Aida F. Ríos

Water masses in the upper and middle North Atlantic Ocean east of the Azores

During the “ANA” cruise in November 1988, Western North Atlantic Water (WNAW) was found linked to the Azores Current (AC) at 23°W, where according to various authors Eastern North Atlantic Water (ENAW) forms a boundary with WNAW east of the Mid-Atlantic Ridge (MAR). This boundary changes position during the year. The vein of Mediterranean Water (MW) moving towards the MAR seems to be connected with the AC, and restricts the penetration of ENAW of subpolar origin (ENAWP). A subsurface front has been found along 42°N, separating ENAW of subtropical origin (ENAWT) that moves northeastwards generating a cyclonic eddy that is confined between the lower limit of the surface layer and approximately the 27.06 isopycnal, and modified ENAWP that moves southwards forming various anticyclonic eddies.

Vivaldi 1991 - A study of the formation, circulation and ventilation of Eastern North Atlantic Central Water

Abstract A synoptic, hydrographic data set comprising 32 full depth CTD casts and 2500 CTD/SeaSoar profiles to 500 m is used to describe the θ/S properties and circulation of Central Water east of the mid-Atlantic Ridge and between 39°N and 54°N. Eastward transport of 20 × 106 m3 s−1 in the North Atlantic Current turns entirely northwards to the west of 54°N, 20°W. This transport consists in the upper layers of Western North Atlantic Water freshened at temperatures below 10°C by mixing with SubArctic Intermediate Water. Northern and Southern branches of the North Atlantic Current are well defined and both turn northwards west of 20°W. A further 10 × 106 m3 s−1 of Eastern North Atlantic Water forms and recirculates anticyclonically to the west of Spain south of the North Atlantic Current and north of 40°N. Eastern North Atlantic Water is most weakly stratified east of 20°W and there is clear correlation between weakly stratified pycnostads and positive salinity anomalies relative to Western North Atlantic Water. Thus Eastern North Atlantic Water is a winter Mode Water in which strong winter cooling has increased the density and hence also the salinity anomaly at a given temperature. Near the southern entrance to the Rockall Trough there is evidence that salinities are also increased by Mediterranean Water influence. Circulation south of the North Atlantic Current is complex. There is no evidence for direct ventilation southwards across 40°N where water properties (θ/S, potential vorticity and CFC-113) and historical data all indicate westward ventilation east of 24°W, with weak southward ventilation occurring further west, in the vicinity of the Azores. The circulation pattern suggested is remarkably similar to that proposed by Helland-Hansen and Nansen in 1926 (The eastern North Atlantic, Geophysiske Publicajoner, 4, 1–76), with anticyclonic circulation of colder Eastern North Atlantic Water north of 40°N meeting warmer water from south of 40°N circulating cyclonically north of the Azores Current. The distribution of pycnostads and θ/S properties between 20°W and 35°W north of the Azores indicates alternate bands of Western and Eastern North Atlantic Water moving eastward and westward respectively, including evidence for westward motion immediately south of the Southern branch of the North Atlantic Current, possibly by westward propagation of anticyclonic eddies containing deep pycnostads.

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Abstract Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September–October) to the cessation (February–May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40° and 43°N have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)—the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

Tracking the variable North Atlantic sink for atmospheric CO2

A Happy Marriage The fluxes of CO2 between the atmosphere and ocean are large and variable, and understanding why the concentration of atmospheric CO2 changes as it does, depends on accurately determining the details of those fluxes. One of the major obstacles in the way of quantifying this exchange is that there are too few measurements available, both temporally and geographically. Watson et al. (p. 1391) report results from a happy marriage of science and commerce—data collected by instruments fitted onto commercial ships plying the waters of the North Atlantic Ocean—that has generated the largest and most comprehensive set of measurements of ocean pCO2 ever collected. These data allow the oceanic CO2 sink to be monitored with unprecedented accuracy and will help researchers precisely map regional interannual air-sea fluxes. Data from instrumented commercial ships reveal substantial interannual variations of carbon dioxide flux between the ocean and the air. The oceans are a major sink for atmospheric carbon dioxide (CO2). Historically, observations have been too sparse to allow accurate tracking of changes in rates of CO2 uptake over ocean basins, so little is known about how these vary. Here, we show observations indicating substantial variability in the CO2 uptake by the North Atlantic on time scales of a few years. Further, we use measurements from a coordinated network of instrumented commercial ships to define the annual flux into the North Atlantic, for the year 2005, to a precision of about 10%. This approach offers the prospect of accurately monitoring the changing ocean CO2 sink for those ocean basins that are well covered by shipping routes.

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.

Displacement of water masses and remineralization rates off the Iberian Peninsula by nutrient anomalies

Temperature, salinity, oxygen, nutrients and COZ data obtained in the areas west and north of the Iberian Peninsula have been analyzed. We assume that the composite parameters such as Broecker’s “NO” and “PO” and our own “CAO” and “SiO” are conservative for our water mass analysis. We demonstrate that the observations can be well represented by mixing between five end-members and estimate the relative proportions of these end-members in waters between 40N and 47N. Furthermore, based upon the differences between the observed and computed concentrations of oxygen and nutrients, the approximate rates of oxygen utilization and nutrient production in each water mass are estimated. The type-values of the “NO,” “

Decadal changes of the θ−S relationship of the Eastern North Atlantic Central Water

Abstract Observations of the eastern North Atlantic mode waters spanning a 20-year period were assembled including previously unpublished observations and historical data, to investigate spatial and temporal variations. Along 42°N, the freshening of salinity on the isopycnal σθ = 27.1 is shown to persist until 1990 and to be followed by an abrupt increase in 1991 to levels not seen since the late 1970s. Some suggestions are made of mechanisms 1970s. Some suggestions are made to mechanisms that may be involved in the variability.

Hydrographic conditions associated with the relaxation of an upwelling event off the Galician coast (NW Spain)

During the GALICIA-XI cruise froIn May 10 to May 14, 1991, the relaxation of a previous strong tipwelling event took place along the Galician coast caused by a shift in the position of the Azores High. The coldest tenperatures and highest nitrate levels were forrod near the coast corresponding to the advection of subsurface waters, particularly of Eastern North Atlantic Water (ENAW). On the basis of thermohaline properties, ENAW of polar (ENAWp) and tropical (ENAWt) origin are distinguished. The highest salinity values found correspond to a saline wedge of ENAW t located off the west coast of Galicia. The relaxation of tipwelling produced a shoreward migration of surface water from oceanic stations which crone into contact with upwelled Eastern North Atlantic Water ti-mn the coastal stations to tbrm a convergence front 28 lan off the coast. Meanwhile, the saline wedge displaced towards both the north and the shore which is thvored by the compression of the tipwelling system to a narrow coastal strip outwardly limited by the convergence zone.

Water masses and distribution of physico-chemical properties in the Western Bransfield Strait and Gerlache Strait during Austral summer 1995/96

In the framework of the FRUELA project, two oceanographic surveys were conducted by R/V Hesperides in the eastern Bellingshausen Sea, western basin of the Bransfield Strait and Gerlache Strait area during December 1995 and January 1996. The main hydrographic structures of the study domain were the Southern Boundary of the ACC and the Bransfield Front. The characteristics and zonation of local water masses are discussed in terms of temperature, salinity, dissolved oxygen, nutrient and inorganic carbon concentrations. Concentration intervals for water mass labelling, on the basis of chemical parameters in addition to the common θ/S-based classification, are defined. Silicate seems to be a very good discriminator for local water masses.

Mixing analysis of nutrients, oxygen and inorganic carbon in the Canary Islands region

Abstract We show the distribution of nutrients, oxygen, total dissolved inorganic carbon ( C T ) and total alkalinity ( A T ) along three sections close to the Canary Islands, between 18°W and the African coast during Meteor 37/2 cruise (January 1997). From the thermohaline properties of Eastern North Atlantic Central Water (ENACW), Mediterranean Water (MW), Antarctic Intermediate Water (AAIW) and North Atlantic Deep Water (NADW), a mixing model has been established based on the water mass description. It can explain most of the variabilities found in the distribution of the chemical variables, including the carbon system, and it is validated through the use of conservative chemical variables like ‘NO.’ From nutrients, oxygen, A T and C T , the chemical characterisation of the water masses was performed by calculating the concentration of these variables in the previously defined thermohaline end-members. The relative variation of nutrient concentrations, resulting from the regeneration of organic matter, was estimated. Close to the African shelf-break, a poleward undercurrent conveying as much as a 11% of AAIW was observed only in the southern section (28.5°N). From the chemical and thermohaline properties of the end-members, a comparison with data from other oceanic regions was made in respect to conservative chemical variables (‘NO’). In addition, a north–south gradient in the ventilation pattern of water masses is observed from the residuals of the model.