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  1. Abstract Baroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slowermore »rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres.« less
  2. A combination of 240 years of output from a state-of-the-art chemistry–climate model and a twentieth-century reanalysis product is used to investigate to what extent sudden stratospheric warmings are preceded by anomalous tropospheric wave activity. To this end we study the fate of lower tropospheric wave events (LTWEs) and their interaction with the stratospheric mean flow. These LTWEs are contrasted with sudden stratospheric deceleration events (SSDs), which are similar to sudden stratospheric warmings but place more emphasis on the explosive dynamical nature of such events. Reanalysis and model output provide very similar statistics: Around one-third of the identified SSDs are preceded by wave events in the lower troposphere, while two-thirds of the SSDs are not preceded by a tropospheric wave event. In addition, only 20% of all anomalous tropospheric wave events are followed by an SSD in the stratosphere. This constitutes statistically robust evidence that the anomalous amplification of wave activity in the stratosphere that drives SSDs is not necessarily due to an anomalous amplification of the waves in the source region (i.e., the lower troposphere). The results suggest that the dynamics in the lowermost stratosphere and the vortex geometry are essential, and should be carefully analyzed in the search formore »precursors of SSDs.« less
  3. Abstract Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability,more »even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.« less
  4. Abstract The Whole Atmosphere Community Climate Model, version 4 (WACCM4), is used to investigate the influence of stratospheric conditions on the development of sudden stratospheric warmings (SSWs). To this end, targeted experiments are performed on selected modeled SSW events. Specifically, the model is reinitialized three weeks before a given SSW, relaxing the surface fluxes, winds, and temperature below 10 km to the corresponding fields from the free-running simulation. Hence, the tropospheric wave evolution is unaltered across the targeted experiments, but the stratosphere itself can evolve freely. The stratospheric zonal-mean state is then altered 21 days prior to the selected SSWs and rerun with an ensemble of different initial conditions. It is found that a given tropospheric evolution concomitant with the development of an SSW does not uniquely determine the occurrence of an event and that the stratospheric conditions are relevant to the subsequent evolution of the stratospheric flow toward an SSW, even for a fixed tropospheric evolution. It is also shown that interpreting the meridional heat flux at 100 hPa as a proxy of the tropospheric injection of wave activity into the stratosphere should be regarded with caution and that stratospheric dynamics critically influence the heat flux at that altitude.