Rain-on-snow (ROS) events can accelerate the surface ablation of sea ice, thus greatly influencing the ice–albedo feedback. However, the variability of ROS events over the Arctic Ocean is poorly understood due to limited historical station data in this region. In this study early melt season ROS events were investigated based on four widely used reanalysis products (ERA-Interim, JRA-55, MERRA, and ERA5) in conjunction with available observations at Arctic coastal stations. The performance of the reanalysis products in representing the timing of ROS events and the phase change of precipitation was assessed. Our results show that ERA-Interim better represents the onset date of ROS events in spring, and ERA5 better represents the phase change of precipitation associated with ROS events. All reanalyses indicate that ROS event timing has shifted to earlier dates in recent decades (with maximum trends up to −4 to −6 d per decade in some regions in ERA-Interim) and that sea ice melt onset in the Pacific sector and most of the Eurasian marginal seas is correlated with this shift. There has been a clear transition from solid to liquid precipitation, leading to more ROS events in spring, although large discrepancies were found between different reanalysis products. In ERA5, the shift from solid to liquid precipitation phase during the early melt season has directly contributed to a reduction in spring snow depth on sea ice by more than −0.5 cm per decade averaged over the Arctic Ocean since 1980, with the largest contribution (about −2.0 cm per decade) in the Kara–Barents seas and Canadian Arctic Archipelago.

The changes in the Arctic precipitation profoundly impact the surface mass balance of ice sheet and sea ice, the extent of snow cover, as well as the land/ice surface runoff in the Arctic, particularly when it occurs in liquid form. Here, we use state‐of‐the‐art models from the Coupled Model Intercomparison Project Phase 5 to project the number of days with rainfall, the intensities and onset dates of rainfall events in the Arctic under the strong emission scenario (RCP8.5). The multi‐model mean shows that rainfall will occur more frequently in the Arctic at the end of this century (2091–2100), with larger increase in the rainy days over the Pacific and Atlantic sectors (up to 12 days/month) during the cold seasons (October–May) and over the Arctic Ocean (up to 14 days/month) during the warm seasons (June–September) as compared with the present day (2006–2015). Greater uncertainty is found in the cold seasons, which mainly comes from the high variability among different models in the Norwegian Sea. Sixty‐seven to ninety‐three percentage of the increases in rainy days is contributed by the local warming and the remainder by the increase in total precipitation. Moreover, at the end of this century, the rainfall in spring will occur much earlier than the present day by more than 1 month, and the extent of rainfall will further expand toward the center of the Arctic Ocean and the inland Greenland in the future. The changes of rainfall intensity on the Arctic land area to the climate warming are more sensitive than that on the Arctic Ocean in warm seasons (May–September). The rainfall will be further strengthened in most of the Arctic continents in summer, with the largest increase in the intensity of ∼2 mm/day along the southwest coast of Greenland. The above results are confirmed by the latest projections from CMIP6 models.

Snow plays an important role in the Arctic climate system, modulating heat transfer in terrestrial and marine environments and controlling feedbacks. Changes in snow depth over Arctic sea ice, particularly in spring, have a strong impact on the surface energy budget, influencing ocean heat loss, ice growth and surface ponding. Snow conditions are sensitive to the phase (solid or liquid) of deposited precipitation. However, variability and potential trends of rain-on-snow events over Arctic sea ice and their role in sea-ice losses are poorly understood. Time series of surface observations at Utqiaġvik, Alaska, reveal rapid reduction in snow depth linked to late-spring rain-on-snow events. Liquid precipitation is key in preconditioning and triggering snow ablation through reduction in surface albedo as well as latent heat release determined by rainfall amount, supported by field observations beginning in 2000 and model results. Rainfall was found to accelerate warming and ripening of the snowpack, with even small amounts (such as 0.3 mm recorded on 24 May 2017) triggering the transition from the warming phase into the ripening phase. Subsequently, direct heat input drives snowmelt, with water content of the snowpack increasing until meltwater output occurs, with an associated rapid decrease in snow depth. Rainfall during the ripening phase can further raise water content in the snow layer, prompting onset of the meltwater output phase in the snowpack. First spring rainfall in Utqiaġvik has been observed to shift to earlier dates since the 1970s, in particular after the mid-1990s. Early melt season rainfall and its fraction of total annual precipitation also exhibit an increasing trend. These changes of precipitation over sea ice may have profound impacts on ice melt through feedbacks involving earlier onset of surface melt.

Shipping volumes in the Northwest Passage are likely to increase under climate change due to the distance advantage over traditional routes and the special strategic location of the Arctic. However, the harsh environment and poor channel conditions may pose a considerable risk to ship navigation. To ensure the safety of ships, understand the navigability of the route, and plan the sustainable use of the Northwest Passage, it is crucial to provide a quantitative risk assessment. Here, we present an analysis of several natural risks faced by ships in the Northwest Passage based on available datasets and use climate model simulations to project the navigability changes. The results showed that: (1) The sea-ice risk to ships in the Northwest Passage has been significantly reduced over the period 1979–2019, and the risk for Polar Class 6 (PC6) ships has decreased more rapidly than for general open-water (OW) ships. The difference in ice-breaking capacity further affects the seaworthy season, with the second seaworthy month being August for OW ships and October for PC6 ships, in addition to the commonly best September. (2) Low visibility is a more common form of adverse weather than strong wind for navigation in the Northwest Passage, mainly on the northern route, although pilotage conditions appear to be improving in September. (3) According to the comprehensive risk map, the distribution of risk is dominated by sea ice. The southern route of the Northwest Passage is superior to the northern route in terms of both sea ice and weather conditions, but there is a risk of shallow water. (4) For the northern route, which has greater transport potential, projections suggest that the sea-ice risk will be steadily lower than any extreme light ice year observed historically whether for the ship class OW or PC6 by 2050, with an increase of 50–80 navigable days, and the navigable period could be from June to January of the following year for PC6 ships by 2100. Our results provide valuable information for ships planning to pass through the Northwest Passage.

The sensitivity of the Arctic precipitation phases (solid and liquid) to the forcings from greenhouse gases (GHGs) and aerosols over 2016–2080 was investigated by using the Community Earth System Model Version 1. Results show that the warming caused by the two forcings results in an increasing trend in total precipitation and a solid‐to‐liquid precipitation transition in the Arctic. Under RCP8.5 scenario, the increased rate of Arctic mean precipitation with global warming forced by aerosol reduction (7.7%/°C) is twice greater than that by increased GHG emission (3.5%/°C). The sensitivity of rainfall to precipitation ratio (RPR) to various forcings is much higher than that of total precipitation in the Arctic. The increased rate of RPR due to global aerosol forcing (8.4%/°C) is approximately 3 times that due to GHG forcing (2.9%/°C) in the Arctic, the differences even larger over Greenland and the eastern Arctic Ocean, resulting in more rainfall in these areas.