Task 3.3: Impact of Arctic Ocean freshening on the Northern Hemisphere climate Didier Swingedouw, Christophe Herbaut, Juliette Mignot, Marie- Noelle Houssais

An impact of freshwater release in the North Atlantic? Rahmstorf et al. 2015 Rahmstorf et al. (2015): Greenland ice sheet (GrIS) melting as a key player for 20th AMOC weakening (if any…) Böning et al. (2016): impact of GrIS melting only clear since 2010s in the Labrador Sea convective activity What is the exact impact of GrIS melting over instrumental era? Böning et al. 2016

Problematic/background On the one hand, lots of hosing experiments show relatively weak impact (THOR, EMBRACE, AMOCMIP) and the present-day release are small (0.01 Sv) On the other hand, hints that it is now playing a role in the last decades (Boning et al. 2016, Robson et al. 2016) or even over the whole 20th century (Rahmstorf et al. 2015). Already consequent melting in the 1930s Former climate models not sensitive enough? (Swingedouw et al. 2013, Liu et al.. 2016). Better now? Close to tipping point for SPG? (cf. Sgubin et al. In press)

Objectives of Task 3.3 Establish the impact over last few decades of the freshwater release in the Arctic-North Atlantic sector on: the ocean stratification the ocean dynamics Estimate the sensitivity to the different fresh water sources: river runoff, Greenland ice sheet melting (not included in CMIP5 models), precipitations… Evaluate the transport pathway of the freshwater release Evaluate the impact of the coupling with atmosphere and ocean model resolution on the ocean response Attribute the recent ocean variability in the North Atlantic

What we propose to do (1/2) Attribution of AMOC changes: historical simulations and coupled reanalysis from AOGCM IPSL-CM6A-LR including GrIS melting (1900-2016) Use of 1/24°regional ocean-only model to estimate robustness of the AOGCM results Which GrIS melting scenario? I think we want to go as realistic as possible? Box & Colgan (2013), Bamber et al. (2013), Fettweis et al. (2016 agreement to provide monthly mean data of GrIS melting runoff), DMI? New CORE II forcing from Japan reanalysis Any other models to participate? (only CNRS for the moment in this action…) Link with WP4: From the coupled reanalysis including melting, launch hindcasts/forecasts also including the melting (task 4.2)

What we propose to do (2/2) Analyse the pathway of GrIS melting and Arctic runoff through release of tracers (passive, Lagrangian) Simulate impact of increased river runoff on the Arctic stratification Estimate possible impact of this change on the freshwater transport from the Arctic to the North Atlantic (e.g. Great Salinity Anomalies, that can strongly impact AMOC, Swingedouw et al. 2015)

AMOC response to Greenland melting AMOC at 26°N We propose that the leakage along the Canary current is key to explain the spread It is related with the representation of the gyres in the models AMOC response(Sv) AMOC changes vs gyres asymmetry Gyres asymetry (°lat) Swingedouw et al., Clim. Dyn., 2013b

Fate of freshwater release Böning et al. 2016 1/20° 1/4° Concentration of tracers after 30 years (1990-2019) ¼° The pathway of freshwater release is dependent on model resolution Böning et al. (2016): passive tracers in ¼° vs. 1/20° Gillard et al. (2016): Lagrangian tracers in ¼o and 1/12o regional NEMO Freshwater leakage pathway? Higher resolution? Gillard et al. 2016 Lagrangian tracers released from different location after 5 years Disko bay 79oN

Thank you! D3.4 (oceanic and climatic impact of freshwater release over the last few decades) due in month 36 The Blue-Action project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 727852

Great Salinity Anomalies Linkage with Arctic Ocean Impact on AMOC variations (e.g. Swingedouw et al. 2015) Changes in stratification can play a role on the linkage between Arctic and North Atlantic.

Bakker et al. GRL 2016 AOGCMs mean

Bakker et al. GRL 2016 AOGCMs mean

An impact of freshwater release in the North Atlantic? Rahmstorf et al. 2015 Rahmstorf et al. (2015): Greenland ice sheet (GrIS) melting as a key player for 20th AMOC weakening (if any…) Böning et al. (2016): impact of GrIS melting only clear since 2010s in the Labrador Sea convective activity What is the exact impact of GrIS melting over instrumental era? Böning et al. 2016

Greenland melting impact Box and Colgan 2013 Attribution of AMOC changes: historical simulations and coupled reanalysis from AOGCM IPSL-CM6A-LR including GrIS melting Use of 1/24°regional ocean-only model to estimate robustness of the AOGCM results Which GrIS melting scenario? Box & Colgan (2013), Bamber et al. (2013), Fettweis et al. (2016 agreement to provide monthly mean data of GrIS melting runoff), DMI? Bamber et al. 2013 Any other models to participate? (only CNRS for the moment in this action…) Link with WP4: melting in hindcasts/forecasts (task 4.2) Sasgen et al. 2012 Rignot et al. 2011

Increased River runoff in the Arctic impact in the Arctic Ocean: stronger stratification with fresher surface and warmer halocline and Atlantic Layer strengthening of the Arctic cyclonic circulation Temperature anomaly Salinity anomaly Sensitivity experiment to increased runoff (Numelin et al., 2016). Redistribution of the fresh water export between CAA export and fram strait export. (Pemberton and Nilsson, 2016). Still true in higher resolution model? (Storage of FW in the Beaufort gyre linked with large scale anticyclonic winds (Giles et al., 2012))

Variability of the freshwater content in the Arctic Ocean: Pathway of FW associated with river runoff linked with the AO (Morison et al., 2012) runoff Storage of FW in the Beaufort gyre linked with large scale anticyclonic winds (Giles et al., 2012)

Including freshwater release in forecasts Böning et al. 2016 Melting of Greenland is affecting Labrador Sea convection (Böning et al. 2016) H2020 Blue-Action European project Construction of a melting scenario based on regional MAR model and iceberg module Applying it to hindcasts and forecasts from new IPSL-CM6 model (hopefully including correct Labrador Sea convection) Greenland surface mass balance from MAR (Fettweis et al. 2013) Iceberg modelled distribution van den Berk & Drijfhout (2015)

WP3 presentation : Increase in freshwater release + potential impact on AMOC (Rahmtorf…), but not clear reconstruction, and disagreement with most reconstruction : decrease is largely due to recovery? Intro : Boning et al. ! (already done the work and deliverable !!!) Can the freshwater release impacted the AMOC earlier on? Understanding ! Pathways of freshwater release! Freshwater leakage + Gillard… etc. Not only the AMOC (Sgubin Detection-attribution of impact of FWF, i.e. ensemble of member included up to date estimate of GrIS melting. FWF release of Box & Colgran from 1900? + nudging in SST?... Compare HR (LOCEAN) and climate pred. System. Do we miss very key process concerning spread, mixing etc (cf. Codron...). Use of tracers (Lagragian, passive?) Already well done in Gillard et al. (2016) ! What more? Reproduce the results with HR. Any strong differences... Location of relase seem to play a lot! Then WP4: hidncasts + forecasts including it since can play a big role from the early XXI and possibility of abrupt changes + observed cold blob whose attribution is unclear (HF for sure, but only? No => Robson + This very abrupt!) Key ref: Rahmstorf, Jackson, Robson, Boning, Gillard, Lique et al. 2014+ Saenko 2016 HR… Estimate of FWF : Rignot et al. 2011, Yang et al. 2016, Box & Colgman 201?, future: Extrapolation Rignot, Fettweiss ? (+ask him for future), van den Berk & Drijfhout 2011 (with iceberg etc.), other ? => we need to take or construct the most representative FWF release… (see Gillard et al.!)