The Diurnal Cycle of the Ocean Surface Boundary Layer: New Observations and New Models
Stephen E Belcher1, Alan L Grant1, Natasha Sarah Lucas2, Tom Philip Rippeth3, Brodie Pearson4, Jeff Polton5, Matthew Palmer5, Gillian Mary Damerell6 and Karen J. Heywood7, (1)University of Reading, Reading, RG6, United Kingdom, (2)Bangor University, School of Ocean Sciences, Menai Bridge, United Kingdom, (3)Bangor University, School of Ocean Sciences, Bangor, Wales, United Kingdom, (4)Brown University, Providence, RI, United States, (5)National Oceanography Centre, Liverpool, United Kingdom, (6)University of East Anglia, Norwich, United Kingdom, (7)University of East Anglia, Norwich, NR4, United Kingdom
Abstract:
New observations of the turbulent dissipation rate in the upper ocean were made during the OSMOSIS cruise with a microstructure glider in the N.E. Atlantic in September 2012. During periods of relatively low wind the OSBL exhibits a strong diurnal cycle. At night the OSBL is deepened by a combination of cooling and wind-wave forcing. After sunrise, solar radiation warms the water column. In pure wind forcing, the layer would form a stably stratified shear layer with a strong temperature gradient near the surface. With wind and wave forcing the interaction between the Stokes drift of the surface waves and vorticity in the turbulence means that mixing continues to be driven via Langmuir turbulence, leading to a shallow well-mixed layer overlying a diurnal thermocline. This is the structure observed in the cruise data (and also seen by Kukulka et al, 2013).
We perform Large Eddy Simulations forced by the observed surface fluxes, which quantitatively capture the diurnal cycle of the turbulence measured during the cruise when wave-driven Langmuir turbulence is modelled.
We also develop a new prognostic model for the depth of the OSBL, as a first step towards developing a new OSBL parameterisation. It is based on conservation of heat, mean potential energy and turbulent kinetic energy in the layer, and includes wave-driven processes. The model agrees well with the OSMOSIS cruise measurements, including several pronounced diurnal cycles and a period of sustained deepening through wind-wave forcing. Early results demonstrate good comparison observations taken during OSMOSIS by gliders that extend through a whole annual cycle.
