On TPV thinking; Impacts on Arctic sea ice
Arctic sea ice exhibits considerable year to year variability, likely amplifying over the next decades as the ice thins. Problematically, larger year to year sea ice anomalies exhibit larger forecast errors. What processes drive the evolution and prediction of summer sea ice? Cyclonic tropopause polar vortices (TPVs) are common, coherent upper-level potential vorticity anomalies with typical radii of 100 to 1000 km and lifetimes of days to months. This seminar centers on two main questions: how do TPVs impact Arctic surface cyclones, Rossby wave breaking, and seasonal circulations? How do those impacts propagate to summer sea ice loss?
Historical cases and global, coupled (atmosphere, land, sea ice, ocean, and river) MPAS-CESM experiments are used to explore these questions. Event-relative composites of extreme 2-day ice loss events show that surface cyclones associated with TPVs drive considerable sea ice loss. Artificially strengthened central Arctic TPVs in MPAS-CESM drive more intense surface cyclogenesis and enhanced ice motion. Stronger TPVs at lower-latitudes drive more, amplified Rossby wave breaking, into the Arctic dependent on the strength of the polar circulation. Summer TPV tracks suggest an eddy-mean flow perspective with higher TPV densities in the central Arctic during seasonal low pressures. Coupled time-mean, eddy-driven impacts are quantified by strengthening TPVs in polar versus mid-latitudes.
The corresponding sea ice response is direct dynamically but complex thermodynamically. Potential for realizing TPV-associated predictability is also discussed.