Seasonal Variability of the North Atlantic Ocean—a Model Intercomparison

Abstract

The seasonal variability of the North Atlantic Ocean is investigatedthrough a comparison of two different numerical models. One is aprimitive equation model (PEM) forced with monthly wind stress andsurface buoyancy flux. The other is a quasi-geostrophic model (QGM)forced with the monthly wind-stress curl only. Both models includerealistic bottom topography and are solved for spherical geometry.We find a great diversity in the baroclinic Rossby wave responseover the models' latitudinal span. The diversity partly stems fromthe latitudinal variation of the internal Rossby radius and thereforecannot be captured with β-plane models. The QGM is used to investigatethe coupling of barotropic and baroclinic modes over topography,especially its role on the propagation properties of the baroclinicwaves. A comparison of the model solutions reveals that in midlatitudes,the spatial variability of the density distribution and the finiteamplitude aspects of the bottom topography are the most importantcomponents of the PEM that are omitted in the QGM. These componentsallow generation of annual baroclinic Rossby waves at the Mid-AtlanticRidge in the PEM. Other aspects of the seasonal response discussedhere are the variability of sea level and vertically integrated transport.The fluctuations of the surface buoyancy fluxes almost double therms amplitude of the sea level fluctuations in the PEM compared tothe QGM. Scale-selective friction and steeper bottom topography inthe PEM lead to some unexpected differences in the transport fluctuationsof the western boundary currents between the models.