Journal Article

The Arctic Ocean in CMIP6 Models: Biases and Projected Changes in Temperature and Salinity

Koldunov, Nikolay, Hinrichs, Claudia, Danilov, Sergey, Khosravi, Narges, Semmler, Tido, Wang, Qiang, Jung, Thomas

2022

Overview

We examine the historical evolution and projected changes in the hydrography of the deep basin of the Arctic Ocean in 23 climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The comparison between historical simulations and observational climatology shows that the simulated Atlantic Water (AW) layer is too deep and thick in the majority of models, including the multi‐model mean (MMM). Moreover, the halocline is too fresh in the MMM. Overall our findings indicate that there is no obvious improvement in the representation of the Arctic hydrography in CMIP6 compared to CMIP5. The climate change projections reveal that the sub‐Arctic seas are outstanding warming hotspots, causing a strong warming trend in the Arctic AW layer. The MMM temperature increase averaged over the upper 700 m at the end of the 21st century is about 40% and 60% higher in the Arctic Ocean than the global mean in the SSP245 and SSP585 scenarios, respectively. Salinity in the upper few hundred meters is projected to decrease in the Arctic deep basin in the MMM. However, the spread in projected salinity changes is large and the tendency toward stronger halocline in the MMM is not simulated by all the models. The identified biases and projection uncertainties call for a concerted effort for major improvements of coupled climate models. Plain Language Summary Coupled climate models are crucial tools for understanding and projecting climate change, especially for the Arctic where the climate is changing at unprecedented rates. A cold fresh layer of water (aka halocline) has been protecting sea‐ice at the surface from the warm layer of water (aka Atlantic Water layer) which flows underneath and could potentially accelerate sea ice melting from below. Climate change disturbs this vertical structure by changing the temperature and salinity of the Arctic Ocean (in a process known as Atlantification and Pacification) which may lead to additional sea ice basal melting and accelerate sea ice decline. We examined the simulated temperature and salinity in the Arctic Ocean deep basin in state‐of‐the‐art climate model simulations which provided the basis for the IPCC Assessment Report. We found that although there are persistent inaccuracies in the representation of Arctic temperature and salinity, the Arctic Ocean below 100 m is subject to much stronger warming than the average global ocean. On the other hand, the upper Arctic Ocean salinity is projected to decrease, which on average may strengthen the isolation of sea ice from Atlantic Water heat in the Arctic deep basin area. Key Points A too deep and thick Arctic Atlantic Water layer continues to be a major issue in contemporary climate models contributing to the CMIP6 The Arctic Ocean below the halocline is subject to much stronger warming than the global mean during the 21st century The multi‐model mean upper ocean salinity is projected to decrease in the future but with high uncertainty

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Arctic amplification / Arctic climates / Arctic hydrography / Atlantic water layer / Atlantification / Atmosphere / Bias / Climate change / Climate models / Climatology / CMIP6 / Experiments / Heat / Hydrography / Modelling / Ocean circulation / Ocean models / Oceans / Polar environments / Salinity / Salinity effects / Simulation / Temperature / Temperature rise