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This article evaluated the performance of 19 Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating the Arctic sea ice from the perspective of annual cycle, spatial pattern and temporal variation. Based on the evaluation, five models with better comprehensive performance capacity were optimized as the ensemble to project the response of the East Asian winter climate to the sea ice‐free Arctic occurring under the SSP2‐4.5 and SSP5‐8.5 scenarios, respectively. The ensemble projections indicate that the sea ice‐free Arctic is followed by a weakening of the East Asian winter monsoon, which is characterized with the shallower East Asian trough and weaker East Asian jet stream. Concurrently, the winter surface net radiation flux is projected to increase in the East Asian‐western North Pacific region. These changes favour large‐scale warming in the East Asian‐western North Pacific region. Moreover, the warming is more pronounced under the sea ice‐free Arctic of SSP2‐4.5 than under that of SSP5‐8.5. The winter precipitation tends to increase along the East Asian coast from South China to the Sea of Okhotsk. Such an increase is closely associated with the enhancement of low‐level moisture. Due to larger enhancement of moisture, there appears greater increase of precipitation in the monsoon region from South China to Japan under the sea ice‐free Arctic of SSP5‐8.5 compare to that under SSP2‐4.5. Substantial changes are also projected for the temperature and precipitation extremes, such as a general increase in warm days and warm nights and an overall intensification of extreme precipitation in the East Asian‐western North Pacific region. [ABSTRACT FROM AUTHOR]

Based on the reanalysis data for the period 1980–2019, this study reveals that the linkage of autumn sea ice concentration in the Barents-Kara Seas (BKSIC) to the subsequent winter North Atlantic Oscillation (NAO) underwent an interdecadal weakening in the early 1990s. Such a weakening is attributed to the discrepancy in the tropospheric warming resulting from the decrease in the interannual variability of the BKSIC from the former period to the latter period. During the former period (1980–1993), the decrease in autumn BKSIC can induce strong warming in the troposphere through enhancing the in-situ diabatic heating, significantly weakening the circumpolar westerly. The weakening of autumn circumpolar westerly then promotes the upward propagation of the Eliassen-Palm (EP) flux from the troposphere to the stratosphere. Till winter, the EP flux propagates downward from the stratosphere to the troposphere. It leads to significant positive stream function anomalies over the Greenland-Barents Seas region, accompanied with negative stream function anomalies over Western Europe. They are linked by a strong southeastward-propagating wave activity flux and result in a negative NAO phase with its southern center located eastward. During the latter period (1994–2019), due to smaller interannual variability of the BKSIC, its resultant warming of the tropospheric temperature is much weaker, not conducive to the aforementioned stratosphere-troposphere interaction and thus diluting the relationship between autumn BKSIC and the subsequent winter NAO. • Linkage of autumn BKSIC to the subsequent winter NAO underwent an interdecadal weakening in the early 1990s • The weakening is attributed to discrepancy in the tropospheric warming caused by decreased BKSIC interannual variability • The stratosphere-troposphere interactions related to the tropospheric warming during the two periods are different [ABSTRACT FROM AUTHOR]

• PM 2.5 is decreasing and BC is increasing yearly in Nanjing. • The relationships between mass concentrations of BC and wind are discussed. • The contributions of BC to PM 2.5 are analysed. In this study, the black carbon (BC) measurements in the atmosphere of Nanjing, China were continuously conducted from 2015 to 2018 using a Model AE-33 aethalometer. By combining dataset of PM 2.5 , PM 10 , CO, NO 2 , SO 2 , O 3 and meteorological parameters, the temporal variations and the source apportionment of BC were given in this study. The results showed that the PM 2.5 mass concentrations decreased in Nanjing, with an average annual rate of variation of 6.50 μg/(m3⋅year). Differently, the annual average concentrations of BC increased with an average annual variation rate of 214.71 ng/(m3⋅year). The seasonal variations showed the pattern of BC mass concentrations in winter > autumn > spring > summer. The diurnal variations of BC mass concentrations showed a double-peak in all four seasons. The first peak occurred at approximately 7:00 in spring, summer and autumn and around 8:00 in winter. The second peak took place after 18:00. The average AAE (absorption Ångström exponent) was 1.26 with a maximum of 1.35 during wintertime and the lowest (1.12) during summertime. In addition, the AAE was smaller in the daytime than that at night, with a minimum occurring between 13:00 and 14:00. BC and visibility show a good power-function relationship at different humidity levels. The average values of the visibility thresholds of the BC mass concentrations in spring, summer, autumn and winter were 1.326, 5.522, 1.340 and 0.708 μg/m3, respectively. The greater the relative humidity, the smaller the visibility threshold for the BC mass concentrations was. Image, graphical abstract [ABSTRACT FROM AUTHOR]