Tropical Cyclones in Simulations of the Middle Holocene

Abstract:

The environmental conditions that support and sustain tropical cyclones are affected by the amount of solar radiation incident on the tropics, which varies on millennial timescales owing to orbital variations. During the middle Holocene, higher amounts of summer solar radiation 6000 years ago (6ka) increased thermal stability and pools of hot, dry air in the tropical troposphere, rendering the thermodynamic environment less favorable than in modern times. (The opposite response is seen in the Southern Hemisphere, where January-March anomalies yield more conducive conditions 6ka there than today.) Here we compare these changes in environmental conditions to tropical storms simulated by two distinct methods as well as to available geologic evidence from the middle Holocene. We find that storms directly spawned by global climate models respond to the changes as the thermodynamic environment predicts: a reduction 6ka in Northern Hemisphere, with an increase 6ka in the Southern Hemisphere. We derive an empirical genesis index that identifies the best fit between environmental conditions and the response in genesis. We also compare the results to storms generated using the statistical downscaling method pioneered by Emanuel. Here too the events similarly respond to the changes in the environmental conditions, but the amplitude of the changes is smaller than seen in the global climate models. We discuss some possible reasons for the differences as well as their implications for studies applying these methods to 21^st century climate.