Abstract. Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemisphere. In winter, the El Niño–Southern Oscillation (ENSO) is the dominant source of predictability in the tropics and extratropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well
understood. 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 confounding effects of autocorrelation, indirect links and common 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 highlight specific mid-latitude regions that are most strongly
connected 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 teleconnections in (sub-)seasonal forecast models and climate models
and therefore works towards improved sub-seasonal predictions and climate
projections.
South Asian monsoon precipitation in CMIP5: a link between inter-model spread and the representations of tropical convection