Harvard Climate Seminar: Sukyoung Lee


Wednesday, April 18, 2018, 12:00pm


Sukyoung Lee
Professor of Meteorology
Penn State University
Department of Meteorology and Atmospheric Science

Tropically Excited Arctic warMing (TEAM) mechanism: A Theory based on a General Circulation Perspective

Records of past climates show a wide range of values of the equator-to-pole temperature gradient, with an apparent universal relationship between the temperature gradient and the global-mean temperature: relative to a reference climate, if the global-mean temperature is higher (lower), the greatest warming (cooling) occurs at polar regions. Understanding this equator-to-pole temperature gradient is fundamental to climate and the general circulation. Here, a general-circulation-based theory for polar amplification is presented. Recognizing the fact that most of the zonal available potential energy (ZAPE) in the atmosphere is untapped, this theory states that La-Niña-like tropical heating can help tap ZAPE and warm the Arctic by exciting poleward and upward propagating Rossby waves that reinforce the climatological stationary waves.

This theory is supported by observation-based data and idealized model experiments. The theory is also supported by the ongoing multi-decadal trend in the convective precipitation which shows a steady increase over the western tropical Pacific. In contrast, most climate models predict an El-Niño-like response to greenhouse-gas warming. Discrepancies between climate models and the observations are often attributed to internal variability. However, evidence will be presented wherein the models are predicting an El-Niño-like response to greenhouse-gas warming at least in part because the convective parameterizations in climate models overpredict warming in the tropical upper troposphere. The climate models also predict Arctic warming, but the models’ Arctic warming is impacted by the warm bias in the models’ tropics.

It is widely accepted that the primary mechanism for Arctic amplification is ice-albedo feedback and resultant sea-ice decline, but observations lend little support to this mechanism. Instead, energy flux from lower latitudes by Rossby waves plays the key role in warming the Arctic and melting the sea ice.