-axis (Hurrell,
1995; new data available from www.cgd.ucar.edu/cas/jhurrell/indices.html).
There are several observations about NAO which can be driven from this time series (Hurrell, 1995):
The NAO has a kind of „memory“, it shows a so-called interannual variability. This means, that
NAO remains often more than one year in the same phase before it changes again. Furthermore, the
changes from positiv to negative values often seem to happen on longer timescales, which is referred
to as interdecadal variability. For example, the Index from 1940 until about 1965 shows a decreasing
trend, while from then on, NAO Index values seem to increase until about 1995 (Hurrell, 1995).
Nina Bachmann, D-UWIS, [email protected]
4.
5
Spatial Structure of the NAO
The NAO Index can be used to express whether the pressure anomalies betweeen the two pressure
centres are high or low: If the Index is positiv, the NAO is said to be in its positive phase (NAO+)
and if the pressure anomalies are low, there is a negative (NAO-) phase. One must consider, that this
is a simplified model of the states of NAO, because it implies that these two phases are static and
bipolar. In reality, the strength of the phases can differ a lot from each other, which is expressed by a
wide range of values for the NAO Index. The aggregation of all the different phases to only two phases, a NAO+ and a NAO– phase, is used due to the weather and climate characteristics, which are
similar for all states of NAO which have a positive or a negative NAO Index value (Wanner et al.,
2001). In this section, the characteristics of NAO+ and NAO– phases are discussed.
4.1
NAO+ phase
The characteristic of a NAO+ phase is a well developed Icelandic Low and an Azores High. As air
always flows from higher to lower pressure, the pressure gradient between the two regions causes
stronger than normal westerlies from the eastern Atlantic towards the European continent (see figure
2). Atlantic air is wet and mild and in winters with NAO+ phases, precipitation and temperatures in
northern Europe are therefore higher than normal. In contrary, Southern Europe tends to be drier and
colder in NAO+ winters due to the extension of the Azores High (Wanner et al., 2001).
The influence of NAO is not limited to Europe only. The well established Icelandic Low leads to
strong northerly winds over Greenland and northeastern Canada. This causes a decrease in land and
sea surface temperatures over the northwestern Atlantic (Hurrell et al., 2003). Northern Canada is
like northern Europe drier and colder during a NAO+ winter. On the other hand, the Icelandic Low
extends over northern Siberia, where it causes warmer and more humid weather compared to normal
Siberian conditions. This affects river runoff and sea-ice extension: If NAO stays several winters in
its positive phase, sea ice boundary extends farther south at the coast of Canada, while the sea ice
boundary of Siberian ice lies more northern than normal.
Sea-ice extension affects salinity, which in turn affects ocean circulation, at least on longer timescales: The global thermohaline circulation variability seems to be synchronized with interdecadal NAO
variations (Hurrell et al., 2003). During a NAO+ phase, the salinity at the coast of Canada is higher
than normal due to the increased sea ice extension (see figure 2). In this case, the dense and cool water sinks down more rapidely than in NAO– phases. Delworth and Dixon (2000) suggest, that increased deep water formation in the North Atlantic region at the coast of Canada could delay the
slowing down of the thermohaline circulation, which was projected by global climate warming
scenarios. Figure 2 gives an overview of the important processes and of heat and water fluxes which
are dominant during a NAO+ phase.
Nina Bachmann, D-UWIS, [email protected]
354
HEINZ WANNER ET AL.
Figure 9a. Graphical representation of the two modes or states of the NAO, based on discussions
with different researchers and a review of different papers. Surfaces mark SSTs and sea-ice extension, arrows show the flow systems in ocean, atmosphere and rivers, blue and red lines indicate
near surface
sea level
pressures and white rectangles describe characteristic climate conditions or
Figure 2: Schematic
NAO+
phase.
important
processes.
(a) Positive
mode, (b) Negative
Icelandic low and
Azores
high are
well established
[big Lmode.
and H], other, smaller high pressure centres
over the
continents are shown as well [H].
As air always flows from higher to lower pressure, the large pressure difference between Azores High and Icelandic Low c causes stronger than normal westerlies [blue-green arrow] towards northern Europe. The Azores
High also gives rise to stronger than normal esterlies [yellow arrow] from North Africa towards the Atlantic.
Sea surface temperature anomalies as well as sea-ice are marked with colored areas [orange: warm bluegrey: cold; light blue: sea-ice extension]. The possible relationship between sea surface temperatures, sea-ice
and NAO is discussed in chapter 5.
Ocean circulation could also play an important role for NAO [orange arrow for warm water transport; blue
arrow for cold water transport; light-blue arrows for under-ice circulation].
River runoff anomalies which are directly forced by higher or lower precipitation due to NAO are also shown
[blue arrows over land in polar region].
Characterisitc climatological conditions or important processes of the hydrological cycle are marked by white
rectangles and are further explained in the text.
(From Wanner et al., 2001).
6
Nina Bachmann, D-UWIS, [email protected]
4.2
7
NAO– phase
During a NAO– phase, there is normally still a Icelandic Low and a Azores High, but both are rather
weak. Therefore, the westerlies are reduced. Almost all characteristics which were described for
NAO+ are inversed during a NAO– phase and should therefore not be discussed here again (more
details in chapter 4.1).
Very rarely, the pressure distribution is reversed, that means there is high pressure over Iceland and
low pressure over Azores. The NAO Index is then very negative, and there is an easterly wind over
Eastern Atlantic. This was the case in January 1881, 1918 and 1963. This „inversed“ pressure gradients led to extremely severe winters, where, in Switzerland for example, several big lakes were frozen on the surface.
NORTH ATLANTIC OSCILLATION – CONCEPTS AND STUDIES
355
Figure 9b.
Figure 3: Schematic NAO– phase. Same symbols as in figure 2. (From Wanner et al., 2001).
Nina Bachmann, D-UWIS, [email protected]
5.
8
Dynamics and possible mechanisms of NAO
There are different possible mechanisms for the NAO. It‘s still unclear, if the NAO is purely an atmospheric phenomenon, or if the ocean-atmosphere interactions like thermohaline circulation, sea
surface temperature variability or sea-ice are influencing the NAO or if these factors are rather influenced by the NAO (Hurrell et al., 2006). The answer to this questions can be found through modelling of NAO. The type of data input to the models can be varied, for example sea surface temperature time series of the past 100 years can be used to generate a simulated NAO. Then, the simulated
NAO variability is compared to the observed variability. The percentage of how much variability is
explained by the model can then tell about the importance of the input data for NAO fluctuations.
5.1
Atmospheric processes
NAO can be simulated with atmospheric general circulation models (AGCMs), which are fed with
climatological variables like insolation, sea surface temperature, sea ice and snow cover which are
the model‘s boundary conditions. As climatological variables, these forcing fields contain no signal
of the NAO. Nevertheless, the spatial pattern and the amplitude of NAO Index anomalies are well
simulated, which shows that the NAO is not imposed onto the atmosphere by external forcing. Therefore, NAO must be caused by intrinsic forcing of the atmosphere, its land-sea distribution and nonlinear dynamics, at least on short time scales of 10 days (Feldstein, 2000). But if this would be the
only reason for the NAO phenomen, this would still not explain interdecadal trends and interannual
variability shown be NAO Index time series. „At present, there is no consensus on the process or
processes that are most likely responsible for the enhanced interannual variability of the NAO.“
(Hurrell et al., 2006). One theory is, that on longer time scales, the lower stratosphere influences the
troposphere, but how exactly this would cause the flow anomalies in the troposphere, remains still
unexplained.
5.2
Ocean influence
On longer time scales, the NAO could be influenced by the Atlantic ocean, but the inverse pathway,
that the NAO influences the ocean is also possible:
Hurrell et al. (2003) concluded, that Atlantic Ocean responses to the NAO, because the changes in
surface wind patterns through NAO has an influence on heat transfer and freshwater exchange on
Atlantic surface, which could alter the thermohaline and horizontal circulation. This would explain
the interdecadal sea surface temperature anomalies, which were observed over the last 100 years and
correspond to NAO pressure fluctuations: The subtropical gyre warmed below 1000 meters, and the
subpolar gyre cooled, which is consistent with a predominantly positive phase of the NAO (IPCC,
2007). At the same time, trends in salinity towards freshening in the subpolar regions and increased
salinity in the subtropics were observed (IPCC, 2007) and can also be related to NAO influenced
sea-ice and water flow processes which are explained in chapter 4.1. Since the 1995s, there has been
a return to warmer and more saline waters in the subpolar regions, which coincidences with a slight
Nina Bachmann, D-UWIS, [email protected]
9
decrease of NAO Indexvalues. But this return to saltier water has not been long enough to change
the long-term global trend, which indicates a water freshening of the relatively fresher regions
(IPCC, 2007).
As mentioned, the other way is also possible: Sea surface temperature fluctuations could force turbulent heat fluxes at ocean surface, which would have a direct effect on atmospheric boundary layer.
Rodwell et al. (1999) argue, that evaporation, precipitation and atmospheric heating processes depend on sea surface temperature of the Atlantic. They showed, that about 50% of the amplitude of
the long-term variability in the wintertime NAO index can be explained through an AGCM, which is
fed by sea surface temperatures (Rodwell et al., 1999; Hurrell et al., 2006). Czaja and Frankignoul
(1999) have also demonstrated, that North Atlantic sea surface temperature anomalies are followed
by anomalies in the atmospheric circulation (IPCC, 2007).
In conclusion, it‘s still unclear, whether the ocean is the dominant factor for NAO variability, or if
NAO is influencing the ocean on the other hand. Similarly, the role of sea ice and land snow cover is
still unclear: the influence could take place in the atmosphere-land or in the land-atmosphere way. A
ocean-influenced or sea-ice influenced NAO would make predictability of future NAO behaviour
easier and is therefore an important part of actual NAO discussion among researchers (see chapter
8).
2–4 °C below
ng from Scanpe to the Black
ween 50% and
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cken Spain.
his was a comthat had prens since the
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? The study by
is issue2 offers
d rainfall variave long been
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nce in sea-level
ssure adjusted
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of the winteratmospheric
en two states:
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and direction,
and heat and
e Atlantic and
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. No wonhave long been
he NAO ahead
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onsisting of a
ribe the largef momentum,
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cale processes
ormation, pret the land and
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he seasonally
he top of the
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ww.nature.com
evolution does not depend on initial condi- climate prediction. For this purpose,
tions. Thus they are not meant to predict the atmospheric models must be sensitive to the
actual evolution of the atmosphere but prescribed extratropical SST distribution.
D-UWIS, most
[email protected]
10
rather to depict, in a statistical sense,Nina
the Bachmann,
pat- However,
attempts to simulate the
tern and strength of atmospheric variability. atmospheric response to extratropical SST
In Rodwell and colleagues’ study, how- anomalies have been frustrating, yielding
ever, actual SST values over the entire world model-specific results that are inconsistent
ocean Influences
are used, prescribed
to follow
their and generally weak7,10. In what constitutes a
6.
of NAO
on Europe
observed monthly states between 1870 and breakthrough in this research, Rodwell et al.
1997. If atmospheric variability in this model have produced strong evidence for a synintegration is temporally synchronized with chronicity between model-simulated and
observations, the cause is in the ocean.
observed NAO fluctuations, resulting from
NAO
pressure
is the variations
cause for stronger
or weaker
westerlies,
depending
on the strength
Over
most ofdistribution
the world ocean,
specifying
realistic
SST variability
in the
of
pressure
anomalies,
by the
NAOextratropical
Index. The transport
of moisture
in SST
depend
largely onexpressed
heat exchange
with
North Atlantic
Ocean. and mild air through
the atmosphere,
sothe
are south
shapedeastern
by atmosDoes thisEurope
bring uscauses
closerchanges
to predicting
the
these
westerlies and
from
Atlantic towards
in temperature,
pre7,8
pheric
variability
NAO
and
its
effect
on
Europe’s
climate?
Only
.
The
ocean
itself
remains
cipitation and storminess over Europe (Rodwell et al., 1999). Figure 4 shows temperature and rainpassive, sluggishly accommodating fast- up to a point. In the extratropics atmospherfall
anomalies
during toindifferent
NAOheat
phasesicinand
Europe.
SST anomalies evolve together, which
changing
fluctuations
atmospheric
flux to form large-scale SST anomalies.
There are only a few examples of an active
oceanic role in SST variability, the most
a
makes it difficult to specify the SST field
ahead of time. This poses a considerable
obstacle to a prediction algorithm based on
b
Winter 1994—95
Winter 1995—96
1010
1025
1015
1020
1000
Low
1020
1025
1015
High
1035
High
1030
1010
Low
1030
Low
1005
-
1015
1010
1020
1025
High
High
1015
1015
1010
1010
Low
1010
Figure 1 Swings of the North Atlantic Oscillation from one phase to the other affect European climate.
Figure 4: a. NAO+ phase and b. NAO- phase. White contours depict the seasonal mean sea-level pressure
a, During
the winter
1994–95,
the respectively
Icelandic low-pressure
andcolder
Azoresthan
high-pressure
regions in
were
field;
regions
in red of
and
blue were
warmer and
normal; regions
green and brown
both
more
intense
than
usual.
In
this
positive
phase
of
the
NAO,
strong
westerly
winds
(arrow)
respectively experienced higher and lower rainfall than normal. Stronger westerlies during a NAO+ phase
steereda)
Atlantic
winter
storms
eastward
and brought
unusually
(figure
is marked
with
a grey
arrow (from
Kushnir,
1999). wet conditions to northern Europe
and warm temperatures all the way into Siberia. The western Mediterranean was dry and northern
Africa cool. b, In the winter of 1995–96 the NAO swung into its negative phase. A weak North Atlantic
pressure system led to reduced Atlantic influence over northern Europe, with less rainfall and much
colder temperatures. Over the Iberian Peninsula and northern Africa drought gave way to excess
Through
it‘s influence on European weather conditions, NAO is steering indirectly the whole Eurorainfall, as winter storms moved freely into the Mediterranean. (Contours depict the seasonal mean
pean
biosphere.
Menzel
et al.,
(2005)
showed
through the
analysis
of plant
phenology
sea-level pressure field;
regions
in red
and blue
were respectively
warmer
and colder
than
normal; data sets, that
the
progress
ofand
season,
change from
springhigher
to summer,
is rainfall
highly than
depending
regions
in green
brownthe
respectively
experienced
and lower
normal.)on NAO state: in ye-
ars with
a positive
NAOMagazines
Index, theLtd
progress of season from west to the east of Europe
was clearly
© 1999
Macmillan
289
pronounced, while in years with a negative NAO Index the progress was depending on other factors
and showed a more north to south component. The variability in NAO causes the annual variability
in season progress.
George et al. (2004) analysed the direct and indirect effects of NAO weather conditions on four lakes in Britain from data which was recorded between 1961 and 1997. Especially the changes of lake
conditions in the two smaller lakes, Esthwaite Water and Blelham Tarn, could directly be correlated
to NAO Index variation (see figure 5). Through the comparison of meteorological measurements and
NAO Index, they found that in years with positive NAO Index, winter weather conditions in Britain
Nina Bachmann, D-UWIS, [email protected]
11
were extremly mild and wet. Therefore, biosphere activity was increased during this years, which
caused a higher nitrate assimilation in the surrounding catchment of the lakes, and therefore a reduced import of nitrate in the water through leaching processes. Moreover, through higher precipitation
in wet winters, washing out of chlorophyll in lakes with small retention time was increased, leading
to a reduced growth of phytoplankton in spring.
The influence of the NAO index on four lakes
771
Fig. 8 AnFigure
overview5:Correlation
of the site-specific
recorded
with the NAOI.
The horizontal
dashed
shows
P ¼ 0.05.
of correlations
NAO Index
and different
parameters
measured
in lines
british
lakes.
Horizonal
dashed
line shows P=0.05. From the left to the right: Water temperature surface and bottom, dissolved reactive
phosphorus, nitrate concentration, chlorophyll a, phytoplankton (Asterionella).
2 Indirect(temperature)
– where the and
effects
of concentration
the NAO are (negabiological
variables.
A striking
of these
Highest
correlation
of NAOfeature
Index with
physical parameters
nitrate
mediated
by
a
chain
of
interactions
in
system
relationships
is
the
systematic
change
recorded
in
tivly correlated). Biological variables response is weaker and also differs more between thethe
lakes
due to difunder investigation;
the ‘non-significant’
relationships
identified
the 2004).
ferent biological
systems (from
Georgefor
et al.,
3 Integrated – where the NAO influences the
DRP, the biomass of phytoplankton and the timing of
ecosystem in a cumulative way.
the Asterionella maxima. In each case, the calculated
In this paper, we have assembled examples that
coefficients formed an ordered sequence that either
illustrate both the direct and indirect effects of the
ranged from values close to zero to significant positive
NAO on the long-term variations recorded in the
or negative values or from a weak negative values to a
Beniston (1997) compared snow records of Swiss
Alps to the phase of NAO. A country like SwitzerEnglish lakes. A notable feature of the direct effects is
significant positive correlation. These differential
land
is sensitive
changes
in snow amount
andstrength
duration:
which
the
of Its
the electricity
correlationsproduction,
noted between
theis based
responses
demonstrate
thattoeach
lake ‘integrates’
the
temperature
of
the
lakes
and
the
status
of
the
NAO.
differentmostly
elements
of
the
climate
signal
in
functionally
on hydro-power, its Tourism industry and mountain ecosystems, they all depend on snowfound
that the thermal
effects of
differentrich
ways.
In
some
cases, such
as the
year-to-year
winter
seasons.
Warm
winters
tend to beLivingstone
drier than(2000)
the norm
(Rebetez,
1996). Additionally,
the
the NAO were particularly pronounced in northern
variations in the water temperature, the effect is
snow
melts
faster
if the
are milder
(Beniston,
1997). In the
high altitudes
Europe
but the correlations
reported
here for of
theSwitzermediated
by the
direct
response
to temperatures
a single meteoroland, mild
winterssuch
are as
often
bythe
persistent
high
pressure
a positive
English
Lakes
are justsituations,
as strong asassociated
those notedwith
at high
logical variable.
In others,
the caused
timing of
latitudes.
The chemical
effects
noted
hereIndex
provide
a
Asterionella
maximum,
the effect
is more complex
NAO
Index. The
relationship
betweenand
pressure
anomalies
in Zurich
and the
NAO
is shown
in
good example of the indirect effects of the NAO on
may be mediated by variations in the wind speed, the
figure 6. A positive NAO Index almost always
leads to snow spareness in the alpine regions (Benisthe flux of nutrients from the surrounding catchment.
underwater light climate and the flushing effects of
ton, 1997).
The most wide-ranging effect was that noted for the
heavy rain.
residual concentration of winter nitrate and appeared
In a recent review, Ottersen et al. (2001) suggest that
to be relatively independent of the nature of the lake.
the ecological effects of the NAO can be classified into
The DRP and biological effects recorded in the two
three categories:
smaller lakes were also indirect but more site-specific.
1 Direct – where there is a very clear connection
Variations in atmospheric features, like the NAO,
between the response and a particular climatic variare now widely used as proxy indicators of regional
able;
! 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 760–774
Nina Bachmann, D-UWIS, [email protected]
288
12
MARTIN BENISTON
NAO index 3
=
Forecast Error (FE)
RPSS (%)
=
=
Terciles
=
Figure 6: Time Series of
wintertime (December to
February) pressure anomalies for the NAO Index
and for Zurich, Switzerland. A positive NAO Index mostly causes higher
than normal pressure in
Zurich, representative for
the opressure
Leading principal component of North Atlantic (20
N-80oN, anomalies
40oE-90oW)in
the Swiss Alps. Longer
mean sea level pressure.
phases
with high pressure
Mean difference between forecast and actual values for
1972/3-2005/6.
anomalies
during
winter
Rank Probability Skill Score = percentage improvement in Rank
Probability
give
raise
to
higher
temScore over 1972/3-2005/6 climate norm.
peratures in higher alpine
The standard way of dividing the NAO into three categories
each
having
the
regions and therefore less
same probability (33.3%) of occurrence. These categories
are below-normal
snow amount
in the Alps
(lower tercile), near-normal (middle tercile) and above-normal
(upper1997).
tercile).
(from Beniston,
Figure 4. Time series of the North Atlantic Oscillation index and surface pressure in Zürich.
European Winter Temperature, Precipitation and Windspeed Verification
of persistent
highas
pressure
in Switzerland,
sometimes
asso-influences European winter
TheObserved
winter episodes
2006/2007
may serve
an actual
example of
how NAO
ciated
with
blocking
in
the
formal
sense
of
the
term,
are
most
often
accompanied
Northern
Europe
experienced
a
warm,
wet
and
windy
winter
with
temperatures
over(1995;
2oC
weather conditions. The value of the NAO Index for the winter 2006/07
calculated
by Hurrell
by
large average,
positive departures
of temperature;
are also linked
to extended
periods
above
and windspeed
and they
precipitation
over
40%isabove
thehigh
1971/2-2000/1
climate
updated
on
www.cgd.ucar.edu/cas/jhurrell
) wastaken
+2.80,
which
a quite
positive NAO
Index.
of
low
or
negligible
precipitation.
These
two
factors
together,
as
the
main
norm (Figure 1). Southern parts of Europe tended to be drier and less windy than normal,
During on
thissnow,
winter,
Northern
experienced
warm,times
wet of
and
windy winter with temperatures
controls
explain
much ofEurope
the observed
sparsenessa during
positive
consistent
with a positive NAO index. Our forecasts did not verify well this year, predicting
NAO
indices.
of more
than 2 Degrees above average (Saunders and Lea, 2007). Windspeed and precipitation were
near
average
precipitation,
temperature
windspeed
for Europe.
Figure
5 illustrates
the relationship
betweenand
average
annual snow
duration This underprediction is
over
40%
thelarge
climate
norm between
and 2000/01.
In contrary,
was
partly
dueabove
to the
underprediction
of1971/72
the CRU
Had we Southern
predictedEurope
the CRU
for
various
depth
thresholds
at the three climatological
sites NAO
(which index.
have been
drier
less perfectly
windy
than
normal
(see
figure
7).in general,
smoothed
using
a five-point
filter
for the
sake
of clarity)
and the smoothed
average the correct windspeed,
NAOand
index
our
forecasts
would,
have predicted
wintertime
(DJF:
December,
January
and
February
mean)
pressure
field
as
mea- the magnitude of these
temperature and precipitation patterns but would have underpredicted
sured in Zürich for the 50-year period. Different snow duration thresholds have
anomalies.
been chosen to illustrate the correspondance between shifts in snow duration with
shifts in surface pressure. The figure highlights two periods during the record when
average pressures increased and peaks exceeded the 1951–1980 climatological
mean value, i.e., from 1967–1975 and since 1980. The latter period of the record
has been the longest this century where pressure has remained largely above the
30-year climatological average.
It is seen that whenever pressures rise, the snow-pack responds by decreasing
in duration (and depth, not shown here). This is particularly visible since 1982 in
Davos and Montana, already to some extent for the 50-cm threshold (particularly
since 1980) but more so for snow-depth thresholds exceeding 100 cm. For example,
with the strengthening of the wintertime mean surface pressure field in the 1980s,
periods with 150 cm of snow or more at Davos have all but disappeared since 1985
and at Montana since 1990. There have been other extended periods where snow
[56]
Figure 7:Weather conditions during december 2006, january and february 2007. Farbscale shows the difference from long time mean (1971/72 – 2000). Left: Surface temperature anomalies in degrees; Right: Precipitation anomalies in percent (from Saunders and Lea, 2007).
Figure 1. Observed anomalies in 2m air temperature, accumulated precipitation and 10m wind speed across
Europe for the 2006/7 winter. These maps are computed using monthly NCEP/NCAR global re-analysis project
Nina Bachmann, D-UWIS, [email protected]
7.
13
Simulations of future NAO development
NAO and its answer to climate change (or influence on climate change) has been one of the most
important topic in all recent NAO studies. The rise of temperatures in northern Europe during the
last years can to some part been linked to NAO behaviour (Hurrell, 1996). It would be important to
understand NAO for understanding global climate change, and therefore todays research effort is
focused mainly on predictability and modelling of NAO phenomena.
As already mentioned in chapter 3, an intesification of NAO, namely a trend towards higher positive
NAO Index values has been observed over the past few decades, which caused higher temperatures
and higher precipitation over Europe (Hurrell, 1995). This upward trend might be unique in earth
history, although this is not yet clear (Hurrell, 1995; Thompson et al., 2000) and recent NAO Index
values show a more decreasing trend.
Osborn et al. (1999) tried to model decadal NAO Index trend with atmospheric climate models. This
modelling predicted a decrease in NAO Index. But instead of a decrease, the observations showed an
increase of NAO Index. It could be that the model isn‘t able to simulate recent NAO Index development correctly, because it doesn‘t include for example natural variablity of solar radiation and
troposphere-stratosphere interactions. Another possibility is, that recent NAO Index development
was forced by external factors, namely by anthropogenic climate warming.
In the 4th assessment report of the International Panel on Climate Change (IPCC, 2007) the authors
state that many simulations project a decrease in arctic sea level pressure in the 21st century (see
figure 8). This would lead to an increase of the NAO Index, which was confirmed by half of the simulation models investigated by Osborn (2004) and Kuzmina et al. (2005). Although the magnitude
of the modelled pressure trends varies largely (IPCC, 2007), none of the models calculated an increasing pressure trend, which makes lower arctic sea level pressure and therefore a higher index very
probable (Miller et al., 2006; IPCC, 2007). Furthermore, climate models simulations for NAO with
regard to troposhperic processes project an increased index in response to increasing CO2 concentraGlobal Climate Projections
tions, although this response
is not that large and it‘s magnitude depends on the model (Stephenson
et al., 2006; IPCC, 2007).
Figure 8: Simulated mean sea level
pressure by atmospheric general circulation models. The value „0“ indicates todays sea level pressures, values above or below indicate an increased respectively decreased mean
sea level pressure by the end of the
21st century (2080-2099). For the
arctic region, the simulations show a
decrease and for the Azores region a
slight increase of about 1 hPa. Therefore, the NAO Indexwould shift towards more positive values (from
IPCC, 2007).
Nina Bachmann, D-UWIS, [email protected]
8.
Conclusions and Outlook
8.1
Conclusions
14
The temporal and geographic pattern of the big pressure systems over the North Atlantic is well understood. NAO describes the phenoma, that the magnitude of these pressure systems‘ strength oscillates, and the NAO Index can describe this oscillation quite well. Timeseries of the NAO Index were
calculated back to a time span of several hundred years to identify possible regularities in the pressure fluctuation.
Furthermore, there is a consensus among researchers, that NAO significantly influences climate
conditions in the whole North Atlantic region. In this context, NAO recently got of high interest in
climate research (see chapter 7) regarding todays anthropogenic climate change disscussion. As
NAO influences European temperatures and precipitation, as well as it influences the whole North
Atlantic climate, knowledge about this phenomena is important for climate change modeling.
As discussed in chapter 7, recent modelling of NAO showed an increasing trend of the NAO Index
towards more positive values during the 21th century. This would lead to a mild and humid climate
in Europe, which is consistent with climate change simulations that focus on other climate-driving
factors like for example green house gas emissions. The prediction of the models are therefore assumed to be quite reliable, altough they differ largely in magnitude.
8.2
Outlook
Athough the research about NAO already started at the beginning of the 20th century, a lot of open
questions are still unanswered.
The problem in NAO research is that progress depends mainly on the use of reliable models. Until
now, Atmospheric-Ocean General Circulation Models (AOGCMs) have been fed with different parameters like for example sea surface temperatures to simulate NAO, but so far, there aren‘t any
models which are specifically adapted for NAO (Osborn, 2004). To improve NAO models, all processes which drive NAO must be included in the right way. This means that the models must also be
extended to the stratosphere in order to include troposphere-stratosphere interactions.
Until now, it isn‘t even clear which are the mechanisms driving NAO (see chapter 4.1 and 6). To
improve NAO modeling, it is first of all important to know all the underlying processes for NAO,
which should be possible with todays AOGCMs. Once all the processes which drive NAO are defined, models for each of these have to be coupled to create a model which is specifically adapted for
NAO. With this new NAO model, several open questions could be investigated further:
- Sea surface temperature variation seems to be closely related to long-term NAO variations
(Rodwell et al., 1999). If other mechanisms can be excluded from being responsible for NAO,
this is an advantage regarding predictability of NAO: If the Ocean, especially sea surface temperature influences NAO, predictability is possible as there are already projection methods for
Nina Bachmann, D-UWIS, [email protected]
15
changes in sea surface temperature (for example: Sutton and Allen, 1997). Therefore, it is considered essential to make sea surface temperature predictions more reliable in the near future (Hurrell et al., 2004; IPCC, 2007).
- It is important to know how increasing CO2 concentrations in the atmosphere have an influence
on NAO. Although models simulated a response of NAO Index to higher concentrations (see
chapter 7), not every model showed a strong increase in NAO Index, but some of them did. It‘s
essential to investigate, if NAO would react with increasing Index values on higher greenhouse
gas concentrations.
- If NAO Index reaches more and more positive values in the 21st century as predicted by several
authors (IPCC, 2007), it would have a strong effect on European weather phenoma. For European
economists and politics the amplitude of this weather effects must been known to take in account
for example more severe stroms or higher precipitation.
Nina Bachmann, D-UWIS, [email protected]
16
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