The Effect of Solar Radiation on Diurnal Mixed Layer Dynamics

The fluxes of heat between the ocean and the atmosphere are an
important parameter for driving ocean circulation and gas transfer at
the air-sea interface. Numerical models of the ocean require accurate
parameterisation of air-sea fluxes. A commonly used scheme is the
COARE algorithm, which computes heat and gas fluxes from environmental
conditions that are routinely measured using in-situ and remote
sensing methods.

To accurately compute these fluxes, the sea surface
temperature at the interface has to be correctly predicted. The
absorption of solar (shortwave) radiation is a key process in the
thermal stratification of the upper ocean. It is estimated that
between 5 and 20% of the incident solar energy at the ocean
surface is attenuated within the molecular boundary layer, and
between 60 and 90% within the top 10 m of the ocean.
In this presentation, the sensitivity of the COARE algorithm to the
use of different shortwave absorption schemes is tested. The original
model applies a 3-wavelength approximation of radiation attenuation.
Alternative schemes use more wavelength bands, dependency on water-type,
dependency on solar angle and cloud index and/or chlorophyll concentration.

The model output for these parameterisations is compared to in-situ
measurements of diurnal stratification in the mixed layer using a
series of upper ocean profiles of temperature which were carried out
in the Mediterranean and the subtropical Northern Atlantic. The
average chlorophyll concentrations in these regions are 2mg/l and
0.3mg/l respectively, thus it concerns two different water types.
Environmental conditions for the studied cases range from 0 to 8 m/s
winds. Concurrent measurements of upper ocean light profiles,
turbulent dissipation and waves, when available, allow to study the
in-situ effect of the sea state to radiation transmission in the upper
ocean.