Focusing of baroclinic tidal energy in a canyon

Abstract

Strong three‐dimensional focusing of internal tidal energy in the Petite Sole Canyon in the Celtic Sea is analyzed using observational data and numerical modeling. In a deep layer (500–800 m) in the center of the canyon, shear variance was elevated by an order of magnitude. Corresponding large vertical oscillations of deep isotherms and a local maximum of horizontal velocity were replicated numerically using the MITgcm. The elevated internal tidal activity in the deep part of the canyon is explained in terms of the downward propagation and focusing of multiple internal tidal beams generated at the shelf break. The near‐circular shape of the canyon head and steep bottom topography throughout the canyon (steeper than the tidal beam) create favorable conditions for the lens‐like focusing of tidal energy in the canyon's center. Observations and modeling show that the energy focusing greatly intensifies local diapycnal mixing that leads to local formation of a baroclinic eddy.

Partly standing internal tides in a dendritic submarine canyon observed by an ocean glider

“…Internal tides and turbulent mixing processes in the wider Celtic Sea and Bay of Biscay region have been previously investigated (e.g., New, 1988;Green et al, 2008;Sharples et al, 2009;Hopkins et al, 2014), but it is clear from high-resolution numerical modelling studies (Vlasenko et al, 2014;Vlasenko and Stashchuk, 2015) that the internal tide field is highly heterogeneous, three-dimensional, and shaped by local bathymetry. Specifically, three-dimensional focusing of internal tide energy within Petite Sole Canyon (part of the wider Whittard Canyon system) has been observed and modelled by Vlasenko et al (2016). A broad understanding of internal tide dynamics within Whittard Canyon will help the extrapolation of results along the highly corrugated Celtic Sea continental slope and to dendritic canyons worldwide.…”

Three‐Dimensional Dynamics of Baroclinic Tides Over a Seamount

“…These are the necessary conditions for baroclinic tidal beam generation (Vlasenko et al, ). These beams propagate from the bank periphery to its center focusing tidal energy over the top, similar to the mechanism of tidal energy focusing in canyons (Vlasenko et al, ). This mechanism resembles the effect of focusing in optical lenses.…”

Glider observations of enhanced deep water upwelling at a shelf break canyon: A mechanism for cross‐slope carbon and nutrient exchange

“…On the 24 June 2012, a transect of 4 CTD profiles (Figure ) was sampled for nutrients along the axis of the Petite Sole canyon [ Vlasenko et al ., ] adjacent to the canyons A and B in this study. Linear regressions suggested that the variance of the profiles of nitrate + nitrite (N+N), silicate (Si) and phosphate (P) was well explained by the potential density with mean R 2 of 0.96 for N+N, 0.82 for Si and 0.91 for P (p<0.001 for all three regressions).…”

“…They incise the continental margins of all ocean basins and are known hotspots of enhanced deep‐sea sediment and water flux [ Savoye et al ., ], dense water cascades [ Canals et al ., ], benthic biomass [ Duineveld et al ., ] and fishing activity. The complex, irregular topography of the canyons can lead to internal wave generation [ Kunze et al ., ], focusing of internal wave energy [ Vlasenko et al ., ], enhanced diapycnal mixing [ Carter and Gregg , ], and localized up and downwelling [ Allen and Durrieu de Madron , ].…”

“…Previously it has been shown that cross‐slope exchange in this region is dominated through processes related to internal waves, which can be nonlinear and propagate from the shelf‐break across the shelf toward the coast, transporting mass and energy [ Green et al ., ; Inall et al ., ]. The internal wave energy and the resultant mixing along the shelf break is locally focused in the canyons along this slope [ Vlasenko et al ., ]. Furthermore, intense mixing at the shelf break, due to internal waves, is also thought to lead to the exchange phenomenon of “salt lenses,” which transport high salinity water over 100 km onto the shelf [ Hopkins et al ., ].…”