Suppression of baroclinic eddies by strong jets

Abstract

The midlatitude storm tracks are among the most prominent features of extratropical climate. Despite the theoretical expectation, based on baroclinic instability theory, that baroclinic eddies strengthen with jet intensification, there is evidence that this relation breaks when the jet is intense. The most well-known case is the Pacific midwinter minimum in storm-track activity, where eddy activity is diminished in winter compared to fall and spring despite the jet being the strongest in winter. To isolate the effect of jet strength on storm activity, we conduct a series of idealized GCM experiments systematically varying jet intensity. The simulations are analyzed using Lagrangian tracking to understand the response from a single-eddy perspective. The Lagrangian analysis shows that while the response of upper-level eddies is dominated by a reduction in the amount of tracked features, the lower-level eddies' response is also affected by a reduction in their lifetime. Analyzing the jet strength effect on the pairing between the upper- and lower-level eddies, we find that the jet intensification increases the relative speed of the upper-level eddies, breaking the baroclinic wave structure and limiting its growth.We show that the Lagrangian response correlates with a shift in the midlatitude spectrum to lower wavenumbers. This shift settles these results with linear baroclinic instability theory, as under the stronger jet conditions synoptic-scale eddies are predicted to have a suboptimal growth rate. These results can explain the midwinter suppression of storm activity over the Pacific and the difference from the Atlantic response.