Below the ocean surface internal tides contribute to ocean mixing. Mixing influences ocean vitality, the environment and climate and scientists studying the Celtic Sea have recently progressed understanding of the complex processes that drive internal tides over continental shelves.
In a newly published paper, scientists at British research institutes show how internal tides cohere more than 170 kilometers shoreward across the broad continental shelf of the Celtic Sea. Their paper is a result of the first in situ observations of internal tides over the Celtic Shelf and these observations provided the first in situ evidence of internal tide coherence over multiple wavelengths on a sea shelf. Their conclusions will also help improve models of ocean mixing in shelf seas.
Led by Professor Mark Inall at SAMS, the team of physical oceanographers conducted their survey of internal tides along a 250 km section of the Celtic Shelf.
Internal tides consist of internal waves with tidal frequencies below the ocean surface. Internal tides are common in shelf and shallow marginal seas located on continental shelves and are a major source of energy for mixing stratified oceans. Vertical mixing is an important part of the deep ocean buoyancy budget, ocean margin buoyancy and nutrient budgets.
Internal tides also contribute to energy dissipation and this study has thrown more light on that process, which is critical for establishing parameters governing internal tides. It has also confirmed evidence that’s been mounting over the past decade, that the decay of internal wave energy is approximately equal to the measured dissipation rate.
Models of the effects of internal tides at deep, supercritical ridges were established in 2010, but this research has revealed for the first time how the decay rates of internal tides affect vertical mixing of shelf seas.
“Although IT [internal tides] decay scales in shelf seas have some regional dependency, developing greater knowledge of the decay scale is a key step towards developing better models,” said Professor Inall.
The survey was performed from the RRS James Cook towing a specially equipped undulator at 4.4ms-1 . Readings were taken from 5 m below the surface to 10 m above the seabed. The cycle concluded every two minutes to produce a horizontal resolution of 260 m.
The paper, Internal tide coherence and decay over a wide shelf sea, by Mark Inall, Dmitry Aleynik, Tim Boyd, Matthew Palmer, and Jonathan Sharples, concluded that while the sea surface signature of internal tides has been observed previously as a coherent feature over 100’s of km in the open ocean and shelf seas, this study has illuminated the structure and energy of these features beneath the ocean surface.
The Celtic Sea is adjacent to the North Atlantic Ocean, lying off the south coast of Ireland. It is bounded by St George’s Channel and the Bristol Channel to the east as well as parts of Wales, Devon, Cornwall and Brittany. To the south and west it is delineated by the abrupt drop away of the continental shelf to the vaster depths of the North Atlantic Ocean.
Scientists involved in this research represent SAMS, the National Oceonography Centre, Liverpool and the School of Environmental Sciences at the University of Liverpool. The research was funded by UK NERC Oceans-2025 programme.