Fig. 5. Drifter trajectories fragments with westward velocity component, superimposed on regions (shaded) of westward mean velocities.
Mean zonal velocity field (Fig. 4a) consists of two eastward jets (KE at 32-36｡? and SF at 3 7-43｡?), when maximum of its variability (Fig. 4b) is unique at every section and correlates well with KE location. By comparing Figs.4a and b, one can see that SF jet, being almost as fast as KE, is much more stable than the later. This can make difficulties for SF detection from satellite altimetry data.
Surprisingly, five of seven sections demonstrate also mean westward flow between KE and SF. “Wave-like” horizontal structure of velocity field in KE have been recognized before [4], but was attributed to eddy activity. Results of analysis of westward fragments of drifter trajectories (Fig. 5), superimposed on the regions of westward mean velocities, give the idea of possibility of continuous mean westward flow in this area. When most of drifters there demonstrate fast rotation in eddies of both signs, some others were advected smoothly through more than 10°E. The existence of this countercurrent should be clarified with more data and independent methods and measurements.
Peculiarities of circulation in the Subtropical Gyre (STG)
Insufficiency of data and their inhomogeneous distribution limits the parameters that can be reliably derived from drifter ensemble. At the same time, Lagrangian measurements can provide new ideas of some features of oceanic circulation. For example, Fig. 6 shows distribution of eddies in the NWP. It is estimated for eddy-like motion of drifter for more than two circuits about the eddy center. One can see cyclones just north of KE and anticyclones south of it, which are, probably KE detached meanders (rings). Some cyclones are also detected around the beginning of Kuroshio, when central STG is filled with anticyclones. At present, it is not clear if they are related to STCC or analogous to tropical instability waves, famous in the eastern Pacific. Remarkable also are eddies, formed by the North Equatorial Current, flowing around Hawaii Islands.
