Seasonal Variability of the North Atlantic Ocean—a Model Intercomparison

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Abstract
The seasonal variability of the North Atlantic Ocean is investigated through a comparison of two different numerical models. One is a primitive equation model (PEM) forced with monthly wind stress and surface buoyancy flux. The other is a quasi-geostrophic model (QGM) forced with the monthly wind-stress curl only. Both models include realistic bottom topography and are solved for spherical geometry. We find a great diversity in the baroclinic Rossby wave response over the models' latitudinal span. The diversity partly stems from the latitudinal variation of the internal Rossby radius and therefore cannot be captured with β-plane models. The QGM is used to investigate the coupling of barotropic and baroclinic modes over topography, especially its role on the propagation properties of the baroclinic waves. A comparison of the model solutions reveals that in midlatitudes, the spatial variability of the density distribution and the finite amplitude aspects of the bottom topography are the most important components of the PEM that are omitted in the QGM. These components allow generation of annual baroclinic Rossby waves at the Mid-Atlantic Ridge in the PEM. Other aspects of the seasonal response discussed here are the variability of sea level and vertically integrated transport. The fluctuations of the surface buoyancy fluxes almost double the rms amplitude of the sea level fluctuations in the PEM compared to the QGM. Scale-selective friction and steeper bottom topography in the PEM lead to some unexpected differences in the transport fluctuations of the western boundary currents between the models.

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