The Effect of Sea Surface Temperature Variation on Wind/Stress Retrieval W. Timothy Liu & Xiaosu Xie Atmospheric Stability Ocean Viscosity

Definition and Basics Wind is air in motion. Stress is the turbulent transport of momentum. Turbulence is generated by instability caused by vertical wind shear and buoyancy We do not have any large-scale stress measurements; our concept of stress distribution is largely influenced by our wind knowledge. Wind influence stress but does not uniquely define stress; stress depends on ocean temperature and current that drives instability. They have smaller-scale variability than winds. Scatterometer measures roughness which is in equilibrium with stress. The geophysical product is the equivalent neutral wind (U N ). U N, by definition, has an unambiguous relation with surface stress, provided that ocean surface current is negligible, while the relation between actual wind and surface stress depends also on atmospheric density stratification It is generally assumed that, over most ocean, atmosphere is neutral and current is negligible, U N has been used as the actual wind, particularly in operational applications.

The Importance to Wind and Stress Winds  Marine weather forecast to avoid shipping hazard  Monitoring and forecasting hurricane  Monitoring and study of monsoon  Convergence fuels convection that drives circulation  Distribution of wind power for electricity generation Stress  Horizontal current driven by wind distributes heat and carbon stored in ocean  Convergence and vorticity control ocean mixing  Mixing brings short-term momentum and heat trapped in the surface into deep to be stored over time  It brings nutrient and carbon stored in the deep to surface for photosynthesis  Affects air-sea turbulent transfer of heat, moisture, and gases

Weissman et al. (1980, JGR) JPL L-band radar on NASA CV-990

Liu (1984, JPO) Seasat Wind speed differences (m/s) SST (ship) o C U N (SASS)/U N (ship) U N <6 m/s SST (ship) o C U N (SASS)/U N (ship) νe at 15 o C

ERS-1 Freilich (1986)

Ebuchi (1997, JAMSTS)

Quilfen et al. (2001, JAOT) Scatterometer winds (ERS-1 & -2) – NDBC winds

Wind and SST Coupling Lin, I.-I.; W.T. Liu, C.-C. Wu, J.C. Chiang, and C.-H. Sui, 2003: Satellite observations of modulation of surface winds by typhoon-induced upper ocean cooling. Geophys. Res. Lett., Vol. 30(3), 1131, doi: /2002GL

Xie et al. 2002, GRL

QuikSCAT ENW (color) & AMSR-E SST (contour) Liu et al. 2008, GRL

Spatial coherence between scatterometer measurements and SST is ubiquitous, under all kinds of atmospheric circulation and boundary layer conditions. Tropical Instability Waves [Xie et al. 1998;Cronin et al. 2003; Hashizume et al. 2002; Chelton et al. 2004] Kuroshio [Nonaka and Xie 2003] Circumpolar Current [White and Annis 2003; O’Neill 2003] Indian Ocean [Vecchi et al., 2004] Winter outbreak in East China Sea [Xie et al. 2002] Gulf Stream Ring [Park and Cornillon 2002] Typhoon wake [Lin et al. 2003] Numerical model simulation [Yu and Liu, 2003; Song et al. 2004] Failure to find a generally applicable wind (boundary layer) theory. The reason is that at small turbulent scales, factors such as Coriolis force, pressure gradient force, baroclincity, cloud entrainment, boundary height, are not important, and ocean factors that generate turbulence (stress) are neglected.

Center of anticyclonic currents Center of cyclonic currents Strong Temperature Gradient & Current Shear at Ocean Front Kuroshio Agulhas Liu et al. 2007, JC Liu et al. 2008, JO

Collocation of ENW magnitude with SST is inherent in the definition of ENW and turbulent mixing theory. (Liu et al. 2007, JC) Observation from satellite Computed from uniform wind field at 10m Agulhas

Observation from satellite Computed from uniform wind field at 10m Liu et al. 2008, JO Kuroshio

Center of cyclonic currents

ENW is higher than actual wind under unstable condition

Center of cyclonic currents SST-Ta U N -UJan

Center of cyclonic currents SST-Ta U N -UJul

Summary Higher SST, equivalent neutral wind (stress) is higher than actual wind Higher SST implies lower viscosity; scatterometers underestimate of stress and wind Stability and viscosity has opposite effect on wind derived from scatterometer. Stability effect is larger than viscosity effect

A Scatterometer sends microwave pulses to the Earth's surface, and measures the power scattered back. The backscattered power depends on the surface roughness. Over oceans, roughness is caused by small waves assumed to be in equilibrium with the wind stress. Measuring both stress magnitude and direction is a unique capability of a scatterometer. Wind Vectors from Scatterometer