| 15 Sep 2025
Inferring the role of IPO phase dependencies and extratropical internal variability on the tropics
Abstract. Regime dependencies and Granger causal relationships between tropical and extratropical teleconnections are inferred using Bayesian structure learning. Using ERA5 data, an examination of the differences between the learned graphical structures during particular phases of the Interdecadal Pacific Oscillation (IPO) are used to infer the role of the background state on interactions between the major climate teleconnections. These relationships present a clear regime dependency on the phase of IPO. In the positive phase, IPO autocorrelations are weak whereas Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) autocorrelations and the influence of the Madden Julian Oscillation (MJO) are indicative of an enhanced Walker circulation. In contrast, during the negative phase, IPO autocorrelations are strongest with evidence of an enhanced role for extratropical teleconnections on the tropics. Exclusion of MJO removes important tropical extratropical influences while increasing posterior edge weights between ENSO, the IPO and IOD. Our analysis reveals the dependence of the ENSO autocorrelation on the phase of the background IPO state, and the role of the MJO as being key to link the extra tropical tropospheric modes (PNA, NAO) and equatorial surface ocean temperatures (IOD, ENSO) and as a consequence convection.
The manuscript addresses causal relationships between climate mode indices. Even assuming that statistical associations between indices reflecting very distinct spatial and temporal scales of variability is a valid exercise to infer climate dynamics, in my opinion the manuscript needs to be revised for improved clarity and readability. The quality of the figures needs to be substantially improved.
Title: IPO should be replaced by Interdecadal Pacific Oscillation
The structure of section 2 is very confusing, including both data description and results before the actual description of the methodology used to produce the results. It’s not clear why Figure 1a) is not a separate figure and instead is mixed with results in figures 1b)-d). Furthermore, Figure 1a) is poorly described, the legend includes text such as “mw (x5), lp, nh=7, which is not described in the caption (and it’s not clear whether really needed or it’s relevance). What is gained from such smoothed signals in a relatively short time series is not evident, and should be better described in the manuscript.
The data description and it’s presentation in table 1 needs to be improved in order to clarify how each climate index was indeed calculated and at what the temporal resolution. The minimalist caption in Table 1 should be improved. What does the * means in RMM1 and RMM2?
Table 2 should be better described both in terms of the caption itself and in the text. The meaning of the “All years” column in Table 2 is not clear. More importantly, the objective criteria by which the positive and negative IPO phases are identified should be clearly and unambiguously stated.
The posterior probability plots should be improved (Figures 1b)-d), Figures 3, 4, and 5 by adding axes (as in Figure 1b), for consistency), and particularly to improve readability of the posterior probabilities, as in the current configuration it is not possible to effectively distinguish between low and intermediate probabilities. Maybe just providing the two highest probability ranges and distinguishing between them in another way other than width of the line (for example by using solid and dashed or dotted lines) would enable to reader to actually see the results, with the current design it’s almost impossible.