Global warming effects on the Arctic
and Sub-Arctic Seas
Jacques C.J. Nihoul
Modelenvironment, University of Liège, Sart Tilman, B-4000 Liege, Belgium,
[email protected]
Abstract
After a rather hydrostatic approach to global warming (mean earth temperature
increasing, ice melting, sea level raising) one came to realize that the effects of
global warming were more of a hydrodynamic nature and that the ocean dynamics
and its modifications in response to global warming constituted an essential factor.
Taking into account the effect of global warming on ocean temperature distribution
and currents contributed to a large extent to clarify the problem. The next step was
obviously to include the effect of global warming on the atmospheric circulation.
We would like to illustrate this point by briefly discussing the so-called North
Atlantic Oscillation (NAO).
The North Atlantic Oscillation (NAO)
The NAO dictates much of the Climate variability from the eastern seaboard of
North America to Siberia and from the Arctic to the subtropical Atlantic, especially
during the boreal winter (Hurrett et al. 2003).
NAO refers to a redistribution of atmospheric mass between the Arctic and
subtropical Atlantic. The NAO is traditionally described by two weather maps
showing the distribution of sea level pressure (SLP) over the North Atlantic in two
typical “educational” situations: NAO+ and NAO− when the pressure difference
between Lisbon and Reykjavik is respectively positive and negative (Figs. 1, 2).
The NAO is understood to swing from one phase to another to produce large
changes in the mean wind speed and direction over the Atlantic, the heat and
moisture transport between the Atlantic and the neighbouring continents and the
intensity and number of storms, their paths, and the associated weather. Such
variations have a significant impact on the wind and buoyancy-driven ocean
circulation as well as on the site and intensity of water mass transformation (Fig. 3).
J.C.J. Nihoul and A.G. Kostianoy (eds.), Influence of Climate Change on the Changing
Arctic and Sub-Arctic Conditions,
© Springer Science + Business Media B.V. 2009
7
8
J.C.J. Nihoul
NAO +
Russian rivers
father east
60
°
°
60
−CHL
Warmer Altantic
inflow to A.O.
Ice flux+
°
50
°
NwAC Narrow
Fast
+LSW
PRODN
Small
Calanus fin.
stock
+R
AI
N
Storm centre in
Lab-Nordic Seas
L
FST
MAX
+Westerl
50
Min. Baltic
ice
Max. Baltic
inflow
ies
65Mts NAC
Warm
H
+Sa
hara
dust
+Coastal
upwelling
+Trades
cold
Base map: Inst. of Geography, U. Berne
Fig. 1. A schematic of the Atlantic–Arctic sector under NAO positive conditions (Stenseth et al.
2004).
+
In NAO
winter situations, enhanced westerly flow across the North Atlantic
moves warm and moist maritime air over Europe, northerlies over Greenland and
northeastern Canada carry cold air southwards, decreasing SST and land temperatures over the North-West Atlantic, the Labrador Sea ice extends further south
9
Global warming effects on the Arctic and Sub-Arctic Seas
Russian rivers
to Eurasian
basin. +CHL
60
°
60
°
NAO −
Ice flux50
°
°
50
H
NwAC broad
slow
Sea ice
+650k km
+GSDW
PRODN
40
°
Large
Calanus fin.
Stock
Warm
L
Storm
centre off
US coast
+300 k km
Baltic Ice
FST
MIN
LC+1 Sv
50Mts NAC
+18 W
PRODN
Westerlies
+Rain
H
Warm-Trades
Base map: Inst. of Geography, U. Berne
Fig. 2. A schematic of the Atlantic–Arctic sector under NAO negative conditions (Stenseth et al.
2004).
while the Greenland Sea ice boundary is found to the North of its climatological
mean extent. NAO+ winters, associated with chill, dry, northwesterlies across the
Labrador Sea are characterized by deep-reaching convective renewal of LSW and
widespread distribution of chilled SST across the Northwest Atlantic.
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J.C.J. Nihoul
NAO Index (December−March) 1864 −2000
6
4
(Ln−Sn)
2
0
−2
−4
−6
1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Fig. 3. Winter (December–March) index of the NAO based on the difference of normalized SLP
between Lisbon, Portugal, and Stykkisholmur/Reykjavik, Iceland form 1864 through 2000. The
heavy solid line represents the index smoothed to remove fluctuations with periods less than
4 years (Stenseth et al. 2004).
The NAO strange attractors
However, examining records of the fluctuations of the NAO index over more than
a century, one realizes that the jumps from one value of the index to the next value
of the same or different sign occur rather at random, strongly suggesting that the
case study situations, although exemplary, do not actually describe two definite
states of the atmosphere but two strange attractors: the system attracted by NAO+
oscillates in its vicinity until it finds a way out and goes to NAO−, oscillates in its
vicinity until it finds a way out and goes back to the vicinity of NAO+− (Fig. 4).
The records show also a definite trend towards the positive NAO phase in recent
decades and the increased attractiveness of the positive phase has often been
attributed to a global warming effect.
One should however be attentive to the fact that, in addition to possible jumps
between NAO+ and NAO− (with a possible predominance of NAO+ attributed to
global warming) small deviations from the “educational” weather maps may occur
as the system wanders in the vicinity of one or the other strange attractor.
Global warming effects on the Arctic and Sub-Arctic Seas
11
Fig. 4. An educational image of trajectones jumping from one strange attractor to the other.
For instance, NAO+ conditions of the most recent winters have shown a shutdown of Labrador Sea Convection. In just two and three winters, the long-sustained
cooling and freshening of LSW has been largely reversed. A comparison of
Atlantic SLP anomaly pattern between the 1995–1999 period with that for 1999–
2000 shows a slight east and northeast displacement, in the more recent period,
responsible for important differences to the marine climate of the West Greenland
Banks and to the convective center of the Labrador Sea.
The NAO is reminiscent of the pioneer work of Lorenz (1990) who, with the help
of a severely truncated and simplified model of atmospheric dynamics, essentially,
a layer of fluid heated from below, showed that for small values of the Rayleigh
number (a non-dimensional measure of the temperature difference between the
lower layer and the upper layer), heat was transported by conduction; for higher
values of the Rayleigh number, convective cells appeared to transport heat, for
still higher values of the Rayleigh number, two strange attractors appear and the
system may jump from one to the other as described above.
Further increase of the Rayleigh number, however sees the strange attractors
disappear and a limit cycle appear.
Could it be the same bifurcation as the one which we observed in the NAO
Index in the last decades 1970–2000 and that many authors attributed to global
warming?
Even if this is stretching the comparison a little far, one should remark that, in
Lorenz’s model, a further increase of the Rayleigh number makes away with the
limit cycle and the strange attractors reappear.
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J.C.J. Nihoul
Further increase of the Rayleigh number generates an intermittent succession of
periodic regimes and bursts of disorder (Nihoul 2007).
In fact, although models like the Lorentz model and the NAO Index representation are helpful to guide one’s intuition of possible bifurcations of the atmospheric
dynamics they are far too rudimentary to describe the dynamics of the system
where the number of spatial modes generated by nonlinear interactions can be the
determinant factor (Nihoul 2007).
One should thus be extremely careful in interpreting the NAO Index and
presumably also in looking in its variations for global warming indications.
References
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Lorenz EN (1990) Can chaos and intransitivity lead to internannual variability. Tellus, 42 A,
378–389
Nihoul JCJ (2007) Chaos, diversity, turbulence and sustainable development. International
Journal of Computing Science and Mathematics, 1, 1, 107–114
Stenseth NCh, Ottersen G, Hirnel JW, Belgrano A (2004) Marine Ecosystems and Climate
Variation, Oxford University Press, Oxford, 252 pp