Enhanced 20 th century heat transfer to the Arctic simulated in the context of climate variations over the last millennium Johann Jungclaus K. Lohmann, and D. Zanchettin Max Planck Institute for Meteorology, Hamburg NACLIM Meeting 2014

NACLIM CT1 investigates mechanisms of variability in the North Atlantic/Arctic region. Extended Earth System Model simulations help to: relate recent changes to long-term variations on the millennial time-scale discriminate between internally-generated and externally forced variations and between natural and anthropogenic drivers relate spatially sparsely sampled reconstructions to regional and large-scale dynamics in atmosphere and ocean test hypothesis in models Putting recent changes in the context of long-term climate evolution

Atlantic water advection into the Arctic Foraminiferal data and inferred surface water masses in Fram Strait and Voering Plateu (Dylmer et al., 2013) Foraminiferal data and temperature of upper Atlantic Water in Fram Strait (Spielhagen et al., 2011) 20 th century temperature observations in two Svalbard fjords (Pavlov et al., 2011) 2000 CE 1920 CE 2000 CE 0 CE 2000 CE 0 CE

SPG variability over the last millennium Miettinen et al., 2012 RAPID 21-COMCR Simulations and proxy-records underline important role of SPG dynamics also for the MWP-LIA transition

Coupled CMIP5-class Earth system model MPI-ESM Atmosphere: ECHAM6 in T63L47 resolution (1.8° horizontal resolution, stratosphere-resolving) Ocean: MPIOM GR1.5L40 ( km horizontal resolution) “Past1000” ( CE) simulation following PMIP protocol (Schmidt et al., 2011): 3 realizations “historical” simulations following CMIP5 protocol: 6 realizations “PiCtrl” unforced control simulation over 1000 years The model system: MPI-ESM Hor. Resolution (km) Jungclaus et al., 2008, 2013 Resolution of the ocean model in the Nordic Seas

Arctic climate in last millennium simulations Arctic summer temperature anomalies w.r.t Arctic sea-ice extent anomalies w.r.t mean Kaufman et al., 2009 Colored lines: MPI- ESM simulations Arctic2K Shi et al., 2012 Kinnard et al., 2011 Present day minimum

Fram Strait Atl. Water temp in MPI-ESM-P past1000 AW core temp at 78°N, 50m depth (anomaly wrt. preindustrial mean) Spielhagen SIMMAX Spielhagen Mg/Ca Colors: MPI-ESM-P past1000 r1,2,3

Atl. water transport into the Arctic model heat transport to Arctic Temperature fluctuation in Fram Strait are associated with pronounced heat transport variations into the Arctic. Mean heat transport ca. 80 TW, 20 th change is about 40% 30 TW heat input means ca. 2 Wm -2 forcing over the Arctic region Colors: MPI-ESM-P: past1000 r1,2,3; black: ensemble mean

totalmocgyre model heat Atl. water transport into the Nordic Seas Modulation of heat transport variations mainly due to changes in gyre heat transport. 20 th century stands out by strong increase both in gyre and overturning heat transport. However, AMOC at 30°N is weakening over 20 th century!

What happens in the 20th century? Trends in total heat transport (TW/100yr) Changes in oceanic heat transport are significantly larger than internal variability mainly in the sub polar North Atlantic Can these changes be explained by variations in the wind forcing (Sedlaček and Mysak, 2009; Häkkinen and Rhines, 2010)? 5-95%-range for 100-yr trends from control run

What happens in the 20th century? Changes in wind stress curl are less coherent in the North Atlantic compared to the southern Hemisphere and stay within internal variability range Trends in zonally averaged wind-stress curl (10 -9 Nm -3 /100yr)

Trends in gyre (blue), moc (red), and total (black) meridional heat transport in the Atlantic for three simulations HTR MOC HTR GYRE HTR TOTAL What happens in the 20th century? Increase in gyre component overcompenstes reduction in MOC-related heat transport from ca. 50°N Divergence of total heat transport suggests cooling in the SPG region and warming north of 60°N

What happens in the 20th century? Trends in overturning circulation (left) and barotropic streamfunction (right) Contours: pre-industrial mean state, shading: trend in Sv/100yr Large-scale changes both in horizontal circulation and overturning: strengthening of the Sub Polar Gyre and of the overturning cell in the Nordic Seas; weakening of the AMOC south of 60°N. Redistribution of heat-transport convergence and cooling of the SPG region and warming of the Nordic Seas/Arctic

What happens in the 20th century? Model simulated trend No record of AMOC is avialable for the 20th century, neither for SPG strength One indication, that the proposed mechanism may have been at work, is the apparent cooling of the sub-polar North Atlantic over the 20th century in the observational datasets (see also Drijfhout et al., GRL, 2012)

EOF1 ~24% EOF2 ~15% EOF3 ~9% SPG variability over the last millennium EOFs of barotropic stream function (annual data) Time series of PC3 (red) and gyre heat (blue) (31-yr running mean) Note: EOF1: “Intergyre –mode”, associated with NAO EOF2: Subpolar-subtropical gyre mode, associated with EAP EOF3 ?

Model simulations show pronounced variations in ocean heat transport into the Arctic and associated changes in the Atlantic Water temperature in Fram Strait Simulations confirm magnitude of 20 th warming and the unprecedented character of these changes in the context of the last millennium Increasing heat transfer in 20 th century can be related to large- scale trends in the overturning and gyre circulation in the sub polar North Atlantic, probably as a result of global warming; local changes in wind stress (curl) appear less important Modulation in northward heat transfer also important over pre- industrial times, but role of external forcing (solar, volcanic..) is unclear Need of ensemble simulations and single-forcing experiments Summary and conclusions

The research leading to these results has received funding from the European Union 7th Framework Programme (FP ), under grant agreement n NACLIM