Identification of the surface state influence in representing the Arctic warming by coordinated atmosphere-only simulations (D3.1)

Summary:

Arctic amplification metrics: The surface air temperature (SAT) anomalies, trends and variability have been used to quantify the Arctic amplification (AA). The use of different metrics, as well as the choice of dataset can affect conclusions about the magnitude and temporal variability of AA. We reviewed the established metrics of AA to see how well they agree upon the temporal signature of AA and assess the consistency in these metrics across commonly-used datasets which cover both the early and late 20th century warming in the Arctic. We find the NOAA 20th Century Reanalysis most closely matches the observations when using metrics based upon the trends, variability and the amplification of SAT anomalies in the Arctic, and the ERA 20th Century Reanalysis is closest to the observations in the SAT anomalies and variability of SAT anomalies.  The largest differences between the century-long reanalysis products and observations are during the early warming period. In the modern warming period, the high density of observations strongly constrains all the reanalysis products, whether they include satellite observations or only surface observations. Thus, all the reanalysis and observation products produce very similar magnitudes and temporal variability in the degree of AA during the recent warming period (Davy et al., 2018).

Sea ice free Arctic contributes to the projected warming minimum in North Atlantic: Projected global warming is not spatially uniform and one of the minima in warming occurs in the North Atlantic (NA). Several models from the Coupled Model Intercomparison Project Phase 5 even projected a slight NA cooling in 2081–2100 relative to 1986–2005. Here we show that, by coupled model simulations, an autumn (September to November) sea-ice free Arctic contributes to the NA warming minimum by weakening the Atlantic meridional overturning circulation (Suo et al., 2017). 

Design and finalize the set-up for the atmosphere-only model coordinated experiments: Four multi-model atmospheric coordinated experiments have been designed and finalized in order to reveal the climate impacts in the Northern Hemisphere of the warming Arctic. The atmospheric experiments will use daily SST and sea-ice concentration from 1979 to 2014 from the CMIP6 HighResMIP protocol. The first experiment uses daily varying SST and sea ice concentration. A second experiment will use varying SST while keeping climatological sea ice concentration. The difference between the two simulations will allow investigating the impacts of the warming Arctic. This protocol is built from HighResMIP and will allow investigating the influence of using high horizontal resolution. Two supplementary experiments will use daily varying SST and sea ice after removal of the multidecadal Pacific or Atlantic variability. These two experiments have never been performed before and will also allow distinguishing the specific impacts of the oceanic multi-decadal variability.

Completed the coordinated experiments by the nine of ten atmosphere-only models: All the four coordinated experiments have been completed and model outputs were made available to all Blue-Action partners.

Warm Pacific can intensify the Arctic warming: Multi-model simulations from Blue-Action showed that the transition from warm Pacific to cold Pacific (Pacific Decadal Oscillation from positive phase to negative phase) during 1979-2013 damped the Arctic warming. The Arctic warming likely will be intensified in the forthcoming decades given the PDO switched to positive phase since 2014.