Geoscience Reference
In-Depth Information
the 1970s, and the Gulf Stream intensity shows an oscillation opposite to that of the tem-
perature. This means that a decreasing temperature in the Labrador Sea may be associated
with a strengthening of the Gulf Stream and vice versa. There appears to be some con-
sistency with the oscillation of the NAO. However, how this ocean-atmosphere interaction
works is still subject to scientific debate .
Increasing transport in the Gulf Stream and North Atlantic circulation may lead to a
deepening of the thermocline, due to the geostrophic constraint, and a compression hori-
zontally of the current system (Stommel,
1958
). This would lead to a withdrawal or weak-
ening of warm surface water from northern parts of the region and a cooling there. This
in turn leads to an increased pressure and weakening of the westerlies. The northern, po-
lar, wind system may then enhance the transport of cold water towards Iceland-Greenland.
At the same time the decreasing westerlies would lead to a weakening of the wind-driven
circulation with a corresponding rise of the thermocline, forcing the warm surface water
radially outwards from the Sargasso Sea region. This in turn leads gradually to a change in
the pressure difference and a strengthening of the westerlies. The system could in this way
be sustainable.
A low NAO corresponds with weak westerlies. This may allow the relatively high
saline water from the south to penetrate relatively further north than during a high NAO
index, which implies strong westerlies. These will force the surface layer water to deviate
towards the east and south, decreasing the penetration of this water northwards. Coupled
ocean-atmosphere model simulations seem consistent with this conceptual interpretation.
But how does this fit with the possible decrease of the meridional over-turning, also called
the thermohaline circulation, referred to in
Section 3.2
? Warming periods may coincide
with an increase of heat transfer towards the north and increasing NADW formation. This
in turn leads to a gradual increase of the atmospheric pressure difference, an increasing
NAO, as experienced since the 1960s, and strengthening of the westerlies, leading to a
gradual decrease of the penetration northwards of the high saline water from the south. In
parallel, the fronts between different air and water masses at middle and high latitudes have
a tendency to shift southwards. This generates a southward displacement of the intertropic-
