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Abstract:
Predictability on seasonal time scales over the North Atlantic-Europe
region is assessed using a seasonal prediction system based on an
initialized version of the Max Planck Institute Earth System Model
(MPI-ESM). For this region, two of the dominant predictors on seasonal
time scales are El Nino-Southern Oscillation (ENSO) and sudden
stratospheric warming (SSW) events. Multiple studies have shown a
potential for improved North Atlantic predictability for either
predictor. Their respective influences are however difficult to
disentangle, since the stratosphere is itself impacted by ENSO. Both El
Nino and SSW events correspond to a negative signature of the North
Atlantic Oscillation (NAO), which has a major influence on European
weather.
This study explores the impact on Europe by separating the stratospheric
pathway of the El Nino teleconnection. In the seasonal prediction
system, the evolution of El Nino events is well captured for lead times
of up to 6 months, and stratospheric variability is reproduced with a
realistic frequency of SSW events. The model reproduces the El Nino
teleconnection through the stratosphere, involving a deepened Aleutian
low connected to a warm anomaly in the northern winter stratosphere. The
stratospheric anomaly signal then propagates downward into the
troposphere through the winter season. Predictability of 500-hPa
geopotential height over Europe at lead times of up to 4 months is shown
to be increased only for El Nino events that exhibit SSW events, and it
is shown that the characteristic negative NAO signal is only obtained
for winters also containing major SSW events for both the model and the
reanalysis data.
