Mixing in the continental slope: study case Gulf of Cadiz

• Main Author: Alvarado Bustos, Ruben
• Other Authors: Huthnance, John, Williams, Ric
• Format: Text
• Language: English
• Subjects: North Atlantic
• Online Access: http://livrepository.liverpool.ac.uk/3253/; http://livrepository.liverpool.ac.uk/3253/1/PhD_Thesis_0611_RAB.pdf; http://livrepository.liverpool.ac.uk/3253/4/AlvaradoBustosRub_June2011_3253.pdf

Internal waves in the ocean are the principal generators of mixing in the abyssal ocean and regions of rough topography. The present research work diagnoses the influence of internal wave activity over the Continental slope in the Gulf of Cadiz. Mediterranean Outflow (MOW) and North Atlantic Central Water (NACW) are the main baroclinic flows over the study region and diapycnal mixing acts in each layer. Semi-diurnal internal tides and a continuous MOW flow are observed on the slope. The MOW flow is persistent reaching >0.40 ms-1, but varies in strength with the tides. The Internal wave field in the Gulf of Cadiz can play an important role affecting the MOW signal over the continental slope; MOW can be displaced by the internal tide. Internal waves are generated by tides and MOW flow interacting with the bottom, the two most energetic sources locally. Also MOW bottom stress provides strong diapycnal mixing, providing well-mixed conditions in the MOW. Internal tides can transfer energy on the slope and can cause turbulence. A critical slope characteristic for semidiurnal internal waves occurs over the continental slope with local background stratification where MOW travels as an undercurrent. Diapycnal mixing is found to be enhanced inside the MOW with a diapycnal diffusivity O(7x10-4 m2s-1), and at the MOW-NACW interfaces, reaching O(2x10-4 m2s-1) for the upper interface and O(5x10-4 m2s-1) for the lower interface. Fine-structure methods are used to estimate mixing using CTD measurements; however most of the records came from temperature profiles (XBTs) and an alternative approach to diagnose the strain uses temperature profiles with inferred salinity (using temperature-salinity relations from the CTDs). Applying the strain method using temperature with inferred salinity profiles provides a plausible approximation of the strain spectrum and the mixing estimates, with uncertainties similar to those diagnosed using CTD measurements. Mixing estimates are also diagnosed using a large scale box model, where a salinity budget is applied to study gain and loss over the Gulf of Cadiz. Box model results confirm that salinity is diapycnally transferred from the MOW layers into the neighbouring NACW layers. The advective and diffusive transfers of salt along the layer are much larger than the diapycnal transfer. The inferred diffusivities from the box model are broadly in accord with the estimates from strain.