Workshop: Stratospheric predictability and impact on the troposphere

Recent studies have proposed that regional Arctic sea-ice anomalies influence
planetary wave propagation into the stratosphere and so affect the strength and
location of the stratospheric polar vortex. The polar stratosphere, in turn, is
known to significantly influence polar and midlatitude tropospheric weather
patterns, including affecting the likelihood of cold-air outbreaks and blocking
events. This `stratospheric pathway' connecting Arctic sea-ice and midlatitude
weather is therefore an important potential source of predictability over
seasonal time scales.

Here I will discuss some recent and ongoing work aimed at understanding the
mechanisms and robustness of the connection between Arctic sea-ice and the polar
stratosphere. First I will discuss an analysis of decadal trends using two large
ensembles of historical simulations with coupled climate models. This reveals
that while several ensemble members simulate a weakening and shift of the
stratospheric polar vortex similar to that observed, there is no robust
connection between polar vortex strength and regional or pan-Arctic sea-ice
anomalies in either ensemble. This is somewhat surprising given that several
recent modelling studies with imposed regional sea-ice anomalies have found such
a connection. I will then discuss some ongoing work, aimed at understanding this
discrepancy, using an idealised atmospheric model. This focuses both on the
impact of the model's representation of the stratosphere on the
sea-ice-stratosphere pathway, as well as the effect of sea-ice anomalies
imposed in different regions of the Arctic.