Eddy-driven air-sea interaction and feedback in the western Arabian - - PowerPoint PPT Presentation

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

Arabian Sea

• Cold, salty, deep ML by the Findlater Jet
• Unstable boundary current, coastal

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

Findlater Jet

Xie et al. 2006 TRMM summer rainfall climatology

Findlater Jet

Eddy-driven air-sea interactions thru wind stress

τ = ρ CD (Ua− Uo) |Ua − Uo|

Ua = Uab + UaSST

Composites in the Southern Oceans

1R ¬1R ¬3R 3R 1R ¬1R ¬3R 3R

7 8 9.8 10.0

SST Wind speed Anti-Cyclone Frenger et al. 2013

surface current 10m wind

Positive correlation between JJA high-passed SST & wind

Eddy-driven Ekman pumping

τ = ρ CD (Ua− Uo) |Ua − Uo|

Dipole SST and SSH

An anticyclonic eddy in the Southern Ocean (Chelton 2013)

10m wind Ua = Uab + UaSST

2 2 1 1 –1 –2 –2 –1 2 1 –1 –2

Downwelling

τ

Upwelling

Ua

Affect the propagation

surface current Uo = Uob+Uoe

τ

2 2 1 1 –1 –2 –2 –1 Monopole

Affect the amplitude

Upwelling

Uoe

• Ekman velocities of 2-3 m/day over the

cold filament, persisting >1 month

25km SCOAR regional coupled model simulation for the Indian Ocean

Seo et al. 2008: Modeling of Mesoscale Coupled Ocean-Atmosphere Interaction and its Feedback to Ocean in the Western Arabian Sea. Ocean Modell.

Wek SST & SSH

Eddy-driven Ekman pumping in the AS

m/day

Wek = ∇×τ ρ0 f

Wek = ∇×τ ρ0( f +ζ)

ζ/f w at the bottom of ML

• SST and surface current both

important for Ekman pumping

Relative effects of eddy-driven air-sea interaction via SST and surface current?

Quantifying the effect of eddy-driven air-sea coupling

• Seo et al. 2007, 2014
• 7 km O-A resolutions

WRF or bulk physics

τ (Q & FW)

Ocean

6-h NCEP FNL monthly SODA

WRF ROMS

Scripps Coupled Ocean-Atmosphere Regional Model

6-h coupling

Atmosphere

SST & Usfc

Smoothing of mesoscale SST and Uo Utot Te Ue Tb Ttot Ub

Exp τ formulation includes

CTL Tb Te Ub Ue noTe Tb Te Ub Ue noUe Tb Te Ub Ue

τ=ρCD(Ua-Uo)|Ua-Uo|

Summertime EKE in the CCS

• 42% reduction of EKE by Uo effect, but Ua 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.

Includes both Te & Ue Without Ue effect Without Te effect AVISO EKE

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

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

JAS 2005-2010 EKE budget noTe CTL noUe

˜ Wtot = Wcur + WSST = r ⇥ ˜ τ τ τ ρo (f + ζ) | {z }

˜ Wc

• 1

ρo (f + ζ)2 ✓ ˜ τ y ∂ζ ∂x ˜ τ x∂ζ ∂y ◆ | {z }

˜ Wζ

+ β˜ τ x ρo (f + ζ)2 | {z }

˜ Wβ

+ r ⇥ τ τ τ 0

SST

ρo (f + ζ) | {z }

WSST

. Wtot = 1 ρo r ⇥ ✓ τ τ τ (f + ζ) ◆ r ⇥ τ τ ⇥ r

Stern 1965 Gaube et al. 2015

Eddy-driven Ekman pumping velocity

background wind stress WLIN

Wζ Wβ WSST

Curl-induced linear Ekman pumping Vorticity gradient-induced nonlinear Ekman pumping β Ekman pumping (negligible) SST induced Ekman pumping

WSST = ∇× # τ SST ρo f +ζ

( )

≈ αc∇cSST ρo f +ζ

( )

Chelton et al. 2007

Estimating eddy SST-driven Ekman pumping velocity

WSST = ∇× # τ SST ρo f +ζ

( )

≈ αc∇cSST ρo f +ζ

( )

noTe noUe OBS αc=0.8 αc=0.6

▽cTʹ[°C per 100km]

CTL αc=0.6 αc=0.1

▽×τ′

[Nm-2 per 107m]

JAS 2005-2009: OBS based on QuikSCAT wind stress and TRMM SST Chelton et al. 2007

JAS 2005-2009: OBS based on AVISO SSH & QuikSCAT wind stress

m/day

Estimated Ekman pumping velocity

WTOTe =

∇×τ b ρ0( f +ζ)

WLIN

! " # $ # + ∇×τ e ρ0( f +ζ)

WSST

! " # $ # − τ b ×∇ζ ρ0( f +ζ)2

Wζ

! " # $ #

. 513

OBS CTL

JAS 2005-2009

m/day

WTOTe =

∇×τ b ρ0( f +ζ)

WLIN

! " # $ # + ∇×τ e ρ0( f +ζ)

WSST

! " # $ # − τ b ×∇ζ ρ0( f +ζ)2

Wζ

! " # $ #

. 513

Estimated Ekman pumping velocity

noTe noUe

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
• bserved 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