To facilitate an easy presentation, all variables and equations in the remaining part of the paper have been non-dimensionalized by using \(\sqrt{1/(c\beta)}\) for the time scale and \(\sqrt{c/\beta}\) for the length scale, where β is the beta parameter evaluated at the equator and \(c=\sqrt{gh_e}\), where g is gravity and he is the equivalent depth of the shallow-water system. A convenient set of meridional basis functions in the equatorial domain is that of the PCFs, ψn(y), centered at the equator. Any horizontal instantaneous flow field on a discrete grid may be expanded as \begin{equation} \lambda'(x,y)=\sum_{k=-K}^K\sum_{n=0}^N\chi_{k,n}^\lambda \psi_n(y)e^{ikx} , \ \ (1)\end{equation} where Λ is a dummy flow variable, which can be zonal (u) or meridional (v) wind or geopotential height (φ), and the prime denotes the departure field from its zonal mean. Here, zonal wavenumber k runs from -K to K, the Nyquist sampling wavenumber. The meridional mode number n is a non-negative integer, and N is the largest meridional mode considered, either by choice or constraint due to meridional resolution. By projecting a flow field Λ'(x,y) onto ψn(y)eikx, the projection coefficient χk,nΛ may be easily retrieved.
The same PCFs also appear in the construction of free solutions to the linearized equatorial β-plane shallow-water equations associated with the so-called "Matsuno modes" (Matsuno, 1966; Longuet-Higgins, 1968). Each meridional mode supports at most three distinct wave classes, whose frequencies are the roots of the equatorial dispersion relation for that mode number n. As a result, we can alternatively expand an instantaneous flow using the equatorial wave solutions, the Matsuno modes, as basis functions. The dispersion relation for equatorial waves is \begin{equation} \omega^2-k^2-\dfrac{k}{\omega}=2n+1 . \ \ (2)\end{equation} The dispersion relation yields three distinct real roots for each pair of zonal wavenumber k and meridional mode number n (n≥ 0), corresponding to a pair of eastward- and westward-propagating mixed Rossby-gravity waves for n=0 or a triplet of eastward- and westward-propagating inertia-gravity waves and a westward-propagating Rossby wave (n>0). Note that for n=0, the third root does not correspond to a physical solution since its eigenvector does not satisfy the boundary conditions. Also, Eq. (2) has one special root for n=-1; namely, ω=k, corresponding to a (eastward-propagating) Kelvin wave. As in (Matsuno, 1966), we use ωk,n(j) with the root index j=1,2,3 to denote these roots, following the convention j=1 for Kelvin (K), eastward-propagating mixed Rossby-gravity (EMRG) or eastward-propagating inertia-gravity (EIG) waves; j=2 for westward-propagating mixed Rossby-gravity (WMRG) or westward-propagating inertia-gravity (WIG) waves; and j=3 for westward-propagating Rossby (R) waves. Then, the Matsuno modes of (n,j,k) can be written as \begin{align} U_{k,n}^{(j)}(x,y)&=\dfrac{i}{2}[A_{k,n}^{(j)}\psi_{n+1}(y)-B_{k,n}^{(j)}\psi_{n-1}(y)]e^{ikx} ,\ \ (3a)\\ V_{k,n}^{(j)}(x,y)&=\psi_n(y)e^{ikx} ,\ \ (3b)\\ \Phi_{k,n}^{(j)}(x,y)&=\dfrac{i}{2}[A_{k,n}^{(j)}\psi_{n+1}(y)+B_{k,n}^{(j)}\psi_{n-1}(y)]e^{ikx} , \ \ (3c)\end{align} where $$ \left[ \begin{array}{l@{\quad}l} A_{k,n}^{(j)}=\dfrac{\sqrt{2(n+1)}}{\omega_{k,n}^{(j)}-k} & {\rm for}ñ\geq 0\\ A_{k,n}^{(1)}=1 & {\rm for}ñ=-1 \end{array} \right. $$ and \begin{equation} B_{k,n}^{(j)}=-\dfrac{\sqrt{2n}}{\omega_{k,n}^{(j)}+k}\quad {\rm for}ñ\geq 0 . \ \ (4)\end{equation} Note that in equations (3a), (3b), and (3c), Uk,n(j)(x,y), Vk,n(j)(x,y), and Φk,n(j)(x,y) are, respectively, zonal velocity, meridional velocity, and geopotential height fields of a Matsuno mode (n, j, k). Bk,n(j) do not exist (or are zero) for all n<1 and Ak,-1(j) exists only for j=1, and all other Ak,-1(j) are undefined (or zero) since there are no roots at n=-1 other than ωk,-1(1)=k for Kelvin waves. The fields defined in Eq. (3) for each (n,j) pair of a given k serve as a set of basis functions for their respective variables; namely, \begin{align} u'(x,y)&=\sum_{k=-K}^K\sum_{n=-1}^N\sum_{j=1}^3\alpha_{k,n}^{(j)}U_{k,n}^{(j)}(x,y) ,\ \ (5a)\\ v'(x,y)&=\sum_{k=-K}^K\sum_{n=-1}^N\sum_{j=1}^3\alpha_{k,n}^{(j)}V_{k,n}^{(j)}(x,y) ,\ \ (5b)\\ \phi'(x,y)&=\sum_{k=-K}^K\sum_{n=-1}^N\sum_{j=1}^3\alpha_{k,n}^{(j)}\Phi_{k,n}^{(j)}(x,y) , \ \ (5c)\end{align} where αk,n(j) is the coefficient of the (n,j) equatorial wave class for zonal wavenumber k. Because the PCFs, ψn(y), exist only for non-negative n, Eq. (3) indicates that the Matsuno mode for Kelvin waves is represented by Uk,-1(1) and Φk,-1(1) (i.e., v'=0 for Kelvin waves).
We next equate the mathematical construction of the total wave field, Eq. (1), where we use the PCFs as a basis, with the physical construction, Eq. (5), where we write the wave fields using the Matsuno modes as the basis set. After some laborious but otherwise straightforward manipulations, we have \begin{eqnarray} 2A_{k,-1}^{(1)}\alpha_{k,-1}^{(1)}&=&\chi_{k,0}^{\phi}+\chi_{k,0}^u , \ \ (6)\\ \left[ \begin{array}{c@{\quad}c} 2A_{k,1}^{(1)} & 2A_{k,1}^{(2)}\\ 1 & 1 \end{array} \right]\left[ \begin{array}{c} \alpha_{k,0}^{(1)}\\ \alpha_{k,0}^{(2)} \end{array} \right]&=&\left[ \begin{array}{c} \chi_{k,1}^{\phi}+\chi_{k,1}^u\\ \chi_{k,0}^v \end{array} \right],\ \ (7)\\ \left[ \begin{array}{c@{\quad}c@{\quad}c} 2A_{k,n}^{(1)} & 2A_{k,n}^{(2)} & 2A_{k,n}^{(3)} \\ 1 & 1 & 1 \\ 2B_{k,n}^{(1)} & 2B_{k,n}^{(2)} & 2B_{k,n}^{(3)} \end{array} \right] \left[ \begin{array}{c} \alpha_{k,n}^{(1)}\\ \alpha_{k,n}^{(2)}\\ \alpha_{k,n}^{(3)} \end{array} \right]&=& \left[ \begin{array}{c} \chi_{k,n+1}^{\phi}+\chi_{k,n+1}^u\\ \chi_{k,n}^v\\ \chi_{k,n-1}^{\phi}-\chi_{k,n-1}^u \end{array} \right],\ {\rm for}ñ\!\geq\! 1 . \ \ (8)\nonumber\\ \end{eqnarray} By solving Eqs. (6) to (8) for the vector containing the αk,n(j) coefficient(s) from the projections of (u',v',φ') on the PCFs, we obtain the coefficient of Kelvin waves from Eq. (6), the coefficients of the pair of eastward- and westward-propagating mixed Rossby-gravity waves from Eq. (7), and the coefficients of all inertia-gravity and Rossby wave triplets from Eq. (8). Specifically, the application of EWEIF to an instantaneous flow field is done with the following steps: (i) project u',v', and φ' onto the PCFs and perform a zonal FFT on those coefficients to retrieve χk,nu,χk,nv, and χk,nφ for all k and n; (ii) calculate Ak,n(j) and Bk,n(j) for each j, n, and k; (iii) plug the results of (i) and (ii) into Eqs. (6) to (8) and then invert Eqs. (6) to (8) for each k and any desired n to solve the vector of αk,n(j) coefficient(s).
The system of Eqs. (6) to (8) describes a complete set of equations for expanding an instantaneous flow of a shallow-water equation model into its constituent equatorial wave class fields with a presumed equivalent depth uniquely. For this reason, we refer to it as the method of equatorial wave expansion of instantaneous flows (EWEIF). The coefficient αk,n(j) of each Matsuno mode of (n,j) can be calculated uniquely for a given instantaneous horizontal wave field, meaning that a full equatorial wave class field may be reconstructed using \begin{eqnarray} u'^{(j)}_n&=&\sum_{-K}^Ka_{k,n}^{(j)}U_{k,n}^{(j)} ,\nonumber\\ v'^{(j)}_n&=&\sum_{-K}^Ka_{k,n}^{(j)}V_{k,n}^{(j)} , \ \ (9)\\ \phi'^{(j)}_n&=&\sum_{-K}^Ka_{k,n}^{(j)}\Phi_{k,n}^{(j)}\nonumber \end{eqnarray} for each pair of (n,j), with the reminder that we only have j=1 for n=-1 (in which Vk,-1(1)=0); j=1 and 2 for n=0; and j=1,2, and 3 for n≥ 1. The summation over all n and j will enable us to recover the original instantaneous horizontal wave field. In summary, one may follow the procedures below to obtain individual equatorial waves from instantaneous wave fields of (u',v',φ'):
(1) Obtain (χk,nu,χk,nv,χk,nφ), by projecting (u',v',φ') on the PCFs in y and individual zonal waves in x according to Eq. (1). This yields all projection coefficients on the right-hand side of Eqs. (6) to (8);
(2) Invert the linear equations, Eqs. (6) to (8), to obtain αk,n(j), which are "unknowns" in these equations;
(3) Use Eq. (9) to obtain the fields (or maps) of individual waves (a single k, j, and n), or individual wave classes (e.g., a single j and n by summing up over k) or the total field (summing up over all k, j, and n).
Note that the temporal information of Matsuno modes or wave classes in the data is contained in the projection coefficients of (u',v',φ') on the PCFs because instantaneous fields at different times have different values of (χk,nu,χk,nv,χk,nφ). By employing Eqs. (6) to (8) at each instantaneous time, we will obtain the temporal evolutions of all individual wave classes of equatorial waves from the data.
In the next three sections, we will validate and illustrate EWEIF for three scenarios. For the first scenario (section 3), we reconstruct a series of temporally evolving wave fields based on the exact (linear) solution of the equatorial β-plane shallow-water equation model, and then apply EWEIF to isolate the individual waves contained in the constructed wave fields. For the second scenario (section 4), we use a numerical model of a linear equatorial β-channel shallow-water equation model, starting at an arbitrarily specified local perturbation field, to generate temporally evolving wave fields. Then, we apply EWEIF to identify all individual waves from the output fields of the numerical model. For the final scenario (section 5), we briefly detail and present results of the application of the EWEIF method to the ERA-Interim dataset, in which we illustrate its utility in analyzing real data.