Multi-level description of the Arctic polar vortex during March- April -May 2020

Abstract:

The Northern Hemisphere (NH) stratosphere during late winter and early spring of 2020 provides an ideal case for study of the Lagrangian properties of the evolving flow and its connections with the troposphere. The dynamical evolution of the NH polar vortex was punctuated by outstanding events ranging from a polar warming at upper levels, a sudden stratospheric warming, the simultaneous appearance of two cyclonic vortices at middle leves, and an extraordinary deep and extended ozone layer.

The polar upper stratosphere (10 hPa) above the North Pole warmed up by tens of K in a few days around mid March. This episode cannot be formally labeled as stratospheric sudden warming because the westerly zonal mean flow at that level did not reverse directions, but kept marked the beginning of a weakening of the westerly until the final reversal towards the end of April when the vortex split in two in the middle stratosphere after the weakening of the westerly circulation in 50hPa.The evolution of the flow at this level followed the familiar pattern of one vortex becoming weaker than the other, which itself weakened until the summer conditions took over.

The NH spring of 2020 also showed the lowest values of stratospheric ozone on record. Although ozone is not strictly a conservative quantity, it can be taken as a tracer for time scales in the order of days and hence can provide an approximate picture of the behavior of fluid parcels. In the case we are considering, at 50 hPa the lowest values of ozone were observed within one of the vortices resulting from the split while they were considerably higher within the other vortex.

We approach the study by using tools of Lagrangian analysis and ERA-5 data. We argue that the polar warming at upper levels in mid March resulted from a local amplification of the longest planetary wave, that the stratospheric sudden warming was induced by strong synoptic scale disturbances in the troposphere north of North America, and that one of the two simultaneous cyclonic vortices represented the decaying winter circulation while the other had its roots in the tropospheric disturbances. We also provide a justification for the lack of mixing between the air within and without the vortex, which contributed to the persistency of low ozone values.