PNNL

Abstract

The primary sources of recent summer Arctic moistening trends in reanalysis are uncertain. Such uncertainty hinders attribution of observed Arctic warming due to in-season radiative effects from moistening on snow and ice losses. Here we use a combined online numerical water tracer and circulation nudging approach in the Community Earth System Model to track the sources of the water vapor feedback beyond their initial sources. Trends in Northern Hemisphere summertime large-scale circulation have driven moistening of the Arctic over recent decades, having a large impact on the Artic radiative budget, accounting for 94% of the increasing water vapor radiative feedback. We identify two key regions supplying the Arctic water vapor feedback: Northeast North America and western/central Eurasia. In both regions, anticyclonic circulations over the southwest Atlantic and eastern Europe move moisture sourced from the tropical oceans poleward to high latitude land through precipitation in winter and spring. During summer, the land surface releases this moisture through evapotranspiration, and this water vapor is transported by winds into the Arctic. We refer to this sequence of terrestrial moisture storage and release as the land capacitor effect. Thus, the impacts of large-scale circulation changes on poleward moisture transport and diverse land-atmosphere interactions over high latitudes represent the underlying mechanisms of the recent moistening and warming in the Arctic.