1. 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

2. 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?

3. Which models? LOCEAN UVic ESCM OCCAM ORCA 025 POM ECCO2 PIOMAS NAOSIM
RCO
CCSM4
Period
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

4. 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: /- 400 km3/yr Model mean: km3/yr Range:-531 to km3/yr
CAA: Observed: /-330 km3/yr Model mean: km3/yr Range: -846 to -5,617 km3/yr

5. 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

6. 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

7. 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 km3/yr
Total: between km3/yr and 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)

8. 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

9. 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

10. 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

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

12. 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

13. 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%)

14. 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)

15. 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?

16. Questions?