Geoscience Reference
In-Depth Information
Ocean-atmosphere coupling and
climate in western Europe
NEW DEVELOPMENTS
Concern about the risk of future destabilization, and even catastrophic collapse, of the Atlantic meridional overturning
circulation
(MOC) and coupled ocean-atmosphere processes continues to grow, particularly regarding the European
climate. The MOC component of global thermohaline circulation (THC) is responsible for a major part of meridional
heat transfer in the North Atlantic and, therefore, anomalous warmth maintaining western European winter
temperatures 15-20C above the latitudinal average. Northwards oceanic heat flux is equivalent to some 30 per cent
of the regional solar radiation flux and accompanies a major saltwater flux, driven by strong Caribbean heating and
associated atmospheric export of water vapour into the Pacific and Indian Oceans. Surface Gulf Stream (North Atlantic
Drift) water cools by heating the lower atmosphere on its northward progress, intensifying where it encounters cold
(albeit fresh) glacial meltwater in the Davis and Denmark Straits and Nordic Sea (
Figure 11.13
).
Density also increases
through salt expression during the autumn/winter formation of Polar sea ice. As density increases, by whatever means,
water subsides to the Atlantic sea bed to form North Atlantic Deep Water (NADW), overflowing sea-bed sills as it
moves south and drawing in an upper MOC inflow from tropical sources. Principal North Atlantic density-driven mixing
sites are located in the Nordic and Labrador Seas.
Aspects of global warming, forecast to peak in the Arctic basin with temperatures of +4-10
o
C by 2080, are
destabilizing this process. The Greenland ice cap is now considered more vulnerable to rapid melting than it was a
decade ago. Arctic basin permafrost is also melting, enhancing the meltwater flux, and the summer extent of Arctic
sea ice has fallen by 10-15 per cent in fifty years. It is thinning by 5-10 per cent yr
-1
and the Intergovernmental Panel
on Climate Change (IPCC) 2007 fourth Assessment Report forecast that its area may shrink by only 30 per cent by
AD
2080 may be grossly optimistic, according to the latest observations. All three systems help to sustain water
density - and hence NADW formation. Their permanent loss would end this contribution, weakening NADW
formation and with a positive feedback on Arctic warming through the parallel reduction of ocean and land surface
albedo. Moreover, oceanic processes are also coupled to the North Atlantic Oscillation (NAO). Meltwater cooling of
SSTs increases meridional temperature and pressure gradients, thus increasing cyclogenesis. Increased wind flow
boosts the latent and sensible heat transfers to the north-east Atlantic in turn, cooling the ocean at the expense of
land surfaces. This oscillation is normally reversed on annual-decadal time scales but there is a risk that it could
become locked in this mode.
The Atlantic MOC will slow down during the twenty-first century, according to most IPCC models, in response to
global warming. For the next few decades, regional atmospheric warming is likely to exceed any cooling related to
reduced MOC. However, MOC becomes less stable as it slows down and its abrupt collapse beyond 2100 cannot
be ruled out. If that were to happen, Europe could be plunged into a new Ice Age, with devastating human
consequences - and we have been there before.
Figure 11.14
shows reconstructed positions of the Arctic Polar
Front (the southern boundary of cold Arctic waters) at various times during the past 25 kyr. It is apparent that its
latitude is highly sensitive on the north-east Atlantic coastline. The Atlantic MOC we know may be in an anomalous,
unstable state capable of reverting suddenly to a shut-down, long-term equilibrium state.
moves around Earth.
Lunar
tides are stronger than
solar
tides because of the moon's proximity. The
semi-diurnal
tidal model, of two tides each day, is most applicable in
equatorial and mid-latitude waters. Polewards, one tide
progressively dominates, giving
mixed
tides or, in high
latitudes, a single
diurnal
tide (
Figure 11.16
).
The sun adds
47 per cent to tidal pull when both are in line, to form
spring tides
twice during each monthly cycle, but reduces
manner by competition between the gravitational fields
of Earth, moon and sun. Moon and sun create
tidal bulges
on either side of Earth, extended in the plane of maximum
pull (
Figure 11.15
),
but their periodicity is not identical.
By rotating once in twenty-four hours about its own axis,
Earth experiences two tides (periodicity: 12·0 hr) relative
to the sun's 'fixed' position but slightly less than two
(periodicity: 12·42 hr) relative to the moon, which also
