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Aaron Donohoe

Senior Research Scientist

Email

adonohoe@apl.washington.edu

Phone

206-616-3471

Department Affiliation

Polar Science Center

Education

B.A. Physics, Bowdoin College, 2003

Ph.D. Atmospheric Sciences, University of Washington, 2011

Publications

2000-present and while at APL-UW

Near invariance of poleward atmospheric heat transport in response to mid-latitude orography

Cox, T., A. Donohoe, G.H. Roe, K.C. Armour, and D.M.W. Frierson, "Near invariance of poleward atmospheric heat transport in response to mid-latitude orography," J. Clim., 35, 4099-4113, doi:10.1175/JCLI-D-21-0888.1, 2022.

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1 Jul 2022

Total poleward atmospheric heat transport (AHT) is similar in both magnitude and latitudinal structure between the Northern and Southern Hemispheres. These similarities occur despite more major mountain ranges in the Northern Hemisphere, which help create substantial stationary eddy AHT that is largely absent in the Southern Hemisphere. However, this hemispheric difference in stationary eddy AHT is compensated by hemispheric differences in other dynamic components of AHT so that total AHT is similar between hemispheres. In order to understand how AHT compensation occurs, we add mid-latitude mountain ranges in two different general circulation models that are otherwise configured as aquaplanets. Even when mid-latitude mountains are introduced, total AHT is nearly invariant. We explore the near invariance of total AHT in response to orography through dynamic, energetic, and diffusive perspectives. Dynamically, orographically induced changes to stationary eddy AHT are compensated by changes in both transient eddy and mean meridional circulation AHT. This creates an AHT system with three interconnected components that resist large changes to total AHT. Energetically, the total AHT can only change if the top-of-atmosphere net radiation changes at the equator-to-pole scale. Mid-latitude orography does not create large-enough changes in the equator-to-pole temperature gradient to alter outgoing longwave radiation enough to substantially change total AHT. In the zonal mean, changes to absorbed shortwave radiation also often compensate for changes in outgoing longwave radiation. Diffusively, the atmosphere smooths anomalies in temperature and humidity created by the addition of mid-latitude orography, such that total AHT is relatively invariant.

Optimal geometric characterization of forced zonal mean tropical precipitation changes

Donohoe, A., A.R. Atwood, and D.S. Battisti, "Optimal geometric characterization of forced zonal mean tropical precipitation changes," Clim. Dyn., EOR, doi:10.1007/s00382-022-06203-6, 2022.

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25 Mar 2022

The zonal and annual mean tropical precipitation response to paleoclimate and anthropogenic forcing scenarios ranging from the Last Glacial Maximum (LGM), CO2 quadrupling (4XCO2), mid-Holocene, North Atlantic freshwater hosing and volcanic forcing is analyzed in an ensemble of global climate models. Zonally averaged tropical precipitation changes are characterized in terms of three geometric manipulations of the climatological precipitation (hereafter, modes): meridional shifts, intensifications, and meridional contractions. We employ an optimization procedure that quantifies the magnitude and robustness (across different models) of changes in each mode in response to each forcing type. Additionally, the fraction of precipitation changes that are explained by the modes—in isolation and combined—is quantified. Shifts are generally less than 1° latitude in magnitude and explain a small fraction (< 10%) of tropical precipitation changes. Contractions and intensifications are strongly correlated across all simulations with a robust intensification and contraction of precipitation under global warming and a robust reduction and expansion under global cooling during the Last Glacial Maximum. The near constant scaling between contractions and intensifications across all simulations is used to define a joint contraction/intensification (CI) mode of tropical precipitation. The CI mode explains nearly 50% of the precipitation change under 4XCO2 and LGM forcing by optimizing a single parameter. These results suggest the shifting mode that has been extensively used to interpret paleo-rainfall reconstructions is of limited use for characterizing forced zonal mean precipitation changes and advocates for a reinterpretation of past precipitation changes to account for the CI mode.

Contributions to polar amplication in CMIP5 and CMIP6 models

Hahn, L.C., K.C. Armour, M.D. Zelinka, C.M. Bitz, and A. Donohoe, "Contributions to polar amplication in CMIP5 and CMIP6 models," Front. Earth Sci., 9, doi:10.3389/feart.2021.710036, 2021.

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20 Aug 2021

As a step towards understanding the fundamental drivers of polar climate change, we evaluate contributions to polar warming and its seasonal and hemispheric asymmetries in Coupled Model Intercomparison Project phase 6 (CMIP6) as compared with CMIP5. CMIP6 models broadly capture the observed pattern of surface- and winter-dominated Arctic warming that has outpaced both tropical and Antarctic warming in recent decades. For both CMIP5 and CMIP6, CO2 quadrupling experiments reveal that the lapse-rate and surface albedo feedbacks contribute most to stronger warming in the Arctic than the tropics or Antarctic. The relative strength of the polar surface albedo feedback in comparison to the lapse-rate feedback is sensitive to the choice of radiative kernel, and the albedo feedback contributes most to intermodel spread in polar warming at both poles. By separately calculating moist and dry atmospheric heat transport, we show that increased poleward moisture transport is another important driver of Arctic amplification and the largest contributor to projected Antarctic warming. Seasonal ocean heat storage and winter-amplified temperature feedbacks contribute most to the winter peak in warming in the Arctic and a weaker winter peak in the Antarctic. In comparison with CMIP5, stronger polar warming in CMIP6 results from a larger surface albedo feedback at both poles, combined with less-negative cloud feedbacks in the Arctic and increased poleward moisture transport in the Antarctic. However, normalizing by the global-mean surface warming yields a similar degree of Arctic amplification and only slightly increased Antarctic amplification in CMIP6 compared to CMIP5.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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