On the Impacts of Climate Change on Midlatitude North Atlantic Landfalling Cyclones

The influence of the climate change on mid-latitude North Atlantic landfalling autumn storms is investigated using a relatively high resolution atmosphere –ocean coupled model system. The model system consists of the Canadian mesoscale compressible community (MC2) model coupled to the Princeton Ocean Model (POM). Atmospheric boundary conditions for autumn storm simulations by this coupled model system are given by the Canadian Climate Centre model, CGCM2 (Second Generation Coupled Global Climate Model), following the IPCC IS92a scenario conditions. The control and high CO2 boundary conditions are obtained from CGCM2 simulations for years 1975-1994 to represent the present climate, and for conditions expected for years 2040-2059 to represent future climate. In our simulations of landfalling midlatitude autumn cyclones, the impact of climate change is to cause slightly decreased cyclone intensities (about 5 hPa), because of the modest cooling around the storm center, and for cyclone tracks to shift slightly poleward. This study builds on results of Jing and Perrie (2007a, 2007b)and Yao et al. (2007).

The present study differs from previous GCM studies of storms in mid- and high-latitudes in climate change scenario studies in that we have applied a relatively high resolution (0.25 degress) coupled model system to investigate climate change impacts on landfalling storms relative to the North Atlantic coastline. Climate change is represented by high CO2 conditions as simulated by the Canadian Climate Centre CGCM2 model following the IPCC IS92a scenario. Using CGCM2 data and automatic storm detection criteria, we selected landfalling storms from the present climate, represented by 1975-1994, with those of a climate change scenario, represented by 2040-2059, with approximate doubled levels of CO2. The two climatologies (present and future) of landfalling storms were then estimated using the coupled model system and ensemble simulations.

We verified that the storm tracks from the coupled model system compare well with those resulting from CGCM2 estimates. In the climate change scenario, the coupled model simulations of the landfalling storms tend to have weaker intensities than those of the present climate, in terms of maximum U10 winds or minimum sea level pressure. Moreover, in the climate change scenario, the associated mean storm track results show a tendency to move slightly northward, toward the North American coastline compared with the present climate, which is consistent with GCM studies.

In the present climate, consideration of composite storm structures suggests that a secondary U10 maximum occurs on the left side of the storms. In the climate change scenario, this feature increases slightly in spatial extent and stronger winds tend to occur towards the front of the storm, and on both sides of the storm centers relative to the propagation directions. Moreover, for the climate change scenario, maximum U10 in the composite storm is about 1 m/s weaker than that of the present climate. In terms of vertical structure, the climate change scenario suggests enhanced vertical wind shear and upward motion. Although large-scale warming tends to occur in the climate change scenario, dynamic cooling in the troposphere around the storm center is produced by the strong upward motion. This tends to reduce the intensities of landfalling storms, compared to estimates for the present climate.

(1) Jiang, J., and Perrie, W., 2007a The Impacts of Climate Change on Autumn North Atlantic Midlatitude Cyclones, Journal of Climate, Vol. 20, No. 7, pages 1174–1187.

(2) Jiang J., and W. Perrie. 2007b: Climate Change Effects on North Atlantic Cyclones. Submitted to J. Geophys. Res. – Atmospheres.

(3) Yao, Y., Perrie, W., Zhang, W., and Jiang, J., 2007: The characteristics of atmosphere-ocean interactions along North Atlantic extratropical storms tracks. Submitted to J. Geophys. Res. - Atmospheres.