Michel Arhan

The Eastern Boundary of the Subtropical North Atlantic

A quasi-meridional hydrographic section carried out between 60° and 20°N offshore from the European and African continental slopes is analyzed in terms of water masses and zonal transports in and out of the eastern boundary. Outstanding features of the meridional distribution of water masses are focused on, such as the transition between North Atlantic and South Atlantic Central Waters at 20°–25°N, the properties and anticyclonic circulation of the Rockall Channel mode water, and the northern boundary of the large-scale Mediterranean Water plume at about 50°N. An eastward transport of about 11×10 6 m 3 s −1 is found to enter the eastern boundary layer at densities lower than 27.25 and feed southward alongshore currents. The relation of the incoming transport to the water mass distribution and its eventual splitting into several outflowing components are discussed. Apart from the downward entrainment of upper water known to occur in the Gulf of Cadiz, there is no sign of the so-called “eastern boundary ventilation” mechanism in the central water density range. Yet a significant transport is found to escape the winter mixed layer toward the interior of the subtropical gyre, suggesting the horizontal southward currents across the sloping bottom of the mixed layer to be the main cause of ventilation at the eastern boundary.

Warm Water Paths in the Equatorial Atlantic as Diagnosed with a General Circulation Model

Abstract Monthly mean velocity fields from a global ocean general circulation model are used to study the main circulation patterns within the upper 1200 m of the equatorial Atlantic. Some recently developed Lagrangian techniques are used to picture and quantify the routes followed in the model by distinct water mass classes, defined by their initial temperature on model transatlantic sections at 10°S and 10°N. The qualitative description in terms of equatorial pathways of this warm component of the so-called global “conveyor belt” is found coherent with the most recent circulation schemes inferred from direct measurements. Diagnostics emphasize the crucial role of the western boundary current system and that of the equatorial subsurface jets in distributing the flow in the equatorial domain, both for northward-flowing and southward-recirculating warm water masses. As the model tracer fields are constrained to remain close to the observed climatology outside the equatorial strip, the circulation calculate...

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.

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.

Circulation and mixing of Mediterranean water west of the Iberian Peninsula

The spreading of water of Mediterranean origin west of the Iberian Peninsula was studied with hydrographic data from several recent cruises and current measurements from the BORD-EST programme. The vertical breakdown of the “Mediterranean salt” content reveals the dominant contribution of the so-called lower core of the outflow (60%), and the significant fraction (22%) brought downward to levels below 1500 m by diffusion. Intense salinity maxima in the upper core (18%) are only encountered south of 38°N in the vein flowing northward along the continental slope, and at a few stations in the deep ocean. Apart from the coastally trapped vein, other preferred paths of the water mass are revealed by the horizontal distributions of salinity maximum and Mediterranean Water percentage. One is southward, west of the Gorringe Bank, and two northwestward ones lie around 40°N and west of the Galicia Bank. Year-long velocity measurements in the Tagus Basin show westward mean values of 7 × 10−2 m s−1 at 1000 m associated with a very intense mesoscale variability. This variability is related to the pronounced dynamical signature of the outflow which favours instability in any branch having detached from the slope current. From a mixing point of view, the strong interleaving activity occurring near Cape St-Vincent is illustrated, but its contribution to the downstream salinity decrease in the coastally trapped vein is weak. Current and meddy detachment play the dominant role, with a scaling estimate of their associated lateral diffusivity of order 500 m2 s−1. The statistical distribution of the density ratio parameter, which governs double-diffusion at the base of the Mediterranean Water, was found to be very tight around Rπ = 1.3 in the temperature range of 5°C< φ < 8°C. North of 40°N, the presence of a fraction of Labrador Sea Water in the underlying water is shown to decrease that parameter and should favour the formation of salt fingers.

Spreading of Labrador Sea Water in the eastern North Atlantic

The spreading of Labrador Sea Water (LSW) in the eastern North Atlantic south of 55°N is described on the basis of recent, high-resolution hydrographic lines and using an inverse model of that basin general circulation. The isopycnic “salinity anomaly” relative to a standard θ/S curve is used to detect the main branches and limits of dominant influence of the intermediate water masses. The LSW crosses the Mid-Atlantic Ridge at latitudes around 50°N with parts spreading farther eastward and southward. One southward pathway along the eastern side of the Mid-Atlantic-Ridge stands out as an eastern basin counterpart of the western Atlantic “Deep Western Boundary Current.” An entry into the Bay of Biscay in its northern part is also evidenced with striking similitudes between two data sets from 1974 and 1989–1990. At the southern limit of the LSW extension, in the vicinity of the Azores-Biscay Rise, a marked front exists basinwide around 1800 m with the more saline Deep Mediterranean Water (DMW). The inverse model velocity field is consistent with most features of the tracer distributions and allows estimation of LSW transports: 6.3±1.2 Sv of LSW with salinities lower than 34.94 cross the 35°W meridian eastward between 44°N and 54°N, one fourth of which recirculates southwestward in the western basin, while another fourth turns northward across 54°N and the remaining half proceeds southward in the eastern basin. The model circulation suggests that DMW results from mixing between eastern basin LSW and the overlying Mediterranean Outflow Water, most probably through double diffusion. We finally describe several mesoscale structures of anomalously high LSW influence south of the LSW-DMW front, notably one sampled by a RAFOS float. The velocity and potential vorticity fields suggest that these “LSW-eddies” are formed through baroclinic instability at the LSW-DMW front.

The Water Masses of the Central North Atlantic in 1983—84

Abstract Hydrographic surveys carried out in 1983–84 along both sides of the Mid-Atlantic Ridge between 24° and 53°N provide a detailed description of the well-known North Atlantic water masses with particular emphasis on their meridional distribution and zonal dissymetry. In the upper layers the dense horizontal sampling resolves the several narrow Gulf Stream extensions into the ocean interior, giving the image, in the Central Water density range, of a mosaic of mode waters separated by fronts. At intermediate depths a vertical shear in the distributions of the Mediterranean Water and Labrador Sea Water stands out, both water masses having their lower part displaced southwards relative to their upper parts. Bottom waters containing nearly 20 percent of pure Antarctic Bottom Water are observed at 50°N in the eastern basin, in contrast with the western basin where proportions greater than 10 percent were found only south of 36°N along our section. This water mass analysis also gives indications that stron...

Physical speciation of iron in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre

Distributions of total dissolvable iron (TDFe; unfiltered), dissolved iron (DFe; 0.2 μm filtered), and soluble iron (SFe; 0.02 μm filtered) were investigated during the BONUS‐GoodHope cruise in the Atlantic sector of the Southern Ocean (34°S/17°E-57°S/0°, February-March 2008). In the mixed layer, mean values of 0.43 ± 0.28 and 0.22 ± 0.18 nmol L−1 were measured for TDFe and DFe, respectively. In deeper waters, TDFe and DFe concentrations were 1.07 ± 0.68 and 0.52 ± 0.30 nmol L−1, respectively. DFe concentrations decreased from the north (subtropical waters) to the south (Weddell Sea Gyre). In the subtropical domain, dusts coming from Patagonia and southern Africa and inputs from the African continental margin may explain high DFe and TDFe concentrations in surface and intermediate waters. Results from numerical models gave support to these hypotheses. In the Antarctic Circumpolar Current domain, estimation of the median advective time of water masses suggests that sediment inputs from the Antarctic Peninsula, South America margin, and/or South Georgia Islands could be an important source of Fe. Except in the subtropical domain where 0.4-0.6 nmol L−1 of SFe were observed in the upper 1500 m, all stations exhibited values close to 0.1-0.2 nmol L−1 in surface and 0.3-0.5 nmol L−1 in deeper waters. For all stations, colloidal Fe (CFe) was a minor fraction of DFe in surface waters and increased with depth. Colloidal aggregation, sinking of CFe, and assimilation of SFe, followed by rapid exchange between the two fractions, are suspected to occur.

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.