that occurred upstream, in the Iceland Basin, after 1996 due to salinification of the
Northeast Atlantic waters entrained into the overflow.
It was demonstrated [17] that the entrainment salinity increase was associated
with the North Atlantic Oscillation (NAO)-induced weakening and contraction of
the Subpolar Gyre and corresponding northwestward advance of subtropical waters
that followed the NAO decline in the mid-1990s and continued through the
mid-2000s. Remarkably, the deep water freshening reversal was not related to
changes in the overflow water salinity.
Deep-Ocean Salinity Changes and the NAO
Close relationship between the thermohaline properties of the northern North
Atlantic intermediate and deep waters and the winter NAO index on a decadal time
scale (r
2
≈ 0.65, 1950s–2000s, Fig. 4b, c) was revealed [18] from the
observation-based salinity time series for LSW in the Labrador Sea [23] and ISOW
in the Iceland Basin [15, 24]. Persistent NAO decline (amplification) leads to
warming and salinification (cooling and freshening) in the intermediate–deep water
column.
An explanation for the close link between the NAO and the coherent decadal
changes in the intermediate and deep water properties in the region was proposed
[18]. The two factors dominate this link (Fig. 4d): (i) intensity of convection in the
Labrador Sea controlling injection of relatively cold fresh waters into the intermediate layer and (ii) zonal extent of the Subpolar Gyre that regulates the relative
contributions of cold fresh subpolar waters and warm saline subtropical waters to
the entrainment into the Norwegian Sea overflow south of the Iceland–Scotland
Ridge and to the Atlantic inflow to the Nordic Seas. These factors act in phase
leading to the observed coherent thermohaline changes in the intermediate–deep
water column.
Due to weakening of the surface forcing associated with the NAO transition into
neutral to low phase (1950s to mid-1960s, mid-1990s to mid-2000s), convection in
the Labrador Sea weakens diminishing cold fresh water penetration into the
intermediate layer. This results in warming and salinification at the intermediate
depths in the Subpolar Gyre. Concurrently, the Subpolar Gyre contracts allowing
northward advance of warm saline upper-ocean and intermediate subtropical waters
in the northeastern North Atlantic. Northward progression of subtropical waters
increases temperature and salinity at the upper intermediate levels and, correspondingly, increases temperature and salinity of the Northeast Atlantic waters
entrained into the Iceland–Scotland overflow along its pathway to the deep Iceland
Basin. As a result, temperature and salinity at the deep levels increase. The contrary
changes: intensification of deep convection in the Labrador Sea and expansion of
the Subpolar Gyre, caused by amplifying surface forcing (mid-1960s to mid-1990s)
lead to cooling and freshening at the intermediate–deep levels. Additionally, under
high-NAO conditions, deep convection may occur in the Irminger Sea potentially
Ship-Based Monitoring of the Northern North Atlantic Ocean …
419
Northeast Atlantic waters entrained into the overflow.
It was demonstrated [17] that the entrainment salinity increase was associated
with the North Atlantic Oscillation (NAO)-induced weakening and contraction of
the Subpolar Gyre and corresponding northwestward advance of subtropical waters
that followed the NAO decline in the mid-1990s and continued through the
mid-2000s. Remarkably, the deep water freshening reversal was not related to
changes in the overflow water salinity.
Deep-Ocean Salinity Changes and the NAO
Close relationship between the thermohaline properties of the northern North
Atlantic intermediate and deep waters and the winter NAO index on a decadal time
scale (r
2
≈ 0.65, 1950s–2000s, Fig. 4b, c) was revealed [18] from the
observation-based salinity time series for LSW in the Labrador Sea [23] and ISOW
in the Iceland Basin [15, 24]. Persistent NAO decline (amplification) leads to
warming and salinification (cooling and freshening) in the intermediate–deep water
column.
An explanation for the close link between the NAO and the coherent decadal
changes in the intermediate and deep water properties in the region was proposed
[18]. The two factors dominate this link (Fig. 4d): (i) intensity of convection in the
Labrador Sea controlling injection of relatively cold fresh waters into the intermediate layer and (ii) zonal extent of the Subpolar Gyre that regulates the relative
contributions of cold fresh subpolar waters and warm saline subtropical waters to
the entrainment into the Norwegian Sea overflow south of the Iceland–Scotland
Ridge and to the Atlantic inflow to the Nordic Seas. These factors act in phase
leading to the observed coherent thermohaline changes in the intermediate–deep
water column.
Due to weakening of the surface forcing associated with the NAO transition into
neutral to low phase (1950s to mid-1960s, mid-1990s to mid-2000s), convection in
the Labrador Sea weakens diminishing cold fresh water penetration into the
intermediate layer. This results in warming and salinification at the intermediate
depths in the Subpolar Gyre. Concurrently, the Subpolar Gyre contracts allowing
northward advance of warm saline upper-ocean and intermediate subtropical waters
in the northeastern North Atlantic. Northward progression of subtropical waters
increases temperature and salinity at the upper intermediate levels and, correspondingly, increases temperature and salinity of the Northeast Atlantic waters
entrained into the Iceland–Scotland overflow along its pathway to the deep Iceland
Basin. As a result, temperature and salinity at the deep levels increase. The contrary
changes: intensification of deep convection in the Labrador Sea and expansion of
the Subpolar Gyre, caused by amplifying surface forcing (mid-1960s to mid-1990s)
lead to cooling and freshening at the intermediate–deep levels. Additionally, under
high-NAO conditions, deep convection may occur in the Irminger Sea potentially
Ship-Based Monitoring of the Northern North Atlantic Ocean …
419