Decomposing the Drivers of Polar Amplification with a Single Column Model
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Student Seminar Series
Department of Atmospheric & Oceanic Sciences
presents
a talk by
Matthew Henry
PhD candidate
Decomposing the Drivers of Polar Amplification with a Single Column Model
The Arctic amplification of surface temperature change is a common feature of both models and observations. The lapse rate feedback dominates the attribution to forcings and feedbacks of polar surface warming under increased CO2. The surface temperature change attribution method based on top-of-atmosphere (TOA) energy budget changes assumes forcings and feedbacks lead to vertically uniform temperature changes, and accounts for the deviation from vertically uniform warming by assigning it to the lapse rate feedback. Here, we decompose the temperature change while accounting for the vertical structure of the temperature change contributions from each forcing and feedback. An idealized moist atmospheric General Circulation Model (GCM) with slab ocean boundary conditions, no clouds or sea ice is forced with a quadrupling of CO2 concentrations with and without a polar surface heat source. It is shown to have a polar amplified pattern of surface temperature change comparable to comprehensive GCM simulations. Then, we use a single column model to decompose the tropical and polar temperature change from the idealized GCM into contributions from forcings and feedbacks excluding the temperature feedback, by individually perturbing the CO2, water vapor and energy transport. The warming from increasing longwave absorbers (CO2 and water vapor) is bottom-heavy and accounts for most of the surface warming in the absence of a surface heat source, whereas the warming from atmospheric heat transport preferentially warms the mid and upper atmosphere. The warming from CO2 alone is polar-amplified as tropical convection moves the warming maximum to the upper troposphere. Adding the `local' water vapor feedback preferentially warms the tropical surface temperature, which cancels polar amplification. And, adding the atmospheric energy transport and its implied water vapor change contributes to further polar amplification. The surface heat source, which could represent oceanic heat transport convergence or the surface albedo feedback, increases polar surface warming and is only partially compensated by a decrease in dry atmospheric energy transport. While this decomposition is applied to an idealized GCM, we hope it can be generalized to a fully comprehensive GCM.
Wednesday Sep 25/ 2.30 PM/ Room 934 Burnside Hall
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