This paper provides an overview of the United States (U.S.) Department of Energy’s (DOE’s) Energy Exascale Earth System Model version 2.1 with an Arctic regionally refined mesh (RRM), hereafter referred to as E3SMv2.1-Arctic, for the atmosphere (25 km), land (25 km), and ocean/ice (10 km) components. We evaluate the atmospheric component and its interactions with land, ocean, and cryosphere by comparing the RRM (E3SM2.1-Arctic) historical simulations (1950-2014) with the uniform low-resolution (LR) counterpart, reanalysis products, and observational datasets. The RRM generally reduces biases in the LR model, improving simulations of Arctic large-scale mean fields, such as precipitation, atmospheric circulation, clouds, atmospheric river frequency, and sea ice dynamics. However, the RRM introduces a seasonally dependent surface air temperature bias, reducing the LR cold bias in summer but enhancing the LR warm bias in winter. The RRM also underestimates winter sea ice area and volume, consistent with its strong winter warm bias. Radiative feedback analysis shows similar climate feedback strengths in both RRM and LR, with the RRM exhibiting a more positive surface albedo feedback and contributing to a stronger surface warming than LR. These findings underscore the importance of high-resolution modeling for advancing our understanding of Arctic climate changes and their broader global impacts, although some persistent biases appear to be independent of model resolution at 10-100 km scales.

In July 2011, observations of a massive phytoplankton bloom in the ice-covered waters of the western Chukchi Sea raised questions about the extent and frequency of under sea- ice blooms and their contribution to the carbon budget in the Arctic Ocean. To address some of these questions, we use the fully-coupled, high-resolution Regional Arctic Sys- tem Model to simulate Arctic marine biogeochemistry over a thirty-year period. Our re- sults demonstrate the presence of massive under sea-ice blooms in the western Arctic not only in summer of 2011 but annually throughout the simulation period. In addition, sim- ilar blooms, yet of lower magnitude occur annually in the eastern Arctic. We investigate the constraints of nitrate concentration and photosynthetically available radiation (PAR) on the initiation, evolution and cessation of under sea-ice blooms. Our results show that increasing PAR reaching the ocean surface through the sea-ice in early summer, when the majority of ice-covered Arctic waters have sufficient surface nitrate levels, is criti- cal to bloom initiation. However, the duration and cessation of under sea-ice blooms is controlled by available nutrient concentrations as well as by the presence of sea-ice. Since modeled critical PAR level are consistently exceeded in summer only in the western Arc- tic, we therefore conclude that the eastern Arctic blooms shown in our simulations did not develop under sea ice, but were instead, at least in part, formed in open waters up- stream and subsequently advected by ocean currents beneath the sea ice.