Impacts of Vertical Momentum Mixing in an Arctic Ocean Model Youyu Lu 1, Greg Holloway 2, Ji Lei 1 1 Bedford Institute of Oceanography 2 Institute of Ocean Sciences Fisheries and Oceans Canada

Motivation  Necessity to parameterize eddies when grid size Δx > or ~ R (internal Rossby Radius)  Previous AOMIP models (Proshutinsky et al. 2004): 3 models include “Neptune” (e.g., Holloway 2009) 2 models also include increased vertical viscosity (IVV) ν z = 0.03 & 0.05 m 2 s -1 ν z independent of z (Holloway et al. 2007)

Motivation (cont’d)  Theory for eddy-induced vertical viscosity goes back to Charney (1971): Eddy statistical tendencies that act horizontally as A(∂ xx + ∂ yy ) imply acting in vertical as A (f 2 /N 2 )∂ zz  Rhines & Young (1982), Greatbatch & Lamb (1990) proposed ν z = A(f 2 /N 2 )  Holloway (1997) discussed linkage among various eddy parameterization ideas, including IVV and Gent- McWilliam’s “thickness-diffusion” or “eddy-induced tracer advection”.

Motivation (cont’d)  Ferreira et al. (2005) found lateral diffusivity varying similarly to N 2, hence Ferreira & Marshall (2006) proposed ν z = αf 2 Constant α = 2*10 8 m 2 s -1 was obtained through fitting in N Atlantic  IVV ν z is usually tapered to zero near surface. But Zhao & Vallis (2008) found model results not strongly sensitive to details of tapering

This Study: Model  Pan-Arctic ice-ocean model based on NEMO v2.3. Horizontal resolution km, maximum 46 vertical z-levels  Initialized with PHC v3 T-S climatology in January; sea-ice from global ocean reanalysis GLORYS v1  Lateral open boundary condition taken from GLORYS v1; Bering Strait volume transport adjusted to match Woodgate et al. (2005)  CORE Normal Year surface forcing; River runoff: monthly climatology of Barnier et al (2006); surface salinity is restored to monthly climatology with a restoring time of 31 days  Horizontal mixing: bi-harmonic viscosity m 4 s -1 ; along- isopycnal harmonic tracer diffusion 300 m 2 s -1

This Study: Model Experiments  Vertical viscosity & diffusivity computed according to 1.5 level TKE closure scheme  Background diffusivity m 2 s -1  Background viscosity set different for two runs: m 2 s -1 for REF 5×10 -2 m 2 s -1 for IVV  Each run last 10 years; monthly averaged fields saved for analysis

Model Results: Flattening of Isopycnls Density averaged for year 10 across Canada Basin. Color shading and back contours for REF. Pink contours for IVV. Increased viscosity leads to flattened isopycnals.

Model Results: Flattening of Isopycnls N 2 across Canada Basin, left for REF, right for IVV. Significant difference near pycnoline. IVV leads to smaller and smoother vertical gradient of density.

Model Results: Changes in Circulation Across Canada Basin, left for REF, right for IVV. Significant changes in upper 300 m. IVV gets smoother flow, weaker Beaufort Gyre. Deeper “rim currents” similar.

Model Results: Changes in Circulation At 16.3 m, left for REF, right for IVV. IVV gets weaker Beaufort Gyre.

Model Results: Changes in Temperature Across Canada Basin from PHC, REF, IVV & IVV-REF. IVV is closer to PHC. But, in Greenland and Labrador Seas, REF is closer to PHC.

Model Results: Changes in Salinity Across Canada Basin from PHC, REF, IVV & IVV-REF. IVV is closer to PHC. But, in Greenland and Labrador Seas, REF is closer to PHC.

Model Results: Changes in Sea-Ice Sea-ice thickness in March & September from REF, IVV & REF-IVV. IVV gets thinner ice. Hypothesis: weaker Beaufort Gyre => less surface convergence =>more upward heat diffusion heat from intermediate warm layer

Conclusions  In an intermediate resolution, non-eddy resolving Arctic model, increasing background vertical viscosity by 500 times (from to 0.05 m 2 s -1 ) leads to “fine tuning”.  Increased vertical viscosity acts to flattening of isopycnals, reduce strength of boundary current, and reduce sea-ice thickness in Canada Basin.  Reduced sea-ice in Canada Basin seems to be caused by local circulation dynamics, not related to Atlantic inflow.  Further work: quantify differences; compare with high- resolution models