Mixing in the open ocean from the Last Glacial Maximum to present date: tidal dissipation and ocean resonance

Summary Vertical mixing in the ocean causes a vertical transport of water, and with it nutrients, salt, heat and pollutants. It is also one of the main mechanisms behind the large scale ocean circulation, and therefore of immense importance for the climate of our planet. The driving force of the vertical mixing in the deep parts of the open ocean, far away from coasts and the wind-mixed upper kilometre of the water column, is the tide. In order to understand and be able to predict changes in climate, we must look back in time and try to understand, explain and quantify previous changes in climate. In the past 21 000 years, since the Last Glacial Maximum (LGM), the sea level has risen by 115-135 m. A major consequence of this sea level rise has been the flooding of the continental shelf seas. Although the massive expansion of the shelf seas has been described as the most important geological event of recent time, there has been no integrated study of the impact of this event on global ocean circulation, and only a few investigations of its impact on ocean mixing. Whilst the contemporary shelf seas only account for 7% of the surface area of the global ocean, over 70% of the tidal energy dissipation takes place there. In consequence the historical rise in sea level has had a profound effect on the dissipation of tidal energy in shelf seas. This will in turn be reflected in a change in the dissipation of tidal energy in the deep ocean. As the tide provides a major, if not the major, component of the mechanical energy which ventilates the deep ocean through turbulent mixing, the rise in sea level has had a substantial impact on the marine system. Tides therefore do not only have the potential to affect the global climate, by influencing the strength of the oceanic circulation, but they can also influence key global biogeochemical cycles and atmospheric carbon dioxide levels. Within the project we will therefore aim to improve the accuracy of the estimates of the tidally driven open ocean mixing in the historical ocean. This will be done using two computer models of the ocean tides. We will simulate the changes in the global ocean mixing rates caused by the ocean tides from the peak of the last ice-age until today, and try to explain and quantify the changes which occur. This will open for a further understanding of the forcing of our ocean circulation system in the past and today. It will also provide insight into the sensitivity of the ocean mixing system, and how it relates to climate dynamics and climate change.