Abyssal Water Masses Research Articles

The effect of Southern Ocean topography on the global MOC and abyssal water mass distribution

Abstract We investigate the role of Southern Ocean topography and wind stress in the deep and abyssal ocean overturning and water mass composition using a suite of idealized global ocean circulation models. Specifically, we address how the presence of a meridional ridge in the vicinity of Drake Passage and the formation of an associated Southern Ocean gyre influences the water mass composition of the abyssal cell. Our experiments are carried out using a numerical representation of the global ocean circulation in an idealized two-basin geometry under varying wind-stress and Drake Passage ridge height. In the presence of a low Drake Passage ridge the overall strength of the meridional overturning circulation is primarily influenced by wind stress, with a topographically-induced weakening of the mid-depth cell and concurrent strengthening of the abyssal cell occurring only after ridge height passes 2500m. Passive tracer experiments show that a strengthening mid-depth cell leads to increased abyssal ventilation by North Atlantic water masses, as more North Atlantic Deep Water (NADW) enters the Southern Ocean and then spreads into the Indo-Pacific. We repeat our tracer experiments without restoring in the high-latitude Southern Ocean in order to identify the origin of water masses that circulate through the Southern Ocean before sinking into the abyss as Antarctic Bottom Water. Our results from these “exchange” tracer experiments show that an increasing ridge height in Drake Passage and the concurrent gyre spin-up lead to substantially decreased NADW-origin waters in the abyssal ocean, as more surface waters from north of the ACC are transferred into the Antarctic Bottom Water formation region.

Energy and Momentum of a Density-Driven Overflow in the Samoan Passage

Abstract The energy and momentum balance of an abyssal overflow across a major sill in the Samoan Passage is estimated from two highly resolved towed sections, set 16 months apart, and results from a two-dimensional numerical simulation. Driven by the density anomaly across the sill, the flow is relatively steady. The system gains energy from divergence of horizontal pressure work and flux of available potential energy . Approximately half of these gains are transferred into kinetic energy while the other half is lost to turbulent dissipation, bottom drag, and divergence in vertical pressure work. Small-scale internal waves emanating downstream of the sill within the overflow layer radiate upward but dissipate most of their energy within the dense overflow layer and at its upper interface. The strongly sheared and highly stratified upper interface acts as a critical layer inhibiting any appreciable upward radiation of energy via topographically generated lee waves. Form drag of , estimated from the pressure drop across the sill, is consistent with energy lost to dissipation and internal wave fluxes. The topographic drag removes momentum from the mean flow, slowing it down and feeding a countercurrent aloft. The processes discussed in this study combine to convert about one-third of the energy released from the cross-sill density difference into turbulent mixing within the overflow and at its upper interface. The observed and modeled vertical momentum flux divergence sustains gradients in shear and stratification, thereby maintaining an efficient route for abyssal water mass transformation downstream of this Samoan Passage sill.

Flow Pathways of Abyssal Water in the Yap Trench and Adjacent Channels and Basins

Abyssal flow pathways in the Yap Trench and adjacent regions were investigated by computing geostrophic flows and analyzing water properties. Using boundary values of potential temperature, salinity, oxygen, and silicate, the upper and lower Circumpolar Deep Waters (UCPW, LCPW) were identified as the main abyssal water masses in the trench. At the western East Mariana Basin, some of the LCPW enters the southern Mariana Trench, and the remainder reaches the northern basin and then partly turns to the northern Mariana Trench, eventually joining the southern trench. At the Yap-Mariana Junction, some LCPW enters the West Mariana Basin, and the remainder passes the region east of the Yap Trench and then flows or subsides to the northern and southern trenches, respectively. In the northern trench, the LCPW flows north into the West Mariana Basin. The UCPW enters the northern Yap Trench via the East Mariana Basin and the Mariana Trench, reaches the sill of the Yap Trench via the East Caroline Basin and a narrow channel, and enters the southern trench via the East/West Caroline and Philippine Basins. The UCPW and LCPW circulate anticlockwise in the southern trench and in the northern region of it, respectively. The LCPW may be driven by the downwelling in the trench.

Abyssal current and water mass in the Main Gap and an adjacent Small Gap of the Emperor Seamount Chain

From 2016 to 2017, we conducted 1-year mooring current observation at the widest, deepest gap, widely known as the Main Gap, and a narrower, shallower gap just south of the Main Gap, called the Small Gap in this study, in the Emperor Seamount Chain, which divides the Northwest Pacific Basin (NWPB) from the Northeast Pacific Basin. We also conducted two hydrographic sections with a conductivity–temperature–depth sensor and a lowered acoustic Doppler current profiler around the two gaps in 2012 and 2016. At the Main Gap, the abyssal current flowed east–northeastward at a statistically significant mean velocity of 1.3–2.4 cm s−1, while the current was dominated by tidal and inertial variability with a magnitude of 3–4 cm s−1. At the Small Gap, the abyssal current had an eastward mean velocity of 9.4 cm s−1, accompanied by mesoscale variability with a magnitude of 7 cm s−1. These eastward currents carried abyssal water below a depth of 5000 m, which came from the northern part of the NWPB, with a volume transport of 1.6 × 106 m3 s−1 through the Main Gap and 0.5 × 106 m3 s−1 through the Small Gap. Cold, saline, oxygen-rich Lower Circumpolar Deep Water, which occupied the south of the zonal seamount range at 37°N, called the S–E Seamounts in this study, was found near the seafloor north of the S–E Seamounts to the west of the Small Gap; however, it did not extend to the Main Gap or to the east of the Small Gap.

Paleoredox conditions of bottom water in the northern Indian Ocean since 39 ka

Ocean circulation has undergone significant changes during the glacial and interglacial periods. The redox condition at the sediment-water interface is mainly regulated by export productivity and ventilation of abyssal water mass. The changes in the redox condition associated with bottom water circulation in the Indian Ocean are still unclear. The present study assessed multi redox proxies (Mn/Ti, Mo/Ti, U/Ti, V/Ti, Ni/Ca, Cu/Ca, MnEF, UEF, and VEF) and Corg in the sediment core collected from the Arabian Sea (NW Indian Ocean) to investigate the past variations in bottom water redox condition. The bottom water in the Arabian Sea is presently bathed with the Lower Circumpolar Deep Water (LCDW), consisting of a fraction of both North Atlantic Deep Water (NADW) and Antarctic Bottom Waters (AABW). Our results show significant changes in oxygenation conditions in response to changes in ocean circulation. The present study revealed that bottom water in the northern Indian Ocean was more oxic during LGM than the deglacial period. We interpret the variability in the contribution of NADW and AABW influenced the redox condition at the study location. Comparing the current study results with previously reported Nd isotopes suggests that the contribution of AABW to the Indian Ocean increased during LGM, resulting in a higher oxic condition in LCDW.

Reversible scavenging and advection – Resolving the neodymium paradox in the South Atlantic

Significant gaps in our understanding of the oceanic cycling of neodymium (Nd) and the other rare earth elements (REEs) remain despite decades of research. One important observation which has not been adequately explained is that the concentration of dissolved Nd typically increases with depth, similar to nutrient profiles, while Nd isotopes appear to reflect conservative water mass mixing in the intermediate and deep ocean; this has been termed the “Nd paradox”. Here we present a detailed study of the dissolved Nd isotopic composition across a section at 40°S in the South Atlantic, collected by UK GEOTRACES cruise (section GA10). The South Atlantic represents a natural laboratory for our understanding of spatial controls on ocean geochemistry, because of the large variability of inputs, spatial differences in particulate cycling, and horizontal advection and mixing at depth between major northern- and southern-sourced water masses. This variability has also made the South Atlantic a critical region subject to intense investigations that aim at reconstructing past changes in ocean processes, such as changes in biological productivity and deep ocean circulation.Our Nd isotope results from the GA10 section provide observational data show the signal of water mass mixing and reversible scavenging. In the surface ocean (0–600 m), Nd isotopic compositions are distinct between different surface ocean currents and spatially can be tied to various continental sources. In the intermediate ocean (600–2500 m), the vertical Nd isotope distribution exhibits distinct signals of different water masses by horizontal advection, including upper North Atlantic Deep Water and Antarctic Intermediate Water formed in the Atlantic Ocean or the Indian Ocean. The Nd isotope distribution also reflects influence of reversible scavenging that smears the signals downwards in the water column (i.e., offset to more radiogenic values). In the deep ocean below 2500 m, Nd isotope distribution largely follows conservative water mass mixing model. Nd concentration in the deep ocean, however, deviates from conservative mixing and increases constantly with depth. We also observe that Nd isotopes appear to be shifted towards the composition of overlying water masses. These observations suggest that reversible scavenging of Nd onto organic and other types of particles is a major vertical process throughout the water column. We also suggest that this process can resolve the “Nd paradox” of decoupling of Nd concentration and isotopic composition due to mixing dynamics. Because abyssal water masses already have a high Nd concentration, a given amount of Nd added from the vertical process has less of an effect on Nd isotopic compositions in deep water masses than it does for intermediate water masses which have comparatively low Nd concentration.

Mixing-Driven Mean Flows and Submesoscale Eddies over Mid-Ocean Ridge Flanks and Fracture Zone Canyons

AbstractTo close the abyssal overturning circulation, dense bottom water has to become lighter by mixing with lighter water above. This diapycnal mixing is strongly enhanced over rough topography in abyssal mixing layers, which span the bottom few hundred meters of the water column. In particular, mixing rates are enhanced over mid-ocean ridge systems, which extend for thousands of kilometers in the global ocean and are thought to be key contributors to the required abyssal water mass transformation. To examine how stratification and thus diabatic transformation is maintained in such abyssal mixing layers, this study explores the circulation driven by bottom-intensified mixing over mid-ocean ridge flanks and within ridge-flank canyons. Idealized numerical experiments show that stratification over the ridge flanks is maintained by submesoscale baroclinic eddies and that stratification within ridge-flank canyons is maintained by mixing-driven mean flows. These restratification processes affect how strong a diabatic buoyancy flux into the abyss can be maintained, and they are essential for maintaining the dipole in water mass transformation that has emerged as the hallmark of a diabatic circulation driven by bottom-intensified mixing.

Projected Slowdown of Antarctic Bottom Water Formation in Response to Amplified Meltwater Contributions

Abstract The sinking and recirculation of Antarctic Bottom Water (AABW) are a major regulator of the storage of heat, carbon, and nutrients in the ocean. This sinking is sensitive to changes in surface buoyancy, in particular because of freshening since salinity plays a greater role in determining density at cold temperatures. Acceleration in Antarctic ice-shelf and land-ice melt could thus significantly impact the ventilation of the world’s oceans, yet future projections do not usually include this effect in models. Here we use an ocean–sea ice model to investigate the potential long-term impact of Antarctic meltwater on ocean circulation and heat storage. The freshwater forcing is derived from present-day estimates of meltwater input from drifting icebergs and basal melt, combined with RCP2.6, RCP4.5, and RCP8.5 scenarios of projected amplification of Antarctic meltwater. We find that the additional freshwater induces a substantial slowdown in the formation rate of AABW, reducing ventilation of the abyssal ocean. Under both the RCP4.5 and RCP8.5 meltwater scenarios, there is a near-complete shutdown of AABW formation within just 50 years, something that is not captured by climate model projections. The abyssal overturning at ~30°S also weakens, with an ~20-yr delay relative to the onset of AABW slowdown. After 200 years, up to ~50% of the original volume of AABW has disappeared as a result of abyssal warming, induced by vertical mixing in the absence of AABW ventilation. This result suggests that climate change could induce the disappearance of present-day abyssal water masses, with implications for the global distribution of heat, carbon, and nutrients.

A Global Glacial Ocean State Estimate Constrained by Upper-Ocean Temperature Proxies

We use the method of least squares with Lagrange multipliers to fit an ocean general circulation model to the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) estimate of near sea surface temperature (NSST) at the Last Glacial Maximum (LGM; circa 23–19 thousand years ago). Compared to a modern simulation, the resulting global, last-glacial ocean state estimate, which fits the MARGO data within uncertainties in a free-running coupled ocean–sea ice simulation, has global-mean NSSTs that are 2°C lower and greater sea ice extent in all seasons in both the Northern and Southern Hemispheres. Increased brine rejection by sea ice formation in the Southern Ocean contributes to a stronger abyssal stratification set principally by salinity, qualitatively consistent with pore fluid measurements. The upper cell of the glacial Atlantic overturning circulation is deeper and stronger. Dye release experiments show similar distributions of Southern Ocean source waters in the glacial and modern western Atlantic, suggesting that LGM NSST data do not require a major reorganization of abyssal water masses. Outstanding challenges in reconstructing LGM ocean conditions include reducing effects from model biases and finding computationally efficient ways to incorporate abyssal tracers in global circulation inversions. Progress will be aided by the development of coupled ocean–atmosphere–ice inverse models, by improving high-latitude model processes that connect the upper and abyssal oceans, and by the collection of additional paleoclimate observations.

Hamiltonian Structure and a Variational Principle for Grounded Abyssal Flow on a Sloping Bottom in a Mid‐Latitude β‐Plane

AbstractObservations, numerical simulations, and theoretical scaling arguments suggest that in mid‐latitudes, away from the high‐latitude source regions and the equator, the meridional transport of abyssal water masses along a continental slope correspond to geostrophic flows that are gravity or density driven and topographically steered. These dynamics are examined using a nonlinear reduced‐gravity geostrophic model that describes grounded abyssal meridional flow over sloping topography that crosses the planetary vorticity gradient. It is shown that this model possesses a noncanonical Hamiltonian formulation. General nonlinear steady solutions to the model can be obtained for arbitrary bottom topography. These solutions correspond to nonparallel shear flows that flow across the planetary vorticity gradient. If the in‐flow current along the poleward boundary is strictly equatorward, then no shock can form in the solution in the mid‐latitude domain. It is also shown that the steady solutions satisfy the first‐order necessary conditions for an extremal to a suitably constrained potential energy functional. Sufficient conditions for the definiteness of the second variation of the constrained energy functional are examined. The theory is illustrated with a nonlinear steady solution corresponding to an abyssal flow with upslope and down slope groundings in the height field.

Meridional dynamics of grounded abyssal water masses on a sloping bottom in a mid-latitude -plane

Observations, numerical simulations and theoretical scaling arguments suggest that in mid-latitudes, away from the source regions and the equator, the meridional transport of abyssal water masses along a continental slope corresponds to planetary geostrophic flows that are gravity- or density-driven and topographically steered. We investigate these dynamics using a nonlinear reduced-gravity model that can describe grounded abyssal meridional flow over sloping topography that crosses the planetary vorticity gradient. Exact nonlinear steady and time-dependent solutions are obtained. The general steady theory is illustrated for a non-parallel equatorward flow that possesses a single along-slope grounding along the upslope flank of the current (complementing previous work). Four specific nonlinear time-dependent solutions are described. Two initial-value problems are solved exactly. The first initial configuration corresponds to an equatorward abyssal flow that has no cross-slope shear in the along-slope velocity and possesses a single grounding along the upslope flank of the current. The nonlinear time-dependent evolution of this initial current into a non-parallel shear flow is described. The second initial condition corresponds to an isolated radially symmetric grounded abyssal pool or dome. The nonlinear time-dependent evolution of this abyssal dome, which propagates equatorward with unsteady along- and cross-slope velocities while deforming into an elliptically shaped abyssal dome with $\unicode[STIX]{x1D6FD}$-induced diminishing height, is described. Finally, the nonlinear time-dependent boundary-value problem can be solved exactly in which the in-flow boundary condition on the poleward boundary of the mid-latitude domain corresponds to a time-dependent abyssal current with both an upslope and downslope grounding. Two specific time-dependent boundary conditions are examined. The first corresponds to a time-limited surge in the equatorward volume transport in the abyssal current along the poleward boundary. The second configuration corresponds to the nonlinear evolution of a finite-amplitude downslope plume or loop that forms in the abyssal current that is reminiscent of those seen in baroclinic instability simulations.

Volume transport from the Japan Basin to the Yamato Basin in the abyssal Japan Sea inferred from direct current observations

The transport of Japan Basin Bottom Water (JBBW) into the Yamato Basin in the Japan Sea is an important boundary condition for the modification of the abyssal water mass in the Yamato Basin. To estimate the volume transport of JBBW, two year-long observations (October 2011–October 2012 and May 2014–May 2015) were carried out using current meters moored in the deep channel connecting the Japan Basin with the Yamato Basin. The mean transport toward the Yamato Basin from the Japan Basin was estimated to be 7.37 × 104 and 5.15 × 104 m3 s−1, consistent with previous estimates from box model analysis and lowered acoustic Doppler current profiler observations. The time series of JBBW transport showed significant variability. A cause of the variability was bottom-intensified flow fluctuations in the 3- to 15-day period band, which suggests bottom-trapped topographic Rossby waves in the deep channel. In addition, during August–October 2014, notable variation of JBBW transport accompanied significant decreases of potential temperature and dissolved oxygen concentration. Detailed examination of the episodic variations of flows, potential temperature, and dissolved oxygen concentration, together with consideration of sea surface height variations, suggested that rapid northward meandering of the surface subarctic front was another cause of the significant variation in JBBW transport.

Noble gas tracers of ventilation during deep-water formation in the Weddell Sea

To explore the dynamics and implications of incomplete air-sea equilibration during the formation of abyssal water masses, we simulated noble gases in the Estimating the Circulation & Climate of the Ocean (ECCO) global ocean state estimate. A novel computation approach utilizing a matrix-free Newton-Krylov (MFNK) scheme was applied to quickly compute the periodic seasonal solutions for noble gas tracers. MFNK allows for quick computation of a cyclo-stationary solution for tracers (i.e., a spun-up, repeating seasonal cycle), which would otherwise be computationally infeasible due to the long time scale of dynamic adjustment of the abyssal ocean (1000’s of years). A suite of experiments isolates individual processes, including atmospheric pressure effects, the solubility pump and air-sea bubble fluxes. In addition to these modeled processes, a volumetric contribution of 0.28 ± 0.07% of glacial melt water is required to reconcile deep-water observations in the Weddell Sea. Another primary finding of our work is that the saturation anomaly of heavy noble gases in model simulations is in excess of two-fold more negative than is suggested from Weddell Sea observations. This result suggests that model water masses are insufficiently ventilated prior to subduction and thus there is insufficient communication between atmosphere and ocean at high latitudes. The discrepancy between noble gas observations and ECCO simulations highlights that important inadequacies remain in how we model high-latitude ventilation with large implications for the oceanic uptake and storage of carbon.

On the Consumption of Antarctic Bottom Water in the Abyssal Ocean

AbstractThe abyssal ocean is primarily filled by cold, dense waters formed around Antarctica and collectively referred to as Antarctic Bottom Water (AABW). At steady state, AABW must be consumed in the ocean interior at the same rate it is produced, but how and where this consumption is achieved remains poorly understood. Here, estimates of abyssal water mass transformation by geothermal heating and parameterized internal wave–driven mixing are presented. This study uses maps of the energy input to internal waves by tidal and geostrophic motions interacting with topography combined with assumptions about the distribution of energy dissipation to evaluate dianeutral transports induced by breaking internal tides and lee waves. Geothermal transformation is assessed based on a map of geothermal heat fluxes. Under the hypotheses underlying the constructed climatologies of buoyancy fluxes, the authors calculate that locally dissipating internal tides and geothermal heating contribute, respectively, about 8 and 5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of AABW consumption (upwelling), mostly north of 30°S. In contrast, parameterized lee wave–driven mixing causes significant transformation only in the Southern Ocean, where it forms about 3 Sv of AABW, decreasing the mean density but enhancing the northward flow of abyssal waters. The possible role of remotely dissipating internal tides in complementing AABW consumption is explored based on idealized distributions of mixing energy. Depending mostly on the chosen vertical structure, such mixing could drive 1 to 28 Sv of additional AABW upwelling, highlighting the need to better constrain the spatial distribution of remote dissipation. Though they carry large uncertainties, these climatological transformation estimates shed light on the qualitative functioning and key unknowns of the diabatic overturning.

Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes

Abstract The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean–ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60°S. Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to the abyssal ocean, while compensating warming in the deep ocean under heating introduces a decadal-scale variability of the abyssal water masses. This study emphasizes the fundamental role buoyancy plays in controlling AABW, as well as the importance of the inclusion of shelf-sourced AABW within climate models in order to attain the complete spectrum of possible climate change responses.

Radiocarbon profiles of the NW Pacific from the LGM and deglaciation: Evaluating ventilation metrics and the effect of uncertain surface reservoir ages

AbstractDuring the last deglaciation, the ventilation of the subarctic Pacific is hypothesized to have changed dramatically, including the rejuvenation of a poorly ventilated abyssal water mass that filled the deep ocean, and fluctuations in the strength of North Pacific intermediate and deep water formation at millennial timescales. Foraminiferal radiocarbon reconstructions of past ventilation changes in the Pacific are valuable but are hampered by poor carbonate preservation, low sediment accumulation rates, bias from bioturbation, and poorly constrained past surface reservoir age. In this study, we present paired benthic‐planktonic radiocarbon measurements from the Okhotsk Sea and Emperor Seamounts. We take advantage of large contemporaneous peaks in benthic abundances from the last glacial maximum, Bolling‐Allerod (BA), and early Holocene to produce time slices of radiocarbon from 1 to 4 km water depth. We explore the impact of uncertain surface reservoir age and evaluate several approaches to quantifying past ocean radiocarbon distribution using our NW Pacific data and a compilation of published data from the North Pacific. Both the calendar age and the absolute value of an ocean radiocarbon estimate depend on the assumed surface reservoir age. But for a time slice from a small geographical area with radiocarbon‐independent stratigraphic correlation between cores, the shape of a water column profile is independent of surface reservoir age. The NW Pacific profiles are similar in shape to the compilation profiles for the entire North Pacific, which suggests that deglacial surface reservoir age changes across the N Pacific did not diverge dramatically across the areas sampled. The Last Glacial Maximum (LGM) profile >2 km spans a wide range of values, ranging from values similar to today to lower than today. However, by the BA the profile has a similar shape to today. Ultimately, local surface reservoir ages, end‐member water mass composition, and mixing ratios must each be constrained before a radiocarbon activity reconstruction can be used to confidently infer ventilation changes.

Abyssal water mass exchange between the Japan and Yamato Basins in the Japan Sea

[1] Bottom water exchange between the Japan and Yamato Basins in the abyssal Japan Sea is investigated based on data from two research cruises in June 2010 and June 2011. Distributions of water properties and velocity profiles from CTD/LADCP observations revealed a two-layer water mass exchange in a rotating system. The cold Japan Basin Bottom Water (JBBW) with high dissolved oxygen (DO) concentrations flowed into the Yamato Basin along the northern boundary of the channel connecting the basins, while the warmer, lower-DO Yamato Basin Bottom Water (YBBW) extended over the JBBW along the southern periphery of the channel. A benthic front between the JBBW and YBBW was observed in both cruises, though the front shifted to the northeast in 2011 compared with its 2010 location. Further, the bottom flows in the channel were not unidirectional in 2011, though consistent southwestward flows were observed in 2010. The JBBW transport into the Yamato Basin was also larger in 2010 than in 2011. Analysis of historical hydrographic data showed that the benthic front exists mostly in the narrowest region of the channel, though it fluctuates temporarily to the northeast or southwest. The fluctuation of the benthic front appears to be associated with the JBBW transport because the observations in June 2010 and June 2011 were carried out during periods of southwestward extension of cold water and northeast protrusion of warm water, respectively. A relationship between benthic front displacements and mesoscale warm eddy migrations is suggested.

Structure and variability of the abyssal water masses in the Ionian Sea in the period 2003‐2010

This study presents aspects of the spatial and temporal variability of abyssal water masses in the Ionian Sea, as derived from recent temperature, salinity, dissolved oxygen and velocity observations and from comparisons between these and former observations. Previous studies showed how in the Southern Adriatic Sea the Adriatic Deep Water (AdDW) became fresher (ΔS ≈ −0.08) and colder (ΔT ≈ −0.1°C) after experiencing warming and salinification between 2003 and 2007. Our data, collected from October 2009 to July 2010 from two bottom moorings, one within the Strait of Otranto and the other in the northern Ionian, confirm this tendency: a bottom vein of southward‐flowing AdDW, whose temperature and salinity continuously decreased during the observation time, was detected there. Typically, the vein travel time between the two stations ranged between 45 and 50 days. This gave us a temporal estimate for AdDW anomaly propagation towards the Ionian abyss from their Adriatic generation region. The density excess of the observed vein was always enough to enable its existence as a bottom‐arrested current. This evidence confirms that, at that time (2009 and 2010), the Adriatic Sea was greatly contributing to the formation of Eastern Mediterranean Deep Water (EMDW), the bottom water of the Eastern Mediterranean. Hence, based on these results and on the evidence that, from 2003 to 2009, abyssal Ionian waters became saltier and warmer under the time‐lagged influence of AdDW, possible future changes in the EMDW characteristics, as a response to Adriatic variability, are discussed.

Warming of deep and abyssal water masses along the Greenwich meridian on decadal time scales: The Weddell gyre as a heat buffer

The Southern Ocean renders a significant contribution to the global overturning system through the formation of deep and bottom waters. The Weddell Sea is one of the most prominent regions in this respect. Data obtained between 1984 and 2008 from eight repeat hydrographic sections, moored instruments and profiling floats in the Weddell gyre on the Greenwich meridian – almost all of them collected with RV Polarstern – were used to identify variations in the Weddell system. Fluctuations in the water mass properties were detected in the Warm Deep Water, with a temperature maximum in the 1990s and a minimum in 2005, but also significant variations occurred in the Weddell Sea Deep and Bottom Waters, whereas the Warm Deep Water is dominated by decadal variations; the average temperature and salinity of the whole water column is subject to a positive trend over 24 years. The variations of the water mass properties are induced by variations of the inflow of Circumpolar Deep Water at the boundary. Due to asymmetric wind forcing at the northern and the southern limb of the gyre, variable in- and outflows occur at the open boundaries. Internal processes redistribute heat and salt in the gyre resulting in a long-term increase of the temperature and salinity in the whole water column. The transfer of heat to deeper layers assigns to the Weddell gyre the role of a buffer, with potential impact on the global climate.

Secuencias únicas 16SrRNA de Eurythenes gryllus (Crustacea: Amphipoda: Lysianassidae) de la planicie abisal del Golfo de México

Amphipods of the species Eurythenes gryllus were collected at 2 locations on the abyssal plain (~3 400 m) of the Gulf of Mexico in order to test whether or not these scavenger amphipods are isolated in this peripheral sea or show connectivity by their predominant swimming behavior, moving horizontally along the abyssal water masses in the region. Partial sequences of the mitochondrial 16S rRNA gene from 2 individuals of E. gryllus were determined and showed small differences when compared to sequences of other amphipods of the same species from the Atlantic Ocean (3.6 to 3.9%) and Pacific Ocean (4.0 to 4.1%) and increasing (4.2 to 9.5%) when compared to sequences of specimens from sites of less than 500 m. The largest differences (18%) were observed when the sequences were compared to that of Eurythenes from the Tongue of the Ocean in spite of its closer geographical distance in the region. Isolation in the deep Gulf of Mexico could be attributed to limited genetic exchange with the western tropical Atlantic through the Caribbean over the 2 040 m deep sill and inexistent at abyssal depth through the Florida Straits.