Abstract This paper examines the processes that drive Arctic anomalous surface warming and sea ice loss during winter-season tropospheric energy flux events, synoptic periods of increased tropospheric energy flux convergence ( F trop ), using the NASA MERRA-2 reanalysis. During an event, a poleward anomaly in F trop initially increases the sensible and latent energy of the Arctic troposphere; as the warm and moist troposphere loses heat, the anomalous energy source is balanced by a flux upward across the tropopause and a downward net surface flux. A new metric for the Arctic surface heating efficiency ( E trop ) is defined, which measures the fraction of the energy source that reaches the surface. Composites of high-, medium-, and low-efficiency events help identify key physical factors, including the vertical structure of F trop and Arctic surface preconditioning. In high-efficiency events ( E trop ≥ 0.63), a bottom-heavy poleward F trop occurs in the presence of an anomalously warm and unstratified Arctic—a consequence of decreased sea ice—resulting in increased vertical mixing, enhanced near-surface warming and moistening, and further sea ice loss. Smaller E trop , and thus weaker surface impacts, are found in events with anomalously large initial sea ice extent and more vertically uniform F trop . These differences in E trop are manifested primarily through turbulent heat fluxes rather than downward longwave radiation. The frequency of high-efficiency events has increased from the period 1980–99 to the period 2000–19, contributing to Arctic surface warming and sea ice decline.
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Stratospheric and Tropospheric Flux Contributions to the Polar Cap Energy Budgets
Abstract The flux of moist static energy into the polar regions plays a key role in the energy budget and climate of the polar regions. While usually studied from a vertically integrated perspective ( F wall ), this analysis examines its vertical structure, using the NASA-MERRA-2 reanalysis to compute climatological and anomalous fluxes of sensible, latent, and potential energy across 70°N and 65°S for the period 1980–2016. The vertical structure of the climatological flux is bimodal, with peaks in the middle to lower troposphere and middle to upper stratosphere. The near-zero flux at the tropopause defines the boundary between stratospheric ( F strat ) and tropospheric ( F trop ) contributions to F wall . Especially at 70°N, F strat is found to be important to the climatology and variability of F wall , contributing 20.9 W m −2 to F wall (19% of F wall ) during the winter and explaining 23% of the variance of F wall . During winter, an anomalous poleward increase in F strat preceding a sudden stratospheric warming is followed by an increase in outgoing longwave radiation anomalies, with little influence on the surface energy budget of the Arctic. Conversely, a majority of the energy input by an anomalous poleward increase in F trop goes toward warming the Arctic surface. Overall, F trop is found to be a better metric than F wall for evaluating the influence of atmospheric circulations on the Arctic surface climate.
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- NSF-PAR ID:
- 10226102
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 34
- Issue:
- 11
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 4261 to 4278
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract This study quantifies the contribution to Arctic winter surface warming from changes in the tropospheric energy transport (
F trop) and the efficiency with whichF tropheats the surface in the RCP8.5 warming scenario of the Community Earth System Model Large Ensemble. A metric for this efficiency,E trop, measures the fraction of anomalousF tropthat is balanced by an anomalous net surface flux (NSF). Drivers ofE tropare identified in synoptic‐scale events during whichF tropis the dominant driver of NSF.E tropis sensitive to the vertical structure ofF tropand pre‐existing Arctic lower‐tropospheric stability (LTS). In RCP8.5, winter‐meanF tropdecreases from 95.1 to 85.4 W m−2, whileE tropincreases by 5.7%, likely driven by decreased Arctic LTS, indicating an increased coupling betweenF tropand the surface energy budget. The net impact of decreasingF tropand increasing efficiency is a positive 0.7 W m−2contribution to winter‐season surface heating. -
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JCLI-D-20-0722.1
