Herlé Mercier

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.

An inverse model of the eastern North Atlantic general circulation and thermocline ventilation

An inverse model is applied to a set of high-quality hydrographic measurements gathered between 1981 and 1993 in the eastern North Atlantic, between 24°N and 54°N and east of 35°W. The method seeks a density field as close as possible to the hydrographic data and an absolute velocity field in exact geostrophic and hydrostatic balances, satisfying as well as possible the planetary vorticity balance, the Ekman dynamics at the surface and mass, salt and heat conservations in a topto-bottom integrated form. A solution is found that departs only slightly from the hydrographic data and that presents reasonably small constraint residuals. The solution is discussed in terms of the eastern North Atlantic general circulation and thermocline ventilation during the observational period. The North Atlantic Current appears to be composed of several branches, and its influence extends down to more than 2500 m. Two-thirds of its total transport recirculates northward across 54°N. The Azores Current appears as an almost zonal current around 33°N that, while recirculating southward, is fed from the north by the southward recirculation of the North Atlantic Current. The total eastward transport of these two currents above the Mid-Atlantic Ridge amounts to 58 ± 11 Sv, far more than predicted using the Sverdrup relation, and more than most of the previous estimates. It is shown that the interaction between the deep circulation and the bottom topography allows such a transport to be compatible with the planetary vorticity balance. Using the modeled circulation, thermocline ventilation is thoroughly studied. The subduction rate patterns show where the lightest varieties (27.0 < σθ < 27.4) of Subpolar Mode Water (SPMW) are mainly formed and subducted. A total of 11.9 ± 0.5 Sv of SPMW formation is estimated in the modeled area, while the total subduction transport amounts to 7.7 ± 0.5 Sv, of which 2.5 ± 0.4 Sv is SPMW subduction. Eddy diffusion along the isopycnals is estimated as playing no important role in SPMW subduction and in the first stages of its subsequent southward transport, but seems to be the dominant process by which that water crosses the Azores Front south of 36°N. The Lagrangian, annual buoyancy budget of the winter mixed layer is finally diagnosed from the model fields, and it is confirmed that SPMW is formed where the mixed-layer loses buoyancy, and subducts as soon as the mixed layer gains buoyancy.

Volume budget of the eastern boundary layer off the Iberian Peninsula

Data from a hydrographic array carried out in May 1989 offshore of the Iberian Peninsula are used to determine the vertical distribution of zonal transports near the ocean eastern boundary (the so-called eastern boundary kinematic condition). Eastward transports of Central Water, Bottom Water and, more surprisingly, of Mediterranean Water, are found, complemented by westward flows at the deep-intermediate and deep levels. The top-to-bottom integrated estimate is 2 × 106 m3 s−1 (±1.2 × 106 m3 s−1) eastward across the studied 5° long meridional segment at 12°30′W. The meridional transports in the 300 km wide boundary layer are northward at all levels, and their vertical integral amounts to six times the zonal one. The finding of a net poleward flow of Central Water is discussed with regard to the notion of a southward Portugal Current. Seasonality of the eastern boundary processes is proposed as a clue to the apparent contradiction and is also suggested to influence the phenomenon of eastern boundary ventilation sometimes invoked in, or inferred from, theoretical studies. The pronounced bathymetric features of the region have strong effects on the details of the boundary layer circulation. At intermediate levels, three meridional advective paths carry the Mediterranean Water away from the west Iberian region: the northward slope undercurrent off Cape Finisterre and two branches in opposing directions west of the Galicia and Gorringe Banks. The meridionally averaged zonal transport of this water mass being eastward, it is argued that a direct entry of Mediterranean Water into the ocean interior can occur, at Iberian latitudes, only through westward meddy propagation. Hydrographic estimates of the eddy salinity transport across 12°30′W are indeed westward. A scaling analysis of the advection-diffusion equation of salinity at the level of Mediterranean Water is proposed, which includes these elements.

The water masses along the western boundary of the south and equatorial Atlantic

A quasi-meridional hydrographic section located offshore from South America from 50°S to 10°N, and three shorter transverse lines to the continental slope, are used for a descriptive study of the water masses along the western boundary of the South and Equatorial Atlantic. At the upper and intermediate levels, the tracer analysis provides geographical limits of the wind-driven circulation regimes, and a comparison of the tracer values at the continental slope and along the meridional section shows where the boundary currents originate. At depths shallower than about 200 m, the subdivision of the subtropical gyre into two cells separated by the Subtropical Countercurrent near 28°S, that was pointed out in a previous study, is corroborated. South of this front, a warm variety (∼18°C) of Subtropical Mode Water in the inner recirculation of the Brazil Current appears, despite its limited extent, as a southern counterpart of the North Atlantic 18°C water. At the deep levels, the Upper Circumpolar Water and Upper North Atlantic Deep Water enter the South Atlantic in a significantly overlapping density range. The ensuing lateral encounter of both water masses occurs at 26°S near the western boundary, where most of the boundary flow of the latter water is stopped and deflected seaward by the base of the subtropical gyre. Other tracer anomalies signal significant eastward escapes of North Atlantic Deep Water: within two jets at about two degrees of latitude on either side of the equator, in another narrow current at 10°S, and at 34°S. The latter latitude marks the confluence, and eastward deflection, of the opposite boundary currents of Lower North Atlantic Deep Water and Lower Circumpolar Water. Near the bottom of the Argentine Basin, the Weddell Sea Deep Water that flows westward north of the Zapiola Ridge is more recently ventilated than the water carried by the boundary current near the Falkland Escarpment. While a part of it flows anticyclonically around the ridge, another part turns equatorward and enhances the southern property signatures of the water farther north.

Transports across the 2002 Greenland-Portugal Ovide section and comparison with 1997

The first Ovide cruise occurred in June–July 2002 on R/V Thalassa between Greenland and Portugal. The absolute transports across the Ovide line are estimated using a box inverse model constrained by direct acoustic Doppler current profiler velocity measurements and by an overall mass balance (±3 Sv, where 1 Sv = 106 m3 s−1) across the section. Main currents are studied and compared to the results of the similar Fourex section performed in August 1997 and revisited here. The meridional overturning cell (MOC) is estimated in two different ways, both leading to a significantly lower value in June 2002 than in August 1997, consistent with the relative strength of the main components of the MOC (North Atlantic Current and deep western boundary current). It has been found that the MOC calculated on density levels is more robust and meaningful than when calculated on depth levels, and it is found to be 16.9 ± 1.0 Sv in 2002 versus 19.2 ± 0.9 Sv in 1997. The 2002 heat transport of 0.44 ± 0.04 × 1015 W is also significantly different from the 0.66 ± 0.05 × 1015 W found in 1997, but it is consistent with the much weaker integrated warm water transport across the section than in 1997.

Hydrographic sections across the Atlantic at 7°30N and 4°30S

Abstract Transatlantic hydrographic sections along 7°30N and 4°30S, and shorter meridional ones along 35°W and 4°W in the intervening latitudinal range, provide a basin-wide description of the Atlantic water masses at their crossing of the equator. The water masses belonging to either the cold or warm segment of the global thermohaline cell enter the equatorial region mostly in the form of western boundary currents. The ways they leave it are more varied. The Ekman drift and a geostrophic western boundary current cause the export of near-surface water to the North Atlantic. A part of the southern Salinity Maximum Water, regarded as the shallowest warm water component, is thought to follow this route after experiencing strong property modification in the equatorial upwelling. The underlying South Atlantic Central Water divides into two northward paths, a direct one along the south American continental slope, hardly observed in the data because of an intense variability in the western half of the 7°30N line, and a longer one through the eastern basin, taken by water of the equatorial thermostad. There is no trace of such as eastern northward route for the Antarctic Intermediate Water, which is apparently forced northward from the equatorial region through the highly variable circulation of the western basin. The deep western boundary currents carrying southward the upper and middle components of the North Atlantic Deep Water experience a first partial shift to the eastern boundary on crossing the equator. At deeper levels, a part of the lower North Atlantic Deep Water also bifurcates eastward at the equator, but loses its identity through vertical mixing with the Antarctic Bottom Water in the equatorial fracture zones. The newly formed homogeneous bottom water proceeds eastward in the Guinea Basin, with further indication of an overflow into the Angola Basin. Beside the North Atlantic Deep Water, the deep layer of the equatorial region contains a lower-oxygen component, most clearly present between the middle and lower cores of the North Atlantic Deep Water. Previous results on this water are substantiated, namely, an arrival from the southeast, and northwestward crossing of the equator offshore from the deep western boundary current of northern water. A further northward progression of the southern water requires that the equatorial branching of the southward deep boundary current be only intermittent. A comparison of the temperatures along 7°30N in 1993 with those obtained at 8°N during the International Geophysical Year, 36 years before, reveals a net warming of the intermediate and upper deep waters, and cooling of the bottom water. This result is similar to that obtained at 24°N by other authors, yet there are signs of a southward propagation of a deep cold anomaly in the western basin, which had reached 24°N in 1992, but not yet 7°30N in 1993.

Transport of Bottom Water in the Romanche Fracture Zone and the Chain Fracture Zone

Abstract Two moored arrays deployed in the Romanche Fracture Zone and Chain Fracture Zone in the equatorial Atlantic Ocean provide two-year-long time series of current and temperature in the Lower North Atlantic Deep Water and the Antarctic Bottom Water. Total time-averaged transport of Antarctic Bottom Water (potential temperature θ < 1.9°C) across the Mid-Atlantic Ridge amounts to 1.22 × 106 m3 s−1 eastward with a standard deviation of ±0.25 × 106 m3 s−1. A time-averaged transport of 0.36(± 0.23) × 106 m3 s−1 eastward is found for the Lower North Atlantic Deep Water in the 1.9° < θ < 2.1°C temperature range, but this may represent only a fraction of the total flow of this water mass across the ridge. Contributions of the Romanche Fracture Zone and Chain Fracture Zone to the Antarctic Bottom Water transport are similar, while the Chain Fracture Zone has the greater share of Lower North Atlantic Deep Water transport. Semiannual and annual periods are detected in the transport time series and together expl...

The disparate evolution of three Agulhas rings in the South Atlantic Ocean

Hydrographic sections carried out in January–March 1995 across the pathway of Agulhas rings in the Cape Basin are used for a brief description of the mesoscale thermohaline variability in this region and a detailed study of three rings that were identified in the data. The three eddies exhibited remarkably diverse dimensions, vertical structures, and water mass characteristics. One of them, R1, was located near the Agulhas retroflection, had a diameter of 200 km, a maximum azimuthal speed of 0.40 m s−1, core oxygen values in excess of 260 μmol kg−1 and was characterized by a well-developed thermostad of 11.6°C. A second ring, R2, at 31°30′S, 9°W, by contrast, had a diameter of about 500 km, a core temperature of 17.1°C, and azimuthal speeds of 0.50 m s−1, suggesting a very different history. A third ring, R3, at 26°S, 9°W, although farthest north of the three, had characteristics similar to ring R1, but with a deeper thermostad. Satellite altimetric data allow one to infer the natural histories of these vortices. Ring Rl detached from the retroflection at the beginning of March 1994 and spent the whole of the subsequent winter south of 42°S. This could explain the estimated heat loss of 620 W m−2. The two rings observed at 9°W were spawned as one feature in April 1993, but interaction with the Erica seamount split it into two eddies; R3 being stalled in the retroflection region for the winter, while R2 moved off rapidly into the South Atlantic, by contrast, retaining most of its heat. These histories account for the observed differences between the rings. They also demonstrate that the interaction of Agulhas rings with their environment, including ambient water masses, the overlying atmosphere, and the bottom topography, is critical to their eventual hydrographic characteristics and the manner in which they contribute to the transport of heat and salt from the Indian to the South Atlantic Ocean.

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).

Effects of the Mixed Layer Time Variability on Kinematic Subduction Rate Diagnostics

Abstract An eddy-resolving primitive equation general circulation model is used to estimate water-mass subduction rates in the North Atlantic Ocean subtropical gyre. The diagnostics are based on the instantaneous kinematic approach, which allows the calculation of the annual rate of water-mass subduction at a given density range, following isopycnal outcrop positions over the annual cycle. It is shown that water-mass subduction is effected rapidly (∼1–2 months) as the mixed layer depth decreases in spring, consistent with Stommel’s hypothesis, and occurs mostly over the area of deep late-winter mixed layers (≥150 m) across the central North Atlantic in the density range 26 ≤ σ ≤ 27.2. Annual subduction rates O(100–200 m yr–1) are found south and east of the Gulf Stream extension in the density range of subtropical mode waters from roughly 26.2 to 26.6. In the northeastern part of the subtropical gyre, annual subduction rates are somewhat larger, O(250 m yr–1), from a density of about 26.9 east of the Nort...