Lagrangian Rotating Contracting Structures

TL;DR

This paper introduces Lagrangian rotating contracting structures (LRCS), regions in unsteady flows characterized by contraction and intrinsic rotation, identified using objective diagnostics like LAVD and contraction tests, applicable to atmospheric and oceanic flows.

Contribution

The paper proposes a new method combining LAVD and contraction criteria to reliably identify rotating contracting regions in complex unsteady flows, independent of geometric assumptions.

Findings

LRCS are present in atmospheric and oceanic flows.

Inertial effects enhance rotation in mesoscale flows.

Finite-time contraction helps isolate materially organized regions.

Abstract

We identify materially defined regions in unsteady two-dimensional flows that combine finite-time contraction with elevated accumulated intrinsic rotation along trajectories, which we term \emph{Lagrangian rotating contracting structures} (LRCS). These regions are detected using existing objective diagnostics -- the Lagrangian-averaged vorticity deviation (LAVD) together with direct tests of material contraction -- without relying on the geometry of LAVD level sets. In strongly deforming flows, LAVD maxima need not correspond to vortical regions or be enclosed by regular level sets, rendering geometry-based identification unreliable. Nevertheless, regions exhibiting inward spiraling motion and contraction can be extracted by combining LAVD with a contraction criterion. Applications to atmospheric and oceanic flows show that such behavior arises both in twisted LAVD fields generated at submesoscales and in mesoscale flows where it is enhanced by inertial effects, with finite-time contraction providing the dynamical constraint that isolates materially organized regions with elevated intrinsic rotation.