Intense natural circulation variability associated with stratospheric sudden warmings, vortex intensifications, and final warmings is a typical feature of the winter Arctic stratosphere. The attendant changes in transport, mixing, and temperature create pronounced perturbations in stratospheric ozone. Understanding these perturbations is important because of their potential feedbacks with the circulation and because ozone is a key trace gas of the stratosphere. Here, we use Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2), reanalysis to contrast the typical spatiotemporal structure of ozone during sudden warming and vortex intensification events. We examine the changes of ozone in both the Arctic and the tropics, document the underlying dynamical mechanisms for the observed changes, and analyze the entire life cycle of the stratospheric events – from the event onset in midwinter to the final warming in early spring. Over the Arctic and during sudden warmings, ozone undergoes a rapid and long-lasting increase of up to
The wintertime Arctic stratosphere is characterized by a number of dynamical, chemical, and physical processes that are coupled to each other in intriguing ways. For example, extreme stratospheric circulation events from the interaction (or lack thereof) of upward-propagating, planetary-scale Rossby waves with the polar vortex create a pronounced dynamical variability in the Arctic. A large concentration of ozone is another important characteristic of the Arctic stratosphere. Ozone is an effective absorber of solar radiation and an important player in the coupling between the chemistry, radiation, and dynamics. The diabatic heating from ozone impacts the temperatures and the winds, and the induced dynamical transport and photochemical reactions again impact the ozone. The feedback between ozone and the circulation may sustain the circulation anomalies and modify the stratospheric sensitivity to external forcings (Hartmann et al., 2000). Ozone is also important for the protection of life on Earth by absorbing harmful ultraviolet radiation. Taken together, ozone is a crucial stratospheric constituent, and understanding the factors that influence its distribution is a critical goal of climate research.
Ozone in the Arctic lower stratosphere is mostly controlled by transport. The transport intensifies in the winter hemisphere (Randel, 1993; Randel et al., 2002), creating a springtime total ozone maximum at high latitudes. The seasonality of the transport is associated with an intensification of the upward-propagating Rossby waves in winter. At times, the bursts of waves and their interaction with the polar vortex are strong enough to create so-called major stratospheric sudden warming events (SSWs; McIntyre, 1982; Limpasuvan et al., 2004; Polvani and Waugh, 2004), arguably the most important form of stratospheric circulation events. In the process, polar temperatures increase rapidly, reverse the climatological Equator-to-pole temperature gradient, and cause the normal westerly flow of the vortex to become easterly (Scherhag, 1952). SSWs occur in about 2 of every 3 years (Butler et al., 2017), most often in January or February (Horan and Reichler, 2017).
Past studies pointed out the close coupling between the stratospheric dynamics and Arctic ozone (e.g., Leovy et al., 1985; Ma et al., 2004), with a positive correlation between polar ozone tendencies and the stratospheric wave driving (Randel et al., 2002). The coupling leads to enhanced poleward ozone transport during SSWs and creates persistent ozone anomalies in the lowermost stratosphere (Butler et al., 2017; Hocke et al., 2015). De la Cámara et al. (2018b) showed that the initial increase in ozone after SSWs is mainly driven by isentropic eddy fluxes associated with the enhanced wave driving, while the subsequent recovery of ozone can be attributed to the competing effects between cross-isentropic advection and irreversible isentropic mixing.
It is perhaps less well known that the influence of SSWs on ozone can also
influence the tropics. Randel (1993) demonstrated how vertical transport
from the 1979–1980 SSW affected tropical ozone in the lower stratosphere and
how the changes in ozone were correlated with temperatures in the upper
stratosphere. The SSW-related influences on the tropics also imprint on the
variability in temperature and water vapor there (Gómez-Escolar et al.,
2014; Tao et al., 2015). However, the SSW effect on tropical ozone is
superimposed on the effects from the quasi-biennial oscillation (QBO), which are downward-propagating westerly and easterly zonal wind anomalies with a cycle of
The winter Arctic stratosphere not only witnesses occasional SSWs. A sustained lack of stratospheric wave driving can create the opposite events to SSWs, i.e., so-called vortex intensification events (VIs). VIs are characterized by an unusually strong and cold polar vortex (Limpasuvan et al., 2005) and reduced transport of ozone into the pole region (Isaksen et al., 2012). The extreme cold during VIs favors halogen-induced chemical ozone depletion, which, in combination with the weakened transport, leads to record-low levels of ozone that can be comparable in magnitude to the southern hemispheric counterpart (Isaksen et al., 2012; Manney et al., 2011). A good example is the most recent winter 2019–2020, which experienced an exceptionally strong, cold, and persistent Arctic stratospheric polar vortex, and which led to record-breaking Arctic ozone depletion.
Another important class of stratospheric circulation events is
stratospheric final warming events (FWs). FWs occur every year at the end of
winter, representing the final breakdown of the polar vortex due to the
seasonal increase in solar heating. FWs are often triggered by pulses of
increased wave activity and can be considered as SSWs that conclude the
winter season (Black and McDaniel, 2007). There also exists an interesting
temporal relationship between FWs, SSWs, and VIs, namely that FWs that are preceded by SSWs in the same winter tend to occur significantly later than the mean FW date (
While the aforementioned studies have started to investigate the response of ozone in the Arctic to SSWs, the response of ozone in the tropics, and also to VI and FW events, has received little attention so far. This study intends to fill this gap and refine the existing knowledge about the spatiotemporal relationship between ozone and a range of Arctic stratospheric circulation events using a modern, observation-based perspective. We achieve this by taking a comparative approach that contrasts the often opposing ozone behavior between SSWs and VIs and between the Arctic and the tropics. Time is another distinctive aspect of this study, as we cover the entire life cycle of the stratospheric circulation events from the event onset in the middle of winter to the date of the FW at the end of winter. We also clarify the role of the associated dynamical and photochemical processes in changing ozone. Overall, our goal is to provide an up-to-date, observation-based view of the global natural dynamic-driven variability in stratospheric ozone. This is not only of interest in its own right but also provides an observational baseline for ozone behavior during stratospheric circulation events that can be used for the validation of coupled chemistry–climate models.
This paper is structured as follows. In Sect. 2, we describe the data and methods used in this study. In Sect. 3, we demonstrate the ozone response in the Arctic, while in Sect. 4 we continue our discussion of the tropics. A summary and conclusion are provided in Sect. 5.
We use 1980–2018 daily fields from the MERRA-2 reanalysis (Bosilovich et
al., 2015) at a horizontal resolution of 1.5
In defining SSWs and FWs, we follow the widely used prescription by Charlton
and Polvani (2007). An SSW is detected when the zonal mean zonal wind at 10 hPa and 60
Our definition of midwinter VIs is also based on U1060, but we first
low-pass filter the data, using 20 d running means. A midwinter VI occurs
when the smoothed daily U1060 anomaly during January or February exceeds 1
standard deviation (16 m s
As shown in Table 1, our definitions lead to 15 SSWs and 8 VIs. For SSWs, the mean central date and the associated FW date are 3 February and 26 April, respectively, leading to a mean length of time of 83 d (ranging from 54 to 117). VIs have a mean central date on 23 January and an associated FW date on 2 April. This translates into a mean length of time of 70 d (ranging from 44 to 91). Note that SSWs are longer in the length of time than VIs, consistent with the findings by Hu et al. (2014) that SSW winters are associated with FW dates that are, on average, late compared to the climatological mean FW date.
Central dates
We use a 180 d running mean window to smooth the zonal mean equatorial
(
The changes in zonal mean ozone (
The ozone tendency
We use
Traditional composites take the averages of various events centered on specific dates (e.g., Butler et al., 2017). However, in the present study, we are interested in the behavior of ozone during the entire life cycle of
stratospheric circulation events, beginning in December before the onset and
ending with the FW at the end of winter. Our interest in this rather long
period is rooted in the fact that the events and their ozone anomalies can
be quite persistent, and that the FW represents yet another perturbation to
the preexisting ozone fields. Since each event and FW occur at different
dates, it is useful to measure the time between the central date of an event
and its associated FW. This is denoted as the length of time. Since the
length of time differs from event to event, we somewhat modify the
traditional compositing technique. Our approach is based on the mean central
date of all selected SSWs (or VIs;
We begin our discussion of how Arctic ozone evolves during SSWs and VIs by presenting some key dynamical quantities, which will then guide the interpretation of our subsequent results. Figure 1 shows the evolution of composite anomalies in the stratospheric wave driving (Fig. 1a, b), the vertical component of the residual circulation (Fig. 1c, d), and temperature (Fig. 1e, f) over the life cycle of SSWs (Fig. 1a, c, e) and VIs (Fig. 1b, d, f).
SSW
SSWs (Fig. 1a, c, e) are typically preceded by enhanced stratospheric wave
driving, starting at a negative lag of
VIs (Fig. 1b, d, f) are, in many respects, opposite to SSWs. As explained in Limpasuvan et al. (2005), VIs evolve relatively slowly and result from the sustained lack of stratospheric wave driving, leading to the gradual strengthening and cooling of the vortex. As shown by Fig. 1b, the wave driving is anomalously small, starting several weeks before onset and minimizing at about 1 week after onset. This is different from SSWs, as the wave driving during SSWs changes much more abruptly during onset. Long after the onset of VIs, the wave driving increases again, first more intermittently and then more systematically during the FW. We note that the magnitude of the wave driving associated with the FW is quite large and comparable to that of SSWs during onset. This may be attributable to the sustained suppression of wave driving during VI onset, contributing to the enhanced release of wave activity after the event and a relatively early FW. Also, the relatively strong polar vortex after VIs (not shown) is conducive to upward-propagating wave activity into the stratosphere.
As for SSWs, changes in the Arctic
The above-described dynamical perturbations are associated with significant changes in the transport of stratospheric ozone and its temperature-dependent photochemical reaction rates. As has been shown to some extent before (Butler et al., 2017; de la Cámara et al., 2018b; Hocke et al., 2015), and as we will show in more detail next, this has major consequences for the distribution of stratospheric ozone.
We first examine the composite evolution of Arctic column ozone (i.e., the
vertically integrated ozone amount) during SSWs (Fig. 2a). Red and gray
shading indicate the deviation of the column ozone from its climatology
(thick black curve), and the green line shows the percent column ozone
anomaly with respect to climatology. Before onset, there is a subtle
decrease in column ozone, presumably related to the anomalously strong and
cold vortex during this time (Fig. 1e) and the reduced ozone transport into
the polar regions. Within the first 10 d following the SSW onset, the
column ozone anomalies rapidly increase by
Next, we examine the evolution of Arctic ozone during VIs (Fig. 2b, d, f).
Column ozone (Fig. 2b) is anomalously negative over the entire VI life
cycle, minimizing at about
We now explore the role of the dynamical mechanisms that create the changes in ozone. From the TEM tracer transport equation Eq. (1), it is clear that several processes are involved. Figure 2e–j present the total time tendencies of ozone (Fig. 2e–f) and the contributions to it from vertical advection (Fig. 2g–h) and eddy flux convergence (Fig. 2i–j). The horizontal advection term is generally small and therefore omitted. For better orientation, the red and blue contours reproduce a constant ozone mixing ratio anomaly from Fig. 2c and d.
Arctic ozone composites during
The negative Arctic ozone anomalies in early winter, before SSWs, are partly
the result of reduced eddy flux convergences (Fig. 2i) and vertical
transport (Fig. 2g). The strong positive ozone tendencies close to the
onset of SSWs, which are responsible for the increase in ozone after SSWs,
result mainly from the convergence of eddy fluxes (Fig. 2i; see also de la
Cámara et al., 2018b), triggered by the enhanced wave driving associated
with SSWs (Fig. 1a). The downward transport of ozone by the enhanced
residual circulation also contributes to the positive tendencies during
onset, in particular in the lower stratosphere (Fig. 2g). After SSWs, the
suppressed planetary wave activity leads to a sustained reduction in eddy
transport and, hence, negative ozone tendencies in the middle and lower
stratosphere. At the same time, the vertical advection of ozone is
anomalously negative in the middle stratosphere after SSWs. Both effects
lead to the gradual decay of the strongly positive ozone anomalies right
after onset and eventually create the abovementioned banded structure of
negative ozone in the middle stratosphere. Overall, this indicates that the
decrease in midstratospheric ozone after SSWs is mainly of dynamical
origin, consistent with de la Cámara et al. (2018b). We note that this
does not support the ideas of Sagi et al. (2017), who argue that the ozone
decrease is due to chemical reactions involving
The VI-related total Arctic ozone tendencies (Fig. 2f) are mostly equal but
opposite in sign to that of SSWs. VIs are passive events that develop
gradually by radiative cooling out to space, and the related negative ozone
anomalies appear long before the actual onset (Fig. 2d), which is related to periods of negative tendencies before and during VI onset (Fig. 2f). The tendencies are related to reduced eddy transport in the upper half (Fig. 2j) and reduced vertical advection in the lower half of the stratosphere (Fig. 2h). Ozone in the upper stratosphere slowly recovers towards climatology, mostly due to increases in eddy transport associated with pulses of planetary waves that restore the vortex back to normal. However, the positive eddy transport is counteracted by the photochemical effect as the temperature is anomalously warm in this layer (Fig. 1f). In contrast, the negative ozone
anomalies in the lower stratosphere are sustained by reduced vertical
advection (Fig. 2h) until mid-March. We also examined the source term
We now turn our attention to the tropics, defined as the
Composite anomalies for
In comparison with the SSWs, the variations in
Understanding the changes in tropical ozone in response to Arctic
stratospheric circulation events is complicated by the simultaneous
influences from the QBO. To disentangle the two effects, we first examine
how the vertical structure of tropical ozone changes in response to the QBO.
Figure 4a shows the vertical cross section of tropical ozone anomalies
(
Composites for QBO events.
Figure 4b demonstrates that SSWs and VIs occur during virtually any phase of
the QBO. However, as shown by the mean timing of the events (V and S markers
on the right), there is a slight preference for SSWs to occur during the
easterly QBO phase and VIs during the westerly QBO phase, a possibility that
was discussed by Dunkerton et al. (1988). To filter out possible QBO
influences from the tropical ozone, we define the QBO ozone signal as the
mean ozone anomalies over days
As in Fig. 2, except for tropical ozone (
Figure 5 presents composite anomalies and composite anomalous tendencies in
tropical ozone during SSWs and VIs. The variations in tropical column ozone
are rather small and amount to only
During VIs (Fig. 5b), there are small tropical column ozone anomalies, which
are mostly positive (
The dynamical mechanisms that create the changes in tropical ozone are
dominated by vertical advection associated with changes to the residual
circulation (Randel, 1993). Enhanced tropical upwelling during SSW onset
(Fig. 3a) combined with a vertical background of ozone mixing ratios that
maximize in the middle stratosphere create positive tendencies above 10 hPa
and negative tendencies below 10 hPa (Fig. 5g). Following the reversal of
the residual circulation anomalies at about 10 d after onset (Fig. 3a),
the vertical advection term leads to oppositely signed ozone anomalies
starting at about mid-February. During VIs, the tropical ozone tendencies
(Fig. 5f) are mostly small. There are negative tendencies from vertical
advection (Fig. 5h) in the upper stratosphere and during onset, owing to the
weakened meridional circulation from the VI. However, these negative
tendencies are compensated by the chemical source term (not shown),
leading, overall, to little change in ozone. As expected, the tropical ozone
tendencies during the FW of VIs (Fig. 5f) are mostly due to vertical
advection (Fig. 5h) and compensating influences from the source term
We used MERRA-2 reanalysis to document the composite spatiotemporal ozone response to Arctic circulation events. While the ozone response in the Arctic to sudden stratospheric warming (SSW) events has already been the target of some previous studies (Butler et al., 2017; de la Cámara et al., 2018b; Hocke et al., 2015), we took a more holistic approach and studied stratospheric ozone in the Arctic and the tropics, and we considered not only SSWs but also vortex intensification (VI) and final warming (FW) events.
In the Arctic, the onset of SSWs leads to a rapid increase in total ozone by
There are also some limitations to this study. In terms of the mechanisms,
we were mostly focused on the various dynamical effects in changing ozone.
However, chemical effects are likely to also play some role in perturbing
ozone, in particular in the chemically dominated upper stratosphere. We were
unable to investigate the chemical effects because of the large
uncertainties associated with the chemical term in the MERRA-2 reanalysis,
but we suspect that the dynamics are, overall, more important than the
chemistry. This is supported by Isaksen et al. (2012), who found that the
chemical effect explained only 23 % of the Arctic ozone loss during the VI from 2011. Nevertheless, it would be interesting to evaluate the relative
contributions from the dynamics and the chemistry in changing ozone during
SSWs and VIs, using output from a range of coupled chemistry climate models (CCMs), similar in spirit to de la Cámara (2018b) for SSWs, using the Whole Atmosphere Community Climate Model (WACCM). We also did not explicitly consider so-called downward planetary wave coupling events (DWCs; Lubis et al., 2017), which are relatively short-lived events (
One of the novel results of this study is that FWs that follow VIs induce a surprisingly strong ozone response, which resembles, in many respects, that of midwinter SSWs. Another relatively new aspect of this study is that Arctic circulation events also perturb ozone in the tropics, which is most pronounced during SSWs and early FWs after VIs. This adds to an increasing body of evidence that the mean meridional circulation communicates the effects of Arctic stratospheric circulation events into the lower latitudes. This leads to the notion that the Arctic circulation extremes have an almost global reach, as also evidenced by their impacts on equatorial stratospheric temperatures (Dhaka et al., 2015) and tropospheric equatorial convective activity (Kodera, 2006). It still remains to be seen how the tropical circulation is affected by the combined heating effects from the tropical ozone and the meridional circulation.
Recent studies have suggested that the dynamical coupling between the stratosphere and the troposphere, and the surface impact of this coupling, is simulated more strongly in models with interactive ozone chemistry (i.e., CCMs; Haase and Matthes, 2019; Li et al., 2016; Romanowsky et al., 2019), suggesting that intraseasonal variations in ozone are important for the prediction of short-term climate. The results from our study could serve as a reference for the validation of CCMs. Simulations with CCMs, in turn, could be used to clarify some of the still open questions of the present study, in particular about the response of tropical ozone during VIs and the relative role of photochemistry in changing ozone during the circulation events.
MERRA-2 reanalysis data are available online via NASA's Goddard Earth Sciences Data and Information Services Center archive
(
HJH performed the analysis and wrote the paper. TR designed the study, provided guidance in the interpretation of the results, and reviewed the paper.
The authors declare that they have no conflict of interest.
We thank the Department of Atmospheric Sciences at the University of Utah for its support. The use of computer infrastructure from the Center for High Performance Computing at the University of Utah is gratefully acknowledged. We also acknowledge NASA for providing the MERRA-2 reanalysis. Finally, we acknowledge the three anonymous reviewers, Sandro Lubis, and the editor, Peter Haynes, for their constructive comments which helped to improve the paper.
This paper was edited by Peter Haynes and reviewed by three anonymous referees.