Causes of AMOC slowdown in highresolution

Transcription

1 Causes of AMOC slowdown in highresolution MPI-ESM Dian Putrasahan; Jin-Song von Storch; Johann Jungclaus; Katja Lohmann; Helmuth Haak; Daniela Matei; Oliver Gutjahr PRIMAVERA 2 nd GA Meeting Nov. 29 th, 2016

2 Challenge: Unstable / low AMOC in high resolution MPI-ESM HR (T127/TP04) XR (T255/TP04)

3 Challenge: Sea ice formation over Labrador Seas Shutdown of deep convection Sea ice fraction MLD

4 Possible reasons for AMOC slowdown: Zonal wind stress Dynamical effect from weakening of surface winds Increased freshwater flux (less evaporation) Increased heat flux (less latent heat loss from presence of sea ice) Heat flux Freshwater flux

5 Challenge: High computational cost for running eddypermitting/resolving GCMs Strategy: Use flux-adjusted HR (T127/TP04) runs to mimic XR (T255/TP04) runs HR_orig = meanhr + variabilityhr Flux adjustment = meanxr every coupling time step HR_XRadjust = HR_orig + flux adjustment = meanxr + variabilityhr Expt. name XR_orig (T255/TP04) HR_orig (T127/TP04) HR_XRalladjust HR_XRbuoyancy HR_XRwinds XR_1.5winds Flux adjusted to XR Control run at high resolution (pre-industrial) Control run at low resolution (pre-industrial) momentum, freshwater and heat fluxes only freshwater and heat fluxes only wind stress over water High resolution with 1.5*(wind stress over water)

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8 Effect on surface salinity XR vs HR buoyancy effect wind stress effect

9 Proposed AMOC weakening mechanism: Weaker winds, reduced wind stress curl over subpolar North Atlantic Weaker SPG Weaker AMOC Freshening /cooling of upper subpolar North Atlantic Surface freshwater input into subpolar North Atlantic Salinity-dominated density changes Sea ice formation Reduced heat loss Shutdown of deep convection in subpolar North Atlantic

10 Proposed AMOC weakening mechanism: Weaker winds, reduced wind stress curl over subpolar North Atlantic Weaker SPG Weaker AMOC Freshening /cooling of upper subpolar North Atlantic Surface freshwater input into subpolar North Atlantic Salinity-dominated density changes Sea ice formation Reduced heat loss Shutdown of deep convection in subpolar North Atlantic

11 Proposed AMOC weakening mechanism: Weaker winds, reduced wind stress curl over subpolar North Atlantic Weaker SPG Weaker AMOC Freshening /cooling of upper subpolar North Atlantic Surface freshwater input into subpolar North Atlantic Salinity-dominated density changes Sea ice formation Reduced heat loss Shutdown of deep convection in subpolar North Atlantic

12 Proposed AMOC weakening mechanism: Weaker winds, reduced wind stress curl over subpolar North Atlantic Weaker SPG Weaker AMOC Freshening /cooling of upper subpolar North Atlantic Surface freshwater input into subpolar North Atlantic Salinity-dominated density changes Sea ice formation Reduced heat loss Shutdown of deep convection in subpolar North Atlantic

13 Proposed AMOC weakening mechanism: Weaker SPG XR 1.5windstress experiment Weaker winds, reduced wind stress curl over subpolar North Atlantic Weaker AMOC Freshening /cooling of upper subpolar North Atlantic Surface freshwater input into subpolar North Atlantic Salinity-dominated density changes Sea ice formation Reduced heat loss Shutdown of deep convection in subpolar North Atlantic

14 Effect on surface salinity XR vs HR buoyancy effect wind stress effect XR 1.5winds

15 Impact on sea ice and deep convection: MLD Sea ice fraction MLD Sea ice fraction

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17 Summary: Slowdown in AMOC with increased atmospheric resolution from T127 to T255 Winds play a major role Weaker winds, reduced wind stress curl over subpolar North Atlantic Weaker SPG Weaker AMOC Freshening /cooling of upper subpolar North Atlantic Surface freshwater input into subpolar North Atlantic Salinity-dominated density changes Sea ice formation Reduced heat loss Shutdown of deep convection in subpolar North Atlantic WP6 madness talk by Katja Lohmann THANK YOU!