Tropical Cyclones and Climate Change in a High Resolution General Circulation Model, HiGEM

How tropical cyclone activity might change due to anthropogenic climate change is assessed using the UK's High Resolution Global Environment Model (HiGEM) (N144, ~90km atmosphere and ~40km ocean). Tropical cyclones are identified in experiments for a 30 year doubled CO2 and a quadrupled CO2 and compared to those identified in simulations for the present day available as an ensemble of nine 30 year simulations. Tropical cyclones are identified and tracked using an objective feature tracking method (Bengtsson et al, 2007). HiGEM shows good skill in replicating the climatology of cyclone track densities and the structure of tropical cyclones compared to observations and re-analysis products. It is also able to produce a realistic relationship of the cyclone distribution with El Niño Southern Oscillation (ENSO). HiGEM produces cyclones with intensities, in terms of winds, that are comparable to those produced by tracking ERA-40 but less than ERA-Interim and International Best Track Archive for Climate Stewardship (IBTrACS).

On the large scale the mean climate change signals in the tropics are generally consistent with the simulations of the Coupled Model Intercomparison Project (CMIP3) models showing an increase in vertical wind shear over the tropical North Atlantic caused by sea surface temperature (SST) warming less than the tropical average and a reduction in the Walker circulation. Tropical cyclones are shown to decrease in frequency globally and in most basins with significant changes in the quadrupled CO2 simulation. However, the North Indian and Central North Pacific regions show an increase in frequency. The intensity of tropical cyclones is seen to increase with increasing CO2 concentration, but only becomes significant in the quadrupled CO2 simulation. The change in tropical cyclone activity is attributed to the change in large scale dynamical parameter such as vertical wind shear and upward motion, as well as, thermodynamic parameters including SST and relative humidity, which vary between basins. The rising branch of the Hadley cell weakens in the North Atlantic with increasing CO2, yet intensifies and shifts slightly southward in the East Pacific changing the location of the tropical cyclone tracks.

HiGEM's realistic simulation of ENSO is used to investigate the impact of different types of El Nino, e.g. modoki, and how these may change with anthropogenic climate change.