Mechanisms of Climate Variability and Change

University Of Washington, Seattle WA

Investigators

Abstract

This work addresses basic physical mechanisms that determine mean climate, climate variability and the atmospheric circulation response to greenhouse gas-induced global warming. Much of the research focuses on the interaction of cloud radiative effects and atmospheric circulation. One issue to be addressed is the close association over the western equatorial Pacific between atmospheric heating due to the radiative effects of clouds and associated water vapor and energy export from the region. The PI hypothesizes that the radiative heating of the atmosphere by clouds and vapor anomalies plays a deciding role in horizontal energy export, in part because cloud radiative heating in the upper troposphere increases the requirement for export. The research further considers the possible effect of upper-tropospheric cloud radiative heating in confining the intertropical convergence zone (ITCZ) to a narrow strip near the equator. A role for cloud radiative effects in confining the ITCZ would be of interest given the persistent double-ITCZ bias in climate models, in which models incorrectly produce an ITCZ on either side of the equator. Further research considers the role of cloud radiative effects in determining key features of the atmospheric circulation response to global warming, including the expansion of the Hadley cell (accompanied by an expansion of the dry regions in the subtropics) and the poleward shift of the midlatitude jet streams commonly found in model simulations. A related issue to be addressed is the dipole in shortwave cloud radiative feedback over the Southern Ocean, where the feedback is negative in lower latitudes (where it thus serves to counteract greenhouse warming) but positive in higher latitudes. Previous work by the PI and his collaborators suggests that the positive feedback at higher latitudes is due to cloud brightening as warmer temperatures favor liquid droplets over ice particles in clouds. Two other topics to be addressed are the mechanisms which produce El Nino-like conditions in the equatorial Pacific sector, and the contribution of natural variability to the increasing sea ice cover trend around Antarctica and the decline of sea ice in the Arctic. The work is conducted through analysis of observations and a variety of numerical model simulations, for example the experiments conducted in the Clouds On Off Climate Intercomparison Experiment are examined to understand the role of cloud radiative effects in energy export and ITCZ confinement. The project has societal broader impacts due to the value of improved understanding of the climate system for anticipating and responding to the likely consequences of climate variability and change. In addition, the project supports three graduate students, thus promoting the development of the next generation of climate science researchers. The PI also engages in outreach to the public and stakeholders through a variety of venues.