THE SENSITIVITY OF HURRICANE FREQUENCY TOITCZ CHANGES AND RADIATIVELY FORCED WARMINGIN AQUAPLANET SIMULATIONS

Timothy Merlis, Ming Zhao, and Isaac Held

Motivation/Approach• Most modeling studies project decreases in the globally

averaged frequency of tropical cyclones (e.g., Knutson et al., 2010).

• Instead of using “genesis indices” – empirical relations between the frequency of storm genesis and the larger scale circulation and thermodynamic structure of the atmosphere- uses a global atmospheric model that simulates TC genesis directly.

• Simple boundary conditions and configuration to isolate sensitivities.

The Model…• GFDL HiRAM with ~50km resolution, 32 vertical levels

• Aquaplanet configuration, no seasonal or diurnal cycle

• 20-m slab ocean, providing some heat capacity and a source of water vapor. The temperature of the slab is predicted by the model.

The Model…(2)

Zhao et al (2009)

Model used by Merlis et al., except w/ prescribed SSTs, realistic boundary conditions, seasonal forcing…

Breaking hemispheric symmetryMost robust hydrologic responses in climate change

Response to warming=> Increased horizontal moisture fluxes=> Poleward expansion of the subtropics

Response to differential warming of the two hemispheres=> tropical rainbelts move to warmer hemisphere

Kang et al (2008)

ITCZ Shi[f]ts in Response to Asymmetric Forcing

Frierson and Hwang (2012)

Paleo-example with NH Extratropical cooling

Wang et al. (2006)

Explicitly simulated (& tracked) “hurricanes”

Perturbed Radiation (w/ unchanged ocean heat flux)Very large solar changes! Roughly Earth’s insolation in ~800 million years

Perturbed Radiation (unchanged ocean heat flux)(2)

Model Produces increased TC frequency in warmer climates, contrary to comprehensive experiments.

Increase consistent w/ displaced ITCZ

Perturbed Radiation + Q-Flux

Reduction in hurricane frequency with warmingif ITCZ position is unchanged.

Recap so far…In these particular idealized simulations, response of hurricane frequency to warming involves 3 very different mechanisms, each controlled by three distinct set of physical mechanisms:

1) The dependence of ITCZ latitude on warming; 2) The dependence of N on warming with fixed ITCZ latitude, and;3) The dependence of N on ITCZ latitude at fixed tropical mean temperature.

So,

Uniform Warming (fixed ITCZ):

ITCZ Shift (fixed Radiative Forcing):

ITCZ Shift Per Unit Warming (fixed asymmetry):