The effect of an increased freshwater flux on the meridional overturning circulation of the North Atlantic: : a numerical study

Abstract

As global warming causes both glaciers and sea ice to decay, increased freshwater flux into the North Atlantic is expected. In addition precipitation at high latitudes is predicted to increase under global warming as well, and thus contributing to an additional increased freshwater flux. These changes can be crucial to the production of deep water taking place in the Nordic Seas and the heat transport into this region. As this heat transport is believed to be an important contributor to the mild climate of North Europe, it is speculated that this could induce a cooling of the climate of this region.

In this thesis the impact of an increased freshwater flux on the meridional overturning circulation in a three-dimensional ideal version of the North Atlantic is investigated by performing several numerical experiments in which the freshwater flux is altered. The energetics driving the circulation are analyzed by the use of the streamfunction in depthdensity coordinates, as proposed by Nycander et al. (2007). The experiments have been conducted for two different basins, one with constant depth and one including bottom topography. The modern terrain following vertical coordinate model ROMS is the model used to conduct these experiments. Traditionally, numerical studies of ocean climate have been performed by the use of geopotential vertical coordinate models. To justify our choice of model the experiments described in Marotzke (1997) and Nycander et al. (2007) were recreated by the use of ROMS. A comparison of the results shows sufficient agreement between the results from the different model types to justify its use for this purpose.

The experiments with an increased freshwater flux were consistent in predicting a weakening of the meridional overturning circulation, and thus reduced deep water production. Associated with this weakening is a decrease of surface layer temperatures at the northern boundary and a heating of the remaining ocean domain. A shutdown of the deep water production at the northern boundary and reversal of the overturning circulation is predicted for very large freshwater fluxes. Fluxes of this magnitude are however deemed highly unlikely. An interesting result is that the presence of bottom topography makes the meridional overturning circulation more resistant to changes in the freshwater flux, and only a weakening of the circulation is predicted in these experiments. The analysis of the energetics driving the circulation shows that it is chiefly thermohaline driven. The spatial resolution of the ocean basin in question along with the density intervals used to calculate this streamfunction may however be sources of error in this calculation. Parameterization of subgrid processes in the model may also affect the results.