Impact of stratification and climatic perturbations to stratification on baroclinic and barotropic tides

Recently there has been a great deal of interest in ocean mixing, primarily because ocean mixing is thought to control the strength of the oceanic meridional overturning circulation. An important source of ocean mixing is breaking internal waves, and an important energy source for oceanic internal waves is the astronomical tide. The ocean climate process team discussed in this session aims to develop parameterizations of internal-wave driven mixing which are suitable for the altered climates of our anticipated future as well as for the present climate. For this reason it is of interest to ask how climatic changes in the oceanic stratification have altered and will alter the ocean tides. By solving the Sturm-Liouville vertical mode problem with a present-day profile of oceanic temperature and salinity as well as the same profile altered by the observed temperature changes occuring over the last 135 years (Roemmich et al. 2012), we find that the square of the phase speed of the first mode baroclinic mode internal tide has changed by about 5-6% over that time frame. We alter the g' and layer depths of a two-layer numerical model of the global barotropic and baroclinic tides such that the phase speed changes by this amount. We will show the changes taking place in the baroclinic tide as well as in the barotropic tide. The barotropic tide changes because the phase speed of the barotropic tide depends on stratification (Gill 1982); this in turn affects the resonance of barotropic tides. The results of the numerical simulations are compared to a two-layer analytical model of tides in a forced-damped closed basin, which also predicts a sensitivity of both barotropic and baroclinic tides to stratification and to climatic perturbations to the stratification. Finally, the numerical results are compared to tide gauge observations of secular changes in tides occurring over the last century or so.