Downward propagation of internal tidal energy into the Bay of Biscay

doi:10.1016/0198-0149(89)90148-9

Deep Sea Research Part A. Oceanographic Research Papers

Volume 36, Issue 5, May 1989, Pages 735-758

https://doi.org/10.1016/0198-0149(89)90148-9 Get rights and content

Abstract

The generation and propagation of internal tidal energy from the continental shelf break is determined by the gradient of the topography and the static stability of the water column. This paper describes field observations in the Bay of Biscay, and after discussing the above parameters, presents convincing evidence for the existence of a beam of internal tidal energy propagating downward, from a source region on the upper continental slopes, into the deep ocean interior along a theoretical ray path. Although this phenomenon has been reported previously in the laboratory, and described by theoretical models, it has never before, to our knowledge, been observed in nature. The observations are compared with the results from a numerical model, and good agreement is obtained. This gives confidence in the model as well as in the interpretation of the observations.

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Particle fluxes and their drivers in the Avilés submarine canyon and adjacent slope, central Cantabrian margin, Bay of Biscay
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The Avilés Canyon in the central Cantabrian margin is one of the largest submarine canyons in Europe, extending from the shelf edge at 130 m depth to 4765 m depth in the Biscay abyssal plain. In this paper we present the results of a year-round (March 2012 to April 2013) study of particle fluxes in this canyon and the adjacent continental slope. Three mooring lines equipped with automated sequential sediment traps, high-accuracy conductivity-temperature recorders and current meters allowed measuring total mass fluxes and their major components (lithogenics, calcium carbonate, opal and organic matter) in the settling material jointly with a set of environmental parameters. The integrated analysis of the data obtained from the moorings together with remote sensing images and meteorological and hydrographical data has shed light on the sources of particles and the across- and along margin mechanisms involved in their transfer to the deep.
Our results allow interpreting the dynamics of the sedimentary particles in the study area. Two factors play a critical role: (i) direct delivery of river-sourced material to the narrow continental shelf, and (ii) major resuspension events caused by large waves and near bottom currents developing at the occasion of the rather frequent severe storms that are typical of the Cantabrian Sea. Wind direction and subsequent wind-driven currents largely determine the way sedimentary particles reach the canyon. While westerly winds favour the injection of sediments into the Avilés Canyon mainly by building an offshore transport in the bottom Ekman layer, easterly winds ease the offshore advection of particulate matter towards the Avilés Canyon and its adjacent western slope principally through the surface Ekman layer. Furthermore, repeated cycles of semidiurnal tides add an extra amount of energy to the prevailing bottom currents and actively contribute to keep a permanent background of suspended particles in near-bottom waters.
High contents of lithogenics in settling particles at the three mooring stations confirm that riverine inputs are the principal source of particles to the Avilés Canyon, including the lowermost canyon, and the adjacent open slope. Primary production also has a strong influence on the amount and the composition of particulate matter, with more than 30% of the total mass flux being of biogenic origin (organic matter, opal and calcium carbonate).
Internal tides near the Celtic Sea shelf break: A new look at a well known problem
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Internal waves generated by tides in the Celtic Sea were investigated on the basis of in situ data collected at the continental slope in July 2012, and theoretically using a weakly nonlinear theory and the Massachusetts Institute of Technology general circulation model. It was found that internal solitary waves generated over the shelf break and propagated seaward did not survive in the course of their evolution. Due to the large bottom steepness they disintegrated locally over the continental slope radiating several wave systems seaward and transforming their energy to higher baroclinic modes. In the open part of the sea, i.e. 120 km away from the shelf break, internal waves were generated by a baroclinic tidal beam which was radiated from the shelf break downward to the abyss. After reflection from the bottom it returned back to the surface where it hit the seasonal pycnocline and generated packets of high-mode internal solitary waves. Another effect that had strong implications for the wave dynamics was internal wave reflection from sharp changes of vertical fluid stratification in the main pycnocline. A large proportion of the tidal beam energy that propagated downward did not reach the bottom but reflected upward from the layered pycnocline and returned back to the surface seasonal pycnocline where it generated some extra higher mode internal wave systems, including internal wave breathers.
Reprint of: Chlorophyll enhancement in the central region of the Bay of Biscay as a result of internal tidal wave interaction
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A multi-sensor satellite approach based on ocean colour, sunglint and Synthetic Aperture Radar imagery is used to study the impact of interacting internal tidal (IT) waves on near-surface chlorophyll-a distribution, in the central Bay of Biscay. Satellite imagery was initially used to characterize the internal solitary wave (ISW) field in the study area, where the “local generation mechanism” was found to be associated with two distinct regions of enhanced barotropic tidal forcing. IT beams formed at the French shelf-break, and generated from critical bathymetry in the vicinities of one of these regions, were found to be consistent with “locally generated” ISWs. Representative case studies illustrate the existence of two different axes of IT propagation originating from the French shelf-break, which intersect close to 46°N, − 7°E, where strong IT interaction has been previously identified. Evidence of constructive interference between large IT waves is then presented and shown to be consistent with enhanced levels of chlorophyll-a concentration detected by means of ocean colour satellite sensors. Finally, the results obtained from satellite climatological mean chlorophyll-a concentration from late summer (i.e. September, when ITs and ISWs can meet ideal propagation conditions) suggest that elevated IT activity plays a significant role in phytoplankton vertical distribution, and therefore influences the late summer ecology in the central Bay of Biscay.
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Citation Excerpt :
The MLD requires a definition, this is often arbitrarily set to the depth at which there is −0.5 °C change in temperature. Due to winter mixing (see for example Somavilla et al. 2009), the MLD in northern the Bay of Biscay can be >500 m in February/March (Pingree and New, 1989; winter MLD are important considerations for water mass properties, winter nutrient levels, mode water properties, Brunt Vaisala Frequency and internal tide propagation from critical slope topography, where upper layer properties may also be introduced from below, but not for the euphotic surface layer production. The spring bloom can be initiated within a surface euphotic layer that may only be 10–20 m deep with vertical temperature differences of as little as 0.1 or 0.2 °C.
The flow of upper-layer surface water and circulation for the Bay of Biscay, continental slope and in the wider region of the NE Atlantic is presented, as well as the seasonality of flow and internal tides. The marine plankton environments of Biscay Ocean, Biscay Eddies, Biscay Slope and Biscay Shelf are defined. The Shelf region (Armorican and Celtic) is further divided into Stratified Shelf, Frontal and Tidally Mixed. Seasonal distributions of chlorophyll a are given for all environment from in situ measurements and remote sensing data. Mixing and stabilisation of surface water in the euphotic layer for the start of the spring bloom using in situ profiling measurements is examined. Some regional responses for the slope current, dinoflagellate blooms and interannual variations in spring diatom numbers with respect to weather and climate in the Bay of Biscay and around the British Isles are suggested and discussed. An example of the Eastern European Ocean Margin continental slope response to winter weather (sea level atmospheric pressure forcing) resulting in warm winter water in the southern Bay of Biscay (Navidad, with eddy production) and off the Shetland continental slopes (the warm-water supply route to the Arctic) is given from the slope climate observation series.
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Citation Excerpt :
Production of internal tide occurs where seafloor slope matches the internal tide characteristic (a function of Brunt–Vaisala frequency, background tidal forcing and local inertial period, e.g. Pingree and New, 1989). Upper slopes of the Bay of Biscay at depths of 300–400 m have been shown to be critical to the semidiurnal tidal forcing at the Celtic–Armorican slopes (Pingree and New, 1989) and southern Biscay (Azevedo et al., 2006), coinciding with the depth of the Brunt–Vaisala minimum of the pycnostand in the Southern Bay of Biscay. In the Avilés Canyon area the upper slope also has many critical regions (not shown).
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Downward propagation of internal tidal energy into the Bay of Biscay

Abstract

Deep-Sea Research

Progress in Oceanography

Deep-Sea Research

Deep-Sea Research

Estuarine and Coastal Marine Science

Deep-Sea Research

Tidal motion in submarine canyons—A laboratory experiment

Journal of Physical Oceanography

Spectral characteristics of internal waves in the ocean

Deep-Sea Research

Tidal interactions in a region of large bottom slope near northwest Africa during JOINT-1

Deep-Sea Research

Measurement and analysis of ocean currents

The generation of long nonlinear internal waves in a weakly stratified shear flow

Journal of Geophysical Research