The influence of the 2009 Arctic vortex split on the distribution of NO near the polar winter mesopause

The polar vortices play a central role in vertically coupling the Sun-Earth system by facilitating the descent of reactive odd nitrogen (NO_x = NO + NO₂) produced in the atmosphere by energetic particle precipitation (EPP-NO_x). Downward transport of EPP-NO_x from the mesosphere-lower thermosphere (MLT) to the stratosphere inside the winter polar vortex is particularly impactful in the wake of prolonged sudden stratospheric warming events. This work is motivated by the fact that state-of-the-art global climate models severely underestimate this EPP-NO_x transport in the Arctic. As a step toward understanding the transport pathways by which MLT air enters the top of the polar vortex, we explore the extent to which Lagrangian Coherent Structures (LCS) impact the geographic distribution of NO near the polar winter mesopause in the Whole Atmosphere Community Climate Model eXtended version with Data Assimilation Research Testbed (WACCMX+DART). As a proof-of-concept, we show that an LCS formed atop one of the split polar vortex lobes during the 2009 SSW. We then demonstrate that the LCS acted to confine air with elevated NO to high latitudes as the air descended into the top of the polar vortex. For the case study shown, the model is in reasonable agreement with SABER temperatures and SOFIE NO. These results present a new conceptual model of transport in the polar winter mesosphere whereby regional-scale long-lived LCS act to sequester elevated NO_x at high latitudes until the air descends to lower altitudes.