228
with the scales of atmospheric forcing responsible. More specifically, it has
to do with the North Atlantic Oscillation [NAO] and its time-dependence,
and as such, we will argue that the convective history of the Labrador Sea
is determined by the combined indirect effects of the two cells which make
up the NAO pattern. The direct effects of these two cells must therefore
be described first.
A version ofthe NAO pattern - a pressure-anomaly centre over Greenland with a band of opposite sign to its south - is shown in Figure 13.
Classically, the NAO index is simply a measure of the pressure difference
between Iceland and the Azores so that a high-index pattern [indicating
strong midlatitude westerlies]' is one of a strong Iceland Low with a strong
Azores Ridge to its south, while in the low-index pattern [as Figure 13] the
signs of these anomaly- cells are reversed.
Among its rich blend of frequencies, the NAO index has exhibited a
considerable long-term variability [Figure 14; see also Rodgers 1984], with
interdecadal minima in the 1880's and 1960's. We have already seen in
Figure 5 the progressive evolution of the high-pressure anomaly cell at
Greenland over much of this century to its maximum in the 1960's and in
fact the "version of the NAO pattern" shown in Figure 13 is that which
rather frequently occurred during winters of the '60's, when the high pressure anomaly cell over Greenland was partnered by a cell of abnormally
low pressure centred over the Southeastern USA.
Dickson and Namias [1976] describe the accompanying change in the
storm climate off the US eastern seaboard. Essentially, from the late 1950's
to 1970, a regime of low winter air temperatures [Figure 15] and the refrigerating effect of an extreme snowcover [see Dickson and N amias, 1976, their
Figure 5] acted to steepen the strong winter land:sea temperature gradient
[Figure 16] with the effect of spinning-up more storms than normal in a
narrow band following the main coastal baroclinic zone [Figure 17]. It is
important in the present context that this increased storm activity was
also accompanied by a faster development rate of storms to occlusion so
that storms reached their maximum development by Newfoundland rather
than [as more-normally] at Iceland and thereafter tended to track east on a
southerly trajectory towards Europe or northwest into Arctic Canada. The
net effect was that the area of maximum storm frequency was withdrawn
> 700 n mi to the southwest [Dickson and Namias op cit, their Figure 8],
so that storm activity over the Labrador Sea during the 1960's was minimal
as a result [See Figure 17].
with the scales of atmospheric forcing responsible. More specifically, it has
to do with the North Atlantic Oscillation [NAO] and its time-dependence,
and as such, we will argue that the convective history of the Labrador Sea
is determined by the combined indirect effects of the two cells which make
up the NAO pattern. The direct effects of these two cells must therefore
be described first.
A version ofthe NAO pattern - a pressure-anomaly centre over Greenland with a band of opposite sign to its south - is shown in Figure 13.
Classically, the NAO index is simply a measure of the pressure difference
between Iceland and the Azores so that a high-index pattern [indicating
strong midlatitude westerlies]' is one of a strong Iceland Low with a strong
Azores Ridge to its south, while in the low-index pattern [as Figure 13] the
signs of these anomaly- cells are reversed.
Among its rich blend of frequencies, the NAO index has exhibited a
considerable long-term variability [Figure 14; see also Rodgers 1984], with
interdecadal minima in the 1880's and 1960's. We have already seen in
Figure 5 the progressive evolution of the high-pressure anomaly cell at
Greenland over much of this century to its maximum in the 1960's and in
fact the "version of the NAO pattern" shown in Figure 13 is that which
rather frequently occurred during winters of the '60's, when the high pressure anomaly cell over Greenland was partnered by a cell of abnormally
low pressure centred over the Southeastern USA.
Dickson and Namias [1976] describe the accompanying change in the
storm climate off the US eastern seaboard. Essentially, from the late 1950's
to 1970, a regime of low winter air temperatures [Figure 15] and the refrigerating effect of an extreme snowcover [see Dickson and N amias, 1976, their
Figure 5] acted to steepen the strong winter land:sea temperature gradient
[Figure 16] with the effect of spinning-up more storms than normal in a
narrow band following the main coastal baroclinic zone [Figure 17]. It is
important in the present context that this increased storm activity was
also accompanied by a faster development rate of storms to occlusion so
that storms reached their maximum development by Newfoundland rather
than [as more-normally] at Iceland and thereafter tended to track east on a
southerly trajectory towards Europe or northwest into Arctic Canada. The
net effect was that the area of maximum storm frequency was withdrawn
> 700 n mi to the southwest [Dickson and Namias op cit, their Figure 8],
so that storm activity over the Labrador Sea during the 1960's was minimal
as a result [See Figure 17].