Figure 4.

Wavelet analysis of the transient relationship between cholera incidence in Ghana and SOI. (a) (i) the analysed time-series: the cholera incidence (solid lines) and the SOI (dashed lines). The incidence series are square root transformed, and all series are filtered and normalized before analyses. (a) (ii) the Fourier spectrum for the cholera incidence series (solid line) and for the SOI (dashed line). The y-axis describes period (in year) as for other y-axis of (b–d) graphs. (b) (i) wavelet power spectrum (S_x(a, τ)) of the cholera incidence. The colours code for power values from dark blue, low values, to dark red, high values. On this graph, the white line shows the maxima of the undulations of the wavelet power spectrum. (b) (ii) the average wavelet spectrum (${\overline{S}}_x(a)$) of cholera incidence. (c) as in (b) but for the SOI. (d) (i) Wavelet coherence between cholera incidence in Ghana and the SOI. The colours are coded a dark blue, low coherence and dark red, high coherence. (d) (ii) Classical Fourier coherence between cholera incidence and the SOI. On (b–d), the dotted-dashed lines showed the α=5% significant levels computed based on 1000 bootstrapped series and on the two-dimensional graphs the cone of influence, which indicates the region not influenced by edge effects, is also shown.