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Effects of tides on water masses and mixing in the Arctic Ocean

Maria Luneva, Yevgeny Aksenov ,

James Harle, Jason Holt

NEMO-shelf pan-Arctic model (NOC Liverpool)

• NEMOv3.2 with shelf processes included coupled with

LIM2-evp V3.6-CICE +MEDUSA

• ¼ resolution based on ORCA-025 tripolar grid (7-15km)-

3.5km regular grid

• Terrain following s-levels on the shelf, resolving BBL + z

levels below (50 levels s-z 75)

• Tides: geopotential 15 harmonics + boundary conditions

from OTPS-7 .2 (1/4) inverse model

• BC and IC – from global ORCA025-LIM2 ORCA0083

• gls vertical mixing : k-eps with Kantha Clayson94 closure

Canuto 2015 closure ( double diffusion included)

• Vppm- vertical piecewise parabolic advection with small

numerical diffusivity

• Smagorinsky horizontal mixing

• Model runs: 1978-2007 (1990-2014) with/without tides

Forcing: DFS5.1

Effect of tides on sea ice.

•Simulations with tides (solid lines)

demonstrate stronger reduction in ice

volume (~15%) and extent (~5%) in

comparison with simulation without

tides (dashed lines); predicts right trends

in comparison with PIOMAS.

Tidal effects decrease ice thickness

mostly everywhere from 10cm to 1m

in Central Arctic, and up to 2m in the

Canadian Archipelago

Luneva et al., 2015 JGR

Effects of tides on the sea surface salinity and river runoff pathways

On a multi-decadal scale

tides significantly affect

the water mass and

freshwater pathways of

river runoff with net

surface salinity increase

by 1-2 PSU, improving

the prediction of heat

content and surface

salinity

Figure: SSS in simulations (NT)

without and (T) with tides and their

differences (T-NT)

Upper panel: winter: March-April

Lowe panel: summer , September

Effects of tides on sea-ice—atmosphere exchange:

Opening and closing of leads due to

tide

Winter: increase heat loss to

atmosphere and ice formation:

Summer: change the albedo and

radiation flux to water

Ocean to ice heat flux differences (T-

NT) and changes in non-solar flux to

atmosphere mostly identical, except

late December.

But: there are strong changes in

downward solar radiation:

opening –closing of ice leads decrease

the albedo and increase heat flux to

water.

Effects of tides on the mixing:

•Tides increase the mixed layer depth

in the polar regions resulting in the

stronger entrainment of warm Atlantic

waters to the surface layer and melting

of ice.

•Tides increase the shear stresses in the

halocline resulting in a burst of

turbulence and mixing.

Figure 1: Difference in ice thickness in

simulations with and without tides after a

year of integration

Figure 2: Difference in the mixed layer depth

Figure 3. Ratio of vertical diffusivities in

simulations with and without tides: 2 orders

higher in Fram Strait (transect shown in

Figure 1.

Figure 1 Figure 2

Figure 3

Greenland Fram St. Svalbard Barents Sea

Tidal shear

Tides induce a strong shear stresses with maximum located below the

surface and over bottom topography anomalies.

As M2 and S2 frequencies are very close to inertial, potential depth of

Ekman layer can be very deep: he~u*/|f-w | or ( he~(Av/|f-w |)1/2 )

Tides induce quasi-steady ageostrophic circulations with strong vertical motions.

Due to nonlinear effects and vertical shear of tidal

velocity tides induce quasi –steady ageostrophic

overturning circulations across jet currents, effecting

•vertical mixing;

•water mass transformations;

• nutrient supply form the deep waters to surface layers.

Figure3: Streamfunction and vertical velocity reconstructed from

nonlinear tidal forcing using phases and amplitudes of tidal velocities

from model output:

Figure1. Monthly mean vertical

velocity and temperature,

simulations with tides

Figure2.simulations without tides