Mediation of sea level from the open ocean to the coast

Sea level is rising globally, threatening the world coastlines. In this context, it is of paramount importance to understand the physical mechanisms driving spatiotemporal coastal sea-level changes. The adaptation of the coastal sea level to seawater density changes in the open ocean remains, for example, rather poorly understood. The present talk is a contribution towards this understanding. In the flat-bottomed open ocean, density horizontal gradients yield alone the presence of geostrophic baroclinic circulation and spatial variations in steric sea level. At the margins of oceanic basins, the situation is very different. The presence of continental slopes is a vorticity barrier hindering baroclinic geostrophic transport towards the coast and accumulation or removal of water there. In addition, the steric sea level vanishes at the coast where the seafloor depth is zero. In the low-frequency limit, how coastal sea level can be impacted by open ocean density spatiotemporal changes is hence non-trivial and must involve ageostrophic mechanisms. Here, we show that seawater density gradients generate large along-slope currents because of the Joint Effect of Baroclinicity and Relief (JEBAR). The latter currents are slowed down by bottom friction, which in the process transmits the sea level – originally of open ocean and steric origin – to the coast as manometric changes. The framework used is the Arrested Topographic Wave theory extended to a baroclinic ocean.