The life cycle of cyclones, dry intrusions and cold fronts and their role in air-sea interaction

Air-sea interaction in the midlatitudes is modulated by the passage of extratropical cyclones and their trailing fronts. Particularly strong ocean heat loss (both sensible and latent) is observed in the post-cold frontal region. In this region, airmasses within the dry intrusion (DI) airstream descend slantwise from the upper troposphere towards the cold trailing front. As the cyclone case-to-case variability is high, understanding the co-occurrence of DIs, cold trailing fronts and cyclones is important for understanding the variability of surface fluxes, especially in regions not usually associated with frequent frontal activity.

 

A climatological study quantifying the co-occurrence of fronts and DIs (Raveh-Rubin and Catto, 2019) found the presence of DIs to be associated with stronger surface heat fluxes. Here the climatological study is extended to account for the cyclone life-cycle by using feature-based identification and tracking in the ERA-Interim dataset, for the 1979-2018 winters. We focus on the relationship between extratropical cyclone characteristics, DIs and cold fronts, their co-evolution throughout the lifetime of a cyclone, and consequently their impact on air-sea interaction.

 

We show that 65-80% of the extratropical cyclones in the storm track region are matched with DIs, mainly during the early stages of the intensification period. Furthermore, cyclones associated with DIs are longer lived, induce up to 50% stronger precipitation in the frontal regions, and up to 60% stronger evaporation, especially in the DI region of influence, compared to non-DI cyclones. These transient events of strong evaporation induced by DIs account up to 40% of the observed climatology, demonstrating the significant role transient weather systems play in the air-sea interaction, at times through a fairly remote influence of the cyclones.