1. A first assessment of changes in the Arctic Ocean freshwater
content from the 1990s to the IPY years -
comparing observations and model results
M. Karcher, B. Rabe,
with contributions from
R. Gerdes, F. Kauker, T. Kikuchi, R. Krishfield,
S. Pisarev, U. Schauer, J. Toole (in alph. order)

2. Objectively mapped FWC based on salinity
Outline
Objectively mapped FWC based on salinity
obervations from the 1990s and the 2000s
Simulated FWC from a NAOSIM simulation
Discussion on possible sources for changes
from observations and modeling

3. Data sources:
Further data were taken from the December 2008 update of the World Ocean Database 2005 (Boyer et al., 2006).

4. JAS Freshwater Content (FWC) surface to 34.0 (Sref: 35)
Two step ‘Objective Mapping‘ (600km and 300km scale)

5. + national projects
NAOSIM
Freshwater
content

6. NAOSIM Experimental setup
BASICS:
MOM 2 based ocean model coupled to Hibler-type sea-ice model
0.25o resolution on rotated spherical grid
30 levels, open southern and Bering Sea boundary
Arctic and subarctic runoff included , SSS restoring (timescale: ½ year)
0.8 Sv prescribed Bering Strait inflow
I.C. AND FORCING:
Initial condition: PHC (Steele et al., 2001)
Forcing NCEP: all forcing data from NCAR/NCEP reanalysis

7. JAS Freshwater Content (FWC)
surface to 34.0 (Sref: 35)
NAOSIM simulation

8. JAS Freshwater Content (FWC)
surface to 34.0 (Sref: 35)
Mapped observations
NAOSIM
3400 km3
6100 km3 (> 500m depth)
minus

9. Thickness change Δh
Salinity change ΔS

10. Model thickness change Δh Model ocean stress curl change Δwcurl
Model agrees with thickness increase, hints a cause: increased EP as suggested by Proshutinsky 09 , here
...points at enhanced EP as source for thickness increase,
in Canadian Basin, supporting Proshutinsky et al. (2009)

11. net thermodynamic ice growth
Simulated
net thermodynamic ice growth
Δhice
...increased melt or reduced freezing in CB and Chukchi area

12. Arctic Decadal Freshwater Content Changes
104 km3
FWC (to 34), rel 35.0
Total Arctic Ocean
Arctic Ocean > 500m depth
„Beaufort Gyre“ > 500m depth
FWC 70s-90s-00s reduction and regain
BG rather constant until 90s
Then loss and enourmous increase to 08
Unaccounted „BG“ area in the west
Periods of loss and gain strikingly
similar to windstress curl
Exception in 70s
Reduction of FWC in Arctic ca km3 70s to 90s, almost regained until 2008
FWC in BG rather constant in 50s-80s, loss in mid90s – early 00s ca km3, enormous gain of ca 6000 km3 in last years, highest ever in 2008
Striking similarity with Proshutinsky‘s idea of loss and gain with wind regimes with exceptions in 60/70s when total Arctic is accounted for
Reason probably unaccounted Beaufrort Sea to Lomonosov Ridge area.
Comparsion with historic decadal data seems contradictory to model for comp to 90s and 00s.
Question of usefullness of historic data for this purpose?! E.g. accuracy of depth of isohalines in old data?!
[Proshutinsky et al., 2009]

13. Arctic Freshwater export (rel. 34.8)
Fram
Bering
Barents
CA 1/CA 2

14. NAOSIMDAS Salinity 60m 'free run' salinity observations
Sep07 30 31 32 33 35 34 36
'optimized run'
Data streams used currently: WODB05 (CTD,BTL, no XBT), additional CTD sections (Pisarev), ARGO floats, ITPs, ice concn (met.no)
Test phase: assim. window 07/ /2008
Sep07

15. Main conclusions:
Arcticwide FWC change between and
km3
Large contribution of Chukchi/Makarov area
Increase of EP: deepening of BG
Reduced mean salinities: more net icemelt, runoff redistribution
Decadal perspective:
Regain of FWC loss of 90s, large BG contribution
Rather constant BG vs. total Arctic before 90s: „BG“ definition
Role of windstress curl for BG ‚breathing‘ confirmed
MOVIE

16. Ice thickness Jul/Aug/Sep 2007
'observed' ice concentration
'free run'
'optimized run'
SSM/I-SSMIS EASE-Grid (NSIDC)
less ice with assim.
more ice with assim.

18. Top 80 m ocean surface curl (Amerasian Basin)

19. Split of changes in hfw between periods into components
with
, Period 1:

20. Thickness change
contribution
Salinity change
contribution
Sources?