Eddy-wind interaction in the California Current System effects on - PowerPoint PPT Presentation

Eddy-wind interaction in the California Current System — effects on eddy kinetic energy and Ekman pumping Hyodae Seo Woods Hole Oceanographic Institution KIOST December 19, 2014

Eddy-wind interaction via SST τ = ρ C D (U a − U o ) |U a − U o | Uniform eastward wind over an anticyclonic eddy in the Southern Ocean (Chelton 2013) a b Surface temperature Dipole Ekman velocity SST and SSH with contour interval = 0.5 cm da and height surface temperature 2 2 0.5 6 d D ⊖ 1 1 0.25 s 3 τ 0 0 0 0 U ⊕ –0.25 –1 –3 –1 –0.5 –6 –2 –2 –2 –1 0 1 2 –2 –1 0 1 2 2 Ekman pumping anomaly 90° out of phase with SSH → Southward propagation of an eddy (e.g., Dewar and Flierl 1987)

Eddy-wind interaction via SST τ = ρ C D (U a − U o ) |U a − U o | 10m wind U a = U ab + U aSST Uniform eastward wind over an anticyclonic eddy in the Southern Ocean (Chelton 2013) a b Surface temperature Dipole Ekman velocity SST and SSH with contour interval = 0.5 cm da and height surface temperature 2 2 0.5 6 d D ⊖ 1 1 0.25 s 3 Correlation (SST & wind): high-passed τ 0 0 0 0 U ⊕ –0.25 –1 –3 –1 –0.5 –6 –2 –2 –2 –1 0 1 2 –2 –1 0 1 2 2 Ekman pumping anomaly 90° out of phase with SSH → Southward propagation of an eddy Satellite observations: Xie 2004 (e.g., Dewar and Flierl 1987)

Eddy-wind interaction via SST τ = ρ C D (U a − U o ) |U a − U o | 10m wind stronger wind over warmer SST U a = U ab + U aSST Uniform eastward wind over an anticyclonic eddy in the Southern Ocean (Chelton 2013) a b Surface temperature Dipole Ekman velocity SST and SSH with contour interval = 0.5 cm da and height surface temperature 2 2 0.5 6 Uab d D ⊖ 1 1 0.25 s 3 Correlation (SST & wind): high-passed τ 0 0 0 0 U ⊕ –0.25 –1 –3 –1 –0.5 –6 –2 –2 –2 –1 0 1 2 –2 –1 0 1 2 2 Ekman pumping anomaly 90° out of phase with SSH → Southward propagation of an eddy Satellite observations: Xie 2004 (e.g., Dewar and Flierl 1987)

Eddy-wind interaction via current τ = ρ C D (U a − U o ) |U a − U o | Upwelling at the center of an anti-cyclonic eddy: decaying of an anticyclonic surface current eddy U o =U ob + U oe W e = τ /[ ρ (f+ ζ )] a b SST and SSH Surface temperature with contour interval = 0.5 cm da with contour interval = 0.5 cm da Monopole Ekman velocity Dipole Ekman velocity and height surface temperature 2 2 2 6 0.5 τ 6 d D ⊖ 1 1 1 3 0.25 3 s τ U ⊕ 0 0 0 0 0 0 U ⊕ –3 –1 –3 –0.25 –1 –1 τ –6 –6 –2 –0.5 –2 –2 –2 –1 0 1 2 2 –2 –1 0 1 2 2 –2 –1 0 1 2 surface current Feedback to ocean would be different!

Eddy-wind interaction: SST and current τ = ρ C D (U a − U o ) |U a − U o | 10m wind U a = U ab + U aSST surface current U o =U ob + U oe resulting wind stress τ ≈ τ b + τ SST + τ cur

Eddy-wind interaction: SST and current τ = ρ C D (U a − U o ) |U a − U o | 10m wind U a = U ab + U aSST surface current U o =U ob + U oe resulting wind stress τ ≈ τ b + τ SST + τ cur Relative effects of τ SST and τ cur on the ocean? foci of this study: EKE and Ekman pumping

- τ coupling effect weakens the eddies: SST an idealized ocean model by Jin et al. (2009) uncoupled SST coupled SST uncoupled EKE coupled EKE Wall 25% reduction of EKE - τ coupling with SST Upwelling - τ coupling reduces the alongshore wind stress, baroclinic - SST instability and offshore Ekman transport.

U o - τ coupling effect also damps the EKE: an OGCM study by Eden and Dietze (2009) uncoupled EKE coupled EKE • 10% reduction in EKE in the mid-latitudes and ~50% in the tropics • Primarily due to increased eddy drag ( τʹ · u ʹ , direct effect) • Change in baroclinic and barotropic instability (indirect effect) of secondary importance

Result from previous studies and goal of this study •Previous studies considered either SST or U o in τ formulation in ocean-only models and saw weakened eddy variability. •This study examines the relative magnitudes of SST and u sfc effects in a fully coupled regional model.

Regional coupled model Scripps Coupled Ocean-Atmosphere Regional Model • Seo et al. 2007, 2014 Ocean Atmosphere WRF or bulk physics • An input-output based τ (Q & FW) coupler; portable & flexible WRF 6-h coupling ROMS SST & U sfc • 7 km O-A resolutions & matching mask 6-h NCEP FNL monthly SODA • 6-yr integration (2005-2010) T tot T b T e Smoothing of mesoscale SST and U o (Putrasahan et al. 2013) 5° loess smoothing U e U tot U b (~3° boxcar smoothing) Similar results with different smoothing (e.g, 3° loess smoothing)

︎ ︎ ︎ Experiments τ = ρ C D (U a -U o )|U a -U o | T tot = T b + T e U tot = U b + U e 5° loess filtering ( ≈ 3° boxcar smoothing) Experiments τ formulation τ ation includes es ✔ CTL T b T e U b U e ✔ noT e T b T e U b U e ✔ noU e T b T e U b U e noT e U e T b T e U b U e noU tot T b T e U b U e

Eddy kinetic energy

Eddy kinetic energy JAS 2005-2010 Drifter climatology CTL noT e cm 2 s -2 Marchesiello et al. 2003 noT e U e noU tot noU e • T e no impact • 25% weaker EKE with U e • 30% weaker EKE with U b +U e

Eddy kinetic energy JAS 2005-2010 Drifter climatology CTL noT e cm 2 s -2 Marchesiello et al. 2003 noT e U e noU tot noU e • T e no impact • 25% weaker EKE with U e • 30% weaker EKE with U b +U e

Monthly EKE time-series — CTL = 171 — noT e = 174 25-30% — noU e = 231 summer EKE difference — noT e U e = 230 winter — noU tot = 247 High EKE in summer, low in winter Reduced eddy activity in both seasons!

Eddy kinetic energy budget advection by mean and eddy current (offshore) ! ! ! ! ! ! ! ! Ke t + U ⋅ Ke + # u ⋅ Ke + ∇⋅ ( # u # p ) = ∇ ∇ ! ! u ⋅ ! " ! ! ! − g " w + ρ o ( − " u ⋅ ( " u ⋅ U )) + " " ρ ∇ τ + ε Wind work (P) P e → K e K m → K e EKE source if positive barotropic baroclinic Eddy drag and dissipation conversion conversion ( ε ) if negative (BT) (BC) Upper 100 m average H~fL/N, where f=10 -4 , L=10 4 m, N=10 -2 → H=10 2 m

Summertime EKE budget in CTL BT BC P • P a primary source of EKE. • BC secondary and BT negligible v ′τ y ′ u ′τ x ′ • v’ τ y ’: Source of EKE • v’ is a linear response to nearshore τ y ’ u ′ τ y ′ v ′ • u’ τ x ’: Dissipating EKE τ x ′ • Eddies (via u’) “systematically” oppose τ x ’ in the upwelling zone 150 m average

Summertime EKE budget in CTL along-shore mean BT BC P • P a primary source of EKE. • BC secondary and BT negligible v ′τ y ′ u ′τ x ′ • v’ τ y ’: Source of EKE • v’ is a linear response to nearshore τ y ’ u ′ τ y ′ v ′ • u’ τ x ’: Dissipating EKE τ x ′ • Eddies (via u’) “systematically” oppose τ x ’ in the upwelling zone 150 m average

Cross-shore distribution of EKE and key EKE budget terms • EKE maximum offshore EKE 50 at 150km 50 — CTL [cm 2 s -2 ] 77 — noT e • P maximum near the — noU e coast (20-30 km) by offshore advection BC 0.58 • No significant change in 0.58 BC bet’n CTL noTe 0.51 [10 -5 kgs -1 m -3 ] • Some reduction of BC in noUe P 1.26 • Decreased wind work 1.33 • noU e ➞ CTL: 20% 1.57 reduction cross-shore distance (km)

Eddies increase the eddy drag and reduce the momentum input. u ′τ x ′ = eddy drag eddy drag 42% CTL= - 0.47 stronger [10 -5 kgs -1 m -3 ] noT e = - 0.53 eddy noU e= - 0.33 drag v ′τ y ′ = wind work wind work 16% CTL=1.74 weaker [10 -5 kgs -1 m -3 ] noT e =1.86 wind noU e= 1.90 work

Ekman pumping velocity

Ekman pumping velocity Stern 1965 ✓ ◆ 1 τ τ τ W tot = r ⇥ ( f + ζ ) ρ o  r ⇥ τ τ ⇥ r background wind stress ˜ = W cur + W SST W tot ✓ ◆ τ x r ⇥ ˜ τ τ τ 1 τ y ∂ζ τ x ∂ζ β ˜ + r ⇥ τ τ τ 0 SST = ˜ ∂ x � ˜ + . � (10) ρ o ( f + ζ ) 2 ρ o ( f + ζ ) 2 ρ o ( f + ζ ) ρ o ( f + ζ ) ∂ y | {z } | {z } | {z } | {z } ˜ W SST ˜ ˜ W lin W c W SST W ζ W β W ζ W β Curl-induced linear Ekman pumping Vorticity gradient-induced nonlinear Ekman pumping β Ekman pumping (negligible) SST induced Ekman pumping

SST -induced Ekman pumping velocity COOL COOL WARM ˆ τ θ ˆ ▽ d T= ( ▽ T · τ ) ʹ WARM ▽ c T =( ▽ T × τ ) ʹ · k ˆ ▽ T Positive empirical relationship Kuroshio Gulf Stream ▽ × τ ′ = α C ▽ c T ʹ ▽ × τ′ ≈ α c ∇ c SST W SST = ∇× # τ SST α C α C ( ) ( ) ρ o f + ζ ρ o f + ζ ▽ c T ʹ Chelton et al. 2004

Wind stress curl and cross-wind SST gradient ≈ α c ∇ c SST W SST = ∇× # τ SST ( ) ( ) ρ o f + ζ ρ o f + ζ OBS CTL α c =0.6 [Nm -2 per 10 7 m] α c =0.8 ▽ × τ′ noU e noT e α c =0.1 α c =0.6 ▽ c T ʹ [°C per 100km] JAS 2005-2009; QuikSCAT wind stress and TRMM SST

Ekman pumping velocity JAS climatology OBS W sst W lin W tot W ζ CTL W sst W lin W tot W ζ JAS 2005-2009 m/day

Ekman pumping velocity JAS climatology noT e W sst W lin W tot W ζ noU e W sst W lin W tot W ζ JAS 2005-2009 m/day