Climatology of potential vorticity streamers and associated isentropic transport pathways across PV gradient barriers

The European Centre for Medium‐Range Weather Forecasts reanalysis product ERA‐Interim from 1979 to 2011 is used to investigate potential vorticity (PV) streamers as indicators of Rossby wave breaking (RWB) and the exchange of air masses between the tropics and the extratropics on isentropes between 350 and 500 K. The concept of the strongest isentropic gradients of PV and wind is applied to mark a dynamically relevant PV boundary on every isentrope. An Eulerian algorithm is then used to identify streamers of this PV contour and a trajectory‐based technique to study the exchange across this contour. The climatology of PV streamers reveals a seasonal cycle with a higher frequency in summer than in winter in both hemispheres on all isentropes. During winter, PV streamers are suppressed above 400 K in line with the occurrence of polar vortices. Globally, the highest frequency of PV streamers is found between 380 and 420 K over the eastern North Pacific in summer. Exchange trajectories associated with these PV streamers constitute important atmospheric transport pathways between the tropics and extratropics. In both hemispheres, PV streamers and their associated air mass exchange are controlled by the presence of monsoon anticyclones in summer and by RWB in the vicinity of jet splittings in winter. Cross‐hemispheric transport is observed between 380 and 400 K in the westerly ducts over the Pacific and Atlantic. Equatorward PV streamers are connected with a more frequent exchange than poleward PV streamers, indicating a more reversible poleward and irreversible equatorward RWB. The latter is often connected with a frequent breaking into PV cutoffs. Key Points Climatology of PV streamer and transport pathways above 350 K PV streamer and exchange are controlled by monsoons (JJA) and jet breaks (DJF) Equatorward PV streamers are more frequent than poleward ones