The Response of the Polar Vortex to Tropospheric Temperature Eddies in an Idealized General Circulation Model

A dry-core idealized general circulation model with a stratospheric polar vortex in the northern hemisphere is run with a combination of simplified topography and imposed tropospheric temperature perturbations, each located in the northern hemisphere with a zonal wave number of one. The phase difference between the imposed temperature wave and the topography is varied to understand what effect this has on the occurrence of polar vortex displacements. Geometric moments are used to identify the centroid of the polar vortex for the purposes of classifying whether or not the polar vortex is displaced. Displacements of the polar vortex are a response to increased tropospheric wave activity. Compared to a model run with only topography, the likelihood of the polar vortex being displaced increases when the warm region is located west of the topography peak, and decreases when the cold region is west of the topography peak. This response from the polar vortex is due to the modulation of vertically propogating wave activity by the temperature forcing. When the southerly winds on the western side of the topographically forced anticyclone are collocated with warm or cold temperature forcing, the vertical wave activity flux in the troposphere becomes more positive or negative, respectively. This is in line with recent reanalysis studies which showed that anomalous warming west of the surface pressure high, in the climatological standing wave, precedes polar vortex disturbances.

Differences in the Sub-seasonal Predictability of Extreme Stratospheric Events

Extreme stratospheric events such as sudden stratospheric warming and strong vortex events associated with an anomalously weak or strong polar vortex can have downward impacts on surface weather that can last for several weeks to months. Hence, successful predictions of these stratospheric events would be beneficial for extended range weather prediction. However, the predictability limit of extreme stratospheric events is most often limited to around 2 weeks or less. The predictability also strongly differs between events, and between event types. The reasons for the observed differences in the predictability, however, are not resolved. To better understand the predictability differences between events, we expand the definitions of extreme stratospheric events to wind deceleration and acceleration events, and conduct a systematic comparison of predictability between event types in the European Centre for Medium-Range Weather Forecasts (ECMWF) prediction system for the sub-seasonal predictions. We find that wind deceleration and acceleration events follow the same predictability behaviour, that is, events of stronger magnitude are less predictable in a close to linear relationship, to the same extent for both types of events. There are however deviations from this linear behaviour for very extreme events. The difficulties of the prediction system in predicting extremely strong anomalies can be traced to a poor predictability of extreme wave activity pulses in the lower stratosphere, which impacts the prediction of deceleration events, and interestingly, also acceleration events. Improvements in the understanding of the wave amplification that is associated with extremely strong wave activity pulses and accurately representing these processes in the model is expected to enhance the predictability of stratospheric extreme events and, by extension, their impacts on surface weather and climate.

Stratospheric Final Warmings fall into two categories with different evolution over the course of the year

In early spring the stratospheric zonal circulation reverses from westerly to easterly. The transition, called Stratospheric Final Warming (SFW), may be smooth and late, mainly controlled by the solar radiative heating of the polar region, or early and abrupt with rapid increase of polar temperature and deceleration of the zonal wind, forced by the planetary wave activity. Here we present a study, based on 71 years meteorological reanalysis data. Two composites of radiative and dynamical SFWs have been built. There is a very significant difference in the evolution during the year of polar temperature and 60°N zonal wind between the two composites. The state of the polar vortex on given month is anticorrelated with its state 2 to 3 months earlier. Early winter is anticorrelated with mid-winter and mid-winter with late winter/early spring. The summer stratosphere keeps a memory of its state in April–May after the SFW until late June.

The Life Cycle and Variability of Antarctic Weak Polar Vortex Events

Motivated by the strong Antarctic sudden stratospheric warming (SSW) in 2019, a survey on the similar Antarctic weak polar events (WPV) is presented, including their life cycle, dynamics, seasonality, and climatic impacts. The Antarctic WPVs have a frequency of about four events per decade, with the 2002 event being the only major SSW. They show a similar life cycle to the SSWs in the Northern Hemisphere but have a longer duration. They are primarily driven by enhanced upward-propagating wavenumber 1 in the presence of a preconditioned polar stratosphere, i.e., a weaker and more contracted Antarctic stratospheric polar vortex. Antarctic WPVs occur mainly in the austral spring. Their early occurrence is preceded by an easterly anomaly in the middle and upper equatorial stratosphere besides the preconditioned polar stratosphere. The Antarctic WPVs increase the ozone concentration in the polar region and are associated with an advanced seasonal transition of the stratospheric polar vortex by about one week. Their frequency doubles after 2000 and is closely related to the advanced Antarctic stratospheric final warming in recent decades. The WPV-resultant negative phase of the southern annular mode descends to the troposphere and persists for about three months, leading to persistent hemispheric scale temperature and precipitation anomalies.

OClO as observed by TROPOMI: a comparison with meteorological parameters and polar stratospheric cloud observations

Chlorine dioxide (OClO) is a by-product of the ozone-depleting halogen chemistry in the stratosphere. Although it is rapidly photolysed at low solar zenith angles (SZAs), it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring at twilight conditions because of the nearly linear dependence of its formation on chlorine oxide (ClO). Here, we compare slant column densities (SCDs) of chlorine dioxide (OClO) retrieved by means of differential optical absorption spectroscopy (DOAS) from spectra measured by the TROPOspheric Monitoring Instrument (TROPOMI) with meteorological data for both Antarctic and Arctic regions for the first three winters in each of the hemispheres (November 2017–October 2020). TROPOMI, a UV–Vis–NIR–SWIR instrument on board of the Sentinel-5P satellite, monitors the Earth's atmosphere in a near-polar orbit at an unprecedented spatial resolution and signal-to-noise ratio and provides daily global coverage at the Equator and thus even more frequent observations at polar regions. The observed OClO SCDs are generally well correlated with the meteorological conditions in the polar winter stratosphere; for example, the chlorine activation signal appears as a sharp gradient in the time series of the OClO SCDs once the temperature drops to values well below the nitric acid trihydrate (NAT) existence temperature (TNAT). Also a relation of enhanced OClO values at lee sides of mountains can be observed at the beginning of the winters, indicating a possible effect of lee waves on chlorine activation. The dataset is also compared with CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) polar stratospheric cloud (PSC) observations. In general, OClO SCDs coincide well with CALIOP measurements for which PSCs are detected. Very high OClO levels are observed for the northern hemispheric winter 2019/20, with an extraordinarily long period with a stable polar vortex being even close to the values found for southern hemispheric winters. An extraordinary winter in the Southern Hemisphere was also observed in 2019, with a minor sudden stratospheric warming at the beginning of September. In this winter, similar OClO values were measured in comparison to the previous (usual) winter till that event but with a OClO deactivation that was 1–2 weeks earlier.

Stratospheric Nudging And Predictable Surface Impacts (SNAPSI): A Protocol for Investigating the Role of the Stratospheric Polar Vortex in Subseasonal to Seasonal Forecasts

Major disruptions of the winter season, high-latitude, stratospheric polar vortices can result in stratospheric anomalies that persist for months. These sudden stratospheric warming events are recognized as an important potential source of forecast skill for surface climate on subseasonal to seasonal timescales. Realizing this skill in operational subseasonal forecast models remains a challenge, as models must capture both the evolution of the stratospheric polar vortices in addition to their coupling to the troposphere. The processes involved in this coupling remain a topic of open research. We present here the Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project. SNAPSI is a new model intercomparison protocol designed to study the role of the Arctic and Antarctic stratospheric polar vortices in sub-seasonal to seasonal forecast models. Based on a set of controlled, subseasonal, ensemble forecasts of three recent events, the protocol aims to address four main scientific goals. First, to quantify the impact of improved stratospheric forecasts on near-surface forecast skill. Second, to attribute specific extreme events to stratospheric variability. Third, to assess the mechanisms by which the stratosphere influences the troposphere in the forecast models, and fourth, to investigate the wave processes that lead to the stratospheric anomalies themselves. Although not a primary focus, the experiments are furthermore expected to shed light on coupling between the tropical stratosphere and troposphere. The output requested will allow for a more detailed, process-based community analysis than has been possible with existing databases of subseasonal forecasts.

Twisting turning and pulsating of the Antarctic ozone hole, as revealed by TOMS data

Using data from TOMS!Nimbus7 and Meteor 3, the evolution of Antarctic ozone holes during the southern springs of 1992, 1993, 1994 was studied. At the South Pole, the evolution was mostly smooth, a steady decrease up to about September end and a steady recovery up to about December end. At latitudes near 65° S, the ozone levels (~220 DU) at different latitudes and longitudes showed fluctuations compatible with passing of a noncircular (oval) ! vortex boundary (two ends of a major axis of an ellipse), with a rotation period of -15 days (full rotation period ~30 days) in 1992 and ~17 days (full rotation period ~34 days) in 1994, different from the 2-3 weeks reported by earlier workers. However, the rotation was not with uniform speeds. During a full rotation, the speeds varied sometimes from almost zero (stalling) for a few days to ~20° per day during other intervals. Outside the oval boundary, often there were, depletions with spacings of a few (5-8) days, extending to lower latitudes up to ~30° S, indicating corrugations in the oval boundary, probably due to the effects of synoptic disturbances on total ozone through tropopause pressure changes and/or I ozone mini- holes caused by anticyclonic tropospheric forcing under the southern polar vortex. The shape of the ozone hole changed from elliptic to almost circular and vice versa within a few days and the area also changed by ~15-20%. Thus, the ozone hole was twisting, turning and pulsating, probably due to a varying strength of the wave number 2 component of the wind system prevailing there.