the net 1950s–2000s trends in the water mass salinities are negative implying that
the global factors (e.g., probable intensification of hydrological cycle [10]) may act
on longer time scales.
Decadal Variability of the Deep Western Boundary Current
at Cape Farewell
Recent decadal changes in the Deep Western Boundary Current (DWBC) transport
southeast of Cape Farewell were assessed from the hydrographic data (1991–2007,
Fig. 6a), direct velocity measurements (2002–2006) and satellite altimetry
(1992–2007). Following the approach used in earlier studies (e.g., [27]), we first
determined that the DWBC (σ 0 > 27.80) baroclinic transport (T BC ) referenced to
1000 m depth increased by ∼2 Sv between the mid-1990s (1994–1997) and 2000s
(2000–2007) (Fig. 6b) [19]. In the next step, we quantified velocity changes at the
reference level (1000 m) by combining estimates of the hydrography-derived
velocity changes in the water column and the altimetry-derived velocity changes at
the sea surface [20]. The inferred increase in the southward velocity at 1000 m
above the DWBC in 1994–2007 indicates that the increase in the DWBC absolute
transport was larger but very close to the 2 Sv increase in the DWBC T BC . This
result along with the observed coherence of the DWBC absolute and baroclinic
transport changes between individual observations [20] imply that the DWBC
absolute transport variability in the region is well represented by its baroclinic
component on the decadal and shorter time scales.
The historical record of the DWBC T BC (1955–2007, Fig. 6c) updated after Bacon
[27] shows distinct decadal variability (±2–2.5 Sv) with the transport minima in the
1950s and mid-1990s, maximum in the early 1980s and moderate-to-high transport in
the 2000s. The DWBC T BC decadal variability is consistent with the general pattern of
the recent decadal hydrographic and circulation changes in the northern North
Atlantic. The DWBC T BC anomalies negatively correlate (R = –0.80, 1955–2007)
with the thickness anomalies of LSW at its origin implying a close link between the
DWBC transport southeast of Cape Farewell and LSW production in the Labrador Sea
(Fig. 6d). During the recent three decades (late 1970s–late 2000s), the DWBC T BC
changes were also in-phase with the changes in the strength and zonal extent of the
Subpolar Gyre [20]. In particular, the Gyre weakening at shallow levels in the
mid-1990s–mid-2000s was accompanied by the DWBC strengthening in the Irminger
Sea [19–21]. The results imply that the decadal changes in (i) LSW production,
(ii) SPG strength, and (iii) DWBC transport in the Irminger Sea are linked, representing a complex coherent oceanic response to the decadal variability of the surface
forcing.
422
A. Sarafanov et al.
the global factors (e.g., probable intensification of hydrological cycle [10]) may act
on longer time scales.
Decadal Variability of the Deep Western Boundary Current
at Cape Farewell
Recent decadal changes in the Deep Western Boundary Current (DWBC) transport
southeast of Cape Farewell were assessed from the hydrographic data (1991–2007,
Fig. 6a), direct velocity measurements (2002–2006) and satellite altimetry
(1992–2007). Following the approach used in earlier studies (e.g., [27]), we first
determined that the DWBC (σ 0 > 27.80) baroclinic transport (T BC ) referenced to
1000 m depth increased by ∼2 Sv between the mid-1990s (1994–1997) and 2000s
(2000–2007) (Fig. 6b) [19]. In the next step, we quantified velocity changes at the
reference level (1000 m) by combining estimates of the hydrography-derived
velocity changes in the water column and the altimetry-derived velocity changes at
the sea surface [20]. The inferred increase in the southward velocity at 1000 m
above the DWBC in 1994–2007 indicates that the increase in the DWBC absolute
transport was larger but very close to the 2 Sv increase in the DWBC T BC . This
result along with the observed coherence of the DWBC absolute and baroclinic
transport changes between individual observations [20] imply that the DWBC
absolute transport variability in the region is well represented by its baroclinic
component on the decadal and shorter time scales.
The historical record of the DWBC T BC (1955–2007, Fig. 6c) updated after Bacon
[27] shows distinct decadal variability (±2–2.5 Sv) with the transport minima in the
1950s and mid-1990s, maximum in the early 1980s and moderate-to-high transport in
the 2000s. The DWBC T BC decadal variability is consistent with the general pattern of
the recent decadal hydrographic and circulation changes in the northern North
Atlantic. The DWBC T BC anomalies negatively correlate (R = –0.80, 1955–2007)
with the thickness anomalies of LSW at its origin implying a close link between the
DWBC transport southeast of Cape Farewell and LSW production in the Labrador Sea
(Fig. 6d). During the recent three decades (late 1970s–late 2000s), the DWBC T BC
changes were also in-phase with the changes in the strength and zonal extent of the
Subpolar Gyre [20]. In particular, the Gyre weakening at shallow levels in the
mid-1990s–mid-2000s was accompanied by the DWBC strengthening in the Irminger
Sea [19–21]. The results imply that the decadal changes in (i) LSW production,
(ii) SPG strength, and (iii) DWBC transport in the Irminger Sea are linked, representing a complex coherent oceanic response to the decadal variability of the surface
forcing.
422
A. Sarafanov et al.