255
6. The ventilation history of the west Atlantic is in synchrony with that
of the Greenland Sea since both are driven directly by the twin coupled
cells ofthe North Atlantic Oscillation [NAO]. The NAO index reached
its extreme low-index state - with ridging over Greenland and low
pressure to its south - during the 1880's and 1960's, with the opposite
high index state peaking during the 1920's. Thus during the 1960's,
the Greenland high pressure- anomaly cell and the trough over the
southeastern United States were each the result of a slow amplification
over decades.
7. From the late 1950's to 1970, under the more-southerly of these two
cells, a regime of cold winter air temperatures and extreme snowcover
greatly enhanced the land-sea temperature gradient at the US eastern
seaboard, spinning-up more storms than normal offshore, and causing
them to develop more rapidly to occlusion in a narrow band following
the main coastal baroclinic gradient. The local effect was to draw
the centre of maximum storm activity far to the southwest of normal,
while the remote effect was to reduce storminess over the Labrador Sea
to a postwar minimum. Along the US and Canadian seaboard, shelfwaters chilled to their long-term minimum values in the '60's, while
offshore the cold, stormy conditions caused maximum formation and
ventilation of the 18-Degree Water pycnostad.
8. The changing convective history of the Labrador Sea is explained as
the combined, remote response to the coupled cells of the North Atlantic Oscillation. During the NAO minimum of the 1960's, while an
intense Greenland Ridge directed an increased accession of freshwater
to the Labrador Sea in the form of the Great Salinity Anomaly, and
its partner-cell over the SE USA caused the withdrawal of its winter
storm activity, convection in the Labrador Sea became increasingly
suppressed. LSW production resumed abruptly in winter 1971-72 as
the GSA passed eastwards out of the area and as the cold winter
regime ended at the US east coast. Since then the tendency has been
towards increasing and deepening ventilation of the Labrador Sea [to
> 2300 m in the most recent years] and a general cooling and freshening of LSW, in marked contrast to the contemporaneous tendencies
in the deep Greenland Sea.
6. The ventilation history of the west Atlantic is in synchrony with that
of the Greenland Sea since both are driven directly by the twin coupled
cells ofthe North Atlantic Oscillation [NAO]. The NAO index reached
its extreme low-index state - with ridging over Greenland and low
pressure to its south - during the 1880's and 1960's, with the opposite
high index state peaking during the 1920's. Thus during the 1960's,
the Greenland high pressure- anomaly cell and the trough over the
southeastern United States were each the result of a slow amplification
over decades.
7. From the late 1950's to 1970, under the more-southerly of these two
cells, a regime of cold winter air temperatures and extreme snowcover
greatly enhanced the land-sea temperature gradient at the US eastern
seaboard, spinning-up more storms than normal offshore, and causing
them to develop more rapidly to occlusion in a narrow band following
the main coastal baroclinic gradient. The local effect was to draw
the centre of maximum storm activity far to the southwest of normal,
while the remote effect was to reduce storminess over the Labrador Sea
to a postwar minimum. Along the US and Canadian seaboard, shelfwaters chilled to their long-term minimum values in the '60's, while
offshore the cold, stormy conditions caused maximum formation and
ventilation of the 18-Degree Water pycnostad.
8. The changing convective history of the Labrador Sea is explained as
the combined, remote response to the coupled cells of the North Atlantic Oscillation. During the NAO minimum of the 1960's, while an
intense Greenland Ridge directed an increased accession of freshwater
to the Labrador Sea in the form of the Great Salinity Anomaly, and
its partner-cell over the SE USA caused the withdrawal of its winter
storm activity, convection in the Labrador Sea became increasingly
suppressed. LSW production resumed abruptly in winter 1971-72 as
the GSA passed eastwards out of the area and as the cold winter
regime ended at the US east coast. Since then the tendency has been
towards increasing and deepening ventilation of the Labrador Sea [to
> 2300 m in the most recent years] and a general cooling and freshening of LSW, in marked contrast to the contemporaneous tendencies
in the deep Greenland Sea.