Ángel Rodríguez-Santana

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.

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.