1. Wind-driven halocline variability in the western Arctic Ocean
AOMIP Workshop WHOI
Oct. 21, 2010
Wind-driven halocline variability
in the western Arctic Ocean
Eiji Watanabe1 and An T. Nguyen2
1International Arctic Research Center, Univ. of Alaska Fairbanks
2　UCLA / Jet Propulsion Laboratory

2. Arctic Freshwater Variation
Introduction
Arctic Freshwater Variation
Freshwater variations in the Arctic Ocean is a hot topic in polar studies
Alaska
Rapid freshening occurs in Canada Basin
Proshutinsky et al. [2009]
Freshwater variability is primarily controlled by wind stress on several timescales

3. Wind-driven Ekman Transport
Introduction
Wind-driven Ekman Transport
Robust anti-cyclonic wind induces Ekman downwelling in Canada Basin
Seasonal cycle of Ekman upwelling/downwelling averaged from 1997 to 2004
Siberia
Yang [2009]
Coastal upwelling is enhanced during autumn and winter
Interannual to decadal variability are also detected
Absences of ocean circulation and bottom bathymetry may cause some biases

4. Wind-driven Halocline Variability
Introduction
Wind-driven Halocline Variability
Halocline variability is important from both physical and biochemical aspects
Wind-driven downwelling causes increase in freshwater content
Upwelling provides nutrients and planktons for surface euphotic zone
Precipitation
River water
Ice meltwater
bloom
Mixed layer
freshening
Pacific water
Halocline layer
Chukchi
shelf
Isohaline
layer N P
Atlantic layer
Ocean circulation
Fine-scale mixing
How much does wind forcing account for halocline variability ?

5. Model and Experimental Design
Method
Model and Experimental Design
Coupled sea ice-ocean model : COCO (developed at Univ. of Tokyo)
Model domain : Arctic Ocean and northern North Atlantic
- horizontal resolution : about 25 km / 28 vertical levels
Model bathymetry [m]
Siberia
Canada
Europe
Greenland
Bering
Strait
Basin
North
Atlantic
Boundary condition
- NCEP/NCAR reanalysis daily data
　(wind, temperature, radiation etc.)
- sponge for lateral ocean boundary
- Pacific water inflow at Bering Strait
Initial condition
ocean : PHC T/S, no motion
no sea ice
Integrated from 1979 to 2008
- spin up for 10 yrs using 1979 forcing

6. Decadal Sea Ice and Ocean Variability
Result
Decadal Sea Ice and Ocean Variability
Model reproduces observed major features in sea ice and ocean properties
Sea ice concentration [ - ]
Sea surface height [cm]
AMSR-E
Sep 2007
Ann 2008 10 15
Greenland
Freshening in
Canada Basin
Sea ice widely retreats
in Siberian Arctic
FWC
80s
90s
00s
[m]
13.3
13.2
15.1
Sref = 34.8 psu

7. Freshwater Variation in Canada Basin
Result
Freshwater Variation in Canada Basin
Freshwater variation is broadly consistent with surface wind forcing
80N
75N
70N N C S
Alaska
Freshwater content [m]
Sref = 34.8psu
freshening
Bering
Strait
Net Ekman convergence [m/yr]
convergent
phase
Recent freshening is induced
by shift of Ekman forcing
to convergent phase ?
neutral
phase

8. Halocline Variability
Result
Halocline Variability
Interannual variation in vertical salinity profile [psu]
[m]
Mixed layer 32
Halocline
layer 33 34
North
Atlantic layer
[m] 32 33 34
South
1980
1985
1990
1995
2000
2005
YEAR
Interannual variations in freshwater content are consistent with
shoaling/deepening of isohaline layers at “halocline depth”

9. Wind-driven Halocline Variability
Result
Wind-driven Halocline Variability
Correlation between halocline depth and Ekman pumping
Alaska
75N
Bering
Strait
0.6
0.7
80N
145W
33 psu
[psu]
correlation
Surface Ekman forcing accounts for a major part of
shoaling/deepening of halocline depth north of 75N
Some other processes would control halocline variability in southern basin

10. Pacific Summer Water Result
SLP [Pa], Ekman pumping [m/yr], volume transport [m2/s] (JAS)
Westerly wind
around low SLP
Downwelling
Strong ACC
salinity
[m] 33 34 32
70N
80N
Latitude 31
Easterly wind
around high SLP
Upwelling
Weak ACC
salinity 33 34 32
[m] 31
70N
80N
Latitude
2003
1018
1012
1014
1016
145W
1005
1013
1008
1006
1010
75N
80N
2007
1011
Alaska L H
Barrow
Canyon
Volume transport of Alaskan Coastal Current [Sv]
Changes in stratification due to Ekman forcing and coastal current
occur in same direction, depending on basin-scale wind fields

11. Eddy-resolving modeling
Result
Eddy-resolving modeling
Pacific water content in 2003 and 2007 cases [m]
Canada
Basin
152W
152W
Barrow
Canyon
Barrow
Canyon
Oct. 10
Oct. 10
2003
2007
Westerly wind phase
Easterly wind phase
Eddy-induced Transport
Ekman Transport
Watanabe [2010] (JGR, in revision)

12. Pacific Winter Water Result
Salinity profile in JAS 1994 [psu]
Halocline depth north of Barrow Canyon mouth [m]
COCO
32psu
[m] 31 32 33 34
Alaskan
shelf
33.5psu
Upper halocline depth is outcropped due to upwelling event
and inflow of Pacific winter water when easterly wind prevails
ECCO2 31
Brine rejection scheme
facilitates inflow of
Pacific winter water 32 33 34
Spring down-canyon transport thickens halocline layer and prevents upwelling of intermediate water 71
Latitude 72 73 74 75
145W

13. [ Anti-cyclonic regime ]
Summary
Summary
Wind forcing accounts for a large part of halocline variability
[ Cyclonic regime ]
[ Anti-cyclonic regime ] U D
Wind
Wind
Winter
water
Winter
water D U
Summer
water
Pacific summer water enhances Ekman-driven changes in stratification
Pacific winter water prevents upwelling of intermediate water

14. Influence of Beaufort Gyre
Discussion
Influence of Beaufort Gyre
Beaufort Gyre plays a role in correlation reduction
Ocean horizontal velocity across 145W section [cm/s]
mean
0.5
[psu] 33 32 34
0.6
0.4
1.0
1.6
-0.4
South
Central
westward
eastward
Black contours :
Correlation coefficient
Detailed analysis will be done as a future work

15. Impact of Sea Ice Dynamics
Discussion
Impact of Sea Ice Dynamics
Sea ice dynamics modifies wind stress at ocean surface
Ratio of Ekman convergence due to
ocean surface stress to wind stress
Correlation coefficient
PDF [%]
80N
75N
0.9
Alaska
High correlation between
annual Ekman anomalies
from wind stress and
ocean surface stress
Sea ice reduces wind energy input into ocean surface