238
Reverdin, LDEO, pers. comm.] confirms that the windstress curl over the
Labrador Sea was also at its postwar minimum at this time. By reducing
the Ekman divergence, this reduced curl would have allowed fresh surface
water to spread into the convective centre of the Labrador Sea.
The separate effects of the two main pressure-anomaly centres at the
time of the NAO minimum were therefore to promote an increased accession of fresh surface water to the Labrador Sea while minimising its winter
storminess and cyclonic circulation, and the long hydrographic record at
OWS BRAVO captures the expected net response - reduced winter heatloss, suppressed convection rarely reaching deeper than 1000m [see ERIKA
DAN 1962, Hudson 1966], increasing stratification and decreasing salinity
[by about 0.7 psu at 10 m] in the surface layers during the 1960's, with a
corresponding increase in salinity at depth. [Figure 23, from Lazier,1980].
4.2 The restoration of deep convection in the Labrador Sea
This tight capping of convection ended abruptly in the winter of 1971-72, as
the freshening influence of the GSA passed eastward out of the area [Dickson et al 1988], as the cold regime of winter air temperatures along the US
eastern seaboard came to an abrupt end [see Figure 15] and as severe winter storminess with chilling northwesterly winds and intense windstress curl
were restored over the Labrador Sea [see Figure 22]. The continued monitoring of subsurface hydrographic conditions at OWS BRAVO, close to the
centre of the cyclonic gyre, provides evidence of increased and deepening
ventilation of the Labrador Sea and increasing production of Labrador Sea
Water since then.
The restoration of deep convection was not regular but tended to intensify in two main steps,- - the first from 1972-76, and the second from 1988
[approx.] onward. [Figure 24]. This intensifying convection, and the progressive cooling and freshening in the 1000-1500 db layer of the Labrador
Sea that has taken place since the early '70's are clearly in total contrast to
the contemporaneous tendencies for suppressed convection, warming and
salinification in the deep Greenland Sea, already described. Overall, Lazier
[1995] shows that over the whole 3500 m watercolumn of the Labrador Sea,
a freshening by 0.059 psu, equivalent to the removal of 200 kg of salt per
m 2 , [or the addition of ~ 6 m of freshwater per m 2 !] and a cooling by
0.46°0, equal to the additional heat loss of 8W/m 2 continuously for 26
years, has taken place between 1966 and 1992. The freshening was not
Reverdin, LDEO, pers. comm.] confirms that the windstress curl over the
Labrador Sea was also at its postwar minimum at this time. By reducing
the Ekman divergence, this reduced curl would have allowed fresh surface
water to spread into the convective centre of the Labrador Sea.
The separate effects of the two main pressure-anomaly centres at the
time of the NAO minimum were therefore to promote an increased accession of fresh surface water to the Labrador Sea while minimising its winter
storminess and cyclonic circulation, and the long hydrographic record at
OWS BRAVO captures the expected net response - reduced winter heatloss, suppressed convection rarely reaching deeper than 1000m [see ERIKA
DAN 1962, Hudson 1966], increasing stratification and decreasing salinity
[by about 0.7 psu at 10 m] in the surface layers during the 1960's, with a
corresponding increase in salinity at depth. [Figure 23, from Lazier,1980].
4.2 The restoration of deep convection in the Labrador Sea
This tight capping of convection ended abruptly in the winter of 1971-72, as
the freshening influence of the GSA passed eastward out of the area [Dickson et al 1988], as the cold regime of winter air temperatures along the US
eastern seaboard came to an abrupt end [see Figure 15] and as severe winter storminess with chilling northwesterly winds and intense windstress curl
were restored over the Labrador Sea [see Figure 22]. The continued monitoring of subsurface hydrographic conditions at OWS BRAVO, close to the
centre of the cyclonic gyre, provides evidence of increased and deepening
ventilation of the Labrador Sea and increasing production of Labrador Sea
Water since then.
The restoration of deep convection was not regular but tended to intensify in two main steps,- - the first from 1972-76, and the second from 1988
[approx.] onward. [Figure 24]. This intensifying convection, and the progressive cooling and freshening in the 1000-1500 db layer of the Labrador
Sea that has taken place since the early '70's are clearly in total contrast to
the contemporaneous tendencies for suppressed convection, warming and
salinification in the deep Greenland Sea, already described. Overall, Lazier
[1995] shows that over the whole 3500 m watercolumn of the Labrador Sea,
a freshening by 0.059 psu, equivalent to the removal of 200 kg of salt per
m 2 , [or the addition of ~ 6 m of freshwater per m 2 !] and a cooling by
0.46°0, equal to the additional heat loss of 8W/m 2 continuously for 26
years, has taken place between 1966 and 1992. The freshening was not