Sensitivity of stationary circulations to global warming.

(Collaborators: William Boos, Robert Wills, Tapio Schneider)

Boreal summer is characterized by large zonal asymmetries in rainfall and evaporation in the Northern Hemisphere subtropics; a famous paradigm is the large rainfall contrast that exists between intense monsoon rainfall over India and arid conditions over the Arabian desert. These zonal asymmetries are produced by stationary circulations, which couple the humid climate of monsoon regions with the arid climate of land deserts. Prediction of rainfall changes over these regions with global warming remains uncertain, with large uncertainty among climate models and lack of a comprehensive theory.

In collaboration with Bill Boos at Yale, I devised a novel mechanism for the sensitivity of stationary circulations to global warming and verified its relevance in a set of idealized GCM simulations spanning a wide range of climate scenarios. This mechanism predicts that a lifting of the tropopause level (from global warming) alone could strengthen stationary circulations, thus producing more extreme zonal contrast in hydrologic imbalance. Preliminary results suggest that changes in the strength of subtropical circulation in comprehensive climate simulations [CMIP5] of global warming scenarios are consistent with this mechanism.

(Levine and Boos, 2016; J. Clim.)

Relevant Publications:

Levine X. J., and W.R. Boos, 2016: A mechanism for the response of the zonally asymmetric subtropical hydrologic cycle to global warming. Journal of Climate, 29, 7851-7867. [pdf] [official version]

Wills, R.C., and X. J. Levine, and T. Schneider, 2017: Local energetic constraints on Walker circulation strength. Journal of the Atmospheric Sciences, 74, 1907-1922. [pdf][official version]

Levine X. J., and W. R. Boos (in prep): A First-Baroclinic mode theory for the sensitivity of stationary circulations to global warming in CMIP5 simulations.

Albedo variability in comprehensive climate models.

(Collaborator: William Boos)

Surface albedo, by modulating the energy budget of the atmosphere, exerts a major influence on Earth’s climate. This influence is particularly strong over arid and semi-arid regions of the tropics, on decadal (e.g. the Sahel drought cycle in present day climate) to centennial timescales (the green Sahara in the mid-Holocene). In collaboration with Bill Boos, I recently demonstrated the existence of large intermodel variability in surface albedo in CMIP5 historical simulations. This variability likely results from inconsistent prescription of surface properties (e.g. vegetation type) and differences in radiative transfer parameterizations between comprehensive climate models. Our results suggest that rainfall over many parts of the tropics responds to local and remote changes in surface albedo over vegetated areas of the tropics and midlatitudes.

Relevant Publications:

Levine X. J., and W.R. Boos, 2017: Land surface albedo bias in climate models and its association with tropical rainfall. Geophysical Research Letters, 44, 1-10, doi:10.1002/2017GL072510. [pdf] [SI] [official version]

Sensitivity of Hadley circulation to climate changes.

(Collaborators: Tapio Schneider, Tobias Bischoff)

The Hadley circulation (HC) is a meridional overturning circulation of the tropical troposphere. The variability of its strength and its width on seasonal to centennial timescales regulates surface climate, particularly rainfall distribution, in the zonal-mean. In collaboration with Tapio Schneider at Caltech, I analyzed a large set of idealized climate model simulations to provide a comprehensive description of the HC dynamical regimes under a wide range of climate changes, and I formulated a quantitative scaling for its extent. Currently, I am developing an even simpler model of the tropical atmosphere (axisymmetric model with parametrized eddy mixing), which apparently captures the sensitivity of the HC strength and extent in full GCMs; the relative simplicity of this model is a significant progress provided that it can shed light on the complex behavior of the HC while being amenable to analytical treatment.

(Levine and Schneider, 2011; J. Atmos. Sci.)

Relevant publications:

Levine X. J., and T. Schneider, 2011: Response of the Hadley circulation to climate change in an aquaplanet GCM coupled to a simple representation of ocean heat transport. Journal of the Atmospheric Sciences, 68, 769-783. [pdf] [official version]

Levine X. J., and T. Schneider, 2015: Baroclinic eddies and the extent of the Hadley circulation: An idealized GCM study. Journal of the Atmospheric Sciences, 72, 2744-2761. [pdf] [official version]

Levine X. J., and T. Schneider, 2012: A Single-layer model of the tropical atmosphere: the Hadley circulation over a wide range of climate changes. Dynamics of Earth’s Hadley circulation, X. Levine, Ph.D. Thesis, 76–110.

Bischoff T., T. Schneider, and X.J. Levine (in prep.): The Hadley Circulation under Tropical and Global Warming.

Funded Fellowships and Grants

2013-2014: Yale Climate & Energy Institute Postdoctoral Fellowship: The intensity and extent of Earth’s dry regions under global climate change.

2015-2018: National Science Foundation Grant (co-authored with William Boos): Zonal asymmetries in the low-latitude hydrological cycle over a broad range of climates.