Response of Annular Mode Variability to Changes in the Equator-to-Pole Temperature Gradient

The large equator-to-pole temperature gradient (ΔT) across the midlatitudes supports a meridionally meandering jetstream. The poleward and equatorward shift of the jetstream about its climatological position, deemed the annular modes, are primarily driven by high-frequency eddies but contain a low-frequency persistence in the shifted jet structure that cannot be explained by frictional dissipation alone. With observational timescales lasting a week or more in nearly every month of the year, this persistence underlies its importance to weather forecasting and climate projections. The eddy-mean flow feedback leading to this persistence is associated with the propagation of waves where a baroclinic process provides a low-level source of waves, while a barotropic process is associated with the horizontal propagation and breaking of upper-level Rossby waves. In these breaking regions, large values of effective diffusivity are found where vorticity contours are zonally elongated through contour stretching and filamentation. The low-frequency vacillation of the jetstream generated through the interaction of the barotropic and baroclinic mechanisms provide the persistence associated with the annular modes. Through the use of the finite-amplitude wave activity formalism, the direct effect of the barotropic mixing and baroclinic source can quantified and directly related to the zonal-mean flow.

Models lack the ability to appropriately capture the annular mode timescales, many being overly persistent, leading to concerns of future climate projections of extreme weather events or changes in the vacillation of the jetstream through thermal forcing. Recent studies have suggested that a decrease in ΔT, associated with Arctic Amplification, leads to an amplified midlatitude jetstream with stalled weather systems and more extreme events. Using an idealized, dry general circulation model with Held-Suarez forcing, this study investigates the effects of the magnitude of ΔT on annular mode timescales with focus on the physical mechanisms related to the timescales. By adding heat to the tropical regions, ΔT is increased, whereas by adding heat to the polar regions to mimic the effect of Arctic Amplification, ΔT is decreased. It is shown that with an increase in ΔT, the midlatitude baroclinic zone and jet stream shift poleward, and the persistence associated with the annular modes decreases. Concurrent with this decrease in timescales, is a reduction in the magnitude of effective diffusivity and a decrease in frequency of Rossby wave breaking on the equatorward flank of the climatological jet. This reduction in mixing of vorticity allows the jet shift to more quickly vacillate back toward its climatological position leading to reduced timescales of annular mode variability. The study also finds a relationship between the magnitude of ΔT and the physical mechanisms associated with annular mode timescales, such as tropospheric blocking and midlatitude wave amplitudes.