Impact of wind-surface current covariability on the Tropical Instability Waves Tropical Atlantic Meeting Paris, France October 18, 2006 Tropical Atlantic Meeting Paris, France October 18, 2006 Hyodae Seo Scripps Institution of Oceanography University of California San Diego Markus Jochum (NCAR) Ragu Murtugudde (Maryland) Arthur J. Miller (SIO)

Outline Model Description; the SCOAR Model; Air-Sea Coupling due to TIWs In the Tropical Pacific Ocean;  Atmospheric Wind Response to SST In the Tropical Atlantic Ocean;  Effect of Wind and Surface Current Covariability on to Ocean Summary Model Description; the SCOAR Model; Air-Sea Coupling due to TIWs In the Tropical Pacific Ocean;  Atmospheric Wind Response to SST In the Tropical Atlantic Ocean;  Effect of Wind and Surface Current Covariability on to Ocean Summary

Model Description and Some Examples; Scripps Coupled Ocean-Atmosphere Regional (SCOAR) Model

SCOAR Model (1) Various coupling frequency (3hrs, 1-day, 5- days..) Bulk formula or RSM physics in ABL for momentum, heat and fresh-water fluxes Wind stress relative to ocean currents: Seo, Miller and Roads (2006) J. Climate, in press ` IC and Lateral BC: NCEP/DOE Reanalysis Ocean Atmos Flux-SST Coupler Regional Spectral Model (RSM) Lateral BC: Ocean analysis / climatology Regional Ocean Modeling System (ROMS)     SST Flux Purpose: Examine air-sea coupled feedback arising in the presence of ocean mesoscale eddies, fronts, and filaments.

It is now being used in various regions in the world ocean. Here are some examples… Central America US. West coast SCOAR Model (2) Eastern Tropical Pacific Ocean Tropical Atlantic Ocean Seo et al GRL

In the Eastern Tropical Pacific Ocean... Wind Response to TIW-induced SST

 Coupled System  Gap Winds  Tropical Depressions and Hurricanes  Tropical Instability Waves Evolving SST and wind-stress vector in km ROMS + 50 km RSM Eastern Equatorial Pacific Ocean Domain

Atmospheric Stability Adjustment to the SST Weaker stratification of ABL over warm phase of TIWs.  Stronger wind near surface Weaker stratification of ABL over warm phase of TIWs.  Stronger wind near surface Combined EOF 1 of SST and WS Vector Stronger shear Weaker shear

Atmospheric Feedback?

In the Atlantic Ocean... Effect of Correlation of Wind and Surface Current on the TIWs Pezzi et al. (2002, GRL) : wind-SST coupling reduces variability of TIWs; but why? TIWs SST´ U sfc ´ ´´ ?

Coupling of Wind and Current... 1/4  ROMS + 1/4  RSM 6-year simulations ; Effect of correlation of wind and current 1/4  ROMS + 1/4  RSM 6-year simulations ; Effect of correlation of wind and current Masina et al. 1999; Jochum et al. 2004; EKE Equation

SST, Surface Current and Wind Stress associated with TIWs. Cyclonic surface current driven by TIWs and overlying cross- front wind stress

Meridional Wind and Current over TIW Eddies... COLD WARM BACKGROUND Wind stress are in opposition of phase with current.  TIW meridional currents are slowed down by wind. Wind stress are in opposition of phase with current.  TIW meridional currents are slowed down by wind.

Zonal Wind and Current over TIW Eddies... WARMCOLD BACKGROUND North: opposite  (-) CORR South: aligned  (+) CORR North: opposite  (-) CORR South: aligned  (+) CORR

Correlation of Wind and Current (95%) Wind and current are negatively correlated over TIW region. Wind-current covariability  energy sink to the TIWs. Wind and current are negatively correlated over TIW region. Wind-current covariability  energy sink to the TIWs. Correlation of v sfc and  y Correlation of u sfc and  x

Estimate of energy conversion rate - Barotropic conversion rate of zonal flow - Local wind-current coupling Masina et al. 1999; Jochum et al. 2004;

Estimated Energy via Coupling of Wind and Current At ~2°N, wind contribution to the TIWs amounts to roughly ~40% of the barotropic convergent rate term... Integrated over TIW region (2  S-5  N), contribution can be roughly ~10%. At ~2°N, wind contribution to the TIWs amounts to roughly ~40% of the barotropic convergent rate term... Integrated over TIW region (2  S-5  N), contribution can be roughly ~10%. Latitude Averages over 30  W-10  W for 6 years. barotropic conversion rate Wind-Current Coupling Masina et al. 1999; Jochum et al. 2004; Seo et al submitted to J. Climate

Conclusion; Coupling due to TIWs 2. Wind stress anomaly generated by SST of TIWs slows down the TIW currents; a negative correlation indicates that coupling of wind and current acts as an EKE sink to the TIWs ( ≈ Pezzi et al. 2002).  Over 2  S-5  N, the energy sink amounts to ~10% of the barotropic conversion rate. 1. Stability adjustment of the ABL due to SST changes vertical turbulent mixing of momentum, thus changes near-surface wind.  This generates perturbation wind stress (and heat flux and ) in phase with SST.  It thus implies further feedback to TIWs in terms of wind stress (and heat flux). 1. Stability adjustment of the ABL due to SST changes vertical turbulent mixing of momentum, thus changes near-surface wind.  This generates perturbation wind stress (and heat flux and ) in phase with SST.  It thus implies further feedback to TIWs in terms of wind stress (and heat flux). Coupled model well captures observed associations between undulating SST front and ABL.

Questions or Comments? Thanks!

Observed: -40~-50 W/m 2 /K Latent Heat Flux and SST Sensible Heat Flux and SST Coupling of SST and turbulent heat flux zonally high-pass filtered SST and heat flux from Evolving SST generates perturbations in turbulent heat flux (and also radiation flux).

Coupling of SST and wind stress MODEL ` Chelton et al. 2005