Pablo Sangrà

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.

Eddy and deep chlorophyl maximum response to wind-shear in the lee of Gran Canaria

The physical and biological properties of the warm wake of Gran Canaria were examined during a survey carried out in June 1998. The sampling region was dominated by the presence of a warm triangular region downwind the island and an anticyclonic eddy spun off the island. Convergent and divergent frontal regions were generated by the wind shear zones extending along either side of the sheltered region of the warm wake. With increasing distance from shore, evidence of convergent/divergent frontal regions weakened, but the influence of the eddy increased. Both structures, frontal regions and the eddy, clearly altered the vertical phytoplankton biomass distribution as indicated by chlorophyll-fluorescence. Downwelling on the convergent boundary moved the 26.2 kg m � 3 isopycnal and its associated deep chlorophyll maximum (DCM) below the 1% light zone. Upwelling at the divergent boundary not only elevated the DCM with its associated isopycnal but also, because of the increased light levels, allowed a shift in the DCM to higher (deeper) density surfaces (26.4 kg m � 3 ). However, the highest integrated chlorophyll occurred in the

Temporal variability of mass transport in the Canary Current

Abstract The variability of the Canary Current is investigated using bimonthly expandable bathythermograph sections from Gran Canaria Island to the African coast between November 1996 and September 1998. The geostrophic transport of the easternmost branch of the Canary Current is estimated by integrating the thermal wind equation using the layer of neutral density 27.3 (roughly 600 m depth) as the layer of no motion. The yearly average geostrophic transport of this branch of the Canary Current is 1.8±1.4×10 9 kg s −1 southward. Approximately half of the transport flows through the channel between the islands of Gran Canaria and Fuerteventura, and the other half through the channel between Fuerteventura and the African coast. The total southward geostrophic transport shows significant seasonal variability, ranging from 1.2±0.3×10 9 kg s −1 in May to 2.6±0.1×10 9 kg s −1 in January, although November is the only month with considerable differences in geostrophic transport between 1996 and 1997. There is seasonal northward transport in both channels, during May in the Gran Canaria–Fuerteventura channel and during November in the Fuerteventura–African coast channel. This seasonal pattern is probably linked to autumn weakening of upwelling in the Canary Islands area and the offshore diversion of this flow at Cape Ghir.

Antarctic marine bacterioplankton subpopulations discriminated by their apparent content of nucleic acids differ in their response to ecological factors

Bacterial abundances determined in Drake Passage and Bransfield and Gerlache Straits (Antarctica) in the Austral summer ranged from 0.78 to 9.4×105 cells ml−1, and were positively correlated with standing stocks of Chl a. Two bacterial subpopulations were discriminated based in their different levels of green fluorescence and wide angle light scatter (SSC) per cell after SYTO-13 staining for the first time in Antarctic waters. High nucleic acid (HNA) and low nucleic acid (LNA) subpopulations differed considerably in their response to changes in environmental variables. The apparent content of nucleic acids per cell for the HNA subpopulation (FL1-HNA) showed vertical profiles similar to those of Chl a, including the presence of a maximum at the subsurface chlorophyll maximum. FL1-HNA was positively correlated with Chl a. No similar trends were observed for the LNA fraction. HNA and LNA subpopulations differed in the response of the wide angle light scatter signal to environmental factors as well. SSC-HNA decreased strongly with depth and was positively correlated with Chl a. Again, no similar trends were observed for the LNA subpopulation. The percentage of HNA cells (%HNA) ranged between 35.0 and 76.7% and showed a general tendency to increase with depth. This increase seemed to be larger when the stratification of the water column was higher. Differences in grazing pressure could be responsible of the unexpected vertical distribution of HNA cells. Our results shows that in situ LNA and HNA bacterioplankton subpopulations are under different ecological controls and likely to play different trophodynamic roles in Antarctic waters.

Diapycnal mixing in Gulf Stream meanders

We use historical data [Bane et al., 1981], interpolated to isopycnic coordinates, to examine the possibility of significant diapycnal mixing within the upper thermocline layers of the Gulf Stream. The data consist of 28 air-dropped expendable bathythermograph (AXBT) sections in the northern Blake Plateau distributed in five different surveys done within 8 days. From the data we obtain a separation index between isopycnals j = ρ ∂z/∂ρ, where z is the depth of an isopycnal and ρ is the density; the diapycnal shear ∂υ/∂ρ, where υ is the geostrophic velocity; and the gradient Richardson number Ri. Following Pelegri and Csanady [1994], we postulate that the material derivative of the density, or density tendency wρ = Dρ/Dt, is the result of small-scale instabilities related to near-critical Ri values. We present the distribution of these quantities (j, ∂υ/∂ρ, Ri, and wρ) and the diapycnal convergence/divergence (∂wρ/∂ρ) over isopycnals and on sections normal to the coast. The results show the passage of steep meanders being related to anomalously low j values (strong density gradients) within the upper thermocline layers and the cyclonic filaments of the stream. The statically stable upper thermocline layers, however, are concurrent with large diapycnal shear and turn out to be dynamically unstable, characterized by low Ri and high-density tendencies and diapycnal convergence/divergence. The errors involved in calculating the dependent variables from AXBT data are assessed using both an error propagation approach and a Monte Carlo error simulation. These errors, although significant, are not large enough to modify the observed patterns substantially.

Steep Shelf Stabilization of the Coastal Bransfield Current: Linear Stability Analysis

AbstractIn situ measurements obtained during the 2010 COUPLING cruise were analyzed in order to fully characterize the velocity structure of the coastal Bransfield Current. An idealized two-layer shallow-water model was used to investigate the various instability processes of the realistic current along the coastal shelf. Particularly studied is how the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the surface current) impacts the growth and the wavelength of the unstable perturbations. For small bottom slopes, when the evolution of the coastal current is controlled by the baroclinic instability, the increase of the topographic parameter To yields a selection of smaller unstable wavelengths. The growth rates increase with small values of To. For larger values of To (To ≳ 10, which is relevant for the coastal Bransfield Current), the baroclinic instability is strongly dampened and the horizontal shear instability becomes the dominant one. In this steep shelf regime, the...

Carbon dynamics within cyclonic eddies: insights from a biomarker study.

It is generally assumed that episodic nutrient pulses by cyclonic eddies into surface waters support a significant fraction of the primary production in subtropical low-nutrient environments in the northern hemisphere. However, contradictory results related to the influence of eddies on particulate organic carbon (POC) export have been reported. As a step toward understanding the complex mechanisms that control export of material within eddies, we present here results from a sediment trap mooring deployed within the path of cyclonic eddies generated near the Canary Islands over a 1.5-year period. We find that, during summer and autumn (when surface stratification is stronger, eddies are more intense, and a relative enrichment in CaCO3 forming organisms occurs), POC export to the deep ocean was 2–4 times higher than observed for the rest of the year. On the contrary, during winter and spring (when mixing is strongest and the seasonal phytoplankton bloom occurs), no significant enhancement of POC export associated with eddies was observed. Our biomarker results suggest that a large fraction of the material exported from surface waters during the late-winter bloom is either recycled in the mesopelagic zone or bypassed by migrant zooplankton to the deep scattering layer, where it would disaggregate to smaller particles or be excreted as dissolved organic carbon. Cyclonic eddies, however, would enhance carbon export below 1000 m depth during the summer stratification period, when eddies are more intense and frequent, highlighting the important role of eddies and their different biological communities on the regional carbon cycle.

Viscosity of Liquid Water via Equilibrium Molecular Dynamics Simulations

Molecular dynamics simulations were carried out for liquid water in the NVE ensemble for calculating shear and bulk viscosities. We used two different intermolecular potential functions for the water dimer: the empirical SPCE model and the ab initio NCC one. The results obtained are compared with the available experimental values, and show that for a more accurate description of these macroscopic liquid properties a polarizable (rigid or flexible) interaction potentials should be employed. Such models, based on ab initio data, have been recently developed, and their incorporation for the viscosity calculations is discussed.

Ageostrophic Secondary Circulation in a Subtropical Intrathermocline Eddy

AbstractVertical motions play a key role in the enhancement of primary production within mesoscale eddies through the introduction of nutrients into the euphotic layer. However, the details of the vertical velocity field w driving these enhancements remain under discussion. For the first time the mesoscale w associated with an intrathermocline eddy is computed and analyzed using in situ high-resolution three-dimensional (3D) fields of density and horizontal velocity by resolving a generalized omega equation valid for high Rossby numbers. In the seasonal pycnocline the diagnosed w reveals a multipolar structure with upwelling and downwelling cells located at the eddy periphery. In the main pycnocline w is characterized by a dipolar structure with downwelling velocities upstream of the propagation path and upwelling velocities downstream. Maximum values of w reach 6.4 m day−1. An observed enhancement of chlorophyll-a at the eddy periphery coincides with the location of the upwelling and downwelling cells. A...