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
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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