Alexey Sokov

Mean full-depth summer circulation and transports at the northern periphery of the Atlantic Ocean in the 2000s

A mean state of the full-depth summer circulation in the Atlantic Ocean in the region in between Cape Farewell (Greenland), Scotland and the Greenland-Scotland Ridge (GSR) is assessed by combining 2002–2008 yearly hydrographic measurements at 59.5°N, mean dynamic topography, satellite altimetry data and available estimates of the Atlantic–Nordic Seas exchange. The mean absolute transports by the upper-ocean, mid-depth and deep currents and the Meridional Overturning Circulation (MOCσ = 16.5 ± 2.2 Sv, at σ0 = 27.55) at 59.5°N are quantified in the density space. Inter-basin and diapycnal volume fluxes in between the 59.5°N section and the GSR are then estimated from a box model. The dominant components of the meridional exchange across 59.5°N are the North Atlantic Current (NAC, 15.5 ± 0.8 Sv, σ0 27.55) east of the Reykjanes Ridge, the northward Irminger Current (IC, 12.0 ± 3.0 Sv) and southward Western Boundary Current (WBC, 32.1 ± 5.9 Sv) in the Irminger Sea and the deep water export from the northern Iceland Basin (3.7 ± 0.8 Sv, σ0 27.80). About 60% (12.7 ± 1.4 Sv) of waters carried in the MOCσ upper limb (σ0 27.55) by the NAC/IC across 59.5°N (21.1 ± 1.0 Sv) recirculates westward south of the GSR and feeds the WBC. 80% (10.2 ± 1.7 Sv) of the recirculating NAC/IC-derived upper-ocean waters gains density of σ0 27.55 and contributes to the MOCσ lower limb. Accordingly, the contribution of light-to-dense water conversion south of the GSR (∼10 Sv) to the MOCσ lower limb at 59.5°N is one and a half times larger than the contribution of dense water production in the Nordic Seas (∼6 Sv).

Recent changes in the Greenland–Scotland overflow‐derived water transport inferred from hydrographic observations in the southern Irminger Sea

Recent decadal changes (1955–2007) in the baroclinic transport (TBC) of the Deep Western Boundary Current (DWBC) carrying the Greenland–Scotland overflow-derived waters along the East Greenland slope are quantified from a set of hydrographic sections in vicinity of Cape Farewell. The updated historical record of TBC shows clear 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. Since the mid-1990s, the DWBC TBC has increased by �2 Sv (significant at the 99.9% level), which constitute �20% of the mean absolute transport (9.0 Sv) as obtained from three cruises in 2002–2006. The DWBC TBC anomalies negatively correlate (R = –0.80) with thickness anomalies of the Labrador Sea Water (LSW) at its origin implying a close association, albeit not necessarily causative, between the DWBC transport east of Greenland and the LSW production.

On the Cascading of Dense Shelf Waters in the Irminger Sea

AbstractHydrographic data collected in the Irminger Sea in the 1990s–2000s indicate that dense shelf waters carried by the East Greenland Current south of the Denmark Strait intermittently descend (cascade) down the continental slope and merge with the deep waters originating from the Nordic Seas overflows. Repeat measurements on the East Greenland shelf at ~200 km south of the Denmark Strait (65°–66°N) reveal that East Greenland shelf waters in the Irminger Sea are occasionally as dense (σ0 > 27.80) as the overflow-derived deep waters carried by the Deep Western Boundary Current (DWBC). Clear hydrographic traces of upstream cascading of dense shelf waters are found over the continental slope at 64.3°N, where the densest plumes (σ0 > 27.80) originating from the shelf are identified as distinct low-salinity anomalies in the DWBC. Downstream observations suggest that dense fresh waters descending from the shelf in the northern Irminger Sea can be distinguished in the DWBC up to the latitude of Cape Farewell...

Assessing decadal changes in the Deep Western Boundary Current absolute transport southeast of Cape Farewell, Greenland, from hydrography and altimetry

[1] In earlier studies, the decadal variability of the Deep Western Boundary Current (DWBC) transport in the vicinity of Cape Farewell, Greenland, has been assessed from changes in the baroclinic velocities computed from hydrographic data and referenced to 1000 m depth. The main limitation of using such an estimate as an index for the DWBC absolute transport variability comes from the unaccounted for decadal velocity changes at the reference level (1000 m). These changes may substantially contribute to the DWBC absolute transport variability by compensating for or adding to the baroclinic transport changes. To assess this contribution to variability, we quantify the decadal velocity changes which occurred at 1000 m depth southeast of Cape Farewell since the mid-1990s. The analysis combines estimates of the baroclinic velocity changes in the water column derived from repeat hydrography at similar to 59.5 degrees N and the velocity changes at the sea surface derived from altimetry. An increase in the southward velocity at 1000 m above the DWBC between the periods of 1994-1997 and 2000-2007 is inferred. It indicates that the increase in the DWBC absolute transport was larger than the 2 Sv (1 Sv = 10(6) m(3) s(-1)) increase in its baroclinic component referenced to 1000 m. This result and the observed coherence of the DWBC absolute and baroclinic transport changes between individual observations imply that the DWBC absolute transport variability in the region is underestimated but qualitatively well represented by its baroclinic component on decadal and shorter time scales.

Cessation and partial reversal of deep water freshening in the northern North Atlantic: observation-based estimates and attribution

Abstract Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait OverflowWater salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland—Scotland Overflow Water (ISOW) and in the deep water column (σ0 > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced 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 ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland—Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.

A benthic storm in the northeastern tropical Pacific over the fields of manganese nodules

Abstract Studies in the northeastern tropical Pacific over 5 years have revealed high eddy activity in the region. Measurements show considerable space-time variation in the bottom currents. A near-bottom intensification in the current velocity over the bottom boundary layer (BBL) has been recorded. A benthic storm (BS) lasting about 10 days, at a maximum measured velocity of 13 cm s−1 6 m above the bottom, appeared to be related to eddies extending to the bottom. The action of these eddies can lead to a change in the direction of the bottom currents and an increase in their velocity. This action also manifests itself by a change in the depth of the benthic thermocline, a reduction in its thickness and in the temperature gradient, as well as in a transformation of the Antarctic Bottom Water (AABW) and the emergence of bottom fronts.

On the renewal of Labrador Sea Water in the Irminger Basin

New evidence of Labrador Sea Water renewal as a result of deep convection in the Irminger Basin is obtained on the basis of the analysis of the data of the distribution of the dissolved oxygen concentration over six sections in the Subpolar North Atlantic in March–October of 1997.

Fifty Years since the First Voyage of the R/V Akademik Kurchatov

An event important for domestic oceanologists occurred in December 1966: the new research vessel (R/V) Akademik Kurchatov was commissioned and headed off on its first voyage. For that time, it was a brand-new type of expeditionary vessel, the Soviet Union’s first, fully designed and built as planned by domestic engineers and scientists. Within the country’s research f leet, the Kurchatov stood out significantly from its predecessors and contemporaries. It was equipped with an 8000 HP twin-shaft diesel propulsion unit, which in relation to the vessel’s 6800-t displacement, was close to the powerful diesel icebreakers of that time. This guaranteed a maximum traveling speed of up to 18 knots and helped reduce traveling time between exploration areas during largescale ocean operations. The Kurchatov was the first civil vessel equipped with an active rudder for slow motion down to 3.5 knots, as well as maneuvering devices. Their simultaneous operation gave the unique possibility of holding the ship at a fixed point and conducting research under difficult weather conditions. The Kurchatov had an onboard computer center, a special helo deck, gyrostabilized platforms to operate marine gravimeters, a Meteor radar system to receive data from weather balloons, and a weather rocket launching pad. The vessel could accommodate up to 80 researchers, with 27 research laboratories and 14 sling and cable electric winches for drifting and mobile overboard operations. Among other devices that were new and unusual for a research vessel, the Kurchatov had active roll dampers in the form of pullout hydrofoils.