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Bryn Ronalds and Elizabeth A. Barnes

Abstract

Previous studies have suggested that, in the zonal mean, the climatological Northern Hemisphere wintertime eddy-driven jet streams will weaken and shift equatorward in response to Arctic amplification and sea ice loss. However, multiple studies have also pointed out that this response has strong regional differences across the two ocean basins, with the North Atlantic jet stream generally weakening across models and the North Pacific jet stream showing signs of strengthening. Based on the zonal wind response with a fully coupled model, this work sets up two case studies using a barotropic model to test a dynamical mechanism that can explain the differences in zonal wind response in the North Pacific versus the North Atlantic. Results indicate that the differences between the two basins are due, at least in part, to differences in the proximity of the jet streams to the sea ice loss, and that in both cases the eddies act to increase the jet speed via changes in wave breaking location and frequency. Thus, while baroclinic arguments may account for an initial reduction in the midlatitude winds through thermal wind balance, eddy–mean flow feedbacks are likely instrumental in determining the final total response and actually act to strengthen the eddy-driven jet stream.

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Brett Soderholm, Bryn Ronalds, and Daniel J. Kirshbaum

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The evolution of convective storms over the Black Hills, an isolated mountain ridge in South Dakota and Wyoming and a regional convection hotspot, is investigated using a 10-yr observational climatology and quasi-idealized numerical simulations. Radar-observed diurnally forced mountain-convection events are classified according to their maximum cell-track length and duration, which are quantified using an automated cell-tracking algorithm. Environmental conditions during these events are obtained from operational radiosonde and model-analysis data. These data suggest that mountain-forced convective cells generally struggle to survive in the convectively inhibited flow downwind of the Black Hills. Those cells that do survive downwind prefer environments with strong bulk vertical shear over the 0–6-km layer, which favors organized multicellular or supercellular convection. Under slightly weaker shear, the cells tend to dissipate rapidly as they propagate downwind. Relatively weak winds aloft, when coupled with low-level winds aligned with the long terrain axis, support longer-lived, quasi-stationary cells with flash-flooding potential. The weak winds favor slow cell propagation while the along-ridge flow limits the negative feedbacks of storm outflow on the elevated convergence over the ridge, allowing convection to repeatedly initiate in the same location. The storm evolution is relatively insensitive to the background thermodynamic profile, provided that sufficient moist instability exists to support deep convection. Convection-permitting numerical simulations reinforce that changes in the background wind profile alone can explain the observed variations in cell evolution. They also suggest that the longevity of convective cells downwind of the ridge is sensitive to terrain-induced modifications to the vertical wind shear.

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Bryn Ronalds, Elizabeth Barnes, and Pedram Hassanzadeh
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Bryn Ronalds, Elizabeth Barnes, and Pedram Hassanzadeh

Abstract

Previous studies have found that the most consistent response of the eddy-driven jet to sea ice loss and Arctic amplification in fully coupled general circulation models (GCMs) is a broad region of anomalous easterlies on the poleward flank. In this study, a similar response is noted in a dry dynamical core GCM with imposed surface heating at the pole, and it is shown that in both a fully coupled GCM’s North Atlantic basin and the dry dynamical core, the anomalous easterlies cause an asymmetrical narrowing of the jet on the poleward flank of the climatological jet. A suite of barotropic model simulations run with polar forcing shows decreased jet positional variability consistent with a narrowing of the jet profile, and it is proposed that this narrowing decreases the distance Rossby waves can propagate away from the jet core, which drives changes in jet variability. Since Rossby wave propagation and dissipation is intrinsic to the development and maintenance of the eddy-driven jet, and is tightly coupled to a jet’s variability, this acts as a meridional constraint on waves’ ability to propagate outside of the jet core, leading to the decreased variability in zonal-mean jet position. The results from all three models demonstrates that this relationship is present across a model hierarchy.

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