An Energetics Study of Wintertime Northern Hemisphere Storm Tracks under 4 × CO2 Conditions in Two Ocean–Atmosphere Coupled Models

Abstract

Different possible behaviors of winter Northern Hemisphere storm tracks under 4 ? CO2 forcing are considered by analyzing the response of two of the ocean?atmosphere coupled models that were run for the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), namely the Institut Pierre Simon Laplace?s global coupled model (IPSL-CM4) and the Centre National de Recherches Meteorologiques?s coupled ocean?atmosphere model (CNRM-CM3). It is interesting to compare these models due to their very different responses, especially concerning the North Atlantic storm track. A local energetics study of the synoptic variability in both models is performed, derived from the eddy energy equations, including diabatic terms. The ability of both models to simulate the present-day eddy energetics is considered, indicating no major discrepancies. Both models indicate that the primary cause for synoptic activity changes at the western end of the storm tracks is related to the baroclinic conversion process, due to mean temperature gradient changes in some localized regions of the western oceanic basins, but also resulting from changes in the eddy efficiency to convert energy from the mean flow. Farther downstream, latent heat release during the developing and mature stages of eddies becomes an important eddy energy source especially in terms of changes between 4 ? CO2 and preindustrial conditions. This diabatic process amplifies the upstream synoptic (hence usually baroclinic) changes, with more and/or stronger storms implying more latent heat being released (and the converse being true for weaker synoptic activity). This amplification is asymmetrical for the models considered under the simulated 4 ? CO2 conditions, due to a greater amount of water vapor contained in warmer air and hence the potential for more condensation for a given synoptic activity. The magnitude of the reduced latent heating is attenuated, whereas increased latent heating is strengthened. Ageostrophic geopotential fluxes are also important in relocating eddy kinetic energy, especially in the vertical.