In Chapter 12 of the book, we describe in detail the solution to Laplace’s Tidal Equations (LTE), which were introduced in Chapter 11. Like the solution to the linear wave equation, where there are even (cosine) and odd (sine) natural responses, there are also even and odd responses for nonlinear wave equations such as the Mathieu equation, where the natural response solutions are identified as MathieuC and MathieuS. So we find that in general the mix of even and odd solutions for any modeled problem is governed by the initial conditions of the behavior along with any continuing forcing. We will describe how that applies to the LTE system next:
In Chapter 12 of the book, we provide a short introduction to ocean tidal analysis. This has an important connection to our model of ENSO (in the same chapter), as the same lunisolar gravitational forcing factors generate the driving stimulus to both tidal and ENSO behaviors. Noting that a recent paper  analyzing the so-called long-period tides (i.e. annual, monthly, fortnightly, weekly) in the Drake Passage provides a quantitative spectral decomposition of the tidal factors, it is interesting to revisit our ENSO analysis in the conventional ocean tidal context …
Woodworth and Hibbert  use selective bottom pressure recorder (BPR) measurements to extract the tidal factors. Below we reproduce the power spectrum and tidal model fit of the data from their paper.