Effect of Eddy-Wind Interaction on Ekman pumping and Eddy Kinetic - - PowerPoint PPT Presentation

Effect of Eddy-Wind Interaction on Ekman pumping and Eddy Kinetic Energy in the California Current System: A Regional Coupled Modeling Study Hyodae Seo

Woods Hole Oceanographic Institution Currently visiting Kyushu University

Art Miller & Joel Norris

Scripps Institution of Oceanography

OFES International Workshop

Aizu University, Oct. 2-3, 2014

Surface wind stress Effects of τSST and τCUR on the ocean?

10m wind speed Ua=Uab+UaSST (Chelton et al. 2001)

• cean surface current

Uo=Uob+Uoe resulting wind stress τ ≈ τb +τSST + τob+τoe

τ=ρ CD (Ua− Uo) |Ua − Uo|

25% reduction of EKE with SST

• τ coupling

SST

• τ coupling effect: Jin et al. (2009)

an idealized ocean model with empirical coupling of SST and τ

• Reduces alongshore wind stress, baroclinic

instability and Ekman transport uncoupled EKE coupled EKE

uncoupled SST coupled SST

Wall Upwelling

Uo-τ coupling effect: Eden and Dietze (2009) an OGCM with inclusion of usfc in τ

• 10% reduction in EKE in the mid-latitude and ~50% in the tropics
• Primarily due to increased eddy drag (τʹ·uʹ, direct effect)
• Change in baroclinic and barotropic instability (indirect effect) of

secondary importance

uncoupled EKE coupled EKE

Result from previous studies and goal of this study

• Previous studies considered either SST or usfc in τ

formulation in ocean-only models and saw weakened eddy variability.

• This study examines the relative importance of SST

and usfc (uob vs uoe) in a fully coupled model, where wind speed adjusts to SST.

Regional coupled model

• Seo et al. 2014 (WRF-ROMS)
• An input-output based

coupler; portable, flexible, expandable

• 7 km O-A resolutions &

matching mask

• 6-yr integration (2005-2010)

SST & Usfc

• atmos. states (WRF

PBL/sfc schemes) or

• sfc. fluxes (bulk param)

Ocean

6-h NCEP FNL monthly SODA

WRF ROMS Smoothing of mesoscale SST and sfc current (Putrasahan et al. 2013)

Scripps Coupled Ocean-Atmosphere Regional Model

6-h coupling

Atmosphere

Utot Te Ue Tb Ttot Ub

Experiments Experiments τ τ formulation ation includes es CTL Tb Te Ub Ue noTe Tb Te Ub Ue noUe Tb Te Ub Ue noTeUe Tb Te Ub Ue noUtot Tb Te Ub Ue Ttot = Tb + Te Utot = Ub+ Ue

5° loess filtering (≈ 3° boxcar smoothing)

τ=ρCD(Ua-Uo)|Ua-Uo|

Experiments Experiments τ τ formulation ation includes es CTL Tb Te Ub Ue noTe Tb Te Ub Ue noUe Tb Te Ub Ue noTeUe Tb Te Ub Ue noUtot Tb Te Ub Ue Ttot = Tb + Te Utot = Ub+ Ue effect of mesoscale surface temperature (Te)

5° loess filtering (≈ 3° boxcar smoothing)

τ=ρCD(Ua-Uo)|Ua-Uo|

Experiments Experiments τ τ formulation ation includes es CTL Tb Te Ub Ue noTe Tb Te Ub Ue noUe Tb Te Ub Ue noTeUe Tb Te Ub Ue noUtot Tb Te Ub Ue Ttot = Tb + Te Utot = Ub+ Ue effect of mesoscale surface current (Ue)

5° loess filtering (≈ 3° boxcar smoothing)

τ=ρCD(Ua-Uo)|Ua-Uo|

Experiments Experiments τ τ formulation ation includes es CTL Tb Te Ub Ue noTe Tb Te Ub Ue noUe Tb Te Ub Ue noTeUe Tb Te Ub Ue noUtot Tb Te Ub Ue Ttot = Tb + Te Utot = Ub+ Ue effect of mesoscale surface temperature (Te) and current (Ue)

5° loess filtering (≈ 3° boxcar smoothing)

τ=ρCD(Ua-Uo)|Ua-Uo|

Experiments Experiments τ τ formulation ation includes es CTL Tb Te Ub Ue noTe Tb Te Ub Ue noUe Tb Te Ub Ue noTeUe Tb Te Ub Ue noUtot Tb Te Ub Ue Ttot = Tb + Te Utot = Ub+ Ue effect of total surface current (Utot=Ue+ Ue)

5° loess filtering (≈ 3° boxcar smoothing)

τ=ρCD(Ua-Uo)|Ua-Uo|

Summer surface eddy kinetic energy

NoTeUe CTL noTe noTeUe noUe noUtot

• Te no impact • 25% weaker EKE with Ue • 30% weaker EKE with Ub+Ue

— CTL = 171 — noTe = 174 — noUe = 231 — noTeUe = 230 — noUtot = 247

EKE time-series

6-yr mean

Cross-shore vs depth EKE

34N 41N

CTL-noUe CTL-noUtot CTL-noTe CTL-noTeUe CTL EKE cm2s2 alongshore averages

Eddy kinetic energy budget

Ket + ! U ⋅ ! ∇ ! Ke+ # ! u ⋅ ! ∇ ! Ke+ ! ∇⋅( # ! u # p ) =

−g " ρ " w + ρo(− " ! u ⋅( " ! u ⋅ ! ∇ ! U))+ " ! u ⋅ !" τ +ε

baroclinic conversion (BC) } } } barotropic conversion (BT) wind work (P) (or eddy drag) Significant difference in only P Upper 100 m average

H~fL/N, where f=10-4, L=104m, N=10-2 → H=102m

Exp τ′· u′ CTL 1.33 noTe 1.38 noUe 1.61 noTeUe 1.62 noUtot 1.73

• No significant change associated

with Te

• 17% weaker P with Ue
• 23% weaker P with Ub+Ue

[10-5 kgs-1m-3]

CTL noTe NoUe noTeUe noUtot

Comparison of wind work (P= τʹ·uʹ)

alongshore averages

Cross-shore distribution of EKE and P

50 1.26

• Positive P (u′.τ′) with the maximum near the coast (20-30 km).
• v′ is a linear response to τy′, increasing EKE.

1.26 — CTL — noTe — noUe — noTeUe — noUtot

P

1.26 1.33 1.57 1.59 1.69 50 50 77 73 79

P EKE

• P decreases by 20-25% 100-300 km offshore with Ue+Ub

EKE

50

Zonal and meridional components of wind work

u′.τx′ v′.τy′ CTL=1.74 noTe=1.86 noUe=1.90 noTeUe=1.97 noUtot=2.0 CTL=-0.47 noTe=-0.53 noUe=-0.33 noTeUe=-0.38 noUtot=-0.31 Both directions contribute equally to the decreased P and EKE.

• Decrease in P (or increase in

eddy drag) by u′.τx′ is -0.14

• Decrease in P by v′.τy′is -0.16

Px= u′.τx′ Py= v′.τy′

CTL-NoUe CTL-NoUe CTL CTL-NoTe CTL-NoUe CTL CTL-NoTe

Change in

• ffshore

(onshore) temperature advection by mean current mainly responsible for the cold (warm) SST

Change SST and surface current

Summary

• Examined the relative importance of τSST vs τcurrent in the EKE in the

CCS using a fully coupled SCOAR model.

• Surface EKE is weakened by ~25% due to mesoscale current.
• ~5% further weakening by background current.
• SST has no impact.
• EKE budget analysis: wind work (P= τʹ·uʹ) is weakened with the

mesoscale current (17%) and background current (23%)

• SST has no impact.
• Comparable contribution from zonal (eddy drag) and meridional (wind

work) direction.

• Change in SST pattern is related to change in mean and eddy

horizontal temperature advection.

Thanks!