Cold and Fresh Biases of the Arctic Atlantic Water Layer in CMIP6 Models; Potential Origin and Implications

We examined the Arctic Atlantic Water (AW) layer in the CMIP6 models. Climatological means of temperature and salinity at 400 m depth from multi-model averages are compared with observations, showing significant biases in both variables. Based on the currently available data, we showed that the CMIP6 models have cold and fresh biases in the Arctic AW layer, and warm and saline biases in the East Greenland Current. The temperature biases are comparable to the climate signal magnitude for temperature, predicted by the CMIP6 models for the end of the 21st century. For salinity, the biases are shown to be even more pronounced than the predicted signals. CMIP6 models also show positive sea-level pressure (SLP) and sea-surface height (SSH) biases in the Nordic Seas. We argue that the identified SLP bias leads to an anomalously weak cyclonic gyre circulation in the Nordic seas, as shown through positive SSH bias. This could cause weaker AW inflow through the Fram Strait, which explains the detected hydrography biases in the AW layer. While we do not rule out other possible factors contributing to the weak AW flow to the Arctic Ocean, we suggest that the identified ocean biases within the CMIP6 models are at least partially driven by atmospheric origins.