In recent decades, Arctic sea ice coverage experienced a drastic decline in winter, when sea ice is expected to recover following the melting season. Using observations and climate model simulations, we found a robust frequency increase in atmospheric rivers (ARs, intense corridors of moisture transport) over Barents-Kara Seas and the neighboring central Arctic (ABK) in early winter. The extensive moisture carried by more frequent ARs has intensified surface downward longwave radiation and liquid rainfall, caused stronger melting of thin, fragile ice cover, and slowed the seasonal recovery of sea ice, contributing to the sea ice cover decline in ABK. A series of model ensemble experiments suggests that, in addition to a uniform AR increase in response to anthropogenic forcing, the contribution of tropical Pacific variability is indispensable in the observed Arctic AR changes. These findings have significant implications for understanding the rapidly changing Arctic hydroclimate and the cryosphere.

Anticipated future reductions in aerosol emissions are expected to accelerate warming and substantially change precipitation characteristics. It is therefore vital to identify existing patterns and possible future pathways of anthropogenic aerosol reductions. The COVID-19 pandemic prompted abrupt, global declines in transportation and industrial activities, providing opportunities to study the aerosol effects of pandemic-driven emissions changes. Here, measures of aerosol optical depth (AOD) from two satellite instruments are used to characterize aerosol burdens in 2020 in four Northern Hemisphere source regions (East & Central China, the United States, India, and Europe). In most regions, spring and summer AOD was substantially lower than in previous years. However, in India and East & Central China, the COVID-19 AOD signature was eclipsed by sources of natural variability (dust) and a multi-year trend, respectively, suggesting that COVID-19-related emissions reductions account for substantially less of the 2020 anomalies in these regions than might otherwise be assumed.

Ozone in the troposphere is a pollutant and greenhouse gas, and it is crucial to better understand its transport from the ozone-rich stratosphere. Tropopause folding, wherein stratospheric air intrudes downward into the troposphere, enables stratosphere-to-troposphere ozone transport (STT). However, systematic analysis of the relationship between folding and tropospheric ozone, using data that can both capture folding’s spatial scales and accurately represent tropospheric chemistry, is lacking. Here, we compare folding in both high-resolution (0.25°) reanalysis ERA5 and low-resolution (0.75°) chemical reanalysis CAMSRA over one year. High-resolution folding is dramatically more frequent and significantly better-correlated with tropospheric ozone. In particular, folding of deep tropospheric extent is nearly 100% missing at low resolution, and folding–ozone correlations increase most with resolution along midlatitude storm tracks, where deep folding is most common. Our results imply that STT is more attributable to tropopause folding than implied by low-resolution analysis, likely associated with resolving filamentary, deep folding.

The contribution of individual aerosol species and greenhouse gases to precipitation changes during the South Asian summer monsoon is uncertain. Mechanisms driving responses to anthropogenic forcings needs further characterization. We use an atmosphere-only climate model to simulate the fast response of the summer monsoon to different anthropogenic aerosol types and to anthropogenic greenhouse gases. Without normalization, sulfate is the largest driver of precipitation change between 1850 and 2000, followed by black carbon and greenhouse gases. Normalized by radiative forcing, the most effective driver is black carbon. The precipitation and moisture budget responses to combinations of aerosol species perturbed together scale as a linear superposition of their individual responses. We use both a circulation-based and moisture budget-based argument to identify mechanisms of aerosol and greenhouse gas induced changes to precipitation, and find that in all cases the dynamic contribution is the dominant driver to precipitation change in the monsoon region.