Abstract: From fjords to the abyssal ocean – internal waves and ocean mixing



Authors: Mattias Green

Over the last four decades internal waves have been recognized as a fundamentally important process for the transfer of energy over a wide range of spatial and temporal scales. This talk will touch on observations of processes related to the generation and propagation of internal (tidal) waves in different regimes. The effects of the dissipation and associated vertical mixing on the physical and biogeochemical system will be used to illustrate the importance of internal waves. A combination of observations and process-based models from a small multi-basin micro-tidal fjord show that the internal wave drag outnumbers bed friction as an energy-loss mechanism in the straits, and the internal tide is the main source of energy for mixing the stagnant basin water inside the sill. We also suggest that internal wave generators do not interact, but the question is left open for further investigation.  Data from moorings and vertical profilers sampled at the shelf break in the Celtic Sea and at the seasonally stratified western Irish Sea show large-amplitude solitary internal waves superposed on an internal tide in both areas. However, the horizontal structure of the wave field is very different at the two sites: at the shelf break, waves propagate both across and parallel to the shelf break, whereas in the Irish Sea the internal tide consistently propagates consistently towards the southwest, parallel to the tidal mixing front. Using perturbation theory we then show that the amount of baroclinic energy propagating past the moorings is roughly the same in both regions, and that there is a strong variation of the energy with the spring-neap cycle. The origin and fate of the internal tide in the western Irish Sea is still unclear, but it is suggested that it is generated around Isle of Man. Dissipation measurements from both areas – and data from station further onshelf in the Celtic Sea - suggest that the internal tide can propagate some 100 km before dissipating. The importance of the internal tide for the bio-chemical system in shelf seas is also discussed in terms of the sub-surface chlorophyll maximum.  Internal tidal waves are one of the major energy sources for mixing in the deep ocean – a process which eventually helps controlling the climate by sustaining the meridional overturning circulation. By a combination of tidal modelling and ocean circulation modelling efforts the relation between available (tidal) energy and the overturning circulation is investigated. An increased supply of mechanical energy can indeed sustain a more vigorous and stable MOC, but it was overcome in the past ocean by a fresher upper North Atlantic. The implicit amount of energy in the model is calculated and a modification to existing diffusivity-based vertical mixing schemes is suggested.