Geoscience Reference
In-Depth Information
Eurasia and cooling over the northeastern North American and the northern North
Atlantic. A prominent point of discussion at the time was the mismatch between
this pattern of temperature trends and projections from climate model simulations
with rising levels of atmospheric greenhouse gases, which mostly depict the stron-
gest warming over the Ocean in autumn and winter (see
Figure 9.15
; the pattern of
Arctic amplification that is now clearly emerging in the observations). Since about
the beginning of the twenty-first century, the NAO has varied between its positive
and negative states; recent events of note include strongly negative states during the
winters of 2009/2010 and 2010/2011 and a strongly positive state during the winter
of 2011/2012. The latter followed the second lowest September sea ice extent in the
satellite record.
There is evidence that low-frequency NAO variability involves large-scale ocean
interactions. As reviewed by M. Hoerling, J. Hurrell, and T. Xu (
2001
), whereas the
observed spatial pattern and amplitude of the NAO is typically well-simulated in
AGCMs with fixed climatological annual cycles of solar radiation, trace gas com-
positions, and SST, such simulations do not reproduce interdecadal changes com-
parable in magnitude to those that are observed. This suggests a role of slowly
varying SSTs. M. Rodewell, D. Rodewell, and C. Folland (
1999
) find that much
of the multidecadal variability of the winter NAO over the past fifty years (up to
the time of that study) can be simulated when the observed temporal evolution of
North Atlantic SST distributions is included. They argue that the SST characteris-
tics are communicated to the atmosphere through evaporation, precipitation, and
atmospheric heating processes and that with knowledge of SSTs, it may be possible
to predict European winter climate several years in advance.
Hoerling et al. (
2001
) counter that most evidence actually suggests that anoma-
lous SST and upper-ocean heat content feedbacks to the atmosphere from the extra-
tropics are rather small, implying that a strict focus on the North Atlantic is inappro-
priate. Their modeling study, which focused on the positive NAO trend from 1980s
through the late 1990s, indicates a more important role of progressive warming
of tropical SSTs, especially over the Indian and Pacific Oceans. Follow-on papers
(Hoerling et al.,
2004
; Hurrell et al.,
2004
) strengthen this argument. These ocean
changes alter the pattern and magnitude of tropical rainfall and atmospheric heat-
ing, which in turn forced a trend toward the positive phase of the NAO. C. Cassau
and L. Terray (
2001
) also focus on the tropics but provide a somewhat different
view - they find links between the positive phase of the NAO and the cold phase of
ENSO (La Niña).
D. Dukhovskoy, M. Johnson, and A. Proshutinsky (
2004
) speculate on a role
of internal Arctic processes involving heat and freshwater exchange between
the Arctic Ocean and GIN seas. During the negative phase on the NAO/AO, the
atmospheric heat flux to the Arctic Ocean is small, and the freshwater flux to
the Greenlan-Iceland-Norwegian seas is less than average. With a small heat flux,
the atmosphere over the Arctic Ocean cools, leading to a more anticyclonic circula-
tion and a stronger Beaufort Gyre, increasing the convergence of surface water and
ice. More freshwater is retained in the Beaufort Gyre. Less freshwater outflow to the
