Pacific water transport in the Arctic Ocean simulated AOMIP Workshop #12 @ WHOI Jan. 15, 2009 Pacific water transport in the Arctic Ocean simulated by an eddy-resolving coupled sea ice-ocean model Eiji Watanabe1 and Hiroyasu Hasumi2 1 : International Arctic Research Center, Univ. of Alaska Fairbanks 2 : Center for Climate System Research, Univ. of Tokyo, JAPAN

Motivation Introduction Pacific water is predominant sources of heat, freshwater and nutrients Freshwater content [m] (Sref = 34.8 psu) Schematic image of Pacific water pathway Model Low FWC High-salinity OBS Steele et al. (2004) Steiner et al. (2004) Eddy-induced transport is important for Pacific water pathway ?

Model Description Method Coupled sea ice-ocean model : COCO (developed at Univ. of Tokyo) Sea ice part - momentum equation based on Mellor and Kantha (1989) - 0-layer thermodynamics (Semtner, 1976) / two thickness category - EVP rheology (Hunke and Duckowicz, 1997) Ocean part ：COCO 3.4 (CCSR ocean component model version 3.4) - free surface general circulation model (OGCM) - UTOPIA/QUICKEST for trace advection (Leonard et al., 1994) - turbulent closure scheme (Noh and Kim, 1999) - Smagorinsky’s biharmonic viscosity (Griffies and Hallberg, 2000) - enstrophy preserving scheme (Ishizaki and Motoi, 1999) Model parameter (background value [m2/s] ) - horizontal viscosity : 5.0×10, horizontal diffusivity : 1.0 - vertical viscosity : 1.0×10-4, vertical diffusivity : (0.1 ~ 3.0)×10-4

Experimental Design Method Model domain ： Chukchi shelf and southern Canada Basin - horizontal resolution : about 2.5 km - vertical level : 2 - 5 - 10 - 15 - 20 - 30 - 40 - 50 - 60 - 80 - 100 - 125 - 150 - 175 (25L) [m] - 200 - 250 - 300 - 350 - 400 - 500 - 700 - 1000 - 2000 - 3000 - 4000 Boundary condition - NCEP/NCAR reanalysis monthly 　(wind, temperature, radiation etc.) - lateral : sponge for ocean 　 open for ice - Pacific water inflow at Bering Strait Model bathymetry [m] Bering Strait Chukchi shelf Model topography [m] Northwind Ridge Initial condition - T, S : PHC in March - ocean circulation : static - sea ice : shown later Barrow Canyon Canada Basin Alaska N Integrated for 1 year from Mar to Feb

Pacific Water Transport Result Pacific Water Transport Pacific water inflows into Canada Basin by mesoscale eddy activities Pacific water content [m] Virtual tracer prescribed at Bering Strait Maximum velocity [cm/s] in Barrow Canyon Pacific water transport [Sv] Canada Basin Barrow Canyon Oct Maximum in late summer N Feb

Instability of Barrow Canyon Jet Result Instability of Barrow Canyon Jet Mesoscale eddies are generated by instability of Barrow Canyon jet Northward velocity [cm/s] (Aug) Energy conversion rate [KJ/m2/mon] Baroclinic instability is dominant N APE ↓ EKE Alaska Barrow Canyon Jet strength depends on surface stress and buoyancy flux ? These fluxes are related to sea ice condition ?

Dependence on Sea Ice Condition Result Dependence on Sea Ice Condition Relationship of Pacific water transport with sea ice is focused on Sea ice concentration [ - ] (Sep) Integrated from different initial ice thickness Pacific water transport [Sv] retreat to basin SICE small ice case N MICE medium ice case remain in shelf Shelf-to-basin transport is promoted in smaller ice case LICE large ice case

Barrow Canyon Jet Result Strong jet and sharp density front appear in smaller ice case Pacific water tracer [ - ] and potential density [kg/m3] (Aug) Velocity of Barrow Canyon jet [cm/s] Alaska SICE Contribution of surface stress ? Contribution of surface buoyancy flux ? LICE

Surface stress [Pa] (Aug) Result Surface Momentum Flux Surface stress = wind stress + ice-ocean stress (τIO) Surface stress [Pa] (Aug) Surface stress [Pa] Jet velocity [cm/s] Sea ice margin Slight surface stress N SICE Barrow Canyon MICE Enhanced ice-ocean stress LICE

Pacific water tracer [ - ] and potential density [kg/m3] (Aug) Result Surface Momentum Flux Pacific water tracer [ - ] and potential density [kg/m3] (Aug) trapped along shore SICE diffused westward LICE Pacific water and density profiles are also altered by sea ice-ocean stress

Velocity of Barrow Canyon jet [cm/s] Result Surface Buoyancy Flux No sea ice-ocean stress cases Velocity of Barrow Canyon jet [cm/s] Jet strength is almost same regardless of sea ice conditions Dependence on surface buoyancy flux is negligible

Ocean Freshwater Transport Result Ocean Freshwater Transport Insufficient eddy-induced transport is important factors for salinity bias ? Freshwater transport from Chukchi shelf to Canada Basin Pacific water tracer [m] (Feb) 795 km3/yr SICE : 797 km3/yr LICE : 888 km3/yr N MICE Freshwater transport is larger by being mixed with sea ice meltwater in large ice case

Summary in Eddy-resolving Model Most of Pacific water inflow into Canada Basin by mesoscale eddies. Generation of eddies is accompanied by instability of Barrow Canyon jet. The inflow is promoted in smaller summer ice extent case. Process of jet braking by ice-ocean stress is important. Freshwater transport is larger in large extent case Pacific water content [m] N MICE MICE

Problem on Model Setting Discussion Problem on Model Setting Enormous computer resource is necessary for eddy-resolving modeling Time required for 10 yrs integration (ARSC / 64 cpu) Eddy-resolving model (2.5 km) - 1 year (entire Arctic Ocean) - 1 month (Chukchi ~ Canada Basin) Basin-scale model (25 km) - 3 days (entire Arctic Ocean) Basin-scale model cannot explicitly reproduce mesoscale eddies Fine Resolution Coarse

Subgrid-scale Eddy-induced Transport Discussion Subgrid-scale Eddy-induced Transport Impact of GM diffusion in the Arctic Ocean modeling is evaluated k = 100 m2/s (control case) k = a L2/T (variable GM case) primitive sophisticated Eddy-induced transport Image of isopycnal layer thickness diffusion

Experimental Design Method Model domain ： Arctic Ocean, Greenland Sea, Baffin Bay - horizontal resolution : about 25 km - vertical level : 2 - 5 - 10 - 15 - 20 - 30 - 40 - 50 - 60 - 80 - 100 - 125 - 150 - 175 (25L) [m] - 200 - 250 - 300 - 350 - 400 - 500 - 700 - 1000 - 2000 - 3000 - 4000 Boundary condition - NCEP/NCAR reanalysis daily mean 　(wind, temperature, radiation etc.) - sponge layer in Atlantic side - Pacific water inflow at Bering Strait Canada Siberia Greenland Bering Strait Fram Eurasian Basin Canada Basin Initial T, S : PHC / no sea ice Spin up : 10yrs (restored at all depths) Climatorogical run : 10yrs [CTL] k = 100 m2/s, [VGM] k = a L2/T

Pacific water content [m] Result Pacific Water Pathway Pacific water content [m] CTL VGM Shelf-to-basin transport is underestimated Pacific water spreads over Canadian Basin

Salinity Distribution Result Salinity Distribution Salinity anomaly [psu] (ann, 0 ~ 200m) VGM - PHC CTL - PHC Appropriate representation of eddy-induced transport is essentially important for modeling on Pacific water pathway and freshwater variation

Reason for Remaining Bias Future Work Reason for Remaining Bias High-salinity bias in Canada Basin still remains even in VGM case Pacific water pathway is not completely reproduced yet Local narrow current may also have a key for Pacific water transport Additional local processes should be clarified in future research Pacific water content [m] Potential temperature [C] Image of possible dynamics on local baroclinic current VGM Sumata and Shimada (2007)