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“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