On the Reduced North Atlantic Storminess during the Last Glacial Period: The Role of Topography in Shaping Synoptic Eddies

Recent climate simulations show that the North-Atlantic storminess was reduced during the Last Glacial Maximum (LGM) in comparison with the present-day climate despite having a stronger baroclinicity. An explanation for this counterintuitive result is provided by comparing numerical experiments made with a full climate model and a simplified dry GCM forced by idealized topography and a relaxation in temperature. Two simulations of the climate model forced by LGM and pre-industrial (PI) conditions are compared. Two simulations of the simplified GCM are also compared, one forced with idealized Rockies (idealized PI) and the other with idealized Rockies and idealized Laurentide Ice Sheet (idealized LGM). The North Atlantic storminess is reduced in the LGM climate run compared to the PI climate one although the baroclinicity is stronger in the former run as already shown by previous studies. The storm track is less intense in the idealized LGM run than the idealized PI one despite having similar baroclinicity. In both climate and idealized runs, an energetic budget shows that the reduced storm-track intensity can be explained by a reduced baroclinic conversion which itself comes from a loss in eddy efficiency to tap the available potential energy. The eddies are less efficient because eddies are less meridionally stretched which makes the eddy heat fluxes less well aligned with the mean temperature gradient. It is the mechanical forcing exerted by the southern slope of the Laurentide Ice Sheet topography on the flow that may explain why the waves are less meridionally stretched in the region of maximum baroclinicity. Indeed, as the wind should be parallel to the mountain it is mainly zonal over the southern slope of the Laurentide Ice Sheet topography. This constraint prevents the heat fluxes from being meridionally oriented and hence from aligning with the mean temperature gradient.