causes of reduced north atlantic storm activity in a global climate model simulation of the last...

Causes of Reduced North Atlantic Storm Activity in A

Global Climate Model Simulation of the Last Glacial

Maximum

Aaron Donohoe, David Battisti, Camille Li

University of WashingtonDepartment of Atmospheric Sciences

Last Glacial Maximum (LGM) run in CCSM3 Otto-Bliesner et al. (2006)

•21ka Insolation •Greenhouse Gas Concentration from ice core measurements•Land Ice Topography and coastlines corresponding to a 120 meters sea level depression from ICE-5G reconstruction (Peltier 2004)

Mean State (DJF) Atlantic Jet

Eady Growth Rate

Per Day

Contours =250 hPa Zonal Wind

.31 mdUf

N dz

.

LGM

MODERN

Eddy ActivityVertically Averaged DJF eddy Variances

Contours = 250 hPa Zonal Wind (m/s)

Linear Stability Of JetHypothesis: Strong LGM Barotropic Shear Stabilizes the Jet to Baroclinic growth

This idea is tested with a 2-layer, QG, linear Model

Conclusion: The jet structure stabilizes the LGM as compared to the Modern jet relative to the Eady Growth rate BUT the LGM is still morelinearly unstable to Baroclinic eddies

MODERN

LGM

Feature Tracking Z 300 hPa• Use the feature tracking algorithm of Hodges on 2-10

day band pass filtered 300 hPa geopotential height • 25 years of data give in each run allow us to compute

instantaneous central magnitude and tendency

Feature Tracking By Zonal Location

Atlantic Domain Ensemble Feature Statistics

SEED MAGNITUDE

Conclusions• Storm activity is reduced in the LGM

simulation despite enhanced baroclinicity relative to the modern day

• The narrowness of the LGM jet can not explain the reduced storm activity

• Reduced seeding (magnitude and number) seems to explain the decreased LGM storm activity despite more efficient baroclinic growth

The Eddy Kinetic Energy Budget

Eddy Kinetic Energy Budget

Energy ConversionEddy Kinetic Energy

Reservoir

•Changes in Barotropic Decay between modern and LGM work in the wrong sense to explain reduced LGM storm activity

•Changes in Baroclinic conversion are indicative of differences in eddy kinetic energy reservoir