Figure 2: The seasonal cycle of the monthly stability a(t) as a function of latitude and longitude.
From: A unified nonlinear stochastic time series analysis for climate science
In (a1,b1,c1 and d1) we plot the values of a(t) determined from 133 years of monthly averaged GISS surface air temperature data from 1881 to 2013 discussed in Fig. 1 and in (a2,b2,c2 and d2) we plot the values of a(t) determined from 36 years of NOAA OI SST V2 data from 1981 to 2016. We observe a striking seasonal evolution in the eastern tropical Pacific, which is particularly strong in the longer GISS record. From January to June a strong stabilising process is operative and is reflected in the negative values of a(t), shown here for March (a1,a2) and May (b1,b2). In contrast, from July to December the values of a(t) change sign, signaling a destabilising process, shown here for August (c1,c2) and September (d1,d2). This demonstrates that in the eastern tropical Pacific the stability exhibits a two-season structure, which is a key ingredient in ENSO’s phase locking to the seasonal cycle. Note that, as always included in reanalysis data, in (a2,b2,c2 and d2), we used the land masked data, with 0 (1) over ocean (land), as described at http://www.esrl.noaa.gov/psd/repository/entry/show?entryid=b5492d1c-7d9c-47f7-b058-e84030622bbd.
