Abstract
We address the problem of the oscillatory periods of two observed phenomena in the instability zone in the eastern equatorial Pacific. The first case concerns two high-speed anticyclonic flow with periods of approximately 12 and 15 mean solar days. The second concerns the long waves with an average period of about 25 days. The problem is that, in terms of their respective mean latitudes, these periods are about twice as large as those of the inertial period of one-half pendulum day. As a solution we offer the hypothesis of the half-inertial flow with its period of one pendulum day.
The inertial flow is governed by KV = −f, where K is the path curvature, V the speed of the flow, and f the Coriolis parameter. In contrast, the half-inertial flow is governed by KV = −f/2 with a speed that is the maximum for a given curvature and latitude and approaches twice that of the geostrophic speed.
In terms of the half-inertial flow, the average lone-wave period of 25 days would correspond to a plausible mean latitude of 2.3°N. Efficient at extracting horizontal shear energy, the half inertial flow could also play an important role in the near-surface heat balance.
