Mesoscale eddies and shelf-basin exchange in the western Arctic Ocean AOMIP Workshop #13 @ WHOI Oct. 22, 2009 Mesoscale eddies and shelf-basin exchange in the western Arctic Ocean Eiji Watanabe International Arctic Research Center, Univ. of Alaska Fairbanks

Pacific Water Transport Introduction Pacific Water Transport Pacific water is predominant sources of heat, freshwater and nutrients Mesoscale Eddies Spall et al. (2008)

Our Interest Introduction When and Where are Beaufort shelfbreak eddies generated ? What controls generation period and place ? Are these elements interannually varing ? Satellite Model

Beaufort Shelfbreak Eddies Result Beaufort Shelfbreak Eddies Numerous eddy-like warm water cores appear along Beaufort shelf break MODIS 8-day-mean level-3 sea surface temperature [degC] Sep. 9, 2003 70 km Sea Ice Eddy-like warm cores Alaska Sep. 6-13, 2003

Eddy Pair from Global Imager (GLI) Result Eddy Pair from Global Imager (GLI) GLI Level 1B radiance (460, 545, 660 nm) Eddy Pair ? Sea Ice Beaufort shelfbreak Alaska Coastal Current Ocean Pt. Barrow Cloud Land Jul. 8, 2003

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 Watanabe and Hasumi (2009, JPO) Model domain ： Chukchi Sea and southern Canada Basin - horizontal resolution : about 2.5 km (eddies are explicitly resolved) - 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 data 　(wind, temperature, radiation etc.) - lateral : sponge for ocean 　 open for sea ice - Pacific water inflow at Bering Strait Model bathymetry [m] Canada Basin Chukchi Sea Alaska Barrow Canyon Chukchi Plateau Bering Strait Initial condition - T, S : PHC in March - ocean circulation : static - sea ice : derived from NIC Integrated for 1 year from Mar to Feb

Sea Ice and Sea Surface Temperature Result Sea Ice and Sea Surface Temperature Sea ice area [ - ] and sea surface temperature [degC] AMSR-E & MODIS Sep. 6-13, 2003 CTL Sep. 22 Barrow Canyon Alaska Bering St. Warm water cores Sea Ice The model reproduces warm water cores along the Beaufort shelf break

Pacific Water Pathway Result Eddy-induced transport Alaska Oct. 10 CTL Pacific water content [m] Canada Basin Eddy-induced transport Herald Canyon Barrow Canyon Alaska Oct. 10 CTL Bering St. Eddy-induced transport of Pacific water seems to be dominant

Beaufort Shelfbreak Eddies Result Beaufort Shelfbreak Eddies Ocean velocity [cm s-1] and relative vorticity [s-1] (30 m depth) 50 km Barrow Canyon Oct. 10 CTL

Beaufort Shelfbreak Eddies Result Beaufort Shelfbreak Eddies Ocean velocity [cm s-1] and relative vorticity [s-1] (30 m depth) ED5 ED4 ED2 ED3 ED1 50 km Barrow Canyon Jet Jul. 22 CTL

Beaufort Shelfbreak Eddies Result Beaufort Shelfbreak Eddies Ocean velocity [cm s-1] and relative vorticity [s-1] (30 m depth) ED5 ED6 ED4 ED3 ED1 ED2 50 km Barrow Canyon Jet Aug. 6 CTL

Beaufort Shelfbreak Eddies Result Beaufort Shelfbreak Eddies Ocean velocity [cm s-1] and relative vorticity [s-1] (30 m depth) ED4 ED5 ED1 ED3 ED12 ED7 ED8 ED11 ED9 ED10 50 km Barrow Canyon Jet Sep. 15 CTL

Variation of Relative Vorticity Result Variation of Relative Vorticity Time series of relative vorticity field [s-1] (30 m depth) Continuous Two event Oct Type I Type III ED8 Sep ED9 ED11 ED12 ED13 ED7 Type II ED6 Aug ED2 ED3 ED4 ED5 Jul ED1 Barrow Canyon Simultaneous 160W 140W

Type I Eddy Result Lateral shear of ocean velocity [s-1] (30 m depth) Jet strength [m s-1] Rossby Number > O(0.1) ED6 ED3 ED1 ED2 24 Jet is frequently intensified throughout summer season Aug. 6 CTL Continuous generation of Type I eddy

Background PV Condition Result Background PV Condition Vertical structure of potential vorticity [109 m-1 s-1], relative vorticity [s-1] and potential density [kg m-3] ED3 CTL Jul. 12 Type II 26σ 27σ 28σ -0.1f [m] N ED9 Type III CTL Sep. 4 25σ 26σ 27σ 28σ -0.2 f N 30 km Generated from shelfbreak wall at mid-depth Generated from PV front in surface layer

Beaufort Shelfbreak Jet Result Beaufort Shelfbreak Jet Potential temperature [degC] and eastward velocity [cm s-1] [m] Type II Type III 30 10 20 Warm jet Cold jet 10 5 30 km Jul. 12 Sep. 4 N N CTL CTL Bottom-intensified cold jet Surface-intensified warm jet Shelfbreak jet is capable of producing eddies by baroclinic instability

Nonlinear Evolution of Coastal Current Discuss Nonlinear Evolution of Coastal Current Low PV water outflow from a strait could form anti-cyclonic eddies Kubokawa (1991) Bussol St. Yasuda et al. (2000) Shimada et al. (2001) Barrow Canyon

Potential Vorticity Front Result Potential Vorticity Front PV time series on both sides of shelfbreak [107 m-1 s-1] (0 - 30m) Type II Type III Barrow Canyon Sep. 4 Type III eddies are generated when the PV difference becomes maximum PVS << PVB PVS >> PVB Frontal wave influences Type III eddy generation ?

Summary Summary Origin of Beaufort shelfbreak eddies and timing of their generation Type II Two event Jul Continuous Jul ~ Sep Type III Sep Type I Sea Ice Cover PV front ? Shelfbreak Jet Wind Barrow Canyon Jet Eddy Decay Process Interannual Variation Basin-scale Impact Future Work Coastal Current