Abstract
Observations from the 2014 Arctic Clouds in Summer Experiment indicate that, in summer, warm-air advection over melting sea-ice results in a strong surface melting feedback forced by a very strong surface-based temperature inversion and fog formation exerting additional heat flux on the surface. Here, we analyze this case further using a combination of reanalysis dataset and satellite products in a Lagrangian framework, thereby extending the view spatially from the local icebreaker observations into a Langrangian perspective. The results confirm that warm-air advection induces a positive net surface-energy-budget anomaly, exerting positive longwave radiation and turbulent heat flux on the surface. Additionally, as warm and moist air penetrates farther into the Arctic, cloud-top cooling and surface mixing eventually erode the surface inversion downstream. The initial surface inversion splits into two elevated inversions while the air columns below the elevated inversions transform into well-mixed layers.
Highlights
The Arctic near-surface temperature has increased more than twice as fast as the global average in recent decades (Graversen et al 2008; Francis and Vavrus 2012; Cohen et al 2014)
Several previous studies suggest that warm-and-moist-air intrusions (WaMAI) into the Arctic from the south significantly contribute to Arctic amplification (Woods and Caballero 2016; Johansson et al 2017; Liu et al 2018; Messori et al 2018; Naakka et al 2019a)
ERA5 mostly captures the structure of temperature profiles in the warm-air advection; Atmospheric Infrared Sounder (AIRS) matches the soundings quite well, except on 4 August when the maximum temperature in the surface inversion observed by AIRS is nearly 5 °C lower than in the soundings
Summary
The Arctic near-surface temperature has increased more than twice as fast as the global average in recent decades (Graversen et al 2008; Francis and Vavrus 2012; Cohen et al 2014). Surface warming was induced by warm-air advection, and by increased longwave emissivity of the lower atmosphere This was found by Kapsch et al (2013, 2016), who linked additional incoming longwave surface radiation to an earlier sea-ice melt onset and a lower September ice extent. Tjernström et al (2015) reported on a strong WaMAI in the Siberian Sea in August 2014 during the Arctic Clouds in Summer Experiment (ACSE) They described the formation of a very strong surface-based temperature inversion and persistent fog as the warm air adjusted to the cold surface. The in situ character of the observations during ACSE prohibited an evaluation of the farther air-mass transformation over the ice, and from observations alone it was difficult to close the energy balance and to determine the relative contributions from advection, cloud formation, radiation, and turbulent mixing. To a cloud-capped well-mixed boundary layer as hypothesized? (2) During ACSE the surface inversion remained quasi-stationary for about a week; what boundary-layer or larger-scale processes are mainly responsible for maintaining this very robust feature over time?
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