Eddy-driven air-sea interaction and feedback in the western Arabian Sea Hyodae Seo Physical Oceanography Department Woods Hole Oceanographic Institution NASCar Planning Meeting June 2-3, Reston, VA
Air-sea interaction in the Arabian Sea Findlater Jet TRMM summer rainfall climatology Arabian Sea • Cold, salty, deep ML by the Findlater Jet • Unstable boundary current, coastal Findlater Jet upwelling, and strong eddy activity • Strong eddy-driven air-sea coupling - affecting energetics of the current system, the low-level structure of the FJ, and the monsoon rainfall Xie et al. 2006
Eddy-driven air-sea interactions thru wind stress surface τ = ρ C D (U a − U o ) |U a − U o | current Composites in the Southern Oceans 10m wind SST Wind speed U a = U ab + U aSST 3R 3R Anti-Cyclone 1R 1R Positive correlation between ¬1R ¬1R JJA high-passed SST & wind ¬3R ¬3R 9.8 10.0 7 8 Frenger et al. 2013
Eddy-driven Ekman pumping τ = ρ C D (U a − U o ) |U a − U o | surface current 10m wind U o = U ob +U oe U a = U ab + U aSST An anticyclonic eddy in the Southern Ocean (Chelton 2013) Dipole Monopole SST and SSH 2 2 2 1 1 1 Downwelling τ U a 0 0 0 Upwelling –1 –1 –1 Upwelling τ –2 –2 –2 –2 –1 0 1 2 –2 –1 0 1 2 Affect the propagation Affect the amplitude U oe
Eddy-driven Ekman pumping in the AS 25km SCOAR regional coupled model ∇× τ W ek = ∇× τ W ek = ρ 0 ( f + ζ ) simulation for the Indian Ocean ρ 0 f SST & SSH Wek m/day ζ /f w at the bottom of ML • Ekman velocities of 2-3 m/day over the • SST and surface current both cold filament, persisting >1 month important for Ekman pumping Seo et al. 2008: Modeling of Mesoscale Coupled Ocean-Atmosphere Interaction and its Feedback to Ocean in the Western Arabian Sea. Ocean Modell .
Relative effects of eddy-driven air-sea interaction via SST and surface current?
Quantifying the effect of eddy-driven air-sea coupling Scripps Coupled Ocean-Atmosphere Regional Model • Seo et al. 2007, 2014 Atmosphere Ocean WRF or bulk physics τ (Q & FW) • 7 km O-A resolutions WRF 6-h coupling ROMS SST & U sfc Smoothing of mesoscale 6-h NCEP FNL monthly SODA SST and U o T tot T b T e τ = ρ C D (U a -U o )|U a -U o | Exp τ formulation includes CTL T b T e U b U e U tot U b U e noT e T b T e U b U e noU e T b T e U b U e
Summertime EKE in the CCS CTL noT e noU e Includes both T e & U e Without T e effect Without U e effect AVISO EKE JAS 2005-2010 • 42% reduction of EKE by U o effect, but U a has no strong effect • Changes in baroclinic and barotropic energy conversion are small. • The EKE reduction is largely explained by the enhanced eddy surface drag. EKE budget ! ! p ) = + ρ o ( − ! " u ⋅ ( ! " w + ! " u ⋅ ! " ! ! ! ! ! ! ! ! u ⋅ U )) − g " " ∇ ρ τ + ε Ke t + U ⋅ Ke + # u ⋅ Ke + ∇⋅ ( # u # ∇ ∇
Eddy-driven Ekman pumping velocity ✓ ◆ Stern 1965 1 τ τ τ W tot = r ⇥ Gaube et al. 2015 ( f + ζ ) ρ o r ⇥ τ τ ⇥ r ˜ = W cur + W SST background wind stress W tot ✓ ◆ τ x r ⇥ ˜ τ τ τ 1 τ y ∂ζ τ x ∂ζ β ˜ + r ⇥ τ τ 0 τ SST = ˜ ∂ x � ˜ + � . ρ o ( f + ζ ) 2 ρ o ( f + ζ ) 2 ρ o ( f + ζ ) ∂ y ρ o ( f + ζ ) | {z } | {z } | {z } | {z } ˜ W SST ˜ ˜ W c W LIN W ζ W β W SST W ζ W β Curl-induced linear Ekman pumping Vorticity gradient-induced nonlinear Ekman pumping β Ekman pumping (negligible) SST induced Chelton et al. 2007 Ekman pumping ≈ α c ∇ c SST W SST = ∇× # τ SST ρ o f + ζ ( ) ρ o f + ζ ( )
Chelton et al. 2007 Estimating eddy SST-driven W SST = ∇× # ≈ α c ∇ c SST τ SST Ekman pumping velocity ( ) ( ) ρ o f + ζ ρ o f + ζ OBS CTL [Nm -2 per 10 7 m] α c =0.6 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: OBS based on QuikSCAT wind stress and TRMM SST
513 W TOTe = . ∇× τ b ∇× τ e − τ b ×∇ ζ + Estimated Ekman pumping velocity ρ 0 ( f + ζ ) 2 ρ 0 ( f + ζ ) ρ 0 ( f + ζ ) ! " # # $ ! " # # $ ! # " # $ W LIN W SST W ζ OBS CTL m/day JAS 2005-2009: OBS based on AVISO SSH & QuikSCAT wind stress
513 W TOTe = . ∇× τ b ∇× τ e − τ b ×∇ ζ + Estimated Ekman pumping velocity ρ 0 ( f + ζ ) 2 ρ 0 ( f + ζ ) ρ 0 ( f + ζ ) ! " # # $ ! " # # $ ! # " # $ W LIN W SST W ζ noT e noU e JAS 2005-2009 m/day
Summary and Research Plan • AS is eddy-rich. Understanding dynamics and impact of eddy-driven air-sea interaction (both thermal and momentum) is of my primary interest. • From the NASCar measurements, I am interested in knowing the observed spatial-temporal structure of meso- and submeso-scale eddies and surface Ekman currents. • From regional model simulations, I will examine • Local impact on the energetics and stability of the current system • Influence on the Findlater Jet and the downstream monsoon rainfall
Thanks hseo@whoi.edu
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