Alonso Hernández-Guerra

The Transition Zone of the Canary Current Upwelling Region.

Abstract Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.

The influence of island-generated eddies on chlorophyll distribution: a study of mesoscale variation around Gran Canaria

This study reports hydrographic and biological observations from three cruises where cyclonic and anticyclonic eddies were observed downstream of Gran Canaria island. Based on field data and remote sensing images (AVHRR and CZCS), two mechanisms associated with island- generated eddies, largely responsible for the formation and distribution of chlorophyll around the Canary Islands, are proposed. First, nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies might represent important sources of primary production in the oligotrophic waters of the Canary region. Second, eddies are responsible for the horizontal transport and distribution of chlorophyll originating near the islands or off the African coast. Water with high chlorophyll content, resulting from island stirring or local upwelling at the flanks of the islands, is incorporated into cyclonic eddies in their development and subsequently transported downstream. On the other hand, anticyclonic eddies can also entrain water rich in chlorophyll when interacting with the offshore boundary of the African coastal upwelling. This chlorophyll will be advected southward as the eddy drifts. The recurrence of cyclonic and anticyclonic eddies, together with the presence of upwelling filaments throughout the year, must have important biological consequences in the formation and transport of organic matter in the Canary region. 0 1997 Elsevier Science Ltd. All rights reserved

Island-induced eddies in the Canary Islands

Abstract Cyclonic and anticyclonic eddies were observed downstream of Gran Canaria (Canary Islands), an almost circular island of about 50 km diameter, located in the path of the Canary Current. Temperature data were obtained from five AXBT and one CTD surveys carried out during two years (April, May and December, 1989; February, May and June, 1990), and from NOAA AVHRR (Advanced Very High Resolution Radiometer) sea surface temperature (SST) satellite images. The presence of eddies in most of the surveys and in many SST images suggests that they are common mesoscale features in the flow past the island througout the year. In general, eddy diameter is similar to the width of the island, while the vertical extent is from the near-surface layers down to at least 400 m depth. However, vertical sections across the eddies show distinct patterns in their structures which could correspond to different stages of development. Wakes of relatively warm surface water develop in the lee of the island, interacting with the eddies and affecting their upper mixed layer structure. It is hypothesized that eddies are sequentially spun off from the island with a period ranging from several days to two weeks. If this is the case, they could contribute to the high levels of eddy kinetic energy observed recently downstream of the Canarian archipelago from moored current meters.

The Eastern Boundary Current system between the Canary Islands and the African Coast

To study the EasternBoundaryCurrentsystem off Northwest Africa in detail several CTD/ADCP-sections and long-term mooring work were carried out in the channel between Lanzarote and Africa. The observations are compared with a fine-resolution model, which was developed in the framework of the CANIGO project. The water masses, which are observed in this area, are characterised and classified in density ranges. The current field shows a high spatial and temporal variability with maximum velocities of about 35 cm/s. Seasonal means as well as currents averaged across the channel are only a few cm/s. In the surface water a steady southward flow in the middle of the channel indicates the CanaryCurrent in this area. During fall a strong northward current is observed close to the African shelf. Though the CanaryCurrent strengthens during summer and fall due to an increase of the trade winds, the transport in the channel decreases or turns northward during that time due to the enhanced poleward current at the eastern side. A northward undercurrent with a mean velocity of +2.3 cm/s is observed at the African slope in 950 m depth. The poleward transport of AAIW increases during fall and a strong influence of relatively fresh AAIW is observed during that time. Most of the observations fit well to the results of the CANIGO model, but the occurrence of MW at the bottom of the channel and the corresponding southward flow cannot be resolved by the model.

Evolution of the Alboran Sea Hydrographic structures during July 1993.

Abstract During the ICTIOALBORAN-0793 multidisciplinary oceanographic survey carried out in July 1993 by the Instituto Espanol de Oceanografia (IEO) in the Alboran Sea, some anomalous features were detected. One was the presence of a small cyclonic eddy in the western Alboran Basin, close to the African coast. The upper layer of the eddy consisted of Mediterranean Surface Water and was separated from its supposed source (the northern Alboran Sea) by the Atlantic Jet. Another feature was the probable temporary interruption of the flow of fresh Atlantic Water (S≈36.5) into the eastern Alboran Basin and its replacement by a modified (saltier) Atlantic Water. These features can be explained assuming a time evolution of the surface circulation in the Alboran Sea forced by speed variations in the inflowing Atlantic Water through the Strait of Gibraltar. A collection of satellite images covering the survey period and across-strait sea level difference data, indicative of the geostrophic velocity of the inflow through the Strait, were used to check this assumption. Both sets of data supplied independent but compatible information in the sense that they complemented each other and gave support to the proposed evolving model. Finally, some speculative ideas attempting to correlate the inferred variability in the Alboran Sea with the state of the baroclinic water exchange through the Strait of Gibraltar (maximal or submaximal) are discussed.

Phytoplankton pigment patterns in the Canary Islands area as determined using Coastal Zone Colour Scanner data

Data from the Nimbus-7 Coastal Zone Colour Scanner (CZCS) were analyzed to determine the phytoplankton pigment patterns in the ocean surrounding the Canary Islands. Two different pattterns were observed in this region. One pattern was associated with the influence of the northwest African coastal upwelling system on the ocean surrounding the Canary Islands and the other was associated with individual islands, probably due to the disruptive effect of the Canary Islands on the flow of the Canary Current

Water masses and circulation in the surface layers of the Caribbean at 66°W

A meridional hydrographic section was made in August–September 1997 at 66°W from the coast of Venezuela to Woods Hole aboard the R/V Knorr. In this report, we concentrate on near surface measurements in the Caribbean. The data show two distinct water masses with different origins. From approximately 14°N to Puerto Rico, Caribbean Surface Water and Subtropical Under Water with their source in the North Atlantic are found, as previously observed. From Venezuela to approximately 13°N, a less saline water mass with its source in the Tropics and South Atlantic is found. Within the southern portion of the section, two different velocity patterns are observed, namely, an eastward flow with a subsurface maximum near the coast of Venezuela, and a surface intensified westward jet with Velocities of 130 cm s−1 in midbasin.

Seasonal variability of recurrent phytoplankton pigment patterns in the Canary Islands area

Seasonal variability of recurrent phytoplankton pigment patterns in the Canary Islands area has been determined using historical data from Nimbus-7 Coastal Zone Color Scanner data. The main patterns associated with the islands are cyclonic and anticyclonic eddies downstream of the islands, apparently caused by their effect as a topographic obstacle to the flow of the Canary Current. These eddies are present during all seasons, suggesting that the speed of the Canary Current is always strong enough to develop eddies. The pattern associated with the influence of the coastal upwelling system off north-west Africa consists of upwelling filaments stretching from the African coast to the islands. They are fundamentally present during summer and early fall when strong Trade Winds blow over the area. Both patterns may contribute to the mesoscale activity south of the islands.

Zonal circulation in the NW Atlantic and Caribbean from a meridional World Ocean Circulation Experiment hydrographic section at 66°W

A World Ocean Circulation experiment Hydrographic Program section along 66°W in the North Atlantic was made in 1997. In addition to the usual variables (hydrographic and tracer) measured in WOCE onetime sections, we made lowered acoustic doppler current profiler (LADCP) measurements at nearly all of the stations. The section closed off a portion of the western North Atlantic to the west of the line, making a closed volume for constraining the circulation. In addition, the deep portions of the Caribbean do not communicate with the rest of the basin. By combining mass, silica, and LADCP information as constraints an inverse calculation obtained reveals a strong eastward transport in the Gulf Stream bounded by westward flowing water on either side. Within these energetic flows we see evidence for recently ventilated Classical Labrador Sea Water, which has not reached the Deep Western Boundary Current north of Puerto Rico in any significant amounts. Within the Caribbean our major new finding is a deep cyclonic circulation below sill depth in excess of 100 times the inflow of deep Atlantic water through the major deep sill: the Anegada-Jungfern Passage. The signature of the deep Atlantic source water is most prominent in CFCs and both a bottom and mid-depth maximum are present in the Caribbean to the south of Puerto Rico. Off the coast of Venezuela, however, only the deeper CFC maximum is found. For the entire section the net overturning circulation, heat flux, and freshwater fluxes are all consistent with expectations based on water mass formation and air-sea exchanges to the west of our section, but the annual mean air-sea fluxes of heat and freshwater from Comprehensive Ocean-Atmosphere Data Sets appear somewhat too small in comparison with our single-section result.

Transport variability in the Lanzarote passage (eastern boundary current of the North Atlantic subtropical Gyre)

Observations from a four-year current meter mooring at 28°44′N, 13°28′W in the Lanzarote passage are used to describe the transport variability of the Eastern Boundary Current of the North Atlantic Subtropical Gyre. Three different water masses are found in the passage: North Atlantic Central Water in the upper levels (roughly Full-size image (<1 K)), Antarctic Intermediate Water (roughly Full-size image (<1 K)) and Mediterranean Water in the layer above the bottom at Full-size image (<1 K). The mean southward transport of NACW is Full-size image (<1 K)Full-size image (<1 K) which is the transport of the easternmost branch of the Canary Current. Fluctuations of NACW transport are large, ranging from Full-size image (<1 K) southward to Full-size image (<1 K) northward. Every autumn a consistent northward transport is observed, which may be related with the eastern boundary upwelling dynamics. The mean transports of AAIW and MW are Full-size image (<1 K) northward and Full-size image (<1 K) southward, respectively. Fluctuations of transport of AAIW and MW are large, from 1.0 to Full-size image (<1 K) and from −0.32 to Full-size image (<1 K), respectively. Thus, the mass transports for each water mass show a high standard deviation of comparable magnitude to the mean. This highlights the importance of the temporal variability of the currents in this passage. A remarkable feature of our observations is that the mean transports of NACW and AAIW during an El Nino event are significantly different.