Dominant patterns of interaction between the tropics and mid-latitudes in boreal summer: causal relationships and the role of timescales

Kurzfassung

Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemi- sphere. In winter, the El Niño-Southern Oscillation (ENSO) is the dominant source of predictability in the tropics and ex- tratropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well under- stood. Here, we assess how tropical convection interacts with mid-latitude summer circulation at different intra-seasonal timescales and how ENSO affects these interactions. First, we apply maximum covariance analysis (MCA) between tropical convective activity and mid-latitude geopotential height fields to identify the dominant modes of interaction. The first MCA mode connects the South Asian monsoon with the mid-latitude circumglobal teleconnection pattern. The second MCA mode connects the western North Pacific summer monsoon in the tropics with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. We show that the MCA patterns are fairly insensitive to the selected intra-seasonal timescale from weekly to 4-weekly data. To study the potential causal interdependencies between these modes and with other atmospheric fields, we apply the causal discovery method PCMCI at different timescales. PCMCI extends standard correlation analysis by removing the con- founding effects of autocorrelation, indirect links and com- mon drivers. In general, there is a two-way causal interaction between the tropics and mid-latitudes, but the strength and sometimes sign of the causal link are timescale dependent. We introduce causal maps that show the regionally specific causal effect from each MCA mode. Those maps confirm the dominant patterns of interaction and in addition high- light specific mid-latitude regions that are most strongly con- nected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO and the tropical mid-latitude linkages remain similar. Still, La Niña strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Niño years the Pacific pattern is reinforced. This study paves the way for process-based validation of boreal summer telecon- nections in (sub-)seasonal forecast models and climate mod- els and therefore works towards improved sub-seasonal pre- dictions and climate projections.