Blocking and its response to climate change

Purpose of review: Atmospheric blocking events represent some of the most high-impact weather patterns in the mid-latitudes, yet they have often been a cause for concern in future climate projections. There has been low confidence in predicted future changes in blocking, despite relatively good agreement between climate models on a decline in blocking. This is due to the lack of a comprehensive theory of blocking and a pervasive underestimation of blocking occurrence by models. This paper reviews the state of knowledge regarding blocking under climate change, with the aim of providing an overview for those working in related fields. Recent Findings: Several avenues have been identified by which blocking can be improved in numerical models, though a fully reliable simulation remains elusive (at least, beyond a few days lead time). Models are therefore starting to provide some useful information on how blocking and its impacts may change in the future, although deeper understanding of the processes at play will be needed to increase confidence in model projections. There are still major uncertainties regarding the processes most important to the onset, maintenance and decay of blocking and advances in our understanding of atmospheric dynamics, for example in the role of diabatic processes, continue to inform the modelling and prediction efforts. Summary: The term ‘blocking’ covers a diverse array of synoptic patterns, and hence a bewildering range of indices has been developed to identify events. Results are hence not considered fully trustworthy until they have been found using several different methods. Examples of such robust results are the underestimation of blocking by models, and an overall decline in future occurrence, albeit with a complex regional and seasonal variation. In contrast, hemispheric trends in blocking over the recent historical period are not supported by different methods, and natural variability will likely dominate regional variations over the next few decades

Recent Arctic amplification and extreme mid-latitude weather

The Arctic region has warmed more than twice as fast as the global average — a phenomenon known as Arctic amplification. The rapid Arctic warming has contributed to dramatic melting of Arctic sea ice and spring snow cover, at a pace greater than that simulated by climate models. These profound changes to the Arctic system have coincided with a period of ostensibly more frequent extreme weather events across the Northern Hemisphere mid-latitudes, including severe winters. The possibility of a link between Arctic change and mid-latitude weather has spurred research activities that reveal three potential dynamical pathways linking Arctic amplification to mid-latitude weather: changes in storm tracks, the jet stream, and planetary waves and their associated energy propagation. Through changes in these key atmospheric features, it is possible, in principle, for sea ice and snow cover to jointly influence mid-latitude weather. However, because of incomplete knowledge of how high-latitude climate change influences these phenomena, combined with sparse and short data records, and imperfect models, large uncertainties regarding the magnitude of such an influence remain. We conclude that improved process understanding, sustained and additional Arctic observations, and better coordinated modelling studies will be needed to advance our understanding of the influences on mid-latitude weather and extreme events

Novel technique shows different hydrophobic chemical signatures of exotic and indigenous plant soils with similar effects of extracts on indigenous species seedling growth

Changes to ecosystem abiotic parameters are regarded as possible mechanisms facilitating plant invasion and community composition shifts. This study compared the hydrophobic chemical signatures of soil from exotic bitou bush (Chrysanthemoides monilifera spp. rotundata) invaded, indigenous acacia (Acacia longifolia var. sophorae) dominated and bare sand (unvegetated) habitats using a novel, rapid, capturing technique which utilised Amberlite® XAD4 resin filled bags that were placed in situ. The hydrophobic chemical signature of the bitou bush soil extract was significantly different to the acacia soil and bare sand extracts. High concentrations of 18 sesquiterpenes dominated the hydrophobic signature of the bitou bush extract. Low concentrations of all three extracts did not significantly affect the seedling growth of three indigenous test species under laboratory conditions, however, at higher concentrations, the extracts from soil inhabited by plants, whether exotic or indigenous, similarly inhibited the seedling growth of two species, while seedling growth of the third species was inhibited by extracts from all three soil types. These results do not support the hypothesis that exotic invasive species are more likely to exhibit allelopathic effects than indigenous plant species.12 page(s