Alexandra Jahn NCAR, Boulder, USA Comparison of the simulated Arctic freshwater export variability from different models Alexandra Jahn NCAR, Boulder, USA Collaborators: Y. Aksenov, B.A. de Cuevas, S. Häkkinen, E. Hansen, R. Gerdes, C. Herbaut, M.-N. Houssais, M. Karcher, C. Lique, A. Nguyen, P. Pemberton, L. de Steur, D. Worthen, J. Zhang AOMIP meeting at WHOI, October 20th 2010

Questions: What is the model mean FW export through the CAA and Fram Strait? How does it compare to observations? What are the robust features of the FW export variability across several models?

Which models? LOCEAN UVic ESCM OCCAM ORCA 025 POM ECCO2 PIOMAS NAOSIM RCO CCSM4 Period 1958-2001 1948-2007 1985- 2002 1958- 2004 1961-2008 1992-2008 1948- 2008 1948- 2008 1980-2004 1948- 2006 Regional/global R G Horizontal 20 km 1.8° x 0.9° 1/12° 1/4° 0.35° x 0.45° 18 km 22 km 1° Vertical 46 levels 32 levels 66 levels 26 levels 50 levels 25 levels 30 levels ? 36 Restoring? Yes No Forcing? ERA-40 NCEP ERA-40 and CORE JRA-25 CORE 10 models

FW export: Fram Strait versus CAA 7 have larger CAA FW export 3 have larger Fram Strait FW export km3/yr Fram Strait CAA In the mean, models agree with observations. But: Partitioning of FW export between Fram Strait and CAA is different Total amount of Arctic FW export is different 7 have CAA larger, 3 Fram Strait Fram Strait: Observed: -1990 +/- 400 km3/yr Model mean: -1835 km3/yr Range:-531 to -2660 km3/yr CAA: Observed: -3200 +/-330 km3/yr Model mean: -2632 km3/yr Range: -846 to -5,617 km3/yr

Variability of liquid FW export (for 10 years) LOCEAN Fram Strait Standard deviation Maximum/Minimum Mean RCO PIOMAS POP-CICE NAOSIM OCCAM km3/yr ORCA025 POM UVic ESCM ECCO2 CAA ORCA025 ECCO2 POP-CICE LOCEAN UVic ESCM km3/yr km3/yr NAOSIM PIOMAS POM OCCAM RCO

Fram Strait FW export Runs have several large features in common (large export in late 1960s, early 1980s, mid 1990s, and around 2005), but also show large interannual differences

Fram Strait FW export – EGC Closest values are seen in LOCEAN, OCCAM, POP/CICE Values with highest exports are all the models that have 64-85% of FW export in EGC Models EGC: between -316 km3/yr and -1717 km3/yr Total: between -1054 km3/yr and -2747 km3/yr  FW export in the EGC is between 13% and 85% of the total Fram Strait FW flux Observations (de Steur et al., 2009): EGC: 1060 ± 394 Shelf: 904 ± 652 Total ~ 2000 km3/yr  EGC contributes ~54% of the total liquid Fram Strait FW export Mean of the simulated FW export in similar parts of the Fram Strait section are close to the observations, but spread is very large (2x observational values)

80% 13% PIOMAS 64% 41% 38% 42% Except POM which shows very low FW export in the EGC, the models show that 38-85% of the FW export occur in the EGC If adjusted for the individual location of the EGC, better agreement is expected Percentage of FW export in EGC “section” 46% 85% POP-CICE 84% Rabe et al., 2009

Fram Strait variability Full Fram Strait -7° W to 0.2° E Some of the FW export variability occurs on the shelf in the models

Role of salinity versus volume flux variability – Fram Strait PIOMAS 0.69 0.78 0.94 0.88 0.89 0.35 0.87 0.68 0.96 0.46 0.93 0.33 Variability due to both salinity and volume flux changes (in most models) Dominated by both salinity and volume flux varibaility 0.44 0.38 0.84 0.93 0.80 0.61 FWC driven FW export anomaly Volume flux driven FW export anomaly

Decreasing FW export since ~1990 CAA FW export Higher correlation between export variability in different models than in Fram Strait (r=0.50-0.88) Decreasing FW export since ~1990

Role of salinity versus volume flux variability – CAA 0.35 0.29 0.96 0.29 0.95 0.37 0.98 0.52 Variability very much dominated by volume flux changes 0.95 0.29 0.97 0.32 ORCA possibly wrong – does not add up 0.88 0.63 0.93 0.48 0.99 0.43 FWC driven FW export anomaly Volume flux driven FW export anomaly

Nares Strait versus Barrow Strait Except POP-CICE, which has the lowest resolution, the models consistently show that the FW export through Barrow Strait is larger (~56% to 76%) than through Nares Strait (26% to 44%)

Summary What’s kind of robust? What seems to be robust? Barrow Strait liquid FW export makes up most (~56% to 76%) of the total CAA liquid FW export (Nares Strait makes up 26% to 44%) Periods of maximum liquid FW export in Fram Strait Decreasing liquid FW export in the CAA since ~1990 CAA liquid FW export variability is mainly dominated by volume flux variability, Fram Strait liquid FW export variability is affected by both salinity and volume flux changes What’s kind of robust? CAA liquid FW export is generally larger than Fram Strait liquid FW export (in 7 of 10 models) What’s very variable? Details of the variability in the liquid Fram Strait FW export Contribution of EGC and shelf to Fram Strait FW export 3 models show ~80-85% (OCCAM, ORCA-25, POP-CICE), 1 model shows 64% (RCO), 4 models show between 38%-46% (LOCEAN,Uvic-ESCM, ECCO2, PIOMAS), and 1 model shows 13% (POM)

Next steps: Role of atmospheric forcing for FW export variability? Why is CAA FW export decreasing since ~1990? Any consistent differences between models that we can attribute to resolution, forcing, etc? Sea ice export variability?

Questions?