Following from the previous comment, this is the phase-space (Lissajous) pattern of ENSO model amplitude vs. dAmplitude/dt. This only features the primary dipole LTE modulation of ~2.57, so it pushes the modulation "loop eyes" toward the
center, as indicated by the yellow
The shape is a parallelogram which may reflect hysteresis, via the lag after the seasonal impulse. This is substantiation of a charge-discharge type mechanism, where the lag after the seasonal impulse is the slow discharge, and the tidal force at the impulse defines the charge.
This is what the equivalent looks like for QBO, which features a lower LTE modulation since the QBO is closer to a 0 wavenumber (monopole behavior instead of dipole).
See this for a plasma charge-discharge https://electronics.stackexchange.com/questions/122829/gas-discharge-v-q-lissajous. The chart below is easier to understand because it doesn't have the non-linear Mach-Zehnder-like LTE modulation
There are other examples of this hysteresis-like behavior involving examples of stiction (sticky friction that gets released) and in seismology with the same sticky release on faults but w/o the cyclic behavior. One reason to keep up-to-date with earthquake-related research.