The source of Arctic precipitation in HadGEM3-GC5

Snow and rain are increasing in the Arctic. Climate model projections indicate a 50% increase in precipitation by the end of the century. This increase is driven by increased local evaporation following reduced sea ice coverage alongside enhanced global evaporation and transport into the Arctic. The relative contributions of these sources of Arctic precipitation are directly assessed using water tracers in the HadGEM3 climate model. An idealized warming simulation shows that, whilst remote sources dominate over local sources of Arctic precipitation in all seasons, the proportion of locally ocean-sourced precipitation increases from 15% at the start of the run to 38% at 4 × CO2. Once the Arctic sea ice has been lost, the fraction of precipitation from local ocean sources stops increasing. For each season this state tends to correspond to the time when Arctic precipitation also switches from snow to rain-dominated. The increase in the fraction of precipitation sourced from the Arctic Ocean (locally sourced) occurs in all seasons, except summer. In summer, the precipitation over the Arctic Ocean is instead mostly sourced from land evapotranspiration. Our water tracer method also reveals that a significant amount of Arctic evaporation is exported by the atmosphere out of the Arctic. This highlights that both the import and export of water vapor must be considered when investigating Arctic hydrological changes. Individual precipitation events tend to have multiple evaporative sources, indicating some mixing of vapor. Remotely sourced precipitation tends to dominate the Arctic margins while local Arctic sources dominate the interior.