Abstract
Observed properties of rings are compared to rings produced in a two-gyre wind driven circulation model and in a model of the Agulhas retroflection. Their temporal evolution is discussed in terms of structure and translation rate. In both observations and numerical model results, propagation speeds 2 to 5 times faster than of an equivalent isolated eddy (which is of the order of the long Rossby wave speed) were observed. The decay rate of model rings with a lateral viscosity of 330m2s−1 is found to be faster than in observations. Furthermore, it is observed that the model rings have a coherent structure all the way to the bottom and it seems likely that this may also be the case in real oceanic rings. In the specific case of the model Agulhas ring, the factors influencing its motion and evolution are isolated in a series of subsidiary experiments. It is found that as the ring rounds the tip of Africa, there is only a small influence of the large scale flows on the ring propagation. On the other hand, the presence of the African continent provides an additional westward movement in addition to β. As soon as the ring drifts into the South Atlantic tropical gyre, advection by the large scale flows dominates the ring motion.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 337-356 |
| Number of pages | 20 |
| Journal | Elsevier Oceanography Series |
| Volume | 50 |
| Issue number | C |
| DOIs | |
| State | Published - Jan 1 1989 |
ASJC Scopus subject areas
- Oceanography
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Evolution of Rings in Numerical Models and Observations. / Chassignet, E. P.; Olson, D. B.; Boudra, D. B.
In: Elsevier Oceanography Series, Vol. 50, No. C, 01.01.1989, p. 337-356.Research output: Contribution to journal › Article › peer-review
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TY - JOUR
T1 - Evolution of Rings in Numerical Models and Observations
AU - Chassignet, E. P.
AU - Olson, D. B.
AU - Boudra, D. B.
N1 - Funding Information: One of the major differences between observed and model rings is in their decay rates. The decay rates of the model rings were found to be 4 to 6 times faster than in observed rings, and are apparently strongly influenced by the lateral viscosity. McWilliams and Flierl (1979), in their study of quasigeostrophic isolated vortices, stated that the vortex amplitude decay rate, in the limit of strong nonlinearity, is governed by the frictional coefficient rather than dispersion. The question arises as to what extent the decay is due to Rossby wave radiation (horizontal and vertical) in the above rings. In the case of an inviscid upper layer lens in the same parameter range as one of those studied here (REll), Flierl (1984) found a decay of the order of 7 years (- 2500 days) due to Rossby wave radiation in the lower layer. This is at least one order of magnitude slower than the decay rates obtained in the model. Thus, it seems most likely that, as in McWilliams and Flierl (1979), viscous effects are the predominant energy sink. It is felt that the use of a lateral viscosity of 50 to 100 mas-1 (instead of the current 330 m'8-l) might bring about comparable decay times between the modeled and observed rings. In the framework of this numerical model, such small viscosities could be employed only with a reduction in grid spacing to perhaps one-half the current 20 km. A successful intercomparison between observed and modeled rings provides the opportunity and justification to isolate the factors in the model influencing the ring motion and evolution, which is not possible with observations alone. This has been carried out by examining in some detail the motion and evolution of one E l l ring and comparing with the behavior of a similar ring in each of several subsidiary experiments. It is found that the presence of Africa provides a westward motion in addition to that due to p, and it seems that this owes to interaction between the ring and a high vortidty band along the no-slip boundary. This topic deserves further investigation in a study focused on eddy-wall interaction. There is apparently only a small influence from the external flows on the propagation of the ring during its passage from the Indian to the Atlantic basin. There is no continual flow between the two basins. It is only when the ring rounds the tip of Africa that a leakage is observed between the ring and the African continent. The advection by the large scale flows is found to dominate the motion once the ring drifts into the South Atlantic subtropical gyre. On the one hand, the relative simplicity of the model allowed the possibility to analyze some of the physical mechanisms behind ring propagation, such as advection by the mean flow or boundary influence. On the other, the effects of bottom topography/realistic coastline are also felt to be of importance, and the model's simplicity has, thus far, inhibited their determination. It is, therefore, expected that more insight into ring propagation and evolution can continue to be gained as the realism/complexity of the models is further increased. 6 ACKNOWLEDGEMENTS This work was supported by NSF grants OCE-8502126 and OCE-8600593 and by the Office of Naval Research grant No. N00014-87-G0116. Computations were carried out using the CRAY computers at the National Center for Atmospheric Research (NCAR). NCAR is sponsored by the National Science Foundation. 7 REFERENCES Basset, A.B., 1888: A treatise on hydrodynamics. Dover Publications, 2: 328 pp. Batchelor, G.K., 1967: An introduction to fluid dynamics. Cambridge University Press, 615 pp. Bleck, R. and D.B. Boudra, 1981: Initial testing of a numerical model ocean circulation model using a Hybrid (quasi-Isopycnic) vertical coordinate. J. Phy. Oceanogr., 11, 755-770. Boudra, D.B. and E.P. Chassignet, 1988: Dynamics of Agulhas retroflection and ring for-mation in a numerical model. Part I. The vorticity balance. J. Phys. Oceanogr., 18, 280-303. Boudra, D.B., K.A. Maillet, and E.P. Chassignet, 1989: Numerical modeling of Agulhas retroflection and ring formation with isopycnal outcropping. In Mesoscale Synoptic Coherent Structures in Geophysical Turbulence. J.C.J. Nihoul and B.M. Jamart, Eds. Elsevier, Amsterdam. Submitted. Brown, O.B., P.C. Cornillon, S.R. Emmerson and A.M. Carle, 1986: Gulf Stream warm rings: A statistical study of their behavior. Deep-sea Res., 53, 1459-1473. Chassignet, E.P. and D.B. Boudra, 1988: Dynamics of Agulhas retroflection and ring forma-tion in a numerical model. Part 11. Energetics - and ring - formation. J. Phys. Oceanogr., 18, 304-319. Cheney, R.E. and P.L. Richardson, 1976: Observed decay of a cyclonic Gulf Stream ring. Deeo-Sea Res.. 23. 143-155. COX, M.D., 1979: 'A numerical study of Somali Current eddies. J. Phys. Oceanogr., 9, 312-326. Flierl, G.R., 1977: The application of linear quasigeostrophic dynamics to Gulf Stream rings. J. Phys. Oceanogr., 7,3 65-379. Flierl, G.R., 1984: Rossby wave radiation from a strongly nonlinear warm eddy. J. Phys. Oceanogr., 14, 47-58. Holland, G.J., 1983: Tropical cyclone motion: Environmental interaction plus a beta effect. J. Atmos. Sci., 40, 328-342. Hooker, S.B., 1987: Mesoscale eddy dynamics by the method ofpoint vortices. Ph.D. Thesis, University of Miami, 158 pp. Lamb, H., 1932. Hydrodynamics. 6th ed. Cambridge University press, 738 pp. Lighthill, M.J., 1969: Linear theory of long waves in a horizontally stratified ocean of uniform depth (Appendix). Philos. Trans. R. Soe. London, 265, 85-92. McWilliams, J.C. amd G.R. Flierl, 1979: On the evolution of isolated non-linear vortices, with application to Gulf Stream rings. J. Phys. Oceanogr., 9, 1155-1182. Mied, R.P. and G.J. Lindemann, 1979: The propagation and evolution of cyclonic Gulf Stream rings. J. Phys. Oceanogr., 9, 1183-1206. Nof, D., 1981: On the P-induced movement of isolated baroclinic eddies. J. Phys. Oceanogr., 11, 1662-1672. Nof, D., 1988: Eddy-wall interactions. J. Mar. Res., 46, 527-555. Olson, D.B., 1980: The physical oceanography of two rings observed by the cyclonic ring experiment. 11. Dynamics. J. Phys. Oceanogr., 10, 514-528. Olson, D.B., R.W. Schmitt, M. Kennelly and T.M. Joyce, 1985: A two-layer diagnostic model of the long-term physical evolution of warm-core ring 82B. J. Geophys. Res., 90, 8813-8822. Olson, D.B. and R.H. Evans, 1986: Rings of the Agulhas. Deep-sea Res., SS, 27-42. Rossby, C.G., 1948: On displacements and intensity changes of atmospheric vortices. Mar. Res, 7, 175-187. Sommerfeld, A., 1950: Mechanics of defomable bodies. Academic Press, New York, 396 pp. Smith, D.C, IV, 1986: A numerical study of Loop Current eddy interaction with topography in the western Gulf of Mexico. J. Phys. Oceanogr., 16, 1260-1272.
PY - 1989/1/1
Y1 - 1989/1/1
N2 - Observed properties of rings are compared to rings produced in a two-gyre wind driven circulation model and in a model of the Agulhas retroflection. Their temporal evolution is discussed in terms of structure and translation rate. In both observations and numerical model results, propagation speeds 2 to 5 times faster than of an equivalent isolated eddy (which is of the order of the long Rossby wave speed) were observed. The decay rate of model rings with a lateral viscosity of 330m2s−1 is found to be faster than in observations. Furthermore, it is observed that the model rings have a coherent structure all the way to the bottom and it seems likely that this may also be the case in real oceanic rings. In the specific case of the model Agulhas ring, the factors influencing its motion and evolution are isolated in a series of subsidiary experiments. It is found that as the ring rounds the tip of Africa, there is only a small influence of the large scale flows on the ring propagation. On the other hand, the presence of the African continent provides an additional westward movement in addition to β. As soon as the ring drifts into the South Atlantic tropical gyre, advection by the large scale flows dominates the ring motion.
AB - Observed properties of rings are compared to rings produced in a two-gyre wind driven circulation model and in a model of the Agulhas retroflection. Their temporal evolution is discussed in terms of structure and translation rate. In both observations and numerical model results, propagation speeds 2 to 5 times faster than of an equivalent isolated eddy (which is of the order of the long Rossby wave speed) were observed. The decay rate of model rings with a lateral viscosity of 330m2s−1 is found to be faster than in observations. Furthermore, it is observed that the model rings have a coherent structure all the way to the bottom and it seems likely that this may also be the case in real oceanic rings. In the specific case of the model Agulhas ring, the factors influencing its motion and evolution are isolated in a series of subsidiary experiments. It is found that as the ring rounds the tip of Africa, there is only a small influence of the large scale flows on the ring propagation. On the other hand, the presence of the African continent provides an additional westward movement in addition to β. As soon as the ring drifts into the South Atlantic tropical gyre, advection by the large scale flows dominates the ring motion.
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U2 - 10.1016/S0422-9894(08)70195-8
DO - 10.1016/S0422-9894(08)70195-8
M3 - Article
AN - SCOPUS:77956811490
VL - 50
SP - 337
EP - 356
JO - Elsevier Oceanography Series
JF - Elsevier Oceanography Series
SN - 0422-9894
IS - C
ER -