ISSN 0001-4370, Oceanology, 2014, Vol. 54, No. 6, pp. 688–694. © Pleiades Publishing, Inc., 2014. Original Russian Text © B.N. Filyushkin, M.A. Sokolovskiy, N.G. Kozhelupova, I.M. Vagina, 2014, published in Okeanologiya, 2014, Vol. 54, No. 6, pp. 737–743. 688 Intrathermocline anticyclonic eddies (lenses) are regularly observed at mid-latitudes in the northeastern Atlantic ocean at intermediate depths of 500–1500 m [1, 6, 12]. They are shaped as elliptic structures with horizontal axes from 40 to 100 km and the vertical ones from 0.4 to 0.9 km and filled with warm and salty Mediterranean waters. The difference of the water characteristics in the lens core and the surrounding waters varies from 1 to 4°C in temperature and from 0.3 to 1.0 PSU in salinity depending on the distance of the lens from the region of its formation. The lifetime of such eddies ranges from 3 to 7 years. Roughly 150– 200 eddies may simultaneously occur in this part of the ocean [3, 9]. These lenses are usually detected due to hydrologi- cal area surveys, in vertical sections, and in course of analysis of the profilograph observational data obtained during the Argo Global Oceanographic Project (the name goes back to the legendary ship of the Argonauts in Greek mythology). During the process of these eddies’ interaction with the ambient water, they can transmit their dynamical signal to the surface of the ocean [1, 4, 8]. These vortices are identifiable in altim- etric satellite images of the ocean surface level [4, 8], as well as from anomalies of SST [14, 15]. Thus, the satellite observations allow to obtaining the evolution pattern both of the surface eddies and the processes of merging and separation of intrather- mocline lenses and their interaction with different fea- tures of the bottom relief. Unfortunately, as yet, the correlation of eddy positions obtained from satellite observations and hydrological measurements is insuf- ficiently reliable. For these reasons, it is expedient to use neutral buoyancy floats (NBF) for long-term observations of the eddy travels in the ocean by deploying the NBFs into the lens bodies. A large-scale experiment was ful- filled in 1984–1986 when the displacements of three lenses were monitored for two years by acoustic NBFs [6, 12]. These lenses measured about 100 km in diam- eter and roughly 800 m in thickness, and their cores were located at a depth of about 1100 m. In the present study we examine some theoretical aspects of the lens observations by applying the both methods. We use the three-layer quasi-geostrophic model [2, 3], where the lenses are understood as the middle layer eddy patches, while the NBFs are related to the cen- ters of passive small initially circular domains of fluid belonging either to the lens or to its vicinity. The model parameters are: the average ocean depth being of H = 4000 m, the upper, middle, and lower layers are consid- ered to be H 1 = 600 m, H 2 = 1000 m, and H 3 = 2400 m thick (for the dimensionless layer thicknesses, we have h 1 = 0.15, h 2 = 0.25, and h 3 = 0.6), while the first and the second radii of deformation are Rd 1 = 32 km and Rd 2 = 15 km. These values are characteristic of the northeastern Atlantic. Under such conditions, the middle layer occupies the depths from 600 to 1600 m. We take Rd 1 as the horizontal length scale. Lagrangian Methods for Observation of Intrathermocline Eddies in Ocean B. N. Filyushkin a , M. A. Sokolovskiy b , N. G. Kozhelupova a , and I. M. Vagina c a Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia e-mail: borfil@ioran.ru; nk@ioran.ru b Water Problems Institute, Russian Academy of Sciences, Moscow, Russia e-mail: sokol@aqua.laser.ru c Moscow State University, Moscow, Russia e-mail: iravag@rambler.ru Received April 8, 2014 Abstract—Intrathermocline anticyclonic eddies (lenses) of Mediterranean origin are regularly observed in the Eastern part of the Atlantic Ocean. These eddies are identified both from satellites as altimetry and sea- surface temperature (SST) changes and according to data of neutral buoyancy floats (NBF) placed in the body of a lens. In this paper, in the framework of a three-layer quasi-geostrophic model, using the contour dynamics method, we consider some theoretical aspects of lens movement observations made by acoustic NBF and freely drifting buoys of the Argo project. Direct experimental observation data on the lenses' drift in the North Atlantic qualitatively confirmed the results of our numerical experiments. In particular, it is shown that the spin of the lens has an advective influence on the behavior of NBF at distances of several lens radii. DOI: 10.1134/S0001437014050051 MARINE PHYSICS