Fiz F. Pérez

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.

Hydrographic variability off the Rías Baixas (NW Spain) during the upwelling season

During the Galicia X cruise, from May to October 1989 an intensive collection of hydrographic data was carried out at a single station on the shelf off the western coast of Galicia. It allows us to follow the response of the water column to the intermittent equatorward wind stress during the upwelling season. Upwelling events occur with biweekly periodicity, bringing Eastern North Atlantic Water (ENAW) to the subsurface layer at our station. A trend of the thermohaline properties of the upwelled water to increase in time was observed. This seems to be mostly due to the southwestward displacement of the origin of this water mass during the year. Although the saltier and warmer ENAW is less nutrient-rich, nutrient levels increase because of the rapid remineralization of organic matter from the Rias, which takes place in the bottom water on the shelf.

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.

A precise and rapid analytical procedure for alkalinity determination

Abstract A potentiometric analytical method is proposed for the determination of the alkalinity of seawater. The precision is 0.1% and each determination takes 3 min. The technique is very easy to use, even on board ship since it is carried out in open flasks. A polynomial equation is also proposed which greatly simplifies the theoretical expression.

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,” “

Nutrient mineralization patterns in shelf waters of the Western Iberian upwelling

Abstract A set of hydrographic data collected during several cruises to the NW Iberian upwelling system has been used to assess the patterns of nutrient mineralization over the shelf. Mineralized nutrient ratios (N/P and N/Si) in the lower water column are clearly below the ratios characteristic of freshly upwelled oceanic waters. Nitrogen mineralization, in comparison to phosphorus mineralization, is incomplete, in agreement with phosphates faster regeneration rate and the low residence time of water, which precludes complete mineralization. Silicate shows the highest enrichment as a consequence of the selective concentration of particulate biogenic silica in the near bottom waters. Diffusion of nutrients mineralized in the sediments seems to represent an important contribution. The lowest mineralization of the surveyed area is found to the north of Cape Finisterre, especially with regard to silicate. To the South, the Rias Baixas (four large embayments) return as reducing particulate organic matter (POM) a small fraction of the upwelled nutrients imported from the shelf. However, the out-flowing freshwater contributes to stratification of coastal waters, which favours POM production, accumulation and sedimentation. In addition, high levels of new dissolved organic matter (DOM) in the outflow can also enhance primary production in shelf surface waters. Finally, highest mineralization was found to the south of the River Mino, where there are no embayments and the shelf is wider. Mineralization tends to: (1) increase the potential primary production of this ecosystem by up to 50%, (nitrogen limitation) or up to 80% (silicon limitation); (2) favour the development of phytoplankton assemblages dominated by diatoms (Si enrichment double that of N and P); and (3) buffer the large spatial and temporal differences introduced by the advected nutrients.

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.

New production of the NW Iberian shelf during the upwelling season over the period 1982–1999

New production (NP) is calculated for NW Iberian shelf waters from 421 to 431N (3500 km 2 ), at the fortnight, upwelling-season (March–October) and inter-annual time-scales. The time series used are (1) upwelling rates (daily values of offshore Ekman transport from 1982 to 1999), (2) bottom shelf temperatures (twice a week values from 1987 to 1999), and (3) the nutrient–temperature relationships ofupwelled Eastern North Atlantic Central Water (ENACW) obtained during 14 hydrographic cruises to the study area (between 1977 and 1998). Marked inter-annual variability is observed, both at the fortnight and the seasonal time-scales. Average NP over the upwelling-season ranged from 330 to 815 mg C m � 2 d � 1 (mean, 4907145 mg C m � 2 d � 1 ) in the 1982–1999 period. Large inter-annual changes ofupwelling rates are the reason behind the NP fluctuations: 83% ofthe variability ofNP can be explained by the offshore Ekman Transport ð� QX Þ: NP is compared with satellite-derived net microbial community production (NCP) during the 1998– 1999 upwelling seasons, when SeaWiFS images are available. An average upwelling-season NP/NCP ratio of0.33 was obtained, indicating that 67% of NCP is respired in situ and 33% is exported off-shelf to the surrounding oligotrophic ocean.

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.