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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.

Summary: This Blue-Action task was focused on improving the representation of some of the most important physical processes which contribute to Arctic warming within the climate models used by the consortium. The two processes we addressed were the effect on the atmospheric state of the fracturing of the sea ice cover and turbulence under strongly stable thermal stratification. The creation and development of The work done: We first analysed the results of previously performed large eddy simulations which resolved the turbulence over leads to determine the effect leads have on sensible heat flux from open water. Because of the effect of three-dimensional structures in the turbulent mixing above leads, the heat flux coming from leads can be amplified compared to the fluxes one would get from open water under the same air-sea temperature difference. The amplification effect strongly depends on the width of the lead, with the largest effect occurring for leads of widths around 1.4 km. We assessed the functional sensitivity of this amplification effect to key parameters used in the turbulence-resolving model, including the length scale for the convective boundary layer, which reflects the background stability in the atmosphere. We combined this relation between the amplification effect of heat fluxes as a function of lead width with observed distributions of lead widths. These were taken from the peer-reviewed literature where The key findings: The presence of leads in sea ice dramatically alters the surface energy balance in the Arctic. There is a large seasonal cycle to the effect of the presence of leads, because the flux from the leads depends strongly on the background stability in the atmosphere. In the winter when the atmosphere is often strongly stably stratified, the leads strongly amplify the surface sensible heat flux coming from open water. In the summer there is the opposite effect and the generally weaker atmospheric stability reduces the flux coming from leads.

A brief introduction to BlueAction at theINTAROS and Blue-Action workshop in Beijing, Apr. 18th, 2018

Presentation held at the HiMAC 2017 in Bejing, 3-4 March 2017 About HiMAC2017 The conference will focus on the observations and understanding of the environmental changes in the "High Mountains and northern Cold Regions (HMCR)”, Delegates will discuss the scientific understanding about various environmental changes in High Mountains and northern Cold Regions, connect these changes between High Mountainous and northern Cold Regions, and discuss to foster an observation network and set up the Task Force on High Mountain and Cold Regions under 'Digital Belt and Road (DBAR)' program. The conference will be held over two days. The International Workshop will be held on 3rd, March; the working conference of DBAR Task Force on High Mountain and Cold Regions will be held in Morning of 4th, March; in the afternoon, the delegates will discuss the Essential Cold Region Variables task to support the Group on Earth Observations Cold Region Initiative (GEOCRI). Conference venue: C608, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, New Technology Campus, Tangjialin, Haidian District, Beijing.

Atlantic multidecadal variability (AMV) is known to impact climate globally, and knowledge about the persistence of AMV is important for understanding past and future climate variability, as well as modeling and assessing climate impacts. The short observational data do not significantly resolve multidecadal variability, but recent paleoproxy reconstructions show multidecadal variability in North Atlantic temperature prior to the instrumental record. However, most of these reconstructions are land-based, not necessarily representing sea surface temperature. Proxy records are also subject to dating errors and microenvironmental effects. We extend the record of AMV 90 years past the instrumental record using principle component analysis of five marine-based proxy records to identify the leading mode of variability. The first principal component is consistent with the observed AMV, and multidecadal variability seems to persist prior to the instrumental record. Thus, we demonstrate that reconstructions of past Atlantic low-frequency variability can be improved by combining marine-based proxies. publishedVersion

A 60-year spin up with MICOM is described. Maximum meridional heat transport is 1.5 PW at 12 oN globally, and 0.75 PW at 18 oN in the Atlantic. It is found that detrainment, entrainment and convection control the seasonal cycle of the mixed-layer, whereas diapycnal mixing dominates in the deep, leading to ventilating of the bottom waters., NERSC Technical report no. 155. Funded by G. C. Rieber Climate Institute

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., The Blue-Action project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No 727852.

•Sea-Ice free Arctic contributes to the projected North Atlantic warming minimum by weakening the Atlantic Meridional Overturning Circulation (AMOC). •The weakened AMOC is driven and sustained by the weakened wind stress over the sub polar gyre associated with a weaker Icelandic low in autumn. •The study presented here emphasizes the role of ocean–atmosphere interaction on the sea-ice free impact, implies a lagged teleconnection through the ocean bridge., This work has been supported by the Research Council of Norway through the BlueArc project (207650/E10), the European Union 7th Framework Programme (FP7 2007-2013) through the NACLIM project (308299) and the National Natural Science Foundation of China (41375083). This work also contributes to the NordForsk-funded project GREENICE (61841): Impacts of Sea-Ice and Snow-Cover Changes on Climate, Green Growth, and Society and the EU H2020 Blue-Action project (727852). Computational and storage resources for this work have been provided by NOTUR (nn2343k) and NORSTORE (ns9015k).

Keynote talk in the International Workshop on Cryospheric Change and Sustainable Development. Lanzhou, China 1-2 August 1st, 2017

A short review on Arctic warming impacts

•Sea-Ice free Arctic contributes to the projected North Atlantic warming minimum by weakening the Atlantic Meridional Overturning Circulation (AMOC). •The weakened AMOC is driven and sustained by the weakened wind stress over the sub polar gyre associated with a weaker Icelandic low in autumn. •The study presented here emphasizes the role of ocean–atmosphere interaction on the sea-ice free impact, implies a lagged teleconnection through the ocean bridge.