Suppression of Baroclinic Eddies by Strong Jets

Abstract

The midlatitude storm tracks are of 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 particularly strong. The most known case is the Pacific midwinter minimum in storm track activity. To isolate the effect of jet strength on storm activity, we conduct a series of 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 low wavenumbers. The shift settles these results with linear baroclinic instability theory, as under the stronger jet conditions synoptic-scale eddies are predicted to have a sub-optimal growth rate. These results can potentially explain the midwinter suppression of storm activity over the Pacific and the difference from the Atlantic response.

Dynamical regimes of polar vortices on terrestrial planets with a seasonal cycle

“…Unlike Earth and our simulations, the suppression of baroclinic activity on Mars is confined to lower levels close to the surface (e.g., Lewis et al 2016). Yet, the existence of a midwinter minimum in our simulations aligns with other explanations such as the jet narrowing as it strengthens (Harnik & Chang 2004) or a threshold in the jet strength (Hadas & Kaspi 2021). The differences in characteristics between the planets, and our simulations, need to be further studied to explore how planetary parameters and atmospheric characteristics affect the relation between storms and the polar vortex.…”

“…This phenomenon also appears over the North Atlantic ocean during years with a strong jet (Afargan & Kaspi 2017). The suppression of storm activity during midwinter is in odds with linear models of baroclinic instability, but can be explained when considering the dephasing of the baroclinic wave when the jet is stronger (Hadas & Kaspi 2021). On Mars, similar to Earth, there is also a suppression of baroclinic activity during solstice, which is again the period when the jet is strongest (e.g., Lewis et al 2016;Mulholland et al 2016;Lee et al 2018).…”

“…In addition, the jet on Earth transitions from a merged jet during the transition season to a more subtropical, thermally driven jet during midwinter (e.g., Yuval et al 2018). Despite these differences in characteristics, the explanation for the EKE minimum on Earth proposed by Hadas & Kaspi (2021), who connected the decrease in EKE with a reduction in the number of storms and their lifetime due to a disconnect between the upper and lower levels as the jet strength increases can also apply to our simulations, given the similar jet speed dependence. When comparing the Earth-like and EKEminimum simulations, at lower jet speeds we can see a similar jet strength dependence (Fig.…”

Dynamical Regimes of Polar Vortices on Terrestrial Planets with a Seasonal Cycle

“…In addition, the jet on Earth transitions from a merged jet during the transition season to a more subtropical, thermally driven jet during midwinter (e.g., Yuval et al 2018). Despite these differences in characteristics, the explanation for the EKE minimum on Earth was proposed by Hadas & Kaspi (2021), who connected the decrease in EKE with a reduction in the number of storms and their lifetime due to a disconnect between the upper and lower levels, as the jet strength increases can also apply to our simulations, given the similar jet speed dependence. When comparing the Earth-like and EKE-minimum simulations, at lower jet speeds, we can see a similar jet strength dependence (Figures 8(c) and (d)).…”

“…This phenomenon also appears over the North Atlantic Ocean during years with a strong jet (Afargan & Kaspi 2017). The suppression of storm activity during midwinter is at odds with linear models of baroclinic instability but can be explained when considering the dephasing of the baroclinic wave when the jet is stronger (Hadas & Kaspi 2021). On Mars, similar to Earth, there is also a suppression of baroclinic activity during solstice, which is again the period when the jet is strongest (e.g., Lewis et al 2016;Mulholland et al 2016; Lee et al 2018).…”