Spirals on the sea

Abstract

Spiral eddies were rst seen in the sunglitter on the Apollo Mission 30 years ago; they have since been recorded on synthetic aperture radar (SAR) images and in the infrared. We present a small sample of images. The spirals are broadly distributed over the world's oceans, 10{25 km in size and overwhelmingly cyclonic. Under light winds favourable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. The linear features are wound into spirals in vortices associated with horizontal shear instability, modi ed by rotation, in regions where the shear is comparable with the Coriolis frequency. Two models for concentrating shear are presented: a softened version of the classical sharp Margules front, and the time-dependent Lagrangian model of Hoskins & Bretherton. Horizontal shear instabilities and both frontal models favour cyclonic shear and cyclonic spirals, but for di¬erent reasons.

Application of multifractal analysis to the study of SAR features and oil spills on the ocean surface

“…The topology of tracers on the ocean surface (Carrillo et al, 2001;Diez et al, 2008) as well as the diffusion of pollutants clearly depend on the local characteristics of the turbulent cascades and on the characteristics of the whole energy and enstrophy spectra that in general will not be in local equilibrium in the sense of Kolmogorov (Kolmogorov, 1941(Kolmogorov, , 1962. The dominant scale when stratification and rotation body forces are in equilibrium in the ocean, local shear will transform slicks on the surface to align and follow the local flow, so the resulting pattern is much more varied than previously believed, and sets of eddies and spirals as shown by Munk (Munk et al, 2000;Platonov et al, 2008) clearly dominate the ocean behaviour. The mixing processes at large-scale produce stirring, which maintains large gradients of the tracers, but in order to mix at molecular level in an irreversible fashion the energy has to cascade to the smallest internal scales, which are outside the present-day range of observation (i.e.…”

Improved statistics of sub-mesoscale eddies in the Baltic Sea retrieved from SAR imagery

“…Again, about 94% of the eddies found were cyclonic and also 94% were less than 20 km in diameter. The authors concluded that some of the eddies might have been caused by horizontal shear instability, as proposed by Munk et al (2000), but other factors such as local inhomogeneity of the sea bottom topography, tidal flows, and wind action may have also significantly contributed to the formation of the observed eddies.…”

10.1007/s11447-008-1012-5

“…Note that β(t) is the wavenumber l(t) normalized to k. The dimensionless wavevector magnitude is (20) (19) is considered on the interval -< z < 0 under the condition = 0 at z = -≡ -Hk. Free-surface boundary condition (16) is rewritten as (21) The dimensionless amplitude of surface height deviation is related to the dimensionless amplitudes and as follows:…”

“…(48) Substituting expression (44) into boundary condition (21) and using relations (45) and (48), we obtain a closed system of equations for (t) and (t):…”