10 March 2016

Local Finite-Amplitude Wave Activity as an Objective Diagnostic of Midlatitude Extreme Weather


There is an ongoing debate regarding whether global warming would energize the midlatitude wave activities that are responsible for extreme weather events such as heat waves and cold snaps. Objective metrics are important for quantifying and improving understanding of weather extremes. Department of Energy scientists at Pacific Northwest National Laboratory and their university collaborators worked out a geophysical fluid-dynamics-based metric—local finite-amplitude wave activity (LWA)—to measure the global circulation extremes, their variability, and trends. This metric overcomes the difficulties of conventional metrics that confuse the signal of the background warming with the increased circulation extreme itself. When applied to the 500 hPa geopotential height, the LWA can capture the global frequency distribution of atmospheric blockings, as well as discern the weather patterns associated with cyclonic versus anticyclonic circulations. Using this metric to study internal variability such as the Arctic Oscillation shows a clear tendency for the LWA and local zonal momentum to compensate each other, which is indicative of the constraint from Kelvin circulation conservation. During the period when drastic Arctic sea ice loss was observed, the wave activity does not show evidence of a hemispheric-scale increase in wave amplitude as previous studies claimed to increase weather extremes. Robust regional trends can now be identified and have important implications for regional climate change.

Jian Lu
Cornell University
Chen, G, J Lu, DA Burrows, and LR Leung.  2015.  "Local Finite-Amplitude Wave Activity as an Objective Diagnostic of Midlatitude Extreme Weather."  Geophysical Research Letters 42(24): 10952-10960, doi:10.1002/2015GL066959.

We thank Clare S.Y. Huang and Noboru Nakamura for valuable discussion on FAWA and LWA and for sharing their manuscript on LWA in review. We are grateful for two anonymous reviewers for constructive comments on the manuscript, especially on the interpretation of the LWA for Z500. This work also benefited from discussion with Peter Hess, Steve Colucci, and David Straus. G. Chen and D.A. Burrows are supported by NSF grant AGS-1064079 and DOE grant DE-SC0012374. Jian Lu and L. Ruby Leung are supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research through the Regional and Global Climate Modeling program. PNNL is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. The ERA-Interim (ERA-I) reanalysis data were obtained from http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=pl/. The NCEP-DOE Reanalysis 2 (NCEP2) data were downloaded from http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.html. The monthly mean Arctic Oscillation index was obtained from Climate Prediction Center (http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml).