Abstract
Abstract. We hypothesized that the overwhelming dominance of cyclonic spirals on satellite images of the sea surface could be caused by some differences between the rotary characteristics of submesoscale cyclonic and anticyclonic eddies. This hypothesis was tested by means of numerical experiments with synthetic floating Lagrangian particles embedded offline in a regional circulation model of the southeastern Baltic Sea with very high horizontal resolution (0.125 nautical mile grid). The numerical experiments showed that the cyclonic spirals can be formed from both a horizontally uniform initial distribution of floating particles and from the initially lined-up particles during an advection time of the order of 1 d. Statistical processing of the trajectories of the synthetic floating particles allowed us to conclude that the submesoscale cyclonic eddies differ from the anticyclonic eddies in three ways favoring the formation of spirals in the tracer field: they can be characterized by (a) a considerably higher angular velocity, (b) a more pronounced differential rotation and (c) a negative helicity.
Highlights
Spiral structures that can be treated as signatures of submesoscale eddies are a common feature on synthetic aperture radar (SAR), infrared and optical satellite images of the sea surface (e.g., Munk et al, 2000; Laanemets et al, 2011; Karimova et al, 2012; Ginzburg et al, 2017)
Similar dense packing of the sea surface with submesoscale eddies was observed in Envisat ASAR WSM images of the southeastern Baltic Sea (Karimova et al, 2012)
As stated in the Introduction, this work aimed to investigate the differences between the rotary characteristics of submesoscale cyclonic and anticyclonic eddies, which, in our opinion, would explain the overwhelming dominance of cyclonic spirals on satellite images of the sea surface recorded in the SAR, infrared and optical ranges
Summary
Spiral structures that can be treated as signatures of submesoscale eddies are a common feature on synthetic aperture radar (SAR), infrared and optical satellite images of the sea surface (e.g., Munk et al, 2000; Laanemets et al, 2011; Karimova et al, 2012; Ginzburg et al, 2017). Walter Munk (Munk, 2001) has summarized the formation mechanism of the spirals as follows: “Under light winds favorable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. Horizontal shear instabilities ensue when the shear becomes comparable to the Coriolis frequency. The resulting vortices wind the linear features into spirals.”. Horizontal shear instabilities were shown to favor cyclonic shear and cyclonic spirals for different reasons (Munk et al, 2000) The resulting vortices wind the linear features into spirals.” Horizontal shear instabilities were shown to favor cyclonic shear and cyclonic spirals for different reasons (Munk et al, 2000)
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