Overview of the relationship between cyclone frequency and surface O3
The observed O3 levels at the two sites are first examined in relation to the ERA-Interim reanalysis cyclone
tracks due to its longer time period to match the O3 observations. Here, tracks were not filtered for intensity.
For springtime during the period of observations from 1988 to 2010 at Mace Head, the average number of the total cyclone
tracks associated with O3 > 75th pc (hereafter, high O3), O3 < 25th pc (hereafter, low
O3) or both high and low O3 is shown in Table as a percentage
of the total tracks in each region. In addition, the number of years during 1988–2010 which have more tracks associated
with high O3 than with low O3 and the number of years which have more tracks associated with low
O3 than with high O3 are shown in Table , including the number
of years where this difference is significant (“SGF”) based on the χ2 statistic.
There are higher mean percentages of tracks passing through the North and Center regions associated with high O3
at Mace Head, 52 and 51 %, respectively, than with low O3 at Mace Head, 37 and 41 %, respectively
(Table ). The North region has the greatest number of years (18 out of
23 years, Table ) where the percentage of tracks associated with high
O3 is greater than the percentage of tracks associated with low O3, where 15 of these years have
a significant χ2 difference (North region (SGF), Table ). The
Center region also has a large number of years (17 years) where the percentage of tracks associated with high
O3 is greater than the percentage of tracks associated with low O3, although only about a third of those
years have a significant difference (Table ). In the South region, in contrast
to the North and Center regions, there are more cyclones passing south of Mace Head
(Table ) which are associated with low O3 at Mace Head (53 %) than
with high O3 (45 %). In the South region, there are 16 years which have more tracks associated with
low O3 than with high O3, of which half of these years have significant differences
(Table ). The transition from significantly more cyclones in the North and
Center regions (located within the main storm track region, Fig. ) associated with high O3 at
Mace Head to significantly more cyclones in the South region (generally south of the main storm track region,
Fig. ) associated with low O3 at Mace Head illustrates how O3 levels are sensitive to the
passage of cyclones and their frontal zones. The percent of tracks which have both high and low O3 increased from
12 % in the North region to 20 % in the South region (Table ).
(a) 1000 hPa MACC O3 (color) and horizontal
wind vectors (10 ms-1 reference arrow) and MSLP (solid contours,
5 hPa intervals) on 4 March 2007 at 00:00 UTC, 12 h prior to
maximum ζ850 for N1 cyclone (N1: 52∘ N, 19∘ W).
Parent low (P: 59∘ N, 34∘ W) is located between Greenland
and Iceland. Mace Head indicated by the pink open circle.
(b) Analysis chart for 4 March 2007 at 00:00 UTC from UK Met Office
(available from archive at
http://www.wetterzentrale.de/topkarten/tkfaxbraar.htm, retrieved
19 May 2015) shows the complex fronts associated with the N1 cyclone and the
parent low. Mace Head (pink circle) is located to the north of the N1 warm
front.
The region east of Mace Head shows a slightly higher mean percentage of tracks associated with high O3 compared to
low O3 (48 compared to 45 %, Table ) with 15 out of
23 years marked by more tracks associated with high O3 than low O3 (6 of which were significant,
Table ) and 15 % of the tracks associated with both high and low O3
observations. The West region has only a slightly higher mean percentage of tracks associated with low O3 compared
to high O3 (52 compared to 51 %), with about half of the years (12 out of 23 years) with more tracks
associated with low O3 than high O3 (Table ) and 20 % of
the tracks associated with both high and low O3 observations. Although the differences are small and this
method did not test which direction the cyclone tracks passed through the region, these results suggest that the cyclones
in the East region are associated with more polluted air than cyclone tracks in the West region.
The relationship between cyclone location and surface O3 is also examined for the southern site, Monte Velho
(Table ). The Center region has a greater mean percentage of cyclone tracks
associated with high O3 (55 %) compared to those associated with low O3 (50 %), while in the
remaining regions the difference between the mean percent of tracks associated with high O3 and with low
O3 is small (Table ). The number of these years having more tracks in
the North, Center, and South regions associated with high O3 is almost equal to the number of years which had more
tracks associated with low O3. Very few years have a significant χ2 difference, possibly due to the
percent of tracks associated with both high and low O3 increases (up to 31 % of tracks) the further away from
the main NA storm track (Table ). However, there are a greater number of years
(12 out of 19 years) where the cyclone tracks are associated with low O3 to the west of Monte Velho, and
also 12 years where the cyclone tracks are associated with high O3 when tracks are to the east of
Monte Velho (Table ). Despite the small differences in the mean percent of
tracks (and the small number of significant years) associated with high and low O3 at Monte Velho, this indicates
that the cyclone tracks still play a role in affecting surface O3 measurements at Monte Velho associated with the
temporal variability of O3.
In summary, the O3 measurements at Mace Head and Monte Velho are influenced by cyclone tracks passing in the
vicinity of the observation sites. The relationship between the O3 observed and the cyclone tracks varies at the
two observation sites. In the following sections, several individual cyclone tracks are selected using the MACC
reanalysis and examined using the MACC and MERRA-2 reanalyses and the STFR tracer to identify the possible sources of high
O3 as well as the transport pathways of O3 within the cyclones which can affect the surface O3
observations at Mace Head and Monte Velho.
Cyclone case studies associated with high O3 at Mace Head
The four case study cyclones, selected in Sect. , are shown in Fig. with the
tracks colored by the corresponding MACC reanalysis O3 at Mace Head (N1 and S1 tracks, Fig. a)
and Monte Velho (N2 and S2 tracks, Fig. b).
Synoptic conditions and O3 distribution for the N1
cyclone (d; 59∘ N, 13∘ W) on 5 March 2007
at 00:00 UTC, 24 h after Fig. . High O3 was reported at
Mace Head. O3 (color; note different scales used for different
levels) and horizontal winds (20 ms-1 reference arrow) are shown
on four levels: (a) 300 hPa, (b) 500 hPa,
(c) 850 hPa, and (d) 1000 hPa. In addition,
MSLP (d; solid contours, 5 hPa intervals),
ω (a–c; black contours for positive values indicating
ascent, 15 hPah-1 contour intervals, and white contours for
descent, -5 hPah-1 contour intervals), and 2 PVU
isosurface (a; thick contour) are shown.
The N1 cyclone occurred during March 2007. The N1 cyclone track across the NA is further south than the S1 cyclone track
before it turns northward, remaining to the west of Mace Head. During the time the N1 cyclone is in the North region
(4–7 March 2007), high O3 (> 55 ppbv) occurs for about a day and a half at Mace Head. The S1 cyclone
occurred at the end of April 2012. As the S1 cyclone passes to the south of Mace Head, there is an increase in O3
until it leaves the South region.
In May 2006, the N2 cyclone moves through the North region about 10∘ (approximately 1000 km) to the north
of Monte Velho (Fig. b), so that Monte Velho was likely impacted by the trailing cold front. The N2
cyclone is a good example of how surface O3 levels can vary during the life cycle of a passing cyclone. At the
time of maximum vorticity, the surface O3 at Monte Velho is < 30 ppbv, while just before the N2 cyclone
leaves the North region the O3 concentration at Monte Velho has nearly doubled (Fig. b). The S2
cyclone also occurred in March 2007, shortly after the N1 cyclone. The S2 cyclone originates about 10∘ to the
west of Monte Velho and slowly tracks to the southeast toward and across North Africa (Fig. b). There is
high O3 (> 45 ppbv) at Monte Velho during the majority of the S2 cyclone's life cycle in the South
region.
The following sections will explore in more detail how the dynamics of the case study cyclones influenced the surface
O3 at Mace Head and Monte Velho. The synoptic conditions during different stages in the life cycle of the case
study cyclones are described using the MACC reanalysis data set and, where stated, also using the MERRA-2 reanalysis data
set and the STFR tracer, in order to investigate the importance of time and location of the cyclone relative to the high
and low O3 observations at the monitoring stations.
N1 cyclone-centered MSLP (solid black contours, 5 hPa
contour intervals), 925 hPa system relative horizontal winds
(20 ms-1, reference arrow), and 925 hPa thermal front
parameter (color, 2.5 K100-2km-2 intervals) on
(a) 4 March 2007 at 00:00 UTC and (b) 5 March 2007 at
00:00 UTC. Radial dotted lines are plotted every 45∘ and dotted
circles represent 5, 10, 15, and 20∘ radii from cyclone center. The
cyclone has been rotated in order that the direction of cyclone propagation
is toward the right, indicated by the large grey arrow. NB: geographical
north for (a) and (b) is toward the right of the page.
N1 cyclone: synoptic conditions associated with high O3 at Mace Head
The N1 cyclone is a secondary frontal cyclone which formed over the western NA (42.6∘ N, 50.6∘ W) on
2 March 2007 at 06:00 UTC from the parent low located between Greenland and Iceland. Analysis of the N1's life cycle
indicates there is high O3 already in the lower troposphere over the Labrador Sea (up to 70 ppbv at
1000 hPa) which had descended from the stratosphere as part of the DI airstream of the parent low (not shown).
This O3-rich air appears to be advected toward Mace Head by the N1 cyclone (Figs. a
and ). Nearly 2 days after the N1 cyclone formed, high levels of O3 (> 55 ppbv,
Fig. a) at 1000 hPa are then advected southeastward into the NA behind the cold front of the N1
cyclone (see Fig. b for location of fronts) and northward around the parent low
(Fig. a). The high O3 eventually reaches Mace Head on 5 March 2007 at midnight, after the cold
front has passed over Mace Head (Fig. d), and persists until 6 March 2007 at 12:00 UTC.
At the time of maximum vorticity (ζ850 × 11.56 × 10-5 s-1, 4 March
2007,
12:00 UTC, not shown), the N1 cyclone and its decaying parent low merged at the surface, intensifying the N1 cyclone.
Strong westerly winds developed throughout the troposphere across the NA region as a result of the large pressure
difference between the low-pressure systems in the NA (the N1 cyclone and an upstream low over Newfoundland) and the
Azores High (Fig. d). In addition to the descent (white contour lines indicate ω,
Fig. a–c) of stratospheric O3-rich air within the DI of the N1 cyclone, O3-rich air from
aloft (300 and 500 hPa) has descended toward the surface from the stratosphere as part of DI in the upstream low
(Fig. a–d). The strong westerly winds appear to transport this O3-rich air across the western
NA (Fig. a–d). This results in persistent high O3 observed at Mace Head in the days following
the maximum vorticity of the N1 cyclone (Fig. d), and even after the cyclone has left the North region
(Fig. a).
N1 cyclone-centered O3 (color) and system relative winds
(20 ms-1, reference arrow) on 4 March 2007 at 00:00 UTC:
(a) 500 hPa with ω (10 hPah-1 contour
intervals, with black contours for positive values indicating ascent and
white contours for negative values indicating descent) and
(c) 1000 hPa with MSLP (solid black contours, 5 hPa
contour intervals) and approximate location of the warm (red line) and cold
(blue line) fronts based on Figs. b and a. The cyclone
has been rotated in order that the direction of cyclone propagation is toward
the right, indicated by the grey arrow. The additional thick vertical lines
in (a) and (c) indicate the location of the vertical
transects (b and d) of O3 (color), ω
(5 hPah-1 contour intervals, with black contours for positive
values indicating ascent and white contours for negative values indicating
descent), θe (dotted contour lines, 5 K intervals),
and the 2 PVU isosurface for the dynamical tropopause (thick solid contour):
(b) 10∘ behind the cyclone center and
(d) 4∘ ahead of cyclone center, respectively. The cross
sections are < 40∘ diameter with positive axis values to the
north. The approximate location of Mace Head is indicated by the large black
dot (a, c, d).
The N1 cyclone is a good example of how one cyclone can bring both low and high O3 to Mace Head. Prior to maximum
vorticity, low O3, defined to be less than the MAM 2007 6-hourly averaged O3 25th pc value of
39.1 ppbv, is reported at Mace Head on 4 March 2007 at 00:00 UTC (Fig. a).
Figure a shows there is a sharp contrast in O3 (> 10 ppbv) at the cold frontal
boundary, with higher O3 levels behind the cold front and lower O3 levels ahead of the cold front in the
warm sector between 30 and 50∘ N. This low O3 is advected from the subtropics northward towards Mace Head
parallel to the cold front within the warm sector (Fig. a).
The distribution of O3 within the airstreams of the N1 cyclone can be examined by using a cyclone-centered
coordinate system (Figs. – and S1–S4 in the Supplement). Here, the reanalyses are sampled onto a radial
grid centered on the cyclone track points within a 20∘ (approximately 2000 km) radius with the cyclones
rotated to be moving from left to right in the figures following . For discussion of the cyclone-centered
maps, the unit circle with cardinal points is used instead of using geographical coordinates such that the direction of
cyclone propagation is from “west” to “east” .
N1 cyclone on 5 March 2007 at 00:00 UTC, similar to
Fig. , except 24 h later. The additional thick vertical lines
(a, c) indicate the location of the vertical transects
(b) 2∘ to the north of the cyclone center and
(d) 7∘ behind the cyclone center, respectively. The cross
sections have positive axis values to the east (a, b) and
to the north (c, d). The approximate location of Mace Head
is indicated by the large black dot. The cyclone has been rotated in order
that the direction of cyclone propagation is toward the right, indicated by
the grey arrow.
In the cyclone-centered coordinate system, the fronts are identified using the TFP function (Fig. ). In
Fig. a, the TFP values on the 925 hPa surface on 4 March 2007 at 00:00 UTC show there is
a baroclinic zone (strong horizontal temperature gradient on a constant pressure surface) or front from the N1 cyclone
center toward the west/southwest (relative to the direction of cyclone propagation) as well as another one to the
south/southeast. However, this does not indicate whether the fronts are warm fronts or cold fronts. Using the synoptic
chart in Fig. b, in combination with the system-relative winds (where the speed of the cyclone
propagation has been subtracted from the winds about each cyclone; ) and the MSLP contours in
Fig. , the front trailing from the center of the N1 cyclone toward the west is diagnosed as a cold front
and the weaker baroclinic zone in the bottom right quadrant is a warm front (Fig. a). There is an
additional baroclinic zone associated with the parent low in the top right quadrant as well as other baroclinic zones
within the warm sector of the cyclone that are not seen in the analysis chart (Fig. b). A day later, the
N1 cyclone has become occluded and the baroclinic zones are weak around the cyclone center and along the trailing cold
front which is now further to the south (Fig. b).
The main feature at 4 March 2007 at 00:00 UTC in both the meteorological fields and O3 distribution is the cold
frontal boundary which separates the WCB and DI airstreams (Fig. ). The strong cold frontal boundary
features as the sharp gradient in the 1000 hPa O3 distribution and the curve in the isobars in the
southwest quadrant of Fig. c (similar to Fig. ). The front is identified between -5
and -2∘ throughout the troposphere by a strong gradient in equivalent potential temperature, θe, from
1000 to 400 hPa, located between the high values of θe (warm and moist) to the south of the cyclone center
(-15 to -5∘) compared to the lower values of θe (cool and dry) to the north (-2 to 15∘,
Fig. b). Ahead of the cold front, the WCB airstream transports O3-poor air into the N1 cyclone
(Fig. ). The WCB airstream can be identified by the region of strong ascent (Fig. a)
throughout the troposphere (-5∘, 1000–400 hPa; Fig. b), with the maximum ascent to the
north of Mace Head (maximum ω> 60 hPah-1 at 700 hPa, Fig. a and d). Within
the N1 cyclone, O3-rich air descended toward the surface within the DI airstream; however this is far from
Mace Head (about 1400 km) at this time (Fig. a and b). Stratospheric O3-rich air,
identified here by high levels of O3 (> 100 ppbv) above the dynamical tropopause (thick solid black
contour), reaches down to nearly 500 hPa behind the cold front (-2 to 3∘,
Fig. b). There is further isentropic descent (along constant θe contours) of the relatively
high levels of O3 behind the cold front over a large area between -5 and 15∘, with strong descent of
ω≤-30 hPah-1. This results in high O3 (> 55 ppbv) at 1000 hPa behind
the surface cold front (-5 to 17∘, Fig. b and c). The stratospheric fraction of air behind the
cold front is ≥30 % down to 700 hPa and ≥10 % down to 850 hPa (Fig. S2b).
Synoptic conditions and O3 distribution for the S1 cyclone
(b; 12∘ N, 48∘ W) on 25 April 2012 at 00:00 UTC,
at the time of maximum ζ850. Two levels are shown:
(a) 500 hPa and (b) 1000 hPa. Additional
features are labeled, including the DI airstream (a) and the
downstream decaying parent low to the S1 cyclone (white “P”, b),
and Mace Head is indicated (pink circle, b).
A day later, the cold front associated with the N1 cyclone has passed over Mace Head and high O3 (55 ppbv)
is reported at the surface. There are elevated levels of O3 in both reanalyses: the MACC reanalysis at the
1000 hPa near Mace Head > 60 ppbv (Fig. c) and the MERRA-2 O3 at
950 hPa is 45–50 ppbv (Fig. S3c). Since the cyclone has started to decay, it is more difficult to
identify the fronts and airstreams within the cyclone (Fig. b). The maximum WCB ascent ahead of the N1
cyclone at 500 hPa (ω> 30 hPah-1, Fig. a) is approximately half the value
compared to the day before (ω> 60 hPah-1, Fig. a). However, there are strong
values of positive ω at 500 hPa (Fig. a and d) associated with the upstream cyclones (closest
cyclone is identified as a closed low contour to the northwest of the N1 cyclone in Fig. c). The DI
airstream has also weakened, yet the tropopause is still visibly depressed at 400 and 500 hPa
(Fig. b and d, respectively) and high levels of O3 (> 55 ppbv) are present throughout
the mid- to lower troposphere (Fig. b and d). The O3 originally from the stratosphere has descended
isentropically toward the surface near the location of Mace Head (black dot, near -7∘ in Fig. b
and 3∘ in Fig. d). This is also portrayed in the MERRA-2 O3 in Fig. S3b, d, with
a tongue of large STFR over Mace Head (at 700 hPa, STFR > 40 % and > 30 %, Fig. S4b and d,
respectively). The high O3 associated with the upstream cyclone descends into the mid-troposphere (10∘,
Fig. d) and is associated with high O3 at Mace Head later in the life cycle of the N1 cyclone.
Similar to Fig. (a–d) but also includes an
inset over Ireland and the UK (45–60∘ N,
15∘ W–0∘; e–h, winds not included) for the S1
cyclone (d; 51∘ N, 5∘ W) 18 h later, on 25 April
2012 at 18:00 UTC for four pressure levels:
(a, e) 300 hPa, (b, f) 500 hPa,
(c, g) 850 hPa, and (d, h) 1000 hPa. High
O3 reported at Mace Head.
The MACC and MERRA-2 representations of the N1 storm (Figs. – and Figs. S1–S4,
respectively), including the stratospheric O3 mixing ratios, are similar. In the troposphere, MERRA-2 O3
is expected to be biased low (Sect. ), and while the MERRA-2 troposphere O3 mixing ratios are
lower in the N1 cyclone compared to the MACC reanalysis, there is still relatively higher O3 within the DI
airstream reaching the surface (Figs. S1b and d and S3b–d) as seen in Figs. and . The
higher vertical resolution of MERRA-2 shows the tropopause fold reaches just below 600 hPa (-3∘,
Fig. S1b) which is 100 hPa closer to the surface than shown in the MACC reanalysis (Fig. b).
Since both reanalyses show similar O3 spatial patterns, although not quantitatively the same, we are confident the
O3 reaching Mace Head has stratospheric origin.
Since the N1 cyclone was described in great detail, in the following case studies we will highlight differences compared
to the N1 cyclone.
S1 cyclone: synoptic conditions associated with high O3 at Mace Head
The S1 cyclone originated over New York, USA (43∘ N, 74∘ W), on 22 April 2012 and tracked past Mace Head
through the South region between 24 April and 26 April 2012. High 75th pc 6-hourly averaged O3
(> 45 ppbv) was generally observed at Mace Head while the S1 cyclone was in the South region on 24 April at
12:00 UTC to 26 April 2012 at 06:00 UTC (Fig. a). The center of the S1 cyclone at the time of maximum
vorticity (25 April 2012 at 00:00 UTC), is located almost directly to the south of Mace Head (50∘ N,
10∘ W; Figs. a and b) having merged with a downstream decaying cyclone (“P”,
Fig. b). At this time, Mace Head reported median O3 levels. There is a high-pressure system
located directly to the west, centered at approximately 50∘ N, 45∘ W, which appears to block transport
from upwind North American sources (Figs. b and d). The atmospheric circulation pattern
of high pressure to the west and the S1 cyclone south of Mace Head is very different to the pattern seen during the life
cycle of the N1 cyclone, where there were several cyclones in the NA sector and strong westerly winds due to the pressure
gradient between the cyclones and the Azores High (Figs. and ).
The source of elevated surface O3 (> 55 ppbv) at Mace Head is not associated with descent of air
directly from the lower stratosphere within the DI airstream of the S1 cyclone, as seen in the N1 cyclone. Instead there
is “residual” high O3 in the mid- to lower troposphere (50–60∘ N,
20∘ W–30∘ E;
Fig. a). This O3 is referred to as residual O3 as it descended several days prior to the
S1 cyclone passing near Mace Head (not shown) within the DI of the downstream cyclone (“P”, Fig. b)
prior to its decay. This high O3 appears to become entrained on the north side of the S1 cyclone and advected
cyclonically in the lower and mid-troposphere over northern UK and eastern Europe/Scandinavia (Fig. ).
Eighteen hours after the S1 cyclone reached maximum vorticity (25 April 2012 at 18:00 UTC), high levels of O3
were reported at Mace Head. The MSLP field is still characterized by a blocking high in the western NA and the S1 cyclone
is to the southeast of Ireland (Fig. d and h). Above Mace Head, there is descent (closed white contour,
Fig. c and g) in the region of high O3 (> 55 ppbv) which can facilitate the transport of
O3 from the mid-troposphere toward the surface. A large ridge has formed over the NA, identified at
300 hPa by the anticyclonic wind flow aloft as well as the 2 PVU contour (Fig. a). On either
side of the ridge at 300 hPa, the troughs bring stratospheric air towards lower latitudes than Mace Head
(Fig. a). This allows for O3-rich air to be transported towards the surface at lower latitudes
as part of the DI airstream within such strong cyclones as the S1 cyclone (between 35 and 45∘ N,
Fig. b–d).
Similar to N1 cyclone Figs. and except for the S1 cyclone on 25 April 2012 at 00:00 UTC (see
Fig. ). The approximate location of the occluded (purple line), warm (red line), and cold (blue line) fronts in (c)
are based on the fronts in Fig. S5a and the synoptic analysis chart (not shown). The additional thick vertical lines in
(a) and (c) indicate the location of the vertical transects (b) 2∘ in front of the cyclone center
and (d) 4∘ to the north of cyclone center, respectively. The cross sections have positive axis values to the north
(a, b) and to the east (c, d). The approximate location of Mace Head has been indicated by the large
black dot. The cyclone has been rotated in order that the direction of cyclone propagation is toward the right, indicated by the grey
arrow.
To investigate the residual O3 in more detail, we compare the S1 cyclone airstreams' vertical and horizontal
distribution of O3 at the time of maximum ζ850 (25 April 2012, 00:00 UTC) using the cyclone-centered
coordinate system in both the MACC and MERRA-2 reanalyses (Figs. and ). In
Figs. a and a, the stratospheric O3 within the DI airstream arrives into the
center of the cyclone from the northwest quadrant and is bounded by the WCB ascent to the right of the cyclone's center
(black contours with maximum at 5∘ radius). Most of the northeast quadrant of the S1 cyclone at 500 hPa,
shown in Figs. a and a, is dominated by the residual high levels of O3
(> 55 ppbv; STFR is > 20 %, Fig. S6a) identified above as coming from the downstream decaying cyclone.
The MACC and MERRA-2 O3 mixing ratios throughout the mid-troposphere (above 600 hPa) to lower stratosphere
(200 hPa) are comparable during the S1 cyclone (Figs. and ). The vertical
transport of O3 will be sensitive to the strong gradients of O3 near the tropopause. The MACC O3
within the DI airstream can be as much as 75 ppbv higher than the MERRA-2 O3, as the MERRA-2 reanalysis
O3 values decrease closer to the dynamical tropopause compared to the MACC O3 distribution. Both the MACC
and MERRA-2 O3 have good agreement to independent observations in the upper troposphere/lower stratosphere (UTLS)
region . In the Northern Hemisphere (NH) extratropics, MACC O3 between 500 and
100 hPa has a bias of -10–0 % when compared to ozonesondes profiles and found
that the average MERRA-2 O3 profile over Europe (45–60∘ N, 0–60∘ E) in the spring of 2005
(after the assimilation of MLS and OMI satellite retrievals) captures the vertical structure of the ozonesondes in that
region (r2 > 0.6), although biased low (up to -20 %) in the UTLS.
S1 cyclone on 25 April 2012 at 00:00 UTC, similar to Fig. except using MERRA-2 reanalysis data. Note: (c) is 950 hPa instead of 1000 hPa to minimize the missing data plotted. This occurs since the GEOS-5 model masks orography.
The two vertical cross sections in Figs. b and d and b and d intersect the approximate
location of Mace Head and the residual high O3 in the mid-troposphere that is not associated with the S1 cyclone's
DI airstream. The tropopause is depressed to about 500 hPa throughout the DI, shown close to the cyclone center
(0∘) in Figs. b and b and behind the S1 cyclone (-8∘) in
Figs. d and d. High levels of O3 (> 55 ppbv) are found down to
800 hPa associated with the DI at the cyclone center (Figs. b and b) and within
the isentropic descent behind S1 between -19 and -5∘ (Figs. d and d). There
is no injection of stratospheric air into the troposphere occurring near Mace Head from the S1 cyclone, as the tropopause
remains almost parallel with the isentropes in the transects between 5 and 15∘ in Figs. b
and b and between -4 and 15∘ in Figs. d and d. The MERRA-2
reanalysis shows the DI of the upstream parent low is directly related to the high O3 in the mid-troposphere as
a small closed 2 PVU contour circle – the dynamical tropopause – is above Mace Head at 500 hPa
(Fig. d), which is not seen in the coarser MACC reanalysis (Fig. d). At 500 hPa,
the STFR is > 40 % within the remnant O3 associated with the decaying cyclone to the east of the S1 center
(Fig. S6d). At about 5∘ in both Fig. b and d transects, there is descent
(-5 hPah-1 contour) of this residual high O3 from the mid-troposphere toward the surface over
Mace Head in the MACC reanalysis. Despite the negative vertical velocity above Mace Head in
Fig. b and d, the simulated MERRA-2 O3 does not descend fully to the surface; however the
simulated O3 at the surface at Mace Head in both reanalyses is approximately 45 ppbv.
Synoptic conditions and O3 distribution for the N2 cyclone
at 1000 hPa: MACC O3 (color) and horizontal
wind vectors (10 ms-1 reference arrow) and MSLP (solid contours, 5 hPa intervals) on (a) 18 May
2006 at 18:00 UTC, (b) 21 May 2006 at 06:00 UTC, and (c) 22 May 2006 at 06:00 UTC. The approximate cold front
(b, c; blue line) and Monte Velho (pink open circle) are included.
N2 cyclone: synoptic conditions associated with high O3 at Monte Velho
The N2 cyclone is a strong cyclone which passed north of Monte Velho and advected high O3 to the observation
site. Cyclogenesis of the N2 cyclone occurs off the east coast of Florida on 17 May 2006 at 12:00 UTC (not shown) and
then tracks northeastward (Fig. b). The synoptic conditions at the time of the N2 cyclone are similar to
the N1 cyclone: about a day after cyclogenesis, the N2 cyclone is located just south of Newfoundland and downwind of the
N2 cyclone are a series of low-pressure systems across the NA region as well as a strong Azores High stretching over most
of the NA (Figs. a and S7c). The large pressure gradient results in strong westerly winds at near
45∘ N (Fig. a) throughout the troposphere (Fig. S7). Similar to the N1 cyclone, stratospheric
O3 descends into the troposphere within the low-pressure systems in the NA ahead of the N2 cyclone
(Figs. a and S7). Similar to the S1 cyclone, a high-pressure system develops to the west of the N2
cyclone (Fig. b and c), supporting upper-level descent behind the N2 cyclone's cold front and negating
transport across the NA from North America (Figs. b and c, S8, and S9).
As was seen for the N1 cyclone, the N2 cyclone is also associated with low O3 before high O3 is observed
at the monitoring station due to the passage of a cold front (Fig. b and c). Since the N2 cyclone is similar
to the N1 and S1 cyclones discussed in Sects. and , full details for this case can be found
in the Supplement (Sect. S2.3).
S2 cyclone: synoptic conditions associated with high O3 at Monte Velho
Synoptic conditions for the S2 cyclone (d; 31∘ N, 17∘ W) on the 13 March 2007 at 18:00 UTC, at the time of maximum ζ850. O3 (color; note different scales used for different levels) and horizontal winds (note magnitude of reference arrow changes) are shown on four levels: (a) 300 hPa, (b) 500 hPa, (c) 850 hPa, and (d) 1000 hPa. In addition, MSLP (d; solid contours, 5 hPa intervals), ω (a, b, and c; 5 hPah-1 contour intervals, black contours for positive values indicating ascent (NB: different interval to previous cyclones) and white contours for descent), and 2 PVU isosurface (a; thick contour) are shown. Monte Velho indicated by pink open circle.
The S2 cyclone is a moderate cyclone that formed on 12 March 2007 at 00:00 UTC to the south of a large high-pressure system
which extended over the eastern NA and Europe (not shown). On 13 March 2007 at 18:00 UTC, 42 h later, the S2 cyclone
reaches maximum vorticity (ζ850=7.3×10-5 s-1; 31.3∘ N, 16.5∘ W) and is
still located to the south of the high-pressure system over the eastern NA (Fig. d). At this time, there
are additional strong weather systems in the NA region: a deep Icelandic low, a low over Hudson Bay, and a high-pressure
system over the western NA. Within the S2 cyclone, there is strong descent within the DI airstream between 300 and
850 hPa (Fig. a–c), with maximum descent at 500 hPa (ω< -25 hPah-1,
Fig. b) nearly as strong as the N1 cyclone (Fig. b). High values of O3
(> 80 ppbv) descend from upper levels toward the surface within the DI airstream and advect cyclonically around
the S2 cyclone and anticyclonically within the high-pressure system at lower levels, behind the low-level cold front (850
and 1000 hPa, Fig. c and d). Over the Iberian Peninsula at 850 hPa, there is descent of
O3-rich air likely associated with the large region of descent throughout the troposphere over the Mediterranean
Sea associated with a downstream cyclone (Fig. a–c).
Similar to Fig. except for the S2 cyclone (c; 23∘ N, 1∘ W) nearly 4 days later on the 17 March 2007 at 12:00 UTC. High O3 recorded at Monte Velho. O3 (color; note different scales used for different levels) and horizontal winds (note magnitude of reference arrow changes) is shown on three levels: (a) 500 hPa, (b) 850 hPa, and (c) 1000 hPa. In addition, MSLP (c; solid contours, 5 hPa intervals) and ω (a and b; 5 hPah-1 contour intervals, black contours for positive values indicating ascent and white contours for descent) are shown.
Over the following 4 days, the high O3 at Monte Velho associated with the S2 cyclone persists while the S2
cyclone tracks southeastward (see Fig. b). At upper levels, a closed low detaches from the stratosphere
(not shown). In Fig. a, the descent within the DI of the S2 cyclone can be identified at
500 hPa; however the S2 cyclone can no longer be identified by a closed low-pressure system over northern Africa
in Fig. c. The high-pressure system over the eastern NA moves southwestward with the O3-rich air
which has accumulated within it (Fig. c). There is still descent occurring over the Iberian Peninsula in
the mid-troposphere, which continues to transport relatively high levels of O3 to the surface at Monte Velho
(Fig. a and b).
The cyclone-centered analysis was not performed for the S2 cyclone since (1) the high O3 at Monte Velho prior to
maximum vorticity is associated with a downstream cyclone's DI airstream (as seen in the S1 cyclone) and (2) after the S2
cyclone started to decay the high O3 is associated with subsidence in the high-pressure system behind the S2 cold
front.