Investigating the Baroclinic Annular Mode and its Predictability of Atmospheric Blocking and Extreme Weather Events in the Midlatitudes (Invited Presentation)

Large-scale weather systems are building blocks of intraseasonal atmospheric variability in the mid-latitudes. While synoptic weather systems typically last 3-5 days, atmospheric blocking (the stagnation of the weather systems) can last for 10-20 days and often cause local extreme weather events in the mid-latitudes. Yet, the first-order dynamics of blocking and associated extreme weather remain enigmatic. Little effort has been put into connecting individual blocking events with large-scale coherent modes of variability within midlatitudes and investigating its potential dynamical linkage.

Storms and precipitations in the Southern Hemisphere experience a regular pulsing every 20-30 days called Baroclinic Annular Mode. A high BAM state may indicate an overall strong wave activity on hemispheric-scale with regional enhancement. As BAM can translate to regional scales, we identified an enhanced zonal flux of wave activity and reduced dispersiveness associated with high BAM states that are dynamically conducive to the occurrence of atmospheric blocking, thus BAM can be employed as a new source of predictability for blocking. We will discuss the physical mechanisms governing the Baroclinic Annular Mode and its connection to atmospheric blocking and associated extreme weather events. We will use a hierarchy of idealized models to study the physical factors influencing the connection between BAM and blocking. We will further show that enhanced diabatic heating in a warming climate will lead to enhanced BAM and atmospheric blocking. Given the dynamical connection between moisture-induced diabatic heating and BAM, the enhanced high BAM state in a warming climate suggests a more regular cluster of extreme weather events, which has important implications for mitigating disasters in the mid-latitudes.