Wind-driven halocline variability in the western Arctic Ocean AOMIP Workshop #14 @ 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

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

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

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 ?

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

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

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

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”

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

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

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)

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

[ 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

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] 1979 - 2008 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

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