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 τ = ρ C D (U a − U o ) |U a − U o | ocean surface current U o =U ob +U oe 10m wind speed U a =U ab +U aSST resulting wind stress (Chelton et al. 2001) τ ≈ τ b + τ SST + τ ob+ τ oe Effects of τ SST and τ CUR on the ocean?

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

U o - τ coupling effect: Eden and Dietze (2009) an OGCM with inclusion of u sfc in τ uncoupled EKE coupled EKE • 10% reduction in EKE in the mid-latitude 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 sfc in τ formulation in ocean-only models and saw weakened eddy variability. • This study examines the relative importance of SST and u sfc (u ob vs u oe ) in a fully coupled model, where wind speed adjusts to SST.

Regional coupled model Scripps Coupled Ocean-Atmosphere Regional Model • Seo et al. 2014 (WRF-ROMS) Ocean Atmosphere atmos. states (WRF PBL/sfc schemes) or • An input-output based sfc. fluxes (bulk param) coupler; portable, flexible, ROMS WRF 6-h coupling expandable 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 sfc current (Putrasahan et al. 2013) U e U tot U b

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

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 effect of mesoscale surface temperature (T e )

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 effect of mesoscale surface current (U e )

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 effect of mesoscale surface temperature (T e ) and current (U e )

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 effect of total surface current (U tot =U e + U e )

Summer surface eddy kinetic energy EKE time-series CTL noT e NoT e U e — CTL = 171 — noT e = 174 — noU e = 231 — noT e U e = 230 — noU tot = 247 noU e noT e U e noU tot 6-yr mean • T e no impact • 25% weaker EKE with U e • 30% weaker EKE with U b +U e

Cross-shore vs depth EKE CTL-noT e CTL EKE CTL-noU e CTL-noT e U e cm 2 s 2 CTL-noU tot 41N alongshore averages 34N

Eddy kinetic energy budget ! ! ! ! ! ! ! ! Ke t + U ⋅ Ke + # u ⋅ Ke + ∇⋅ ( # u # p ) = ∇ ∇ ! ! u ⋅ ! " ! ! ! − g " w + ρ o ( − " u ⋅ ( " u ⋅ U )) + " " ρ ∇ τ + ε } } } barotropic baroclinic wind work (P) conversion conversion (or eddy drag) (BT) (BC) Significant difference in only P Upper 100 m average H~fL/N, where f=10 -4 , L=10 4 m, N=10 -2 → H=10 2 m

Comparison of wind work (P= τʹ · u ʹ ) CTL noT e NoU e τ′ · u ′ Exp CTL 1.33 noT e 1.38 noUe 1.61 noT e U e 1.62 noU tot 1.73 noT e U e noU tot [10 -5 kgs -1 m -3 ] • No significant change associated with T e • 17% weaker P with U e • 23% weaker P with U b +U e alongshore averages

Cross-shore distribution of EKE and P 50 EKE P 50 1.26 1.26 — CTL — noT e • Positive P (u ′ . τ′ ) with the maximum near the coast (20-30 km). — noU e - v ′ is a linear response to τ y ′ , increasing EKE. — noT e U e — noU tot EKE P 1.26 50 1.33 50 1.57 77 1.59 73 1.69 79 • P decreases by 20-25% 100-300 km offshore with U e +U b

Zonal and meridional components of wind work P x = u ′ . τ x ′ u ′ . τ x ′ CTL= - 0.47 noT e = - 0.53 noU e= - 0.33 noT e U e = - 0.38 noU tot = - 0.31 • Decrease in P (or increase in eddy drag) by u ′ . τ x ′ is -0.14 v ′ . τ y ′ P y = v ′ . τ y ′ CTL=1.74 noT e =1.86 noU e= 1.90 noT e U e =1.97 noU tot =2.0 • Decrease in P by v ′ . τ y ′ is -0.16 Both directions contribute equally to the decreased P and EKE.

Change SST and surface current CTL-NoU e CTL CTL-NoT e Change in offshore (onshore) temperature advection by mean current mainly CTL-NoU e CTL CTL-NoT e responsible for the cold (warm) SST CTL-NoU e

Summary • Examined the relative importance of τ SST vs τ current 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 · u ʹ ) 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.

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