FY 2022

Arctic Ocean freshwater in CMIP6 coupled models

Wang, S., Q. Wang, M. Wang, G. Lohmann, and F. Qiao

Earth's Future, 10, e2022EF002878, doi: 10.1029/2022EF002878, View online, open access at AGU (external link) (2022)

In this study we assessed the representation of the sea surface salinity (SSS) and liquid freshwater content (LFWC) of the Arctic Ocean in the historical simulation of 31 CMIP6 models with comparison to 39 Coupled Model Intercomparison Project phase 5 (CMIP5) models, and investigated the projected changes in Arctic liquid and solid freshwater content and freshwater budget in scenarios with two different shared socioeconomic pathways (SSP2-4.5 and SSP5-8.5). No significant improvement was found in the SSS and LFWC simulation from CMIP5 to CMIP6, given the large model spreads in both CMIP phases. The overestimation of LFWC continues to be a common bias in CMIP6. In the historical simulation, the multi-model mean river runoff, net precipitation, Bering Strait and Barents Sea Opening (BSO) freshwater transports are 2,928 ± 1,068, 1,839 ± 3,424, 2,538 ± 1,009, and −636 ± 553 km³/year, respectively. In the last decade of the 21st century, CMIP6 MMM projects these budget terms to rise to 4,346 ± 1,484 km3/year (3,678 ± 1,255 km3/year), 3,866 ± 2,935 km³/year (3,145 ± 2,651 km3/year), 2,631 ± 1,119 km³/year (2,649 ± 1,141 km3/year) and 1,033 ± 1,496 km³/year (449 ± 1,222 km³/year) under SSP5-8.5 (SSP2-4.5). Arctic sea ice is expected to continue declining in the future, and sea ice meltwater flux is likely to decrease to about zero in the mid-21st century under both SSP2-4.5 and SSP5-8.5 scenarios. Liquid freshwater exiting Fram and Davis straits will be higher in the future, and the Fram Strait export will remain larger. The Arctic Ocean is projected to hold a total of 160,300 ± 62,330 km³ (141,590 ± 50,310 km³) liquid freshwater under SSP5-8.5 (SSP2-4.5) by 2100, about 60% (40%) more than its historical climatology.

Plain Language Summary. The Arctic Ocean is freshening, rendering strong implications on changes in the Arctic physical and biogeochemical environment. Our knowledge about possible future Arctic changes relies on results from Coupled Model Intercomparison Project (CMIP) models. In this study, we conduct a comprehensive analysis on the Arctic sea surface salinity (SSS), liquid freshwater content (LFWC) and freshwater budget by comparing the new CMIP6 to the previous Coupled Model Intercomparison Project phase 5 (CMIP5) results. An improvement can be found in the SSS simulation in CMIP6, but the large inter-model spread in both CMIP phases makes the improvement insignificant. In CMIP6, the Arctic LFWC remains to be overestimated in the historical simulation. A strong freshening trend is projected in the Arctic Ocean, with the freshwater sources from river runoff and net precipitation persistently increasing in a warming climate. The inflow through the BSO is likely to change from an Arctic freshwater sink to a source around 2050. At the end of this century, the total liquid freshwater storage is expected to rise by 60% under the SSP5-8.5 scenario. In contrast to the increasing LFWC, the solid freshwater content in the form of sea ice is projected to continue declining, with projected summer sea ice vanishing around 2050.