Flood Risk and Vulnerability Analysis Project - Atlantic Climate ...

changes predicted by climate models under various scenarios of increased anthropogenic CO 2
during the 21 st century. The material presented below is summarized from Chapter 5 of the
IPCC 2007 report.
Regarding observed changes in the North Atlantic sub polar gyre, the IPCC reports large salinity
changes in the past 50 years associated with changed inputs of fresh water and with the NAO.
These anomalies called „Great Salinity Anomalies‟ (GSAs) have had time to travel around the
gyre and are observed in the Labrador Sea and the Nordic Seas. They have affected the water
masses formed by deep convection, Labrador Sea Water (LSW) and the North Atlantic Deep
Water (NADW), which alternate between dense, cold types and less dense, warm types. There
was an overall trend towards freshening of the sub polar gyre consistent with the positive phase
of the NAO between 1960 and 1990. Since then, with a shift in the NAO, it has returned to being
saltier and warmer, but not for long enough to have reverted the long term trend. These
observed variations in high latitude salinities also result in variations in the depths and volumes
of water formed by deep convection, possibly affecting the strength of the MOC. Although it is
very likely that the MOC has experienced significant changes over interannual to decadal time
scales up to the end of the 20 th century, there is no coherent evidence of a trend in the mean
strength of the MOC.
The IPCC found that an increase in high latitude temperature and precipitation was a common
feature of all climate projections; both tend to make surface water less dense (warmer and less
salty) which inhibits deep convection with, as a possible consequence, a reduction of the MOC.
Models show a wide range of projections, from a reduction of the MOC by up to 50% or more to
changes indistinguishable from simulated natural variability. The reduction of the MOC
proceeds at the same rate as the simulated warming as it is a direct response to the increase in
ocean surface buoyancy. This may be delayed by a few decades, but not prevented, by a
positive trend in the NAO. No model shows an increase or an abrupt shut-down of the MOC
during the 21 st century. However the possibility of the latter beyond the end of the 21 st century
cannot be excluded. Indeed, overall simulations show a decrease of the MOC over the next
100 years, possibly associated with a significant reduction in the formation of the LSW. Under
the assumption that warming would stop after a century, some models show a recovery of the
MOC and others show the MOC remaining at reduced strength. A reduction of the MOC would
result in a reduction of the heat flux towards high latitudes which would slow down the warming
of the North Atlantic. However, no cooling is observed in the surrounding regions because it is
overcompensated by radiating forcing. This would therefore potentially delay warming of the
region but would not be expected to result in cooling over Newfoundland.
The IPCC reports that many simulations project an increase in the positive phase of both the
AO and the NAO due to anthropogenic warming, although the changes might not be distinct
from the larger multi-decadal internal variability observed during the first decade of the 21 st
century. One consequence of an increase in the positive phase of the AO and the NAO would
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