Characteristics of cold-air outbreak events and associated polar mesoscale cyclogenesis over the north Atlantic region

Terpstra, Annick, Renfrew, Ian A. ORCID: and Sergeev, Denis E. (2021) Characteristics of cold-air outbreak events and associated polar mesoscale cyclogenesis over the north Atlantic region. Journal of Climate, 34 (11). 4567–4584. ISSN 0894-8755

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Equatorward excursions of cold polar air masses into ice-free regions, so-called cold-air outbreak (CAO) events, are frequently accompanied by the development of severe mesoscale weather features. Focusing on two key regions, the Labrador Sea and the Greenland–Norwegian Seas, we apply objective detection for both CAO events and polar mesoscale cyclones to outline the temporal evolution of CAO events and quantify associated mesoscale cyclogenesis. We introduce a novel metric, the CAO depth, which incorporates both the static stability and the temperature of the air mass. The large-scale atmospheric conditions during the onset of CAO events comprise a very cold upper-level trough over the CAO region and a surface cyclone downstream. As the CAO matures, the cold air mass extends southeastward, accompanied by lower static stability and enhanced surface fluxes. Despite the nearly 20° difference in latitude, CAO events over both regions exhibit similar evolution and characteristics including surface fluxes and thermodynamic structure. About two-thirds of the identified CAO events are accompanied by polar mesoscale cyclogenesis, with the majority of mesoscale cyclones originating inside the cold air masses. Neither the duration nor the maturity of the CAO event seems relevant for mesoscale cyclogenesis. Mesoscale cyclogenesis conditions during CAO events over the Labrador Sea are warmer, moister and exhibit stronger surface latent heat fluxes than their Norwegian Sea counterparts.

Item Type: Article
Additional Information: Funding Information: Acknowledgments. Data for this study are provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) and available via the Copernicus Climate Change Service Climate Data Store (CDS). AT is supported by the Research Council of Norway as part of the project DYNAMISM (262110), and by the European Union’s Seventh Framework Programme for research, technological development, and demonstration under the Marie Curie Grant Agreement 608695. IR acknowledges funding from NERC NE/N009754/1 AFIS, part of the Iceland Greenland Seas Project. DS is partly supported by a Science and Technology Facilities Council Consolidated Grant (ST/R000395/1). The code used in this study relies on the following Python libraries: Matplotlib (Hunter 2007), pandas (McKinney 2010), xarray (Hoyer and Hamman 2017), and MetPy (May et al. 2020).
Uncontrolled Keywords: atmosphere-ocean interaction,cold air surges,cyclolysis,extreme events,mesocyclones,polar lows,atmospheric science ,/dk/atira/pure/subjectarea/asjc/1900/1902
Faculty \ School: Faculty of Science > School of Environmental Sciences
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Depositing User: LivePure Connector
Date Deposited: 13 Feb 2021 01:04
Last Modified: 23 Oct 2022 02:13
DOI: 10.1175/JCLI-D-20-0595.1

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