Editorial Type:
Article
Article Type:
Research Article

Enhanced Turbulence Associated with the Diurnal Jet in the Ocean Surface Boundary Layer

Graig Sutherland Department of Mathematics, University of Oslo, Oslo, Norway
School of Physics, and the Ryan Institute, National University of Ireland, Galway, Ireland

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Louis Marié Laboratoire de Physique des Océans, CNRS/IFREMER/IRD/UBO, Plouzané, France

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Gilles Reverdin LOCEAN Laboratory, Sorbonne Universités (UPMC, University of Paris 6)-CNRS-IRD-MNHN, Paris, France

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Kai H. Christensen Norwegian Meteorological Institute, Oslo, Norway

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Göran Broström Department of Marine Sciences, University of Göteburg, Göteborg, Sweden

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Brian Ward School of Physics, and the Ryan Institute, National University of Ireland, Galway, Ireland

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Print Publication:
01 Oct 2016
DOI:
https://doi.org/10.1175/JPO-D-15-0172.1
Page(s):
3051–3067
Restricted access

Abstract

Detailed observations of the diurnal jet, a surface intensification of the wind-driven current associated with the diurnal cycle of sea surface temperature (SST), were obtained during August and September 2012 in the subtropical Atlantic. A diurnal increase in SST of 0.2° to 0.5°C was observed, which corresponded to a diurnal jet of 0.15 m s−1. The increase in near-surface stratification limits the vertical diffusion of the wind stress, which in turn increases the near-surface shear. While the stratification decreased the turbulent dissipation rate ε below the depth of the diurnal jet, there was an observed increase in ε within the diurnal jet. The diurnal jet was observed to increase the near-surface shear by a factor of 5, which coincided with enhanced values of ε. The diurnal evolution of the Richardson number, which is an indicator of shear instability, is less than 1, suggesting that shear instability may contribute to near-surface turbulence. While the increased stratification due to the diurnal heating limits the depth of the momentum flux due to the wind, shear instability provides an additional source of turbulence that interacts with the enhanced shear of the diurnal jet to increase ε within this shallow layer.

Corresponding author address: Graig Sutherland, Department of Mathematics, University of Oslo, Postboks 1053 Blindern, 0316 Oslo, Norway. E-mail: graigors@math.uio.no

Abstract

Detailed observations of the diurnal jet, a surface intensification of the wind-driven current associated with the diurnal cycle of sea surface temperature (SST), were obtained during August and September 2012 in the subtropical Atlantic. A diurnal increase in SST of 0.2° to 0.5°C was observed, which corresponded to a diurnal jet of 0.15 m s−1. The increase in near-surface stratification limits the vertical diffusion of the wind stress, which in turn increases the near-surface shear. While the stratification decreased the turbulent dissipation rate ε below the depth of the diurnal jet, there was an observed increase in ε within the diurnal jet. The diurnal jet was observed to increase the near-surface shear by a factor of 5, which coincided with enhanced values of ε. The diurnal evolution of the Richardson number, which is an indicator of shear instability, is less than 1, suggesting that shear instability may contribute to near-surface turbulence. While the increased stratification due to the diurnal heating limits the depth of the momentum flux due to the wind, shear instability provides an additional source of turbulence that interacts with the enhanced shear of the diurnal jet to increase ε within this shallow layer.

Corresponding author address: Graig Sutherland, Department of Mathematics, University of Oslo, Postboks 1053 Blindern, 0316 Oslo, Norway. E-mail: graigors@math.uio.no
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