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Timothy Merlis , Ming Zhao, and Isaac Held

The sensitivity of hurricane frequency to ITCZ changes and radiatively forced warming in aquaplanet simulations. Timothy Merlis , Ming Zhao, and Isaac Held. Motivation/Approach.

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Timothy Merlis , Ming Zhao, and Isaac Held

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  1. The sensitivity of hurricane frequency toITCZ changes and radiatively forced warmingin aquaplanet simulations Timothy Merlis, Ming Zhao, and Isaac Held

  2. 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.

  3. 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.

  4. The Model…(2) Model used by Merlis et al., except w/ prescribed SSTs, realistic boundary conditions, seasonal forcing… Zhao et al (2009)

  5. Breaking hemispheric symmetry Most 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)

  6. ITCZ Shi[f]ts in Response to Asymmetric Forcing Frierson and Hwang (2012)

  7. Paleo-example with NH Extratropical cooling Wang et al. (2006)

  8. Explicitly simulated (& tracked) “hurricanes”

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

  10. Perturbed Radiation (unchanged ocean heat flux)(2) Model Produces increased TC frequency in warmer climates, contrary to comprehensive experiments.

  11. Increase consistent w/ displaced ITCZ

  12. Perturbed Radiation + Q-Flux Reduction in hurricane frequency with warming if ITCZ position is unchanged.

  13. 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.

  14. So, Uniform Warming (fixed ITCZ): ITCZ Shift (fixed Radiative Forcing): ITCZ Shift Per Unit Warming (fixed asymmetry):

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