A Mechanism for the Recently Increased Interdecadal Variability of the Silk Road Pattern

Abstract

The Silk Road pattern (SRP) teleconnection manifests in summer over Eurasia, where it is associated with substantial temperature and precipitation anomalies. The SRP varies on interannual and decadal scales; reanalyses show an increase in its decadal variability around the mid-1970s. Understanding what drives this decadal variability is particularly important, because contemporary seasonal prediction models struggle to predict the phase of the SRP. Based on analysis of observations and multiple targeted numerical experiments, this study proposes a mechanism for decadal SRP variability. Causal effect network analysis confirms a positive feedback loop between the eastern portion of the SRP pattern and vertical motion over India on synoptic time scales. Anomalies over a larger region of subtropical South Asia can reinforce a background state that projects onto the positive or negative SRP through this mechanism. This effect is isolated and confirmed in targeted numerical simulations. The transition from weak to strong decadal variability in the mid-1970s is consistent with more spatially coherent interannual precipitation variability over subtropical South Asia. Furthermore, results suggest that oceanic variability does not directly force the SRP. Nevertheless, sea surface temperatures in the North Atlantic and the North Pacific may indirectly affect the SRP by modulating South Asian rainfall on decadal time scales.

Dominant patterns of interaction between the tropics and mid-latitudes in boreal summer: Causal relationships and the role of time-scales

“…This finding is also in agreement with previous work showing that, at decadal time-scales, the CGT and Silk Road (SRP) patterns intensify under PDO negative (i.e. La Niña-like) forcing (Stephan et al, 2019). During El Niño summers, the SAM shows a more prominent effect on the tropical Pacific.…”

“…Latent heat release due to strong monsoonal rainfall can influence subtropical and mid-latitude regions via Rossby wave teleconnections. The SAM has been connected to subtropical arid conditions in the North African region via the so-called monsoon -desert mechanism, creating reinforced descending motions over the Sahara during strong SAM phases (Rodwell and Hoskins, 1996;Stephan et al, 2019). This mechanism is fairly well captured by both climate (Cherchi et al, 2014) and seasonal forecast models (Beverley et al, 2019).The SAM is also connected to mid-latitude circulation via its interaction with the circumglobal teleconnection pattern (CGT), a wave pattern with 5 centres of action encircling the northern mid-latitudes and affecting temperature and precipitation there (Ding and Wang, 2005;Kripalani et al, 1997).…”

Tropical and mid-latitude teleconnections interacting with the Indian summer monsoon rainfall: a theory-guided causal effect network approach

“…Thus, we argue that the CGTI region and the midlatitude circulation are important for both the intraseasonal and seasonal variability of the MT rainfall. An explanation for this could be that the seasonal and/or monthly representation of the monsoon-circumglobal teleconnection interaction is actually strongly influenced by mechanisms that act on intraseasonal timescales, similarly to what has been proposed by Stephan et al (2019), who come to a similar conclusion but analysing the causal interaction between the Silk Road pattern and the monsoon-desert mechanism. Here, a hint in this direction is provided by the analysis of the monsooncircumglobal teleconnection link at a monthly timescale: while the spatial correlation between the MT rainfall and the Z200 field at lag 0 strongly resembles the circumglobal teleconnection pattern (when monthly averages are analysed), this similarity disappears when Z200 leads the MT rainfall by 1 month (see the Supplement, Fig.…”

“…However, here we focus on the relationship between the MT rainfall and the circumglobal teleconnection pattern; thus, a link from the MT rainfall towards the Saharan region cannot be inferred from this analysis. Moreover, the causal relationship between the north-eastern Indian rainfall and the downdraft over the north-eastern Sahara related to the monsoon-desert mechanism has already been shown (Stephan et al, 2019).…”

“…For a given set of time series, a CEN reconstructs the likely underlying causal structure by iteratively testing for conditional independent relationships among the input time series. CENs have been applied to the study of stratospheric polar vortex variability (Kretschmer et al, 2016(Kretschmer et al, , 2018, the multi-decadal North Atlantic overturning circulation (Schleussner et al, 2014), the intraseasonal stratosphere-troposphere coupling in the Southern Hemisphere (Saggioro and Shepherd, 2019), and the causal interactions between the ISM, the Silk Road pattern and the monsoon-desert mechanism (Stephan et al, 2019). Although shown to be a useful statistical tool to study teleconnection pathways, a successful application of CEN requires (such as any data-driven method) expert knowledge of the underlying physical processes, including the relevant variables, timescales and temporal resolution (Runge, 2018).…”