Abstract
The declining trend of Arctic September sea ice constitutes a significant change in the Arctic climate system. Large year-to-year variations are superimposed on this sea–ice trend, with the largest variability observed in the eastern Arctic Ocean. Knowledge of the processes important for this variability may lead to an improved understanding of seasonal and long-term changes. Previous studies suggest that transport of heat and moisture into the Arctic during spring enhances downward surface longwave radiation, thereby controlling the annual melt onset, setting the stage for the September ice minimum. In agreement with these studies, we find that years with a low September sea–ice concentration (SIC) are characterized by more persistent periods in spring with enhanced energy flux to the surface in forms of net longwave radiation plus turbulent fluxes, compared to years with a high SIC. Two main atmospheric circulation patterns related to these episodes are identified: one resembles the so-called Arctic dipole anomaly that promotes transport of heat and moisture from the North Pacific, whereas the other is characterized by negative geopotential height anomalies over the Arctic, favoring cyclonic flow from Siberia and the Kara Sea into the eastern Arctic Ocean. However, differences between years with low and high September SIC appear not to be due to different spring circulation patterns; instead it is the persistence and intensity of processes associated with these patterns that distinguish the two groups of anomalous years: Years with low September SIC feature episodes that are consistently stronger and more persistent than years with high SIC.
Highlights
The Arctic sea–ice extent has declined significantly during recent years, with the largest reduction in September (Serreze and Stroeve 2015; IPCC 2013)
3.1 Sea–ice conditions during years of low September sea ice Except for 2003, the September sea–ice concentration (SIC) for the entire Northern Hemisphere is anomalously low during all LIYs, which are identified with respect to the investigation area (Fig. 2)
To investigate the atmospheric circulation patterns associated with the springtime longwave radiation and turbulent fluxes (LWNT) episodes, a Self organizing maps (SOMs) algorithm was applied to all spring days from 1979 to 2012
Summary
The Arctic sea–ice extent has declined significantly during recent years, with the largest reduction in September (Serreze and Stroeve 2015; IPCC 2013). Kapsch et al (2013) showed that positive anomalies of clouds and atmospheric water vapor are present over the part of the Arctic that exhibits the largest sea–ice variability in spring of years with anomalously low September sea–ice concentrations (SICs). Positive anomalies of clouds and water vapor in spring lead to enhanced energy flux to the surface, in the form of more downward longwave radiation (LWD), and an earlier melt onset (Mortin et al 2016; Lee et al 2002) These cloud and water–vapor anomalies are linked to anomalous moisture transport into the Arctic, indicating a remote origin (Kapsch et al 2013; Mortin et al 2016). We seek to answer two major questions: (1) are there specific spring atmospheric circulation regimes that favor atmospheric moisture or heat transport into the Arctic in years with a low September SIC? and (2) are there systematic differences in the character, e.g., duration and strength, of spring atmospheric transport events in years with a low September SIC versus years with a high SIC? In order to approach these questions, we present a characterization of individual events, attempting to provide a step forward in our understanding of important processes governing the large variability of the ice cover
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