Interactions of the Loop Current and
adjacent rings in the Gulf of Mexico
In collaboration with M.Toner and
A.D. Kirwan, University of Delaware, and L. Kantha and J. Choi,
University of Colorado
We apply dynamical systems methodology to
investigate the Lagrangian dynamics of the Loop Current (LC) and the adjacent
mesoscale rings over a 20-day period, starting June 1, 1998.
Observational data (altimetry, drifters) and a model velocity field
(University of Colorado real-time nowcast/forecast version of the
Princeton Ocean Model) were used in the study.
A Lagrangian approach provides detailed information about ring interaction
and reveals features of the ring dynamics that are not apparent
in the Eulerian picture. During the observation period, a new ring was formed
in the meander of the LC and a large anticyclonic ring to the west
was cleaved by a cyclonic eddy.
The coherent structures were identified in the Lagrangian framework
by means of Effective Invariant Manifolds (EIM).
EIMs divide the flow into the regions with distinguished dynamic fate,
acting as boundaries between coherent structures.
Concurrent drifter trajectories are in qualitative agreement with
the results of the Lagrangian analysis based on the model velocity field.
The hyperbolic regions, attendant to the
mesoscale coherent structures provide sufficient stretching/compression
required for the existence of robust, meaningful EIMs. The manifolds
delineate the coherent features and afford a tracking of their evolution.
The dynamics of the manifold pattern describes the interaction between
the LC and the rings in considerable detail.
The amount of water involved in the coherent motion, the character of the
water exchange between the rings, the activity of the small-scale mixing in
different flow regions -- all these
important features of transport can be obtained only within the
Lagrangian approach.
The trajectories of the drifting buoys, passing through
the region provide independent evidence of the existence of the
large anticyclonic ring and of the smaller cyclonic eddy
between the ring and the LC.
The results derived from the model and the experimental drifter data
agree on the qualitative level, but do not coincide in detail.
Since POM assimilates only the Eulerian data, these discrepancies
are not surprising. They manifest the value of
the Lagrangian drifter data, which carries independent information about the
flow. Incorporation of this information into ocean models would be an
important step in improving their fidelity.
Leonid Kuznetsov
email leonid@cfm.brown.edu
