The 2015/16 Northern Hemisphere winter stratosphere appeared to have the greatest potential yet seen for record Arctic ozone loss. Temperatures in the Arctic lower stratosphere were at record lows from December 2015 through early February 2016, with an unprecedented period of temperatures below ice polar stratospheric cloud thresholds. Trace gas measurements from the Aura Microwave Limb Sounder (MLS) show that exceptional denitrification and dehydration, as well as extensive chlorine activation, occurred throughout the polar vortex. Ozone decreases in 2015/16 began earlier and proceeded more rapidly than those in 2010/11, a winter that saw unprecedented Arctic ozone loss. However, on 5–6 March 2016 a major final sudden stratospheric warming (“major final warming”, MFW) began. By mid-March, the mid-stratospheric vortex split after being displaced far off the pole. The resulting offspring vortices decayed rapidly preceding the full breakdown of the vortex by early April. In the lower stratosphere, the period of temperatures low enough for chlorine activation ended nearly a month earlier than that in 2011 because of the MFW. Ozone loss rates were thus kept in check because there was less sunlight during the cold period. Although the winter mean volume of air in which chemical ozone loss could occur was as large as that in 2010/11, observed ozone values did not drop to the persistently low values reached in 2011.
We use MLS trace gas measurements, as well as mixing and polar vortex
diagnostics based on meteorological fields, to show how the timing and
intensity of the MFW and its impact on transport and mixing halted
chemical ozone loss. Our detailed characterization of the polar vortex
breakdown includes investigations of individual offspring vortices and
the origins and fate of air within them. Comparisons of mixing
diagnostics with lower-stratospheric N
Sudden stratospheric warmings (SSWs), which are characterized by abrupt
warming and weakening or reversal of the polar wintertime westerly
circulation
Recent Arctic winters with SSWs have led to different extremes in polar
processing and ozone loss: the 2012/13 NH winter was exceptionally
cold in December, but a major vortex-split SSW in January gave rise to
two unusually strong offspring vortices that moved far into sunlight
Interannual variability in NH winters is also reflected in the timing of
the springtime stratospheric final warming. These events mark the
transition of the stratospheric winter circulation from westerly to
easterly, where it remains until the following autumn. Numerous studies
suggest that the timing of final warmings is related to SSWs earlier in
winter:
As we will show below, the 2015/16 Arctic winter was the coldest on
record (since at least 1979) in the lower stratosphere through January. Minimum temperatures
in the lower stratosphere were far below those in the 2010/11
winter/spring when extensive chemical loss led to record low values of
Arctic ozone in April 2011
In this paper, we analyze meteorological data from the MERRA-2 (Modern Era Retrospective-analysis for Research and Applications) reanalysis and trace gas data from the Aura MLS instrument to give an overview of dynamical conditions and chemical composition in the polar vortex during the 2015/16 winter and to detail the effects of the MFW that shattered the vortex in early March 2016, which curtailed polar processing and limited chemical ozone loss. We focus on transport and mixing during the vortex breakup and its effects on the composition of air that was dispersed from the vortex. A comprehensive picture of the vortex evolution and breakup is obtained using a newly developed package for characterizing multiple vortices. We describe the evolution of the vortex and trace gases through the MFW and associated vortex splitting, focusing on mixing and dispersal of chemically processed air from the vortex.
After describing the datasets and methods used (Sect.
The National Aeronautics and Space Administration's (NASA) Global
Modeling and Assimilation Office (GMAO) MERRA-2
dataset
The Earth Observing System (EOS) Aura satellite was launched in July
2004, in a 98
Equivalent latitude
To investigate the potential for polar chemical processing and ozone loss
during the 2015/16 winter, we use a standard set of polar processing diagnostics
calculated from MERRA-2 data. We primarily make use of diagnostics described
by
Our analysis makes use of a detailed characterization of the 2015/16
stratospheric polar vortex, particularly during the period of time when
the vortex split into multiple offspring. We use the CAVE-ART
(Characterization and Analysis of Vortex
Evolution using Algorithms for Region Tracking) analysis package, which was
developed to comprehensively describe the state of the polar vortex
throughout the winter season.
A paper describing the full details and implementation of CAVE-ART
is in preparation
Such detailed characterizations are particularly useful during vortex-split
SSW events wherein the resulting offspring vortices can vary in
size and strength in ways that ultimately influence polar processing.
For example, a preliminary version of CAVE-ART was used by
EqL time series of MLS data are produced using a weighted average of MLS
data in EqL and time, with data additionally weighted by measurement precision
The diagnostics of mixing and transport barriers described above
represent averages around EqL contours and thus give
information on bulk mixing properties; for example, the strength of
the transport
barrier at the EqL of the vortex edge is an estimate of that barrier
averaged over the entire length of the edges of all vortices present at that time. To examine
regional mixing (e.g., variations along the edge of a vortex, or
differences between individual vortices), we use the function
We use the core of the Lagrangian trajectory diagnostic code described
by
The following list briefly summarizes the diagnostics of transport barriers and mixing used here:
PV gradients: gradients of scaled PV as a function
of EqL reveal the EqL location and sharpness of the vortex edge
because PV increases dramatically between the surf zone and the vortex interior. The function Trace gas gradients: trace gases measured by MLS,
such as N
In addition to identifying transport barriers and mixing regions as
described above, we use the trajectories described above for the
calculation of
Time series of
Figure
Time series of 490 K MLS vortex-averaged N
The 2010/11 winter (blue lines) was not in general characterized by
record low temperatures but rather by an exceptionally prolonged
period, extending into early April, of temperatures below the chlorine
activation threshold (Fig.
In early January 2013 (orange lines), temperatures abruptly rose far
above the chlorine activation threshold during a “vortex-split”
SSW. This event was among the strongest SSWs on record,
with one of the largest abrupt temperature increases, deepest vertical
ranges of wind reversal, and most prolonged periods of easterlies. However,
the exceptional cold prior to that event (Fig. 1a), and exceptional
exposure of the vortex and offspring vortices to sunlight in December
and January (Fig.
The meteorological conditions in 2014/15 (green lines) led to the opposite
extreme of polar processing. A brief minor SSW split the vortex on
5 January 2015, after which temperatures soon dropped below the chlorine
activation threshold again. The resultant rapid chlorine deactivation,
combined with exceptionally strong descent within the vortex (as seen in the record
N
In comparison to these previous recent years with exceptional
combinations of dynamical conditions leading to unanticipated extremes
in Arctic polar processing, the 2015/16 winter (red lines) stands out as
yet another unexpected extreme in variability of the Arctic winter
stratosphere. Minimum temperatures (Fig.
The exceptionally cold conditions resulted in extensive early winter
chlorine activation in 2015/16, with low HCl values in late
December/early January matched only by those in 2012/13
(Fig.
Ozone continued to decrease in the vortex at a rate slightly faster than
that in 2011 until the beginning of March 2016. If uninterrupted, ozone
values would have been expected to drop lower than those in 2011 by
mid-March. Instead, before mid-March, a brief increase of about
0.5 ppmv was followed by about a week of decreasing values and then
slightly increasing values for the rest of the winter
(Fig.
Winter polar processing statistics based on temperatures from
the MERRA-2 reanalysis:
Potential temperature/time series of vortex-averaged (the sum
of all regions identified by CAVE-ART using the 84
Figure
In contrast to 2011 and 2013, during which evidence of renitrification
was seen above 400 K
In the following, we focus on the evolution of the vortex and trace gas
transport during the MFW on the individual isentropic surfaces marked by
horizontal lines in Fig.
Equivalent latitude–time series at 850 K for 2015/16 showing
MERRA-2
Equivalent latitude–time series at 490 K for 2015/16; as in Fig.
Equivalent latitude–time series at 550 K for 2015/16 as in
Fig.
In Figs.
In the lower stratosphere, at 490 K (Fig.
The signatures of mixing vary between trace gases depending on region and
times because of differing horizontal gradients. Evidence of air from
near the vortex edge mixing out into midlatitudes is seen in N
At this level, minimum temperatures rose above the ice PSC threshold in
late February, and the steady increase in H
While the transport barriers seen in sPV gradients and
A somewhat similar evolution is seen at 550 K
(Fig.
Equivalent latitude line plots of indicators of mixing and
transport barriers at 490 K showing individual dates from 24 February
through 15 April (see color bar). The panels show
Figure
Binning
The EqL-based view presented above gives a global perspective on the evolution in vortex area and strength during the MFW period. This averaged view of transport barrier and mixing diagnostics shows that a small area of well-confined vortex air lingered through March, but by early April the transport barrier presented by the vortex edge was greatly weakened, and the potential for mixing was high. In the following, we focus on the synoptic evolution of the vortices and regional aspects of transport and mixing during the MFW period.
Orthographic maps of 850 K MLS CO (first row) and H
Figure
As in Fig.
As in Fig.
Figures
At 550 K (Fig.
Trajectory-based parcel history maps at 490 K showing the
locations of air parcels initialized inside vortex regions as
defined by CAVE-ART on 16 March (row A) and 20 March
(row B). Parcels are colored black (parent), green (offspring-p), or
blue (offspring-s) if they
were inside a valid vortex region on the initialization date (column
2, red labeling); otherwise the parcels are colored grey.
Columns 1, 3, and 4 show the locations of these parcels 12 days
before and 8 and 14 days after initialization, respectively. The
white contours show the vortex regions identified by CAVE-ART
in MERRA-2 data (subsampled to match the 1.25
As in Fig.
The trajectory-based air parcel history maps in Figs.
At 550 K (Fig.
Air from even small offspring vortices in the lower
stratosphere thus remained in distinct confined regions long after the vortex
split in the lower stratosphere. At all levels, examination of the grey parcels – that is, all the
parcels that were outside any vortex on the initialization day –
without the overlaid vortex parcels indicates that few of them were
entrained into vortex regions. Thus, as long as the regions were large enough to be
identified as vortices by CAVE-ART, they remained mostly devoid of air with
extra-vortex origins. This indicates that the mixing during the vortex
break up was largely one-way, with air mixing out of the vortices through
filamentation as they eroded and lost their identity. This result
is consistent with previous studies of dispersal of air from the lower-stratospheric vortex
Vortex characteristics and MLS trace gas averages in individual
vortex regions in the lower stratosphere. Top panels show the area of
each vortex (shading) along with the number of MLS measurement points
inside each vortex on each day (lines/symbols). The second row shows
wind speeds from MERRA-2 averaged around the edge of each
individual vortex. Succeeding rows show averages of MLS N
Figure
The MLS sampling of the large, strong parent vortex in January
through mid-February included 500–700 measurements per day, but both
its area and the number of measurements in it had dropped somewhat at
all levels by 24 February (the start date of the panels in
Fig.
Vortex edge wind speeds show a deep minimum in the period between the
start of the MFW and the split. Wind speeds showed some day-to-day
variability after the split but overall decreased steadily. The
minimum just prior to the split arises largely because the vortex had
already developed into multiple closed circulations that were only
joined immediately prior to the vortex split by a narrow “bridge” with
high PV but low wind speeds. The increase in the variability of the
wind speeds immediately before the split reflects the existence of low
wind speeds along the bridge but high wind speeds elsewhere along the
vortex edges; edge wind speeds increase, and their standard deviations
decrease, once the bridge is broken and the offspring become separated.
As seen above, the offspring at 490 K were short-lived (about 5 and 7 days for the blue offspring-s and green offspring-p vortices,
respectively), with wind speeds along their edges decreasing rapidly. In
fact, as seen in Fig.
The evolution of trace gases in the individual vortices is also consistent
with the picture of mixing and vortex breakup seen above. At 490 K,
N
At 550 K, N
Examination of similar vortex averages of the shorter-lived species
HNO
We have analyzed meteorological fields from the MERRA-2 reanalysis and trace gas data from the Aura Microwave Limb Sounder (MLS) to provide an overview of the exceptionally cold 2015/16 winter and a detailed description of the vortex breakup in a major final SSW (“major final warming” or MFW) that prevented chemical ozone loss from reaching record high values. Our analyses utilized several mixing diagnostics, as well as a new package (CAVE-ART) for characterizing multiple vortex regions.
The 2015/16 Arctic winter was the coldest on record in December
through early February. Lower stratospheric temperatures were at or
near record lows from late December into early February, and far below
average from December through mid-March. A substantial region of
temperatures below the ice PSC threshold was present continuously from
late December through early February, far longer than during any
previously observed Arctic winter: the winter mean volume of air below
the ice PSC threshold was over twice that previously seen. The chemical
ozone loss potential, measured by the commonly used metric of volume of
air below the chlorine activation threshold, was nearly identical to that in
2010/11 (when unprecedented Arctic ozone loss occurred). The evolution
of trace gases from MLS is consistent with the exceptional
meteorological conditions: vortex-wide dehydration was present between
about 410 and 520 K potential temperature, something never before
observed in the Arctic. Denitrification was also exceptional, and
extensive chlorine activation and chemical ozone loss began earlier than
in all but one previous winter (2012/2013, Fig.
That lower-stratospheric ozone loss did not reach the extent of that in spring 2011 was primarily due to the occurrence of an MFW beginning in early March 2016. This event had two critical consequences: first, while the total volume of cold air during the winter was similar to that in 2010/11, that cold period ended significantly earlier in the winter in 2016, when ozone loss was slower due to less sunlight exposure. Second, the sudden vortex breakup in the MFW resulted in rapid dispersal of chemically processed air from the vortex and consequently curtailed chemical processing, which might have lingered for some time if chlorine had remained confined in a relatively large intact vortex and thus deactivated more gradually.
The Arctic winter meteorology in 2015/16 was so anomalous that
extensive study of numerous processes will be needed to fully
characterize its consequences. In this paper we focus on one aspect of
this exceptional winter: a detailed description of the event that
limited ozone loss to an amount that, while larger than typical in the
Arctic, was not unprecedented – the MFW and vortex breakup in early
March. The MFW itself was remarkable: the major SSW criteria were
fulfilled when the vortex was a single elongated entity displaced far
off the pole in the middle stratosphere
In the middle stratosphere (exemplified herein by 850 K), transport and
mixing diagnostics and MLS trace gases show that by the time of the MFW the
vortex had already shrunk, and a strong Aleutian anticyclone and vigorous
surf zone formed, consistent with climatology. In mid-March, about a
week after the MFW began, the vortex split into two very unequal pieces;
the larger parent vortex rapidly sheared out and dispersed, while a small
coherent remnant of the offspring lingered through late March. The evolution of MLS CO and H
The breakup and dispersal of air from the vortex in the lower stratosphere was slower and more episodic, with largest changes in the short period surrounding the vortex split. Some of the specific consequences of the lower-stratospheric vortex evolution (shown here at 490 and 550 K) during the MFW for transport, mixing, and dispersal of chemically processed air are as follows:
At 490 K, two small offspring split off the main vortex in
mid-March, but these persisted for only about a week. At 550 K, the vortex split into two pieces, both of which
remained well defined for over a month after the split. Mixing increased only slightly after the onset of the MFW around
7 March, but extensive mixing occurred in the few days during and after the vortex
split in mid-March. Immediately following the split the total vortex area decreased
by 30 to 40%, with the largest offspring covering about 4 % of
the hemisphere and smaller offspring an additional 1 to 2 % of the hemisphere. Following this period of intensive vortex erosion and mixing, air remained
well confined within the remaining offspring vortices. Abundances of MLS N ClO rapidly decayed in the offspring vortices as a result of a
combination of rapid deactivation and dispersal of vortex air during
the split. The evolution of ozone in the offspring vortices was dependent
on the region within the parent vortex where the air originated,
such that the offspring at 490 K contained higher values
characteristic of the collar of undepleted ozone along the vortex
edge, whereas at 550 K, low ozone values extended farther out into
the vortex edge region and the smaller, but stronger, offspring
vortex carried lower ozone than the parent. The “function
In both the lower and middle stratosphere the mixing following the MFW was primarily via erosion and filamentation of the vortices as long as they remained intact. This resulted in wide dispersal and rapid mixing of air formerly in the vortex, but, in general, transport of extra-vortex air into the vortex regions was rare.
The major final SSW in early March 2016 was a remarkable finale to an
already exceptional Arctic winter. The results presented here suggest the need for many further studies to assess how
well the evolution of the vortex and trace gases throughout the 2015/16 winter fits with
our current understanding of and ability to model lower-stratospheric polar chemical
processes. The 2015/2016 winter also provides a unique addition to the already wide
variety of natural experiments conducted via the immense variability in Arctic polar
vortex evolution, longevity, and breakup.
This new information is important for improving our detailed
understanding of variations in dispersal of ozone-depleted and/or
chemically activated air from the vortex and its implications for
present and future global ozone distributions. Further studies will
include detailed analyses using similar methods to this work comparing
the vortex breakup in 2016 with that in other winters, both Arctic and
Antarctic. This is particularly interesting given reported differences
between years with early and late Arctic final warmings
In light of the 2012/13 winter, when an exceptionally strong
vortex-split SSW resulted in record early winter ozone loss, and the
2014/15 winter, when a very brief, minor SSW resulted in record high
vortex ozone values, the importance of the early and abrupt major final
SSW in limiting ozone loss in spring 2016 once again emphasizes the
complexity of the interactions between these extreme dynamical events
and chemical processes in the stratospheric polar vortex. In each of
these winters, the SSW events had dramatic consequences that were
largely unanticipated. SSW characteristics are also expected to evolve with the
changing climate
The datasets used are publicly available, the
MLS data from
Both authors contributed equally to this paper. G. L. Manney led the analysis of MLS data, Z. D. Lawrence led the analysis of transport and mixing diagnostics and made the supplemental animations, and both G. L. Manney and Z. D. Lawrence prepared the manuscript.
The authors declare that they have no conflict of interest.
We thank members of the MLS team at JPL for data processing/analysis, data management, and computational support (especially Luis Millan, Ryan Fuller, and Brian Knosp), for producing/providing the MLS dataset, and for helpful discussions (especially Nathaniel Livesey, Michelle Santee, and Michael Schwartz). We also thank Ken Minschwaner for helpful discussions and comments on the manuscript. MERRA-2 reanalysis data were provided by NASA's GMAO, led by Steven Pawson, and we especially thank Kris Wargan for his helpful comments and assistance with usage of those. Andreas Dörnbrack, Rémi Thiéblemont, and an anonymous referee provided comments that were very helpful in improving the paper. G. L. Manney's work on this paper was funded by and conducted under a contract from the Aura MLS Project at the Jet Propulsion Laboratory, California Institute of Technology.Edited by: F. Khosrawi Reviewed by: A. Dörnbrack, R. Thiéblemont, and two anonymous referees