Abstract
The speeds of both Arctic surface warming and sea-ice shrinking have accelerated over recent decades. However, the causes of this unprecedented phenomenon remain unclear and are subjects of considerable debate. In this study, we report strong observational evidence, for the first time from long-term (1984–2014) spatially complete satellite records, that increased cloudiness and atmospheric water vapor in winter and spring have caused an extraordinary downward longwave radiative flux to the ice surface, which may then amplify the Arctic wintertime ice-surface warming. In addition, we also provide observed evidence that it is quite likely the enhancement of the wintertime greenhouse effect caused by water vapor and cloudiness has advanced the time of onset of ice melting in mid-May through inhibiting sea-ice refreezing in the winter and accelerating the pre-melting process in the spring, and in turn triggered the positive sea-ice albedo feedback process and accelerated the sea ice melting in the summer.
Highlights
Despite an apparent hiatus in global warming[1,2,3], the Arctic climate continues to experience unprecedented changes
To investigate the relationships among atmospheric longwave radiative forcing, wintertime surface warming and the late spring initial sea-ice melting in the Arctic, we calculated long-term anomalies in spatial and wintertime temporally averaged surface downward longwave radiative flux (LWD), water vapor (WV), cloud area fraction (CFC), skin temperature (SKT), and sea ice concentration (SIC) for the melting onset period[37] between May 16 and June 5
In low-onset ice years (LOIYs), the above-average greenhouse effect from positive cloudiness and water vapor anomalies in the Arctic wintertime season (December to May) can together generate approximately 3.04 W m−2 (0.81 W m−2 from cloudiness and 2.23 W m−2 from water vapor) extra downward longwave radiative forcing, which is almost equal to the surface longwave radiation (LWD) anomaly of 3.22 W m−2. These findings indicate that the greenhouse effect caused by water vapor and cloudiness plays a dominant role in the inter-annual variation of Arctic wintertime LWD, and skin temperature and initial sea-ice melting
Summary
To investigate the relationships among atmospheric longwave radiative forcing, wintertime surface warming and the late spring initial sea-ice melting in the Arctic, we calculated long-term anomalies in spatial (maximum sea-ice coverage north of 60°N) and wintertime (including both winter and spring) temporally averaged surface downward longwave radiative flux (LWD), water vapor (WV), cloud area fraction (CFC), skin temperature (SKT), and sea ice concentration (SIC) for the melting onset period[37] between May 16 and June 5 (days of year 136 to 156). To quantify the contributions of greenhouse effects associated with water vapor and cloudiness to the long-term trend of LWD, and in turn Arctic wintertime warming and initial sea-ice melting, we have calculated the statistical linear trends of all variables for both the winter and spring seasons and estimated the surface downward longwave radiative forcing caused by changes in water vapor and cloudiness. Since only the thin ice over coastal regions begin to melt in late spring[40], and the sea-ice state is influenced by surface wind[10], ocean currents[9], ice albedo feedback[35] and other factors[18], there are still some differences in the spatial patterns of both long-term trend and de-trended anomalies between onset SIC and other four variables
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