250
formed in the winter of 1971-72, when the restoration of intense convection
at BRAVO first redistributed the accumulated fresh water from the surface
layers to intermediate depths [see Figure 24a]. This interpretation is not
without its puzzling aspects, not least the questions of why the freshening
at Rockall was so large [at 0.1 psu, not far short of the entire freshening
observed at BRAVO; see Figure 24], and still so abrupt after an elapsed
spreading-time of 18 years. However, the spreading rate of 0.4 km/d, [0.47
cm/s], about half the lower of recent estimates from the Western Basin,
seems not inappropriate, and what other North Atlantic site than the convective centre of the Labrador Sea could produce such rapid freshening to
1600 m?
Cunningham and Haine [1995 a,b] comment usefully on the fate of the
LSW vintage formed during the second period of intense convection at
BRAVO in the late-'80's. Since a potential vorticity minimum is a characteristic and conserved property of any convectively-formed watermass,
they follow Talley and McCartney [1982] in employing it as an LSW tracer
in their analysis ofresults from the 1991 VIVALDI surveys of the Eastern
Atlantic. Specifically, they find that the range of potential vorticity values
from the VIVALDI area in 1991 [i.e. 1.5-7 x 1O-14/cm/ s] showed no overlap with values from the same area in 1957-64 [8 -16 x 1O-14/cm/ s] when
Talley and McCartney compiled their map [Figure 32]. The LSW in that
area had evidently been completely renewed since the early '60's. Noting
that the minimum salinity at the LSW core [0"1.5 = 34.64] on any of their
zonal VIVALDI sections was the value of 34.866 at 51 ON - the latitude
where the p.v. minimum crosses the Mid-Atlantic Ridge according to both
studies - Cunningham and Haine suggest from the "source function" at
BRAVO [Figure 24] that salinities of this value must have been generated
in the Labrador Sea either between 1975-80 or after 1985. In line with
Top, Clarke and Jenkins [1987] who propose upper limits of 1.4 cm/s and
0.7cm/s for the southward and eastward spreading of LSW, respectively,
Cunningham and Haine discount the spreading rates of mm/s that the former dates would imply, and therefore conclude that the freshest water they
observe on VIVALDI was of the 1986 vintage. Thus at the time of the 1991
VIVALDI survey, the products of the late-'80's convective event were east
of the Mid-Atlantic Ridge but had not then reached the eastern boundary.
[A similar conclusion emerged from CFC analyses of trans-ocean sections
worked during the CONVEX program of 1991; Denise Smythe-Wright pers
comm,1995].
formed in the winter of 1971-72, when the restoration of intense convection
at BRAVO first redistributed the accumulated fresh water from the surface
layers to intermediate depths [see Figure 24a]. This interpretation is not
without its puzzling aspects, not least the questions of why the freshening
at Rockall was so large [at 0.1 psu, not far short of the entire freshening
observed at BRAVO; see Figure 24], and still so abrupt after an elapsed
spreading-time of 18 years. However, the spreading rate of 0.4 km/d, [0.47
cm/s], about half the lower of recent estimates from the Western Basin,
seems not inappropriate, and what other North Atlantic site than the convective centre of the Labrador Sea could produce such rapid freshening to
1600 m?
Cunningham and Haine [1995 a,b] comment usefully on the fate of the
LSW vintage formed during the second period of intense convection at
BRAVO in the late-'80's. Since a potential vorticity minimum is a characteristic and conserved property of any convectively-formed watermass,
they follow Talley and McCartney [1982] in employing it as an LSW tracer
in their analysis ofresults from the 1991 VIVALDI surveys of the Eastern
Atlantic. Specifically, they find that the range of potential vorticity values
from the VIVALDI area in 1991 [i.e. 1.5-7 x 1O-14/cm/ s] showed no overlap with values from the same area in 1957-64 [8 -16 x 1O-14/cm/ s] when
Talley and McCartney compiled their map [Figure 32]. The LSW in that
area had evidently been completely renewed since the early '60's. Noting
that the minimum salinity at the LSW core [0"1.5 = 34.64] on any of their
zonal VIVALDI sections was the value of 34.866 at 51 ON - the latitude
where the p.v. minimum crosses the Mid-Atlantic Ridge according to both
studies - Cunningham and Haine suggest from the "source function" at
BRAVO [Figure 24] that salinities of this value must have been generated
in the Labrador Sea either between 1975-80 or after 1985. In line with
Top, Clarke and Jenkins [1987] who propose upper limits of 1.4 cm/s and
0.7cm/s for the southward and eastward spreading of LSW, respectively,
Cunningham and Haine discount the spreading rates of mm/s that the former dates would imply, and therefore conclude that the freshest water they
observe on VIVALDI was of the 1986 vintage. Thus at the time of the 1991
VIVALDI survey, the products of the late-'80's convective event were east
of the Mid-Atlantic Ridge but had not then reached the eastern boundary.
[A similar conclusion emerged from CFC analyses of trans-ocean sections
worked during the CONVEX program of 1991; Denise Smythe-Wright pers
comm,1995].