“A near-uniform fluctuation dominating sea level and ocean bottom pressure variations across the

Arctic Ocean and the Nordic Seas”

Ichiro Fukumori1, Ou Wang1, William Llovel1, Ian Fenty1, and Gael Forget2

1Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA, USA 2Massachusetts Institute of Technology, Cambridge MA, USA

[Fukumorietal,2015,Prog.Oceanogr.,134,152-172.]

1

•  A near-uniform barotropic fluctuation dominates the Arctic and Nordic Seas.

Overview What

•  The fluctuation is due to bifurcating coastally trapped waves confined to a region of closed f/H contours,

•  Satellites (GRACE, Jason, etc) can provide valuable observations of marginal seas,

•  Adjoint models provide an effective means to identify causal mechanisms.

How

“Correlation does not imply causation” but adjoint does.!

2

Coherent Basin-wide Arctic Variations Peralta-Ferriz et al. (2011)

±5 cm

See also Hughes and Stepanov (2004), Volkov and Landerer (2013), Peralta-Ferriz et al. (2014), Volkov (2014)

Ocean bottom pressure (OBP) time-series (left) & its

regression (right) with winds (vector) and atmospheric

pressure (color)

May 05 Jul 06

3

Spatial Coherence (GRACE)

0 0.2 0.4 0.6 0.8 1.0

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

A)

0-1.0 20-0.6 40

Explained Variance (%)Correlation

-0.2 600.2 800.6 1001.0

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

C)

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

B)

log10(Root-Mean-Square)

1

2

34

RMS Amplitude" Correlation with OBP @ North Pole"

0 0.2 0.4 0.6 0.8 1.0

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

A)

0-1.0 20-0.6 40

Explained Variance (%)Correlation

-0.2 600.2 800.6 1001.0

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

C)

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

B)

log10(Root-Mean-Square)

1

2

34

OBP variance explained by OBP @ North Pole"

Ocean bottom pressure (OBP) variations are nearly uniform in amplitude & phase across the deep Arctic basins. "

4

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

A)

0 0.2 0.4 0.6 0.8 1.0 0-1.0 20-0.6 40

Explained Variance (%)Correlation

-0.2 600.2 800.6 1001.0

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

C)

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

B)

log10(Root-Mean-Square)

Spatial Coherence (ECCO)

RMS Amplitude" Correlation with OBP @ North Pole"

OBP variance explained by OBP @ North Pole"

ECCO OBP variations are comparable to GRACE results. "

5

J t( )≈ ∂J∂φi x,Δt( )δφi x,t−Δt( )

Δt∑

x∑

i∑

Gradient by adjoint

1995 2000Year

-10

-5

0

Equ

ival

ent S

ea L

evel

(cm

)

5

10

2005 2010

Model simulation

Adjoint reconstruction by wind

GRACE

forcing i at location x & time t-Δt

Temporal Variability

Mean Arctic OBP anomaly at time t

Adjoint Gradient Decomposition

Mea

n A

rctic

OB

P

6

Causal Mechanism (location)

1−

var J − ∂J∂φi x,Δt( )δφi x,t − Δt( )

Δt∑

i=wind∑

⎧⎨⎪

⎩⎪

⎫⎬⎪

⎭⎪var J{ }

Fraction of mean Arctic ocean bottom pressure variance explained by winds

at different locations.

135°W

45°W

0°E

45°E

135°E

180

45°N

30°N

15°N

60°N

75°N

-8 0 8 16Expl. Var. Contribution (×10 /km )

-7 2

Peralta-Ferriz et al. (2011) 7

Causal Mechanism (wind direction)

Along-bathymetry

Cross-bathymetry

135°W

45°W

0°E

45°E

135°E

180

45°N

30°N

15°N

60°N

75°N

A)

135°W

45°W

0°E

45°E

135°E

180

45°N

30°N

15°N

60°N

75°N

B)

-8-12-16 1612-4 0 4 8

Model Sensitivity (×10 cm / (N/m ) / km )-2 2 2

∂J∂φwind x,4-weeks( )

8

Wind-driven coastally trapped waves bifurcate at the shallow straits, allowing anomalies to persist in the deep Arctic basins shielded by steep gradients of

planetary potential vorticity f/H.

z

yx

A

B

B’

A’

Causal Mechanism

135°W

45°W

0°E

45°E

135°E

180

45°N

60°N

75°N

-8.1 -7.2

log10(Potential Vorticity)

-6.3 -5.4 9

OBP Response to Wind Perturbation off Greenland

90°W45°W

60°N

30°N

0°N

90°N

0

Ocean Bottom Pressure (mm)

-4-8 4 8 ×10

A)

D) 13 h

B) 4 h

E) 19 h

C) 7 h

F) 106 h

-2

10

1.  A near-uniform barotropic fluctuation dominates sea level and ocean bottom pressure variations across the deep Arctic basins including the Nordic Seas,

2.  The fluctuation is driven by winds along the continental slopes of the Arctic and its neighboring region,

3.  The winds drive coastally trapped waves that bifurcate at the Arctic’s shallow straits and become confined in the Arctic’s deep ocean basins of near-uniform planetary potential vorticity,

4.  Satellites can provide valuable observations of marginal seas,

5.  Adjoint models provide effective means to identify and to quantify causal mechanisms.

Summary & Conclusion

11