Tropical Plumes in the UTLS and the Subtropical Jet Stream

Tropical – Extratropical Interaction has been an important component of Bosart’s research agenda, particularly in the past decade.   His recent work suggests that a significant portion of the energy in the extratropical Rossby wave train (RWT) is derived from upper level tropical energy, injected into the RWT at the jet stream level.  In this presentation, we perform a global study based on the ERA-I reanalysis of the past 36 years to study the origins of the subtropical jet  (STJ) and polar jet streams (PJ) ,  focusing on the role of tropical plumes in the Upper Troposphere-Lower Stratosphere (UTLS) of very low potential vorticity.   Our results suggest that STJs represent the poleward boundary of tropical plumes of low PV.  We argue that these plumes are the real time manifestation of the Brewer-Dobbs circulation, and that they are a principal source of energy driving the Rossby wave train.  It is also argued that the PJs, often or perhaps usually begin as STJs,  and so tropical UTLS plumes, that gradually take on extratropical characteristics, as they grow downward at higher latitudes and interact with low level baroclinicity over a period of 5-10 days.  The implications of this analysis on the global connection of tropical weather to extratropical weather will be discussed at the oral presentation.