The Influence of Extratropical Dry Intrusions on Tropical Temperature and Wind Extremes

Extratropical cyclones are the main system controlling midlatitude weather. In the wake of cyclones are dry air intrusions that descend equatorward, in a slantwise manner, from the upper troposphere towards the near-surface cold front. There, anomalously cold temperatures and strong winds are expected due to cold advection and vertical momentum transport in dry intrusions. However, the association of extratropical cyclones with temperature and wind extremes has not been globally quantified. The goal of this research is to understand the contribution of extratropical cyclones, through dry intrusions, to wind and cold (and compound) extremes, also outside the extratropics. The results show that the strongest association (over 60%) of extreme events to dry intrusion is in the subtropics, where dry intrusions rarely reach (~5%). The role of extratropical cyclones is thus non-local, in the sense that they affect surface extremes away from the cyclone’s centre. We further focus regionally on northwest (sub)tropical Africa and central South America to better distinguish between compound cold-wind events that occur with and without the presence of dry intrusions. This approach reveals that DI-associated extremes last longer and cover larger areas, which indicates higher impact. We then characterize the extratropical dynamics preconditioning the intrusion of dry air into the tropics, highlighting the importance of Rossby wave breaking and the associated anomalous patterns of the midlatitude jet as well as the role of surface cyclones.  The connection of extratropical weather systems to extremes in the subtropics and tropics places the results in the context of extratropical-tropical interaction, with potential implications for the predictability of tropical extremes.