Observations of the multi-year variability of mixing in shelf seas

Abstract. We present 5-year long continuous mixing observations from four mooring locations, spanning 10° degrees of latitude, on the continental shelf of North-Western Australia. The sites are subject to a diverse range of atmospheric and oceanic forcing and each has distinct local bathymetry. We estimated the diapycnal diffusivity and vertical turbulent heat flux at all sites and throughout the water column using a fine-scale mixing length model. There were four dominant peaks in the power spectrum: annual, spring-neap, diurnal, and semi-diurnal. We examined the variability in the mixing at all sites in terms of the seasonal stratification, depth, and tidal range and phase. The highest annual variability occurred at the highest latitude sites which had the greatest seasonal variation in stratification. The mixing also varied with the strength of the dominant tidal constituents (both baroclinic and barotropic). Heat flux was a more useful measure of mixing than diffusivity due to the strong seasonal variations in density stratification on the shelf. Finally, we provide guidance on the determination of the appropriate sampling period to adequately describe the mixing, and discuss the effects of short sampling periods on the ability to describe the long-term mixing.