Projected Changes in CMIP5 Low Level North Atlantic Storm Tracks: Relative Roles of Local and Remote SST

Abstract:

Over the 21^st century, the CMIP5 multi-model ensemble mean North Atlantic storm track changes are projected to undergo an overall poleward intensification and eastward extension. However, considerable spread exists with regard to how individual CMIP5 models represent the magnitude and structure of these storm track changes. In this study, we examine the extent to which ocean forcing affects the ability of CMIP5 models to simulate the behavior of the North Atlantic storm tracks under future climate change scenarios. Emphasis is placed on investigating projected changes within the NorESM/CESM family of models. Despite similarities between the atmospheric models, the projected lower tropospheric storm track changes are markedly different between the coupled CMIP5 output from CCSM4, CESM1-CAM5, NorESM1-M. A suite of AGCM experiments are used to evaluate the sensitivity of the North Atlantic storm tracks to different CMIP5 realizations of projected changes in: 1) SSTs across the North Atlantic and 2) SSTs across the globe. Unlike the upper tropospheric storm track changes, which are strongly influenced by remote SST changes, the lower tropospheric North Atlantic storm tracks are linked to changes in local SST and attendant surface baroclinicity. However, the atmospheric responses in the AGCM simulations are dependent on the choice of the atmospheric model: CAM5 simulations exhibit a higher sensitivity to the underlying SST conditions than CAM4. Analysis will focus on the physical mechanisms through which SST drive the low-level storm track changes. We will also examine the influence of direct radiative forcing associated with increasing greenhouse gas concentrations on our AGCM simulations.