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The sensitivity of hurricane frequency to The sensitivity of hurricane frequency to

The sensitivity of hurricane frequency to - PowerPoint Presentation

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The sensitivity of hurricane frequency to - PPT Presentation

ITCZ changes and radiatively forced warming in aquaplanet simulations Timothy Merlis Ming Zhao and Isaac Held MotivationApproach Most modeling studies project decreases in the globally averaged frequency of tropical ID: 378856

warming itcz fixed frequency itcz warming frequency fixed model heat dependence latitude ocean genesis flux unchanged radiation perturbed response hurricane tropical temperature

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Slide1

The sensitivity of hurricane frequency toITCZ changes and radiatively forced warmingin aquaplanet simulations

Timothy

Merlis

, Ming Zhao, and Isaac HeldSlide2

Motivation/ApproachMost 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.Slide3

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

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.Slide4

The Model…(2)Zhao et al (2009)

Model used by

Merlis

et al., except w/ prescribed SSTs, realistic boundary conditions, seasonal forcing…Slide5

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)Slide6

ITCZ Shi[f]ts in Response to Asymmetric Forcing

Frierson

and Hwang (2012)Slide7

Paleo-example with NH Extratropical cooling

Wang et al. (2006)Slide8

Explicitly simulated (& tracked) “hurricanes”Slide9

Perturbed Radiation (w/ unchanged ocean heat flux)

Very large solar changes! Roughly Earth’s insolation in ~800 million yearsSlide10

Perturbed Radiation (unchanged ocean heat flux)(2)

Model Produces increased TC frequency in warmer climates, contrary to comprehensive experiments.Slide11

Increase consistent w/ displaced ITCZSlide12

Perturbed Radiation + Q-Flux

Reduction in hurricane frequency with warming

if

ITCZ position is unchanged.Slide13

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.Slide14

So,

Uniform Warming (fixed ITCZ):

ITCZ Shift (fixed Radiative Forcing):

ITCZ Shift Per Unit Warming (fixed asymmetry): Slide15