Decay of Agulhas Rings Due to Cross-Frontal Secondary Circulations
Abstract
A series of idealized numerical experiments is presented to study the evolution of an Agulhas ring. The modeled ring is a circular symmetric idealization of ring Astrid, as measured by a cruise in March 2000. In particular, the hypothesis is tested that mixing is mainly due to cross-frontal secondary circulations. Both the role of beta-decay and shear instabilities in setting-up cross-frontal mixing is studied. The largest mixing of ring water with its surroundings occurs in the first months after shedding when the heat loss is maximal. Therefore, it is tested whether buoyancy forcing is able to enhance cross-frontal circulations. It is concluded that for realistic initial conditions the observed decay can be accounted for by adiabatic decay associated with cross-frontal circulations. The adiabatic decay appears to be a nonlinear function of the ring strength, possibly enhanced by the instability of the ring. The relative decay for weaker, stable rings is more than twice as weak. For a wide parameter range all modeled rings are unstable for a mode-2 perturbation and split. Observations suggest that most rings do not split and are not as unstable as suggested by the model. What causes this mismatch and how it affects the decay rate is still unclear. Realistic cooling is observed to slightly retard the split and marginally stabilize the ring. The decay rate is enhanced with typically 30%. For weaker rings the impact of cooling becomes less.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 19, 2001
- Accession Number
- ADP013581
Entities
People
- L. De Steur
- P. J. Van Leeuwen
- S. S. Drijfhout
Organizations
- Royal Netherlands Meteorological Institute