Recent Upper Arctic Ocean Warming Expedited by a Summertime Internal Atmospheric Process

The upper ocean (0-50m) temperature in the Arctic has risen by about half a degree since 1979 with an escalation in the past two decades. This warming has been primarily attributed to increased poleward oceanic heat transport and enhanced atmospheric heat entering the Arctic Ocean as a result of anthropogenically driven sea ice loss. Here, using both observational and modeling analyses, we demonstrate that a multiyear trend in the summertime large-scale atmospheric circulation has led to increased barotropic high pressure over this region. This process, which we ascribe to internal variability rather than anthropogenic forcing, has played an important role in upper ocean warming in summer and fall over the past four decades, especially after 2000. This warming is a result of dynamically driven subsidence in the atmosphere, which warms air and triggers a sea ice-albedo feedback by reducing shortwave reflection. Nudging experiments in which the wind fields are constrained toward the observed state support this mechanism and suggest that internal variability contribution to recent Arctic Ocean warming accounts for about 24% of observed warming over the past four decades and 63% of the accelerated warming from 2000 to 2018. Thus, climate models need to replicate this important internal process in order to realistically simulate Arctic Ocean temperature variability and trends.