Another important geostrophic wave in Lake Ontario is the Poincaré wave. The Poincaré wave has a more complex structure and may be visualized as a combination of two sinusoidal waves with equal amplitude, wavelength and frequency. The two sinusoidal waves travel in directions that form equal angles with the main axis of the basin [7].
Poincaré waves are oscillations in the thermocline across the entire lake and in Lake Ontario (and northern hemisphere) have an anticyclonic phase propagation [1]. Poincaré wave amplitudes do not decrease exponentially away from the shoreline as Kelvin wave do, but are dispersive waves. It is thought that the reflection of Poincaré waves at the shoreline may generate Kelvin waves.
Vertical cross-section of a Poincaré wave in 2D is similar to that of a seiche, but in the horizontal plane, the wave-induced velocity is a clockwise rotation ( in the northern hemisphere) of velocity vectors that results in horizontal orbital transport as opposed to the linear back-and-forth motion of a seiche [6]. Like with seiches large amplitude Poincaré waves occur after storms and they decay with a half-life on the order of several internal periods. Most effective in exciting a given Poincaré wave mode is a wind-stress episode lasting for half a wave period.
In Lake Ontario the lowest modes have periods close to the internal period at this latitude (~17 hours) , which is excited with the highest energy transfer by wind-stress impulses of about 8 hours in duration (which is the typical lifetime of a strong wind-stress episode at mid-latitudes). Observations show that in Lake Ontario during the thermal stratification period the thermocline oscillates with of a periods slightly under the inertial period and can be considered Poincaré waves.