Causal interactions between tropics and mid-latitudes in reanalysis data and S2S forecasts

Abstract:

Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemisphere during boreal summer. At subseasonal (weekly to monthly) time scales, the two dominant coupled modes of tropical – mid-latitude co-variability in reanalysis data sets indicate that the South Asian monsoon is linked with the mid-latitude circumglobal teleconnection pattern, while the western North Pacific summer monsoon in the tropics is linked with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. Using a causal discovery algorithm, we show that these patterns reflect two-way causal interactions between the tropics and mid-latitudes and that the strength and sign of the causal links are timescale dependent. By visualizing those causal associations on a 2-dimensional map, we are able to highlight specific mid-latitude regions that are most strongly connected to tropical convection. The observed tropical-mid-latitude linkages are only mildly affected by ENSO: 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. Building upon the successful application of our methodology to reanalysis data, we finally study how the inferred causal teleconnections are represented in sub-seasonal to seasonal (S2S) prediction models and provide a process-based validation of boreal summer teleconnections. Our findings help to identify possibly under- or over-represented mechanisms in the studied models, and thereby contributes to the development of future improved sub-seasonal forecasts.