Figure 4: The noise intensity N(t) constructed from monthly NOAA OI SST V2 data from December 1981 to January 2016 available at http://www.esrl.noaa.gov/psd/repository/entry/show?entryid=b5492d1c-7d9c-47f7-b058-e84030622bbd, as also described in Fig. 2.
From: A unified nonlinear stochastic time series analysis for climate science

We see that during March (a), April (b) and May (c), a “tongue” of N(t) grows from the boundary of eastern tropical Pacific (which may be related to coastal upwelling) and then extends toward the centre of tropical Pacific. During the same period the monthly stability a(t) is negative in the eastern tropical Pacific, indicating the dominance of stabilising processes.