The effect of wave-induced turbulence on the ocean mixed layer during tropical cyclones: Field observations on the Australian North-West Shelf

Abstract

[1] Field observations of water temperature on the Australian North-West Shelf (Eastern Indian Ocean) with the support of numerical simulations are used to demonstrate that the injection of turbulence generated by the wave orbital motion substantially contributes to the mixing of the upper ocean. Measurements also show that a considerable deepening of the mixed layer occurs during tropical cyclones, when the production of wave-induced turbulent kinetic energy overcomes the contribution of the current-generated shear turbulence. 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. Furthermore, variations of a normally shallow mixed layer depth are observed within a relatively short timescale 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 timescales to influence the depth of the mixed layer.

Surface waves and wave‐coupled effects in lower atmosphere and upper ocean

“… Qiao and Huang [2012]conducted direct comparison of the upper‐ocean mixing due to vertical shear of the mean currents, the mechanism which is routinely attributed with such mixing, and the surface wave‐induced mixing. Based on a series of numerical experiments and comparisons with realistic temperature profiles they concluded that the surface wave‐induced mixing is dominant, and the ocean circulation model can perform well even without the shear‐induced mixing due to the mean current Toffoli et al [2012a], Huang et al [2012b]. and Song et al [2012]investigated particular phenomena and circumstances in the ocean and climate system when the wave‐induced mixing was essential Toffoli et al [2012a]…”

“… Ting et al [2012] demonstrated that, for a given wind speed, the sea drag can grow as much as 25% as a function of the wave directional spreading. Results of Toffoli et al [2012a] indicate that the sea drag depends on water depth and wave steepness, which make the wave profile more vertically asymmetric, and on the concentration of water vapor in the air, which modifies air density and friction velocity. Druzhinin et al [2012]suggest a numerical algorithm to conduct direct numerical simulations of the wind flow over steep waves.…”

Global changes of extreme coastal wave energy fluxes triggered by intensified teleconnection patterns

“…We note that CMIP5 models present some shortcomings in the simulation of tropical cyclones that dominate the extreme WEF in areas such as the subequatorial-tropical eastern Pacific, usually modeling features weaker, larger, and less frequent than observed, and underestimating the peaks mainly due to the low resolution [Camargo, 2013] and to the lack of ocean-atmosphere coupling in many CMIP5 models [Shimura et al, 2016]. Moreover, the complex coupled dynamics of tropical cyclones and waves are not yet fully understood Toffoli et al, 2012;Chen et al, 2013;Janssen et al, 2013;Zappa et al, 2016]. However, they are designed to model properly the large features of geophysical fields that impact on the intensity and frequency of the tropical cyclones [Camargo, 2013].…”

Autonomous Surface Vehicle Measurements of the Ocean’s Response to Tropical Cyclone Freda

“…Toffoli et al (2012) suggest wave-orbital-motioninduced turbulence as a possible process to explain the deepening of the mixed layer during tropical cyclones; however, evidence for such turbulence is based on very limited indirect laboratory studies, and as far as we are aware there is no direct field evidence of such turbulence. It appears that other mechanisms may need to be considered.…”