Figure 5
Observed signatures of NIW: (a, b) near-inertial velocities in zonal and meridional directions, \((U_{NI},V_{NI})\), respectively; (c) wave phase \(\Phi(t,z)=\mathrm{tan^{-1}}(U_{NI},V_{NI})\); (d) \(\Phi(t)\) at depth of 153 m shown by black dots. In (d), blue lines show the linear regression for the phase evolution deduced from horizontal velocity vectors in (c) and expressed by \(\Phi(t)=-2{\uppi}{\upomega}(t-t_{0})\), where \(\upomega=1.05f\) and \(t_{0}\) is the initial time set to 00:00 UTC on September 25. In (e), hodographs of vertical shear vector for the near-inertial horizontal current, \((U_z,V_z)\), are shown, for the periods of 48 h, approximately equivalent to 2.5 inertial cycles. Black dots show the initial point. Ellipses in red show the best-fit regressions with the semi-major and semi-minor axes, \(a\) and \(b\) respectively, which are estimated by the least-square criterion technique. In (f–h), characteristics of wave properties extracted from the horizontal orbits of the ellipse are shown: (f) mean radius \(\left(a+b\right)/2\); (g) major/minor ratio \(r=a/b\); (h) tilt angle \(\upphi\), with positive values in clockwise.
