Finescale Vertical Structure of the Upwelling System off Southern Peru as Observed from Glider Data

Alice Pietri LOCEAN, Paris, France

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Pierre Testor LOCEAN, Paris, France

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Vincent Echevin LOCEAN, Paris, France

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Alexis Chaigneau LEGOS-OMP, Toulouse, France

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Laurent Mortier LOCEAN, Paris, France

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Gerard Eldin LEGOS-OMP, Toulouse, France

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Carmen Grados IMARPE, Chucuito-Callao, Peru

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Abstract

The upwelling system off southern Peru has been observed using an autonomous underwater vehicle (a Slocum glider) during October–November 2008. Nine cross-front sections have been carried out across an intense upwelling cell near 14°S. During almost two months, profiles of temperature, salinity, and fluorescence were collected at less than 1-km resolution, between the surface and 200-m depth. Estimates of alongshore absolute geostrophic velocities were inferred from the density field and the glider drift between two surfacings. In the frontal region, salinity and biogeochemical fields displayed cross-shore submesoscale filamentary structures throughout the mission. Those features presented a width of 10–20 km, a vertical extent of ~150 m, and appeared to propagate toward the shore. They were steeper than isopycnals and kept an aspect ratio close to f/N, the inverse of the Prandtl ratio. These filamentary structures may be interpreted mainly as a manifestation of submesoscale turbulence through stirring of the salinity gradients by the mesoscale eddy field. However, meandering of the front or cross-frontal wind-driven instabilities could also play a role in inducing vertical velocities.

Corresponding author address: Alice Pietri, LOCEAN, 4 Place Jussieu, Case 100, 75252 Paris CEDEX 05, France. E-mail: alice.pietri@locean-ipsl.upmc.fr

Abstract

The upwelling system off southern Peru has been observed using an autonomous underwater vehicle (a Slocum glider) during October–November 2008. Nine cross-front sections have been carried out across an intense upwelling cell near 14°S. During almost two months, profiles of temperature, salinity, and fluorescence were collected at less than 1-km resolution, between the surface and 200-m depth. Estimates of alongshore absolute geostrophic velocities were inferred from the density field and the glider drift between two surfacings. In the frontal region, salinity and biogeochemical fields displayed cross-shore submesoscale filamentary structures throughout the mission. Those features presented a width of 10–20 km, a vertical extent of ~150 m, and appeared to propagate toward the shore. They were steeper than isopycnals and kept an aspect ratio close to f/N, the inverse of the Prandtl ratio. These filamentary structures may be interpreted mainly as a manifestation of submesoscale turbulence through stirring of the salinity gradients by the mesoscale eddy field. However, meandering of the front or cross-frontal wind-driven instabilities could also play a role in inducing vertical velocities.

Corresponding author address: Alice Pietri, LOCEAN, 4 Place Jussieu, Case 100, 75252 Paris CEDEX 05, France. E-mail: alice.pietri@locean-ipsl.upmc.fr
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