Kevin G. Speer

Global Ocean Meridional Overturning

Abstract A decade-mean global ocean circulation is estimated using inverse techniques, incorporating air–sea fluxes of heat and freshwater, recent hydrographic sections, and direct current measurements. This information is used to determine mass, heat, freshwater, and other chemical transports, and to constrain boundary currents and dense overflows. The 18 boxes defined by these sections are divided into 45 isopycnal (neutral density) layers. Diapycnal transfers within the boxes are allowed, representing advective fluxes and mixing processes. Air–sea fluxes at the surface produce transfers between outcropping layers. The model obtains a global overturning circulation consistent with the various observations, revealing two global-scale meridional circulation cells: an upper cell, with sinking in the Arctic and subarctic regions and upwelling in the Southern Ocean, and a lower cell, with sinking around the Antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges.

The Diabatic Deacon Cell

Abstract Southern Ocean air–sea fluxes from the COADS dataset are examined for compatibility between buoyancy gain and northward Ekman transport. An analysis in density classes points to an upwelling of Upper Circumpolar Deep Water and subsequent buoyancy gain from air–sea exchange as water flows northward in the Ekman layer. The Upper Circumpolar Deep Water layer is too shallow to admit southward geostrophic flow along topography, and an eddy mass flux mechanism for southward transport in this layer to replenish the upwelling is advanced, based on observed large-scale potential vorticity gradients. Estimates of the strength and vertical structure of the meridional flow are given using repeated hydrographic sections and a new climatology.

Rates of Water Mass Formation in the North Atlantic Ocean

Abstract North Atlantic air-sea heat and freshwater flux data from several sources are used to estimate the conversion rate of water from one density to another throughout the range of sea surface density. This cross-isopycnal mass flux varies greatly over the ocean, with a maximum of 32.2 × 106 m3 s−1 at σ = 26.1 kg m−3 (toward greater densities) and a minimum of −7.6 × 106 m3 s−1 (toward lesser densities) at σ = 23.0 kg m−3. The air-sea fluxes force water to accumulate in three density bands: one at the lowest sea surface densities generated by heating; one centered near the density of subtropical mode water; and one spanning subpolar mode water densities. The transfer of water to the highest and lowest densities is balanced by mixing, which returns water to the middle density range, and also by boundary sources or sinks. Integrating the cross-isopycnal flux over all densities gives an annual average sinking of about 9 × 1O6 m3 s−1, which presumably escapes across the equator and must be balanced by a s...

Directly measured mid-depth circulation in the northeastern North Atlantic Ocean

The circulation of water masses in the northeastern North Atlantic Ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes. But the ocean circulation at depths below the reach of satellite observations is difficult to measure, and only recently have comprehensive, direct observations of whole ocean basins been possible. Here we present quantitative maps of the absolute velocities at two levels in the northeastern North Atlantic as obtained from acoustically tracked floats. We find that most of the mean flow transported northward by the Gulf Stream system at the thermocline level (about 600 m depth) remains within the subpolar region, and only relatively little enters the Rockall trough or the Nordic seas. Contrary to previous work, our data indicate that warm, saline water from the Mediterranean Sea reaches the high latitudes through a combination of narrow slope currents and mixing processes. At both depths under investigation, currents cross the Mid-Atlantic Ridge preferentially over deep gaps in the ridge, demonstrating that sea-floor topography can constrain even upper-ocean circulation patterns.

Mixing in the Romanche Fracture Zone

The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data. The finestructure of density profiles is described using the Thorpe scalesLT. It is shown from microstructure data taken in the bottom water that the Ozmidov scale LO is related to LT by the linear relationship LO 5 0.95LT, similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water. Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4 8C isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.88C isotherm, emphasizing the role of these passages for bottom-water property modifications.

Antarctic climate change and the environment: an update

We present an update of the ‘key points’ from the Antarctic Climate Change and the Environment (ACCE) report that was published by the Scientific Committee on Antarctic Research (SCAR) in 2009. We summarise subsequent advances in knowledge concerning how the climates of the Antarctic and Southern Ocean have changed in the past, how they might change in the future, and examine the associated impacts on the marine and terrestrial biota. We also incorporate relevant material presented by SCAR to the Antarctic Treaty Consultative Meetings, and make use of emerging results that will form part of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report

Response of the Antarctic Circumpolar Current to Atmospheric Variability

Abstract Historical hydrographic profiles, combined with recent Argo profiles, are used to obtain an estimate of the mean geostrophic circulation in the Southern Ocean. Thirteen years of altimetric sea level anomaly data are then added to reconstruct the time variable sea level, and this new dataset is analyzed to identify and monitor the position of the two main fronts of the Antarctic Circumpolar Current (ACC) during the period 1993–2005. The authors relate their movements to the two main atmospheric climate modes of the Southern Hemisphere: the Southern Annular Mode (SAM) and the El Nino–Southern Oscillation (ENSO). The study finds that although the fronts are steered by the bathymetry, which sets their mean pathway on first order, in flat-bottom areas the fronts are subject to large meandering because of mesoscale activity and atmospheric forcing. While the dominant mode of atmospheric variability in the Southern Hemisphere, SAM, is relatively symmetric, the oceanic response of the fronts is not, show...

Deep circulation in the eastern South Atlantic Ocean

Abstract Tracer properties on sections of closely spaced hydrographic stations across the Angola Basin of the South Atlantic Ocean along Lats 11° and 24°S suggest a three-layer description of the deep circulation there. Below 4 km the basin is closed off in the south, so water enters only from the north; the interior flow is southward, and the western-boundary current (above the eastern flank of the Mid-Atlantic Ridge) is southward at 11°S but northward at 24°S, as required by the Stommel-Arons dynamics. At depths roughly between 2400 and 4000 m the basin seems to be supplied only from the south, the western-boundary current is everywhere northward, and the interior flow is southward. Near the 2-km level the Mid-Atlantic Ridge is too deep to be an effective western boundary; the flow seems to he broadly southeastward across the full basin at l1°S, but at 24°S a topographically guided current flows northward above the ridge to supply southward interior flow. A hydrographic section along the Greenwich meridian illustrates an eastward jet at depths between 1300 and 3200 m that extends near Lat. 22°S from the western-boundary curent to a gap in the Walvis Ridge, through which the jet introduces water to the Cape Basin. Geostrophic estimates of the volume transports of these circulation clements, calculated with reference to zero-velocity surfaces construed from the tracer fields, are consistent in direction with the inferred flow patterns, but the values may be somewhat erroneously high, as they imply dubiously large upward velocities. A tongue of oxygen-poor, nutrient-rich water is found at depths between 3000 and 4500 m at 11°S but not at 24°S. It is strongest at the African continental rise, and extends some 1000 km westward. Its origin is attributed to decay within the ~ediment of detritus originating mainly from the Congo River plume, and its form to the deep horizontal flow field.

Large-Scale Vertical and Horizontal Circulation in the North Atlantic Ocean

Abstract Observations of large-scale hydrography, air–sea forcing, and regional circulation from numerous studies are combined by inverse methods to determine the basin-scale circulation, average diapycnal mixing, and adjustments to air–sea forcing of the North Atlantic Ocean. Dense overflows through the Denmark Strait and Faroe Bank channels are explicitly included and are associated with strong vertical and lateral circulation and mixing. These processes in the far northern Atlantic play a fundamental role in the meridional overturning circulation for the entire ocean, accompanied by an upper cell of mode-water and intermediate-water circulation. The two cells converge roughly at the mean depth of the midocean ridge crest. The Labrador Sea Water layer lies within this convergence. South of the overflow region, model-derived mean diapycnal diffusivities are O(10−5 m2 s−1) or smaller at the base of the thermocline, and diapycnal advection is driven primarily by air–sea transformation on outcropping layers.

Southern Ocean Thermocline Ventilation

Abstract An approximate mass (volume) budget in the surface layer of the Southern Ocean is used to investigate the intensity and regional variability of the ventilation process, discussed here in terms of subduction and upwelling. Ventilation resulting from Ekman pumping is estimated from satellite winds, the geostrophic mean component is assessed from a climatology strengthened with Argo data, and the eddy-induced advection is included via the parameterization of Gent and McWilliams, together with eddy mixing estimates. All three components contribute significantly to ventilation. Finally, the seasonal cycle of the upper ocean is resolved using Argo data. The circumpolar-averaged circulation shows an upwelling in the Antarctic Intermediate Water (AAIW) density classes, which is carried north into a zone of dense Subantarctic Mode Water (SAMW) subduction. Although no consistent net production is found in the light SAMW density classes, a large subduction of Subtropical Mode Water (STMW) is observed. The S...