Hurricanes in Other Climates

Robert KortyTexas A&M

Atlantic Main Development Region SST (ASO mean) and power dissipation (whole season, whole basin)1-3-4-3-1 filter applied to smooth interannual variability

Holland and Bruyere (submitted)

Lines of evidence:

• Geologic evidence (paleotempestology)• Quantitative results from modeling techniques:

Global

Tropical cyclones and climate

Current

Warmer

Lines of evidence:

• Geologic evidence (paleotempestology)• Quantitative results from modeling techniques:

Global (general circulation) Downscaling

Tropical cyclones and climate

Lines of evidence:

• Geologic evidence (paleotempestology)• Quantitative results from modeling techniques:

Global (general circulation) Downscaling

• Thermodynamic controls from large-scale environment Potential intensity (thermal stability of atmosphere) Moisture (entropy) content of tropical atmosphere Vertical wind shear

Tropical cyclones and climate

Knutson et al. 2008

• Atlantic tropical cyclone counts fall in warmer simulation• Distribution is shifted towards stronger events• Global models are unable to simulate intense events…

Tropical cyclones and climate

Knutson et al. 2008

Tropical cyclone climatology in 21st century simulations

• Atlantic tropical cyclone counts fall in warmer simulation• Distribution is shifted towards stronger events• Global models are unable to simulate intense events…

Bender et al. 2010…but higher resolution models show an increase in the frequency of strong events

Potential Intensity of tropical cyclones

• Thermodynamic speed limit to a tropical cyclone’s (TC) intensity

• Vmax set by strength of fluxes from sea to atmosphere and the thermodynamic stability of the atmospheric column above

• Principally determined by level of neutral bouyancy, which sets To.

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Vmax =CkCD

SST −ToTo

(k0* − k)

The Last Glacial Maximum: 21,000 years ago

CO2 was 185 ppmTropical temperatures were 2-3oC cooler than todayAs much as 30oC colder over land where there was ice

Storm Season potential intensity and SST

Longitude

Latit

ude

Storm Season potential intensity and SST

Longitude

Latit

ude

Similar behavior in hot climates

• A series of simulations in which CO2 is progressively doubled.

• These start with preindustrial levels (280 ppm) and roll upward: 560, 1120, 2240, 4480, 8960.

• The hottest of these (8960 ppm, which is 25 preindustrial levels) feature tropical SST > 40oC.

Changes in potential intensity and SST

2240 ppm CO2 - control 8960 ppm CO2 - control

Joint distribution of SST and potential intensity

Joint distribution of SST and To.

Joint distribution of SST and LNB.

What else can affect potential intensity?

• Anything that has an uneven response relative to a moist adiabat. This includes the response to strong volcanic eruptions and, more weakly, to the solar cycle.

• Following the eruption of a low-latitude volcano that explodes aerosols and particles into the stratosphere, surface climate cools over the following year while tropopause temperatures rise.

Simulation of the Last Millennium

Volcanism

Seasons following 1258 eruption

Kuwae (1452)

Tambora (1815)

Seasonal cycle of PI following volcanic eruptions

Zonal annual mean difference in T: Smax-SminModel (CCSM) Reanalysis (NCAR/NCEP)

Orbit varies: does this affect TC environment?

• Date Earth makes its closest approach to the Sun rotates around the calendar. Every 57 years it advances another day.

• 6000 years ago, atmospheric composition was nearly identical to its state through start of industrial revolution ca. 1870, but nearest approach to Sun occurred in September rather than January (today).

Lines of evidence:

• Geologic evidence (paleotempestology)

Tropical cyclones and climate

Photos courtesy of Kam-biu Liu (LSU) and Jon Woodruff (U. Mass.).

Hurricane Carol

Great New EnglandHurricane

Great SeptemberGale

Great Colonial Hurricane

TOA radiation perturbations

Change in Mid-Holocene potential intensity

Change in surface T and s at Mid-Holocene

Mid-Tropospheric entropy content

• Tropical cyclones require a supply of moist, high entropy air to maintain a saturated core.

• Downdrafts carry drier, low entropy air from the middle troposphere into the boundary layer. If inflow is not strong enough, these can choke off incipient events.

• Relevant parameter is sb-sm ~ s*-sm.

• Moist entropy varies with both relative humidity and temperature, and can change with climate owing to large-scale temperature variations even when RH does not.

Saturation Entropy Deficit