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Correlation does not necessarily imply causation, and this is why causal methods have been developed to try to disentangle true causal links from spurious relationships. In our study, we use two causal methods, namely the Liang-Kleeman information flow (LKIF) and the Peter and Clark momentary conditional independence (PCMCI) algorithm, and apply them to four different artificial models of increasing complexity and one real-case study based on climate indices in the North Atlantic and North Pacific. We show that both methods are superior to the classical correlation analysis, especially in removing spurious links. LKIF and PCMCI display some strengths and weaknesses for the three simplest models, with LKIF performing better with a smaller number of variables, and PCMCI being best with a larger number of variables. Detecting causal links from the fourth model is more challenging as the system is nonlinear and chaotic. For the real-case study with climate indices, both methods present some similarities and differences at monthly time scale. One of the key differences is that LKIF identifies the Arctic Oscillation (AO) as the largest driver, while El Niño-Southern Oscillation (ENSO) is the main influencing variable for PCMCI. More research is needed to confirm these links, in particular including nonlinear causal methods. [ABSTRACT FROM AUTHOR]

We apply Causal Effect Networks to evaluate the influence of spring North Atlantic extratropical surface temperatures (SST) on the summer East Atlantic Pattern (EA) seasonal predictability during the 20th century. We find in the ERA-20C reanalysis that a meridional SST gradient in spring (SST index) causally influences the summer EA, with an estimated causal effect expressed by a β -coefficient of about 0.2 (a 1 standard deviation change in spring SST index causes a 0.2 standard deviation change in the EA 3–4 months later). We only find this link to be causal, however, during the period 1958–2008. When performing the analysis on 45-year-long timeseries randomly sampled in this late period, we find the strength of the causal link to be affected by interannual variability, suggesting a potential modulation by an external physical mechanism. In addition to the summer EA, we find that spring SST has an estimated causal effect of about -0.2 on summer 2-metre air temperatures over northwestern Europe, possibly mediated by summer EA. We then use a pre-industrial and a historical simulation, as well as a 30-member initialised seasonal prediction ensemble with MPI-ESM-MR to test the model performance in reproducing the detected causal links in ERA-20C and to evaluate whether this performance might leave an imprint in the model predictive skill of European summer climate. We find that while MPI-ESM-MR is mostly unable to reproduce the causal link between spring SST and the summer EA among the different datasets, the 30-member initialised ensemble can moderately reproduce a causal link between spring SST and summer 2-metre air temperatures over a region west of the British Isles. We perform a predictive skill assessment conditioned on the spring SST causal links for July–August sea level pressure, 500 hPa geopotential height and 2-metre air temperatures for predictions initialised in May. Our results suggest that MPI-ESM-MR's performance in reproducing the spring SST causal links constrains the seasonal prediction skill of European summer climate. [ABSTRACT FROM AUTHOR]

Much of the forecast skill in the mid-latitudes on seasonal timescales originates from deep convection in the tropical belt. For boreal summer, such tropical-extratropical teleconnections are less well understood as compared to winter. Here we validate the representation of boreal tropical – extratropical teleconnections in a general circulation model in comparison with observational data. To characterise variability between tropical convective activity and mid-latitude circulation, we identify the South Asian monsoon (SAM) – circumglobal teleconnection (CGT) pattern and the western North Pacific summer monsoon (WNPSM) – North Pacific high (NPH) pairs as the leading modes of tropical-extratropical coupled variability in both reanalysis (ERA5) and seasonal forecast (SEAS5) data. We calculate causal maps, an application of the PCMCI causal discovery algorithm which identifies causal links in a 2D field, to show the causal effect of each of these patterns on circulation and convection in the Northern Hemisphere. The spatial patterns and signs of the causal links in SEAS5 closely resemble those seen in ERA5, independent of the initialization date of SEAS5. However, the strength of causal links in SEAS5 is often too weak (about two thirds of those in ERA5). By performing a subsampling (over time) experiment, we identify those regions for which SEAS5 data well reproduce ERA5 values, e.g. South-eastern US, and highlight those where the bias is more prominent, e.g. North Africa. We demonstrate that different El Niño – Southern Oscillation phases have only a marginal effect on the strength of these links. Finally, we discuss the potential role of model mean-state biases in explaining differences between SEAS5 and ERA5 causal links. [ABSTRACT FROM AUTHOR]

The Indian summer monsoon (ISM) rainfall activity at sub-seasonal timescales is characterized by the alternation of active and break phases in each ISM season. Both tropical and mid-latitude drivers have been detected to that influence the ISM circulation. The circumglobal-teleconnection (CGT) is an observed circulation pattern in boreal summer, which drives sub-seasonal precipitation and temperature anomalies across the northern mid-latitudes. The CGT is hypothesized to serve as a connector between the mid-latitude circulation and the ISM system. Moreover, the role of the mid-latitude circulation during extreme rainfall events which affect the Himalayan foothills, such as the Pakistan flooding in 2010, has also been detected. Here, we use causal discovery tools to quantify the two-way causal links between the ISM and the CGT. Our analysis shows a robust causal link from the CGT and the North Atlantic Oscillation (NAO) to the ISM rainfall. A reversed link connects the ISM rainfall back to the CGT. We highlight the pathways that explain this link and show that other components of the ISM system, such as vertical transport of moisture, are crucial in propagating the corresponding signal. We thus support the hypothesis of a two-way interaction between the ISM rainfall and the mid-latitude teleconnection. Moreover, our causal discovery tools correctly identify the different tropical and mid-latitude drivers that determine the ISM circulation at 1 to -2-week lead time. [ABSTRACT FROM AUTHOR]