The effect of waves on the mixed layer depth

Mixing of the upper ocean directly affects the air-sea exchange of heat, momentum and gases, but currently wave physics exists only as a remote factor in most climate models. However, large waves that occur in tropical storms and cyclones can contribute in mixing a wider layer of the upper ocean with the cooler deeper parts, exchanging heat and carbon dioxide with the atmosphere which affects weather and climate.

Field observations of water temperature on the Australian North-West Shelf (Eastern Indian Ocean) with the support of numerical simulations demonstrate that the injection of turbulence generated by the wave orbital motion substantially contributes to the mixing of the upper ocean. Particularly, a considerable deepening of the mixed layer tends to occur during tropical cyclones, when the production of wave-induced turbulence kinetic energy overcomes the contribution of the current-generated shear turbulence. This effect is particularly visible in the Figure below, where the isotherms shows a rapid mixing of the upper ocean layer, bringing the water temperature to uniformity (down to depth comparable to half the dominant peak period - about 12s during the tropical cyclone depicted below).

Despite a significant contribution to the deepening of the mixed layer, the effect of a background current and atmospheric forcing are not on their own capable of justifying the observed deepening of the mixed layer through most of the water column.

Variations of a normally shallow mixed layer depth are observed within a relatively short time scale of approximately 10 hours after the intensification of wave activity and vanish soon after the decay of storm surface waves. This rapid development tends also to exclude any significant contribution by wave breaking, as small rates of vertical diffusivity for wave breaking-induced turbulence would require longer time scales to influence the depth of the mixed layer.

Additional details on this study can be found @ http://www.agu.org/pubs/crossref/pip/2011JC007780.shtml

Significant wave height and temperature profile during a tropical cyclone

Acknowledgments
Financial support of the Australian Research Council and Woodside Energy Ltd through the grant LP0883888 is acknowledged. Field observations from North Rankin A Gas Platform were kindly provided by Woodside Energy Ltd (who remains the sole owner of any proprietary rights in the field observations).

Sea drag in finite water depth

The coupling between the atmospheric boundary layer and the ocean surface in large-scale models is usually parametrized in term of the sea drag coefficient, which is routinely estimated as a function of mean wind speed. The scatter of data around such parametric dependences, however, is very significant and imposes a serious limitation on the forecasts and predictions that make use of sea surface drag parametrizations. The analysis of an atmospheric and wave data set collected in finite water depth at the Lake George measurement site (Australia) suggests that this variability relates to a number of parameters at the air-sea interface other than wind speed alone. In particular, results indicates that the sea drag depends on water depth and wave steepness, which make the wave profile more vertically asymmetric, and the concentration of water vapour in the air, which modifies air density and friction velocity. These dependences are used to derive parametric functions based on the combined contribution of wind, waves and relative humidity. A standard statistical analysis confirms a substantial improvement in the prediction of the drag coefficient and sea surface roughness, when additional parameters are taken into account.

http://www.agu.org/pubs/crossref/pip/2011JC007857.shtml