shore to shore”, from the sea surface to the bottom. Based on the unique data set
thus collected, a number of fundamental findings have been achieved. Below, we
briefly summarize the main subjects and results of our research.
The 59.5° N transatlantic section (Fig. 1) was designed for monitoring the
large-scale circulation and thermohaline/chemical properties of oceanic waters at
the northern periphery of the North Atlantic, the region where the warm
upper-ocean waters are transformed by deep convection and mixing into the colder
intermediate and deep waters, the Labrador Sea Water (LSW), Iceland Scotland
Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) (Fig. 1)
transported southward in the lower limb of the Atlantic MOC. Hydrographic data
collected at 59.5° N along with those obtained within the framework of the kindred
projects, primarily the French OVIDE (http://www.ifremer.fr/lpo/ovide), and historical data sets have been used for studying the dense water production [13, 14],
decadal temperature and salinity changes in the intermediate–deep water column
[15–17], causes of these changes [17, 18], the mean state [8] and long-term variability of the large-scale circulation in the region [19–21].
Deep Convection in the Irminger Sea
The oxygen data collected in 1997 in the northern North Atlantic in several sections
ending nearby the southern tip of Greenland provided the observation-based support for the hypothesis [22] that winter convection in the Irminger Sea may penetrate deep into the LSW layer (1000–2000 m), thus causing local renewal of this
water mass. A separate lateral maximum of oxygen concentrations in the deep LSW
layer was detected east of Cape Farewell (59.5° N, 36–40° W): the concentrations
increased (by ∼0.1 ml/l) from the Labrador Sea eastern edge toward the Irminger
Sea (Fig. 2) rather than the reverse, as would be expected if LSW observed in the
Irminger Sea interior in 1997 were solely of advective origin [13].
Reversal of the Deep-Water Freshening
The LSW and Nordic Seas overflow-derived deep waters, ISOW and DSOW,
freshened in the northern North Atlantic during the last three–four decades of the
20th century [9]. Between the 1960s and 1990s, the water column in the region
freshened on average by about 0.03 [10].
The long-term freshening reversed in the mid-1990s [15–17]. The salinification
(and warming) of the intermediate and deep waters since the mid-1990s (Fig. 3) was
much more intense than the preceding freshening. Over nearly a decade (1997–2006),
temperature/salinity in the intermediate–deep water column (σ 0 ≥ 27.45, depths >
500–1000 m) at 59.5° N increased by ∼0.3 °C/0.03–0.04 [16].
Ship-Based Monitoring of the Northern North Atlantic Ocean …
417
thus collected, a number of fundamental findings have been achieved. Below, we
briefly summarize the main subjects and results of our research.
The 59.5° N transatlantic section (Fig. 1) was designed for monitoring the
large-scale circulation and thermohaline/chemical properties of oceanic waters at
the northern periphery of the North Atlantic, the region where the warm
upper-ocean waters are transformed by deep convection and mixing into the colder
intermediate and deep waters, the Labrador Sea Water (LSW), Iceland Scotland
Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) (Fig. 1)
transported southward in the lower limb of the Atlantic MOC. Hydrographic data
collected at 59.5° N along with those obtained within the framework of the kindred
projects, primarily the French OVIDE (http://www.ifremer.fr/lpo/ovide), and historical data sets have been used for studying the dense water production [13, 14],
decadal temperature and salinity changes in the intermediate–deep water column
[15–17], causes of these changes [17, 18], the mean state [8] and long-term variability of the large-scale circulation in the region [19–21].
Deep Convection in the Irminger Sea
The oxygen data collected in 1997 in the northern North Atlantic in several sections
ending nearby the southern tip of Greenland provided the observation-based support for the hypothesis [22] that winter convection in the Irminger Sea may penetrate deep into the LSW layer (1000–2000 m), thus causing local renewal of this
water mass. A separate lateral maximum of oxygen concentrations in the deep LSW
layer was detected east of Cape Farewell (59.5° N, 36–40° W): the concentrations
increased (by ∼0.1 ml/l) from the Labrador Sea eastern edge toward the Irminger
Sea (Fig. 2) rather than the reverse, as would be expected if LSW observed in the
Irminger Sea interior in 1997 were solely of advective origin [13].
Reversal of the Deep-Water Freshening
The LSW and Nordic Seas overflow-derived deep waters, ISOW and DSOW,
freshened in the northern North Atlantic during the last three–four decades of the
20th century [9]. Between the 1960s and 1990s, the water column in the region
freshened on average by about 0.03 [10].
The long-term freshening reversed in the mid-1990s [15–17]. The salinification
(and warming) of the intermediate and deep waters since the mid-1990s (Fig. 3) was
much more intense than the preceding freshening. Over nearly a decade (1997–2006),
temperature/salinity in the intermediate–deep water column (σ 0 ≥ 27.45, depths >
500–1000 m) at 59.5° N increased by ∼0.3 °C/0.03–0.04 [16].
Ship-Based Monitoring of the Northern North Atlantic Ocean …
417