Frozen‐in anticyclones occurring in polar Northern Hemisphere during springtime: Characterization, occurrence and link with quasi‐biennial oscillation

During winter, the polar vortex forms a dynamical barrier in the arctic stratosphere which prevents large scale exchanges between the high latitude and tropical regions. Nevertheless, the occurrence of thin tropical air mass intrusions at the edge of the polar vortex have in fact been detected and modeled. These structures could play an important role in improving our knowledge of the balance between chemical and dynamical processes associated with the ozone budget. After the final stratospheric warming in springtime, the breakdown of the polar vortex occurs and the summer circulation starts to develop. Air mass intrusions from the tropics can be trapped at polar latitudes and persist until August in the anticyclone, advected by summer easterlies. These structures, named “frozen‐in anticyclones” (FrIACs), have already been observed in 2003 and 2005 by MIPAS‐ENVISAT and MLS‐AURA tracer measurements. We present here a new case of FrIAC in 2007 highlighted using MLS‐AURA measurements. In order to better understand the dynamical conditions required for such events and the associated processes, we performed a climatology of tropical air mass intrusions using a potential vorticity contour advection model. This climatology reveals a preferred path for exchanges between the polar and tropical stratospheres. Using ERA‐Iterim wind and temperature reanalysis from ECMWF, we have established links between FrIAC occurrences and Rossby wave activity. There is evidence that FrIACs can exist if no major sudden stratospheric warming occurs during the polar vortex phase and their development seems favorable if the tropical quasi‐biennial oscillation is in the easterly phase.