Climate System Research Center, Geosciences Alan Condron Peter Winsor, Chris Hill and Dimitris Menemenlis Changes in the Arctic freshwater budget in response to extreme NAO wind forcing

Climate System Research Center, Geosciences Where is freshwater stored in the Arctic? ~ 20 m Vertically integrated freshwater content (m) from PHC v3.0

Climate System Research Center, Geosciences Why are we interested in the mechanisms controlling the Arctic freshwater budget? The Arctic is a large (~84,000 km 3 ) freshwater source situated north of climatically sensitive (deep convection) regions in the North Atlantic ocean  potential to impact deepwater formation in the North Atlantic, and also the MOC? [Zhang and Vallis, 2006]. From 1965 – 1995 the North Atlantic appears to have become fresher (by ~19,000 km 3 ) as a result of increases in the export of freshwater from the Arctic [Curry & Mauritzen, 2005; Peterson et al. 2006]. The Great Salinity Anomaly in the 1960s - one of the most persistent and extreme variations in salinity yet observed in the North Atlantic – likely had an Arctic origin [Dickson et al.1988]

Climate System Research Center, Geosciences Atmospheric controls on the retention/export of Arctic freshwater Large-scale changes in Arctic atmospheric circulation shown to play a dominant role on the freshwater budget of the Arctic [e.g. Proshutinsky & Johnson, 1997] The increased freshwater export to the North Atlantic from appears to be related to a persistent POSITIVE phase of the NAO. [Peterson et al. 2006] Above: Changes in North Atlantic FW storage and the NAO [from Peterson et al. 2006]

Climate System Research Center, Geosciences The North Atlantic Oscillation over the Arctic North Atlantic Oscillation (NAO) index (DJFM) is the normalized difference in pressure between Portugal and Iceland NAO- (1969) NAO+ (1989) Anticyclonic High pressure (1024 hPa) Cyclonic low pressure (1010 hPa)

Climate System Research Center, Geosciences Clarifying the link between the NAO and the retention/export of Arctic freshwater to the North Atlantic 1)How will the freshwater budget of the Arctic vary purely with a change in the wind-forcing associated with the different phase (+/-) of the NAO? 2)A number of GCMs predict that the NAO will become increasingly positive under a global warming scenario [Stephenson et al. 2006]. What will happen to the freshwater budget of the Arctic if the same phase of the NAO persists for several decades?

Climate System Research Center, Geosciences Model set-up (MITgcm) Massachusetts Institute of Technology general circulation model (MITgcm) 3-dimensional, hydrostatic, free surface, numerical ocean circulation model. 1/6 degree (~18-km) horizontal resolution. 50 vertical levels. Regional arctic configuration (Menemenlis et al) Coupled to a viscous-plastic sea ice rheology model (Losch et al. 2010) Initial hydrography (T, S) from Polar Hydrographic Climatology (PHC) 3.0 (Steele et al. 2001) NO Salinity or Temperature restoring Forcing is from NCEP 6-hourly reanalysis

Climate System Research Center, Geosciences The MITgcm cube-sphere configuration Velocity (m/s) MITgcm. Global set-up at 1/6 degree (18km) Regional Model

Climate System Research Center, Geosciences Good agreement between freshwater budget in climatology and control run (Serreze et al, 2006) OBSERVED MODEL (Control)

Climate System Research Center, Geosciences NAO forcing experiments Integrate model for 10 years. Exp 1: repeatedly force with NAO positive (1989) wind field. Exp2 : repeatedly force with NAO negative (1969) wind field. All other variables (2 m air-temp, humidity, precip etc) vary with time (6 hourly) from Jan 1992 to Dec 2001 NAO- (1969) NAO+ (1989) Anticyclonic High pressure Cyclonic Low pressure

Climate System Research Center, Geosciences Changes in the vertically integrated freshwater content YEAR 10: 1% loss in Arctic freshwater storage ~ 82,000 km 3 YEAR 10: 15% loss in Arctic freshwater storage ~70,500 km 3 YEAR 1: freshwater = 83,131 km 3 20m 17m 18.5m

Climate System Research Center, Geosciences Annual (10-yr) mean freshwater transport (km 3 yr -1 ) Fram Strait Barents SeaBering Strait Canadian Arch. NAO NAO Difference NAO NAO Difference LIQUID ICE There is an increased freshwater export to North Atlantic during NAO + forcing

Climate System Research Center, Geosciences Mechanism: Freshwater storage modulated by expansion/contraction of the Beaufort Gyre in responding to whether the atmospheric circulation regime is anticyclonic/cyclonic [e.g Proshutinsky et al., 2002]. NAO +  cyclonic circulation  reduced ekman convergence  Beaufort gyre spins down  freshwater release NAO -  anticyclonic circulation  ekman convergence  freshwater retained/stored

Climate System Research Center, Geosciences Exp. 2: Long-term evolution and recovery Two longer integrations were performed: 1) Repeatedly force the model with NAO+ for 30 years. 2) Switch forcing to NAO- after 10 years. The Arctic reaches a new freshwater depleted equilibrium after years of NAO+ forcing The Arctic recovers virtually its entire freshwater content (to within 1%) when the forcing is switched to NAO-

Climate System Research Center, Geosciences The pathway of freshwater away from the Beaufort gyre 1)Tracer advected primarily through the Canadian Archipelago 2)Confined to the shelf after it leaves the Arctic, as a buoyant coastal flow.

Climate System Research Center, Geosciences Beaufort gyre acting as a way to slow down the release of freshwater to the North Atlantic from abrupt (1 year) meltwater pulses. Applications to past, abrupt climate change 1 year flood

Climate System Research Center, Geosciences Conclusions Positive NAO  export of freshwater to the North Atlantic, analogous to the period Negative NAO  Freshwater remains in Beaufort Gyre. After years of positive NAO  Arctic reaches a new freshwater depleted state. Switching the forcing back to a negative NAO (after 10 years of NAO+ forcing) allows the freshwater content to recover in only 20 years. A freshwater tipping point is not reached. At 1/6 degree, very little Arctic freshwater penetrates into the center of the Nordic and Labrador Seas.