A near-uniform fluctuation dominating sea level and ocean bottom pressure variations across the Arctic Ocean and the Nordic Seas

Transcription

1 A near-uniform fluctuation dominating sea level and ocean bottom pressure variations across the Arctic Ocean and the Nordic Seas Ichiro Fukumori 1, Ou Wang 1, William Llovel 1, Ian Fenty 1, and Gael Forget 2 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA, USA 2 Massachusetts Institute of Technology, Cambridge MA, USA [Fukumori et al, 2015, Prog. Oceanogr., 134, ] 1

2 What Overview A near-uniform barotropic fluctuation dominates the Arctic and Nordic Seas. How 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. Correlation does not imply causation but adjoint does.! 2

3 Coherent Basin-wide Arctic Variations Peralta-Ferriz et al. (2011) ±5 cm Ocean bottom pressure (OBP) time-series (left) & its regression (right) with winds (vector) and atmospheric pressure (color) May 05 Jul 06 See also Hughes and Stepanov (2004), Volkov and Landerer (2013), Peralta-Ferriz et al. (2014), Volkov (2014) 3

4 Spatial Coherence (GRACE) Ocean bottom pressure (OBP) variations are nearly uniform in amplitude & phase across the deep Arctic basins. " A) RMS Amplitude" Correlation with North Pole" B) OBP variance explained by North Pole" C) log10(root-mean-square) Correlation Explained Variance (%) 4

5 Spatial Coherence (ECCO) ECCO OBP variations are comparable to GRACE results. " RMS Amplitude" Correlation with North Pole" OBP variance explained by North Pole" A) B) C) log10(root-mean-square) Correlation Explained Variance (%) 5

6 Temporal Variability Mean Arctic OBP Equivalent Sea Level (cm) Model simulation GRACE Adjoint reconstruction by wind Year Adjoint Gradient Decomposition J ( t) i x Δt J φ ( i x,δt) δφ ( i x,t Δt) Mean Arctic OBP anomaly at time t Gradient by adjoint forcing i at location x & time t-δt 6

7 Causal Mechanism (location) Fraction of mean Arctic ocean bottom pressure variance explained by winds at different locations. 1 var J i=wind Δt J φ i x,δt ( ) δφ ( x,t Δt) i { } var J 30 N 15 N Peralta-Ferriz et al. (2011) Expl. Var. Contribution ( 10 /km ) 7

8 Causal Mechanism (wind direction) Alongbathymetry Crossbathymetry A) B) φ wind J ( x,4-weeks) 30 N 30 N 15 N 15 N Model Sensitivity ( 10 cm / (N/m ) / km ) 8

9 Causal Mechanism 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. x z y A B B A log10(potential Vorticity) 9

10 OBP Response to Wind Perturbation off Greenland A) B) 4 h C) 7 h 90 N D) 13 h E) 19 h F) 106 h 90 W 30 N 0 N Ocean Bottom Pressure (mm) -2 10

11 Summary & Conclusion 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. 11