Lagrangian analysis of the northern polar vortex split in April 2020 during development of the Arctic ozone hole

<div>The evolution of the Northern Hemisphere stratosphere during late winter and early spring of 2020 was punctuated by outstanding events both in dynamics and tracer evolution. It provides an ideal case for study of the Lagrangian properties of the evolving flow and its connections with the troposphere. The events ranged from an episode of polar warming at upper levels in March, a polar vortex split into two cyclonic vortices at middle and lower levels in April, and a remarkably deep and persistent mass of ozone poor air within the westerly circulation throughout the period. The latter feature was particularly remarkable during 2020, which showed the lowest values of stratospheric ozone on record.</div><div> </div><div>We focus on the vortex split in April 2020 and we examine this split at middle as well as lower stratospheric levels, and the interactions that occurred between the resulting two vortices which determined the distribution of ozone among them. We also examine the connections among stratospheric and tropospheric events during the period.</div><div> </div><div>Our approach for analysis will be based on the application of Lagrangian tools to the flow field, based on following air parcels trajectories, examining barriers to the flow, and the activity and propagation of planetary waves. Our findings confirm the key role for the split played by a flow configuration with a polar hyperbolic trajectory and associated manifolds. A trajectory analysis illustrates the transport of ozone between the vortices during the split. We argue that these stratospheric events were linked to strong synoptic scale disturbances in the troposphere forming a wave train from the north Pacific to North America and Eurasia.</div><div><strong> </strong></div><div><strong>Reference:</strong><strong> </strong>J. Curbelo, G. Chen,  C. R. Mechoso. Multi-level analysis of the northern polar vortex split in April 2020 during development of the Arctic ozone hole. Earth and Space Science Open Archive. doi: 10.1002/essoar.10505516.1</div><div> </div><div><strong>Acknowledgements:</strong> NSF Grant AGS-1832842, RYC2018-025169 and EIN2019-103087.</div>

Role of Gravity Waves in a Vortex-Split Sudden Stratospheric Warming in January 2009

The role of gravity waves (GWs) in a sudden stratospheric warming (SSW) event that occurred in January 2009 (SSW09) is investigated using the MERRA-2 dataset. Nearly 2 weeks prior to the central date (lag = 0), at which the zonal-mean zonal wind at 10 hPa and 60°N first becomes negative, westward GW drag (GWD) is significantly enhanced in the lower mesosphere and stratosphere. At 5 days before lag = 0, planetary waves (PWs) of zonal wavenumber 2 (ZWN-2) in the stratosphere are enhanced, while PWs of ZWN-1 are weakened, which are evident from the amplitudes of the PWs and their Eliassen–Palm flux divergence (EPD). To examine the relationship between PWs and GWs, a nonconservative GWD (NCGWD) source term of the linearized quasigeostrophic potential vorticity equation is considered. A ZWN-2 pattern of the NCGWD forcing is developed around z = 55–60 km with a secondary peak around z = 40 km just before the PWs of ZWN-2 in the stratosphere began to enhance. A significant positive correlation between the NCGWD forcing in the upper stratosphere and lower mesosphere (USLM; 0.3–0.1 hPa in the present data) and the PWs of ZWN-2 in the stratosphere (5–1 hPa) exists. This result demonstrates that the amplification of the PWs of ZWN-2 in the stratosphere before the onset of SSW09 is likely related to the generation of PWs by GWD in the USLM, which is revealed by the enhanced downward-propagating PWs of ZWN-2 into the stratosphere from above.

Parallel Comparison of Major Sudden Stratospheric Warming Events in CESM1-WACCM and CESM2-WACCM

After the recent release of the historical runs by community Earth system model version 2–the whole atmosphere community climate model (CESM2-WACCM), the major sudden stratospheric warming (SSW) events in this model and in its previous version (CESM1-WACCM) are compared based on a modern reanalysis (JRA55). Using the World Meteorological Organization (WMO) definition of SSWs and a threshold-based classification method that can describe the polar vortex morphology, SSWs in models and the reanalysis are further classified into two types, vortex displacement SSWs and vortex split SSWs. The general statistical characteristics of the two types of SSW events in the two model versions are evaluated. Both CESM1-WACCM and CESM2-WACCM models are shown to reproduce the SSW frequency successfully, although the circulations differences between vortex displacement SSWs and vortex split SSWs in CESM2-WACCM are smaller than in CESM1-WACCM. Composite polar temperature, geopotential height, wind, and eddy heat flux anomalies in both the two models and the reanalysis show similar evolutions. In addition, positive Pacific–North America and negative Western Pacific patterns in the troposphere preceding vortex displacement and split SSWs are observed in both observations and the models. The strong negative North Atlantic oscillation-like pattern, especially after vortex split SSW onset, is also identified in models. The near-surface cold Eurasia–warm North America pattern before both types of SSW onset, the warm Eurasia–cold North America pattern after displacement SSW onset, and the cold Eurasia–cold North America pattern after split SSW onset are consistently identified in JRA55, CESM1-WACCM, and CESM2-WACCM, although the temperature anomalies after the split SSW onset in CESM2-WACCM are somewhat underestimated.

A New Interpretation of Vortex-Split Sudden Stratospheric Warmings in Terms of Equilibrium Statistical Mechanics

Vortex-split sudden stratospheric warmings (S-SSWs) are investigated by using the Japanese 55-year Reanalysis, a spherical barotropic quasigeostrophic (QG) model, and equilibrium statistical mechanics. The statistical mechanics theory predicts a large-scale steady state as the most probable outcome of turbulent stirring, and such a state can be computed without describing all the details of the dynamics. The theory is applied to a disk domain that is modeled on the polar cap north of 45°N in the stratosphere. The equilibrium state is obtained by computing the maximum of an entropy functional. In the range of parameters relevant to the winter stratosphere, this state is anticyclonic. By contrast, cyclonic states are quasi-stationary states corresponding to saddle points of the entropy functional. These results indicate that the mean state of the stratosphere associated with the polar vortex is not close to an equilibrium state but to a quasi-stationary state. The theoretical calculations are compared with the results of a quasi-static experiment in which a wavenumber-2 topographic amplitude is increased linearly and slowly with time. The results suggest that the S-SSW can be qualitatively interpreted as the transition from the cyclonic quasi-stationary state toward the anticyclonic equilibrium state. The polar vortex splits during the transition toward the equilibrium state.

Polar processing in a split vortex: early winter Arctic ozone loss in 2012/13

A sudden stratospheric warming (SSW) in early January 2013 caused the polar vortex to split. After the lower stratospheric vortex split on 8 January, the two offspring vortices – one over Canada and the other over Siberia – remained intact, well-confined, and largely at latitudes that received sunlight until they reunited at the end of January. As the SSW began, temperatures abruptly rose above chlorine activation thresholds throughout the lower stratosphere. The vortex was very disturbed prior to the SSW, and was exposed to much more sunlight than usual in December 2012 and January 2013. Aura Microwave Limb Sounder (MLS) nitric acid (HNO3) data and observations from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) indicate extensive polar stratospheric cloud (PSC) activity, with evidence of PSCs containing solid nitric acid trihydrate particles during much of December 2012. Consistent with the sunlight exposure and PSC activity, MLS observations show that chlorine monoxide (ClO) became enhanced early in December. Despite the cessation of PSC activity with the onset of the SSW, enhanced vortex ClO persisted until mid-February, indicating lingering chlorine activation. The smaller Canadian offspring vortex had lower temperatures, lower HNO3, lower hydrogen chloride (HCl), and higher ClO in late January than the Siberian vortex. Chlorine deactivation began later in the Canadian than in the Siberian vortex. HNO3 remained depressed within the vortices after temperatures rose above the PSC existence threshold, and passive transport calculations indicate vortex-averaged denitrification of about 4 ppbv; the resulting low HNO3 values persisted until the vortex dissipated in mid-February. Consistent with the strong chlorine activation and exposure to sunlight, MLS measurements show rapid ozone loss commencing in mid-December and continuing through January. Lagrangian transport estimates suggest ~ 0.7–0.8 ppmv (parts per million by volume) vortex-averaged chemical ozone loss by late January near 500 K (~ 21 km), with substantial loss occurring from ~ 450 to 550 K. The surface area of PSCs in December 2012 was larger than that in any other December observed by CALIPSO. As a result of denitrification, HNO3 abundances in 2012/13 were among the lowest in the MLS record for the Arctic. ClO enhancement was much greater in December 2012 through mid-January 2013 than that at the corresponding time in any other Arctic winter observed by MLS. Furthermore, reformation of HCl appeared to play a greater role in chlorine deactivation than in more typical Arctic winters. Ozone loss in December 2012 and January 2013 was larger than any previously observed in those months. This pattern of exceptional early winter polar processing and ozone loss resulted from the unique combination of dynamical conditions associated with the early January 2013 SSW, namely unusually low temperatures in December 2012 and offspring vortices that remained well-confined and largely in sunlit regions for about a month after the vortex split.