A weak El Niño during 2014–2015 boreal winter developed as a strong
boreal summer event in 2015 which continued and even enhanced during the
following winter. In this work, the detailed changes in the structure,
dynamics, and trace gases within the Asian summer monsoon anticyclone (ASMA)
during the extreme El Niño of 2015–2016 is delineated by using Aura Microwave
Limb Sounder (MLS) measurements, COSMIC radio occultation (RO) temperature,
and National Centers for Environmental Prediction (NCEP) reanalysis products. Our analysis concentrates only on the summer
months of July and August 2015 when the Niño 3.4 index started to exceed values of 1.5. The results show that the ASMA structure was quite different in
summer 2015 as compared to the long-term (2005–2014) mean. In July, the
spatial extension of the ASMA is greater than the long-term mean in all
the regions except over northeastern Asia, where it exhibits a strong
southward shift in its position. The ASMA splits into two, and the western
Pacific mode is evident in August. Interestingly, the subtropical westerly
jet (STJ) shifted southward from its normal position over northeastern Asia,
and as a result midlatitude air moved southward in 2015. Intense Rossby wave
breaking events along with STJ are also found in July 2015. Due to these
dynamical changes in the ASMA, pronounced changes in the ASMA tracers are
noticed in 2015 compared to the long-term mean. A 30 % (20 %) decrease
in carbon monoxide (water vapor) at 100 hPa is observed in July over most of
the ASMA region, whereas in August the drop is strongly concentrated at the
edges of the ASMA. A prominent increase in O3 (> 40 %) at
100 hPa is clearly evident within the ASMA in July, whereas in August the
increase is strongly located (even at 121 hPa) over the western edges of the
ASMA. Further, the temperature around the tropopause shows significant
positive anomalies (∼ 5 K) within the ASMA in 2015. The present
results clearly reveal the El-Niño-induced dynamical changes caused
significant changes in the trace gases within the ASMA in summer 2015.
Introduction
The Asian summer monsoon anticyclone (ASMA) is a distinct circulation system
in the upper troposphere and lower stratosphere (UTLS) during Northern
Hemisphere boreal summer centered at ∼ 25∘ N and
extending roughly between 15 to 40∘ N (Park et al., 2004; Randel
et al., 2010). It is encircled by the subtropical westerly jet stream to the
north and by the equatorial easterly jet to the south (Randel and Park,
2006). It is well recognized that the ASMA circulation is a prominent
transport pathway for troposphere pollutants to enter the stratosphere
(Randel et al., 2010). Previous studies have concluded that deep convection
during the summer monsoon can effectively transport the pollutants, aerosols, and
tropospheric tracers from the boundary layer into the UTLS region (Vogel et
al., 2016; Santee et al., 2017). These transported pollutants, tracers and
aerosols become confined in the ASMA and, consequently, affect the trace gas
composition in the UTLS region (Randel et al., 2010; Hossaini et al., 2015). It is clearly evident from the
previous studies that the ASMA has higher concentrations of tropospheric
tracers such as carbon monoxide (CO), hydrogen cyanide (HCN), and methane
(CH4) and lower concentrations of stratospheric tracers including ozone
(O3) and nitric acid (HNO3) (Park et al., 2004, 2008; Li et al., 2005; Randel et al., 2010; Vernier et al., 2015, 2018; Yan and Bian, 2015; Yu et al., 2017; Santee et al., 2017). The
comprehensive study on the climatological composition within the ASMA can be
found in Santee et al. (2017). The Asian summer monsoon (ASM) convection and orographic lifting are
the primary mechanisms for the higher concentrations of the tropospheric
tracers in the ASMA (Li et al., 2005; Park et al., 2009; Santee et al.,
2017). Apart from these trace gases a strong persistent tropopause-level
aerosol layer called the “Asian Tropopause Aerosol Layer” (ATAL) also existed
between 12 to 18 km within the ASMA, and it was first detected from the
CALIPSO measurements (Vernier et al., 2011).
Similarly, higher concentrations of water vapor (WV) within the ASMA during
the summer monsoon is well documented in the literature (Gettelman et
al., 2004; Park et al., 2007; Randel et al., 2010; Bian et al., 2012; Xu et
al., 2014; Jiang et al., 2015; Das and Suneeth, 2020). It is well known that
most of the WV enters the stratosphere through the tropical tropopause
(Fueglistaler et al., 2009), and the temperature present at the tropical
tropopause strongly controls the WV entering the lower stratosphere (LS). It
is also well documented that several processes such as convection, the strength
of the Brewer–Dobson circulation, El Niño–Southern Oscillation (ENSO),
and quasi-biennial oscillation (QBO) are responsible for the WV transport to
the UTLS region (Holton et al., 1995; Dessler et al., 2014; Jiang et al.,
2015). Other factors such as gravity waves and horizontal advection can also
influence the WV transport in the UTLS region. For example, Khan and Jin (2016) studied the effect of gravity waves on the tropopause and WV over
the Tibetan Plateau and reported that the gravity wave is the source for the WV
transport from the lower to higher altitudes. Recently, Das and Suneeth (2020) reported about the distributions of WV in the UTLS over the ASMA
during summer using 13 years of Aura Microwave Limb Sounder (MLS) observations.
They concluded that WV in the UTLS region inside the central part of ASMA is
mostly controlled by horizontal advection and much less by the local
process and tropopause temperature in both summer and winter.
Convection during the summer monsoon is one of the major sources to
transport the boundary layer pollutants into the UTLS region (Randel et al.,
2010). It is a well established fact that the ENSO has a strong influence on
convection and circulation changes over the Asian monsoon region (Kumar et
al., 1999; Wang et al., 2015; Gadgil and Francis, 2016). Enhanced
(suppressed) convection over the Asian monsoon region is generally observed in
the cold phase of ENSO (warm phase of ENSO) known as La Niña (El
Niño). Few studies exist to date on the impact of ENSO on the
ASMA trace gas composition changes and its dynamical changes. For example,
Yan et al. (2018) reported the influence of ENSO on the ASMA with a major
focus on how the ENSO winter signal propagates into the following seasons.
They showed the weaker O3 transport into the tropics during the onset
of the ASMA after boreal winter El Niño events, but the difference
between El Niño and La Niña composites becomes insignificant in the
summer. In another study, Tweedy et al. (2018) demonstrated the impact of
boreal summer ENSO events on O3 composition within the ASMA in
different phases of ENSO events. They reported that the ASMA forms earlier
and stronger in the La Niña period that leads to greater equatorward
transport of O3-rich air from the extratropics into the northern
tropics than during El Niño periods. Recently, Fadnavis et al. (2019)
reported higher concentrations of aerosol layers observed in the ATAL region
during the El Niño period over the northern part of South Asia. However,
the above-mentioned studies are mainly focused on changes in the ASMA with
respect to ENSO on seasonal scales or the mature stage of the monsoon (combined mean
of July and August).
Based on the above-mentioned studies, it can be concluded that the ENSO also
has a strong influence on the ASMA structure and its composition. The recent
2015–2016 El Niño event was recorded as an extreme and long-lasting event
in the 21st century (Avery et al., 2017). It
started as a weak El Niño during 2014–2015 boreal winter, and it developed
as a strong boreal summer El Niño event in 2015 (Tweedy et al., 2018).
Further, this strong boreal summer event continued and was significantly
enhanced until the boreal winter of 2015–2016. In this event, several unusual
changes occurred in the tropical UTLS region including the strong
enhancement in the lower stratosphere WV (higher positive tropopause
temperature anomalies) over the Southeast Asian and western Pacific regions
(Avery et al., 2017) and anomalous distribution of trace gases in the UTLS
region (Diallo et al., 2018; Ravindra Babu et al., 2019a). In a similar way, the
response of different trace gases (O3, HCl, WV) to the disrupted
2015–2016 quasi-biennial oscillation (QBO) associated with the 2015–2016 El
Niño event is also reported by Tweedy et al. (2017). Dunkerton (2016)
discussed the possible role of the unusually warm ENSO event in 2015–2016 to the
QBO disruption by triggering the extratropical planetary waves. Therefore,
in the present study, we investigated the detailed changes observed in the
ASMA 2015 particularly by focusing on the structure, dynamics, and trace
gas variability within the ASMA in July and August 2015 by using satellite
observations and reanalysis products. The present research article is
organized as follows. The database and methodology adopted in this study are
discussed in Sect. 2. The results and discussions are illustrated in
Sect. 3. Finally, the summary and conclusions obtained from the present
study are summarized in Sect. 4.
Database and methodologyMicrowave Limb Sounder measurements
In the present study, version 4.2 of Aura MLS measurements of CO, O3, and
WV are utilized. The MLS data of July and August in each year from
2005 to 2015 are considered. The vertical resolution for CO is in the
range of 3.5–5 km from the upper troposphere to the lower mesosphere and the
useful range is 215–0.0046 hPa. The horizontal resolution for CO is about
460 km at 100 hPa and 690 km at 215 hPa. For WV, the vertical resolution is
in the range of 2.0–3.7 km from 316 to 0.22 hPa, and the along-track
horizontal resolution varies from 210 to 360 km for pressure greater than
4.6 hPa. For O3, the vertical resolution is ∼ 2.5 km, and
the along-track horizontal resolution varies between 300 and 450 km. The
precision (systematic uncertainty) for WV is ∼ 10 %–40 %
(∼ 10 %–25 %), for O3∼ 0.02–0.04
(∼ 0.02–0.05) ppmv, and for CO ∼ 19 ppbv
(30 %). More details about the MLS version 4 level 2 data
can be found in Livesey et al. (2018).
COSMIC radio occultation measurements
To see the changes in the tropopause temperature and height within the ASMA,
we used high-resolution, post-processed products of level 2 dry temperature
profiles obtained from Constellation Observing System for Meteorology,
Ionosphere, and Climate (COSMIC) radio occultation (RO). Each month of July
and August from 2006 to 2015 is considered. The data are downloaded from the
COSMIC Data Analysis and Archive Center (CDAAC) website. We used 200 m
vertical resolution temperature profiles in the study. Details of the
temperature retrieval from the bending angle and refractivity profiles
obtained from the RO sounding are represented well in the literature
(Kursinski et al., 1997; Anthes et al., 2008). The COSMIC temperature have a
precision of 0.1 % between 8 and 25 km (Kishore et al., 2009; Kim and Son, 2012). The temperature accuracy in the UTLS is better than 0.5 K for
individual profiles and ∼ 0.1 K for averaged profiles (Hajj et
al., 2004). It is noted that for individual RO temperature profiles, the
observational uncertainty estimate is 0.7 K in the tropopause region,
slightly decreasing into the troposphere, and gradually increasing into the
stratosphere (Scherllin-Pirscher et al., 2011a). For monthly zonal-averaged
temperature fields, the total uncertainty estimate is smaller than 0.15 K in
the UTLS (Scherllin-Pirscher et al., 2011b). Overall, the uncertainties of
RO climatological fields are small compared to any other UTLS observing
system for thermodynamic atmospheric variables. Note that these data are
compared with a variety of techniques including GPS radiosonde data and had a good correlation particularly in the UTLS region (Rao et al., 2009;
Kishore et al., 2009). The COSMIC RO profiles have been widely used for
studying the tropopause changes and its variabilities (Kim and Son, 2012;
Ravindra Babu et al., 2015; Ravindra Babu and Liou, 2021).
National Centers for Environmental Prediction (NCEP) reanalysis data
We also utilized monthly mean geopotential height (GPH) and wind vectors
(zonal and meridional wind speed) from the NCEP-DOE Reanalysis 2 (Kanamitsu
et al., 2002), covering the same time period as the MLS observations
(2005–2015). NCEP-DOE Reanalysis 2 is an improved version of the NCEP
Reanalysis 1 model that fixed errors and updated parametrizations of
physical processes. The horizontal resolution of NCEP-DOE Reanalysis 2 is
2.5∘× 2.5∘.
Apart from the above-mentioned data sets, we also used European Centre for
Medium-Range Weather Forecasts (ECMWF) interim reanalysis potential
vorticity (PV) data particularly at the 350 K isentropic surface in July and
August 2015 (ERA-Interim; Uppala et al., 2005; Dee et al., 2011).
Methodology
Daily available MLS profiles of O3, CO, and WV in each month are
constructed and gridded by averaging the profiles inside bins with a
resolution of 5∘ latitude × 5∘ longitude. The
following equation is used to estimate the relative change in percentage.
Relative change in percentage=xi-x‾x‾×100,
where xi represents the monthly mean of July and August in 2015, and
x‾ is the corresponding monthly long-term mean which is calculated by using the data from 2005 to 2014.
Temporal evolution of observed Niño 3.4 index data from January
2005 to December 2016.
Spatial distribution of geopotential height obtained from NCEP-DOE
Reanalysis 2 data during July 2015 at (a) 100 hPa and (b) 150 hPa
superimposed with wind vectors at the respective corresponding levels.
Subplots of (c) and (d) are the same as (a) and (b) but for the month of
August. The solid black contour lines represent the ASMA region at 100 hPa (16.75 km GPH contour).
Results and discussion
It is well reported that the ASMA is highly dynamic in nature with respect
to its position and shape. It also varies at different timescales, i.e.,
day-to-day, weekly, and monthly scales, caused by internal dynamical
variability (Randel and Park, 2006; Garny and Randel, 2013; Pan et al.,
2016; Nützel et al., 2016; Santee et al., 2017). The intensity and
spatial extension of the ASMA are prominent in July and August when the
monsoon was in the mature phase (Santee et al., 2017; Basha et al., 2020).
It can be noticed that the 2015–2016 El Niño event was one of the
strongest boreal summer events that occurred in the entire MLS data record
(Tweedy et al., 2018). In this event, the Niño 3.4 data exceeded +1.5
in July and +1.8 in August (Fig. 1). Therefore, in the present study, we
mainly focused on ASMA behavior and trace gas changes within the ASMA on
monthly scales particularly in July and August 2015 which represent strong
El Niño.
Structure and dynamical changes in ASMA during 2015
In general, in the studies looking at monthly or seasonal timescales related to
the thermodynamical features in the ASMA, the anticyclone region is mostly
defined from the simple constant GPH contours at different pressure levels
(Randel and Park, 2006; Yan et al., 2011; Bergman et al., 2013; Basha et
al., 2020). Previous researchers used different GPH contours at 100 hPa to
define the anticyclone region. For example, Yan et al. (2011) used 16.7 km,
Bergman et al. (2013) used 16.77 km, and recently Basha et al. (2020) used
16.75 km GPH contours as the anticyclone region. In a similar manner, we also
defined the ASMA region based on GPHs obtained from NCEP-DOE Reanalysis 2 at 100 hPa and considered the 16.75 km GPH contour as the anticyclone region.
Spatial distribution of geopotential height anomalies obtained
from NCEP-DOE Reanalysis 2 data during July 2015 (a) at 100 hPa and (b) 150 hPa superimposed with wind vectors at the respective corresponding levels.
Subplots of (c) and (d) same as (a) and (b) but for the month of August. The
solid white contour lines represent the ASMA region at 100 hPa (16.75 km GPH contour) observed in 2015 whereas the black color lines represent the
mean of 2005–2014.
The spatial distribution of GPH at 100 and 150 hPa for the month of July
(August) is shown in Fig. 2a and b (Fig. 2c and d). The corresponding
monthly mean winds at respective pressure levels are also shown in Fig. 2. The solid black line represents the ASMA region at 100 hPa
based on 16.75 km GPH contour. The GPH distribution in Fig. 2 shows clear
distinct variability in the ASMA spatial structure between July and August
at both pressure levels. For example, at 100 hPa, the maximum GPH center was
located over the western side in July, whereas it was located near to the
Tibetan region in August. Interestingly the ASMA itself separated into two
anticyclones (16.75 km GPH contour; solid black line in the figure) in August
compared to July. The center of the small anticyclone was located over the
northwestern Pacific near 140∘ E with the closed circulation indicated
by the wind arrows.
Further, we compared the ASMA structure in 2015 with the referenced long-term
mean. For this, we obtained the GPH anomalies by subtracting the background
long-term mean (2005–2014) from 2015. Figure 3 shows the
latitudinal–longitudinal distribution of GPH anomalies (shaded color) along
with wind vectors depicting the circulation pattern at 100 hPa, as well as at 150 hPa during July and August. The white (black) color contour represents 16.75 km GPH at 100 hPa for the corresponding month in 2015 (long-term mean). The
GPH anomalies at both pressure levels show quite different features in July
and August. A clear wave-like structure can be observed from the GPH
anomalies. In July, the GPH anomalies exhibit strong negative maxima over
25–40∘ N, 90–120∘ E and positive maxima over 40–50∘ N,
60–80∘ E regions. The 16.75 km GPH contour lines in the ASMA region
exhibit higher extensions in all the directions except over the northeastern
edges of the ASMA in July compared to the long-term mean. At the same
location (northeastern edges), the ASMA exhibits a pronounced southward
extension in July. Distinct features of GPH anomalies are noticed in August
as compared to July. In August, the strong negative GPH anomalies are
situated over the west and northeastern edges of the ASMA.
Spatial distribution of monthly mean zonal winds obtained from
NCEP-DOE Reanalysis 2 data at 200 hPa in July during (a) 2005–2014 and (b) 2015. Subplots of (c) and (d) same as (a) and (b) but for the month of
August. The solid white contour lines represent the ASMA region at 100 hPa (16.75 km GPH contour).
It is well known that the subtropical westerly jet is an important
characteristic feature of the ASMA (Ramaswamy, 1958), and thus its changes
during 2015 are also investigated. As the peak intensity of the westerly jet
was located at 200 hPa (Chiang et al., 2015), we focused mainly on 200 hPa
zonal wind changes in July and August. Figure 4a and c (Fig. 4b and d)
show the spatial distribution of the long-term (2015) monthly mean zonal wind at
200 hPa during July and August. In general, the subtropical westerlies are
located near to ∼ 40∘ N latitude during the mature phase
of the monsoon period (Chiang et al., 2015). Compared to the long-term mean, a
significant weakening of the subtropical westerlies is noticed in 2015.
Further, a strong southward shift in the westerlies is observed over the
northeastern Asia region. This southward shift is moved even up to 30∘ N
in both months. From zonal wind at 200 hPa (Fig. 4) and wind vectors at 100 and 150 hPa (Fig. 2), it is clear that anomalous changes have occurred in
the subtropical westerlies over the northeastern parts of the ASMA around
30–40∘ N, 90–120∘ E during July and August 2015. The southward shift
in the westerlies is strongly associated with the southward extension of the
ASMA over the northeastern side of the ASMA (Fig. 2). This is strongly
supported by the previous findings of Lin and Lu (2005) for which they showed
the southward extension of the South Asian high could lead to the southward
shift of the westerlies.
ERA-Interim-observed spatial distribution of potential vorticity
(PV) on a 350 K isentropic surface in potential vorticity units (PVUs; 1 PVU = 10-6 K m2 kg-1 s-1): (a) monthly mean of July and (b) monthly mean of August 2015. The solid white contour lines represent the ASMA region at 100 hPa (16.75 km GPH contour).
Same as Fig. 5 but for the weekly distribution of PV in July 2015. Magenta colored arrows indicate the regions of RWB.
From the GPH and winds observations, it is clear that pronounced changes are
evident in the dynamical structure of the ASMA in 2015, and also relatively
different features are noticed between July and August. Interestingly
the ASMA itself separated into two anticyclones during August 2015, and the
separation exactly coincided with the strong negative GPH anomalies and
southward meandering of subtropical westerlies over the northeastern side of
the ASMA. The West Pacific (WP) mode of the anticyclone is visible in
August. The split of the anticyclone and the formation of the WP mode are in
agreement with previous studies reported by a few researchers earlier (e.g.,
Honomichl and Pan, 2020). The presence of the WP mode may be due to the
eastward eddy shedding of the ASMA system in the process of its sub-seasonal
zonal oscillation (Honomichl and Pan, 2020) or Rossby wave breaking (RWB) in
the subtropical westerly jet (Fadnavis and Chattopadhyay, 2017). Fadnavis
and Chattopadhyay (2017) also identified the split of ASMA into two
anticyclones: one over Iran and another over the Tibetan region due to the
RWB in the June 2014 monsoon period. To see any signatures of these RWBs in 2015,
we further analyzed the RWBs through the ERA interim reanalysis potential
vorticity (PV) data. Based on previous studies, it is reported that RWBs can
be identified from the PV distribution at a 350 K isentropic surface (Samanta et
al., 2016; Fadnavis and Chattopadhyay, 2017). We used 350 K isentropic
surface PV data in July and August 2015 in the present analysis.
Figure 5a and b show the distribution of ERA interim monthly mean PV at the
350 K isentropic surface during July and August 2015. It can be seen that,
during July and August 2015, clear RWB signatures are evident near 100∘ E.
It is noted that the equatorial advection of high PV values with a steep
gradient and the southward movement of PV from the westerly jet are the
basic features of the RWB (Vellore et al., 2016; Samanta et al. 2016). These
features are clearly exhibited in Fig. 5 with higher PV values extending up
to ∼ 30∘ N in both months over the 100∘ E region. The
location of this RWB is significantly correlated with a southward meandering
of westerlies and strong negative GPH anomalies. However, the observed RWB
signatures in both months are from monthly mean PV data. Further, to see the
clear signatures of these RWBs, we made weekly based analyses for the month of July.
For this we considered 1–7 July as week 1 and 8–14 July as week 2 so on. The
weekly mean distribution of 350 K isentropic surface PV during July is shown
in Fig. 6. The magenta colored arrows which are shown in Fig. 6
represent the RWB events during July 2015. A clear signature of air with
high values of PV traverses from the extratropics to ASMA as is evident in
Fig. 6. At weekly scales, clear RWB signatures are observed over the
anticyclone region. For example, in week 1 and week 2, the RWB signatures
are evident over the northern region of the ASMA. However, in week 3 and
week 4, these RWB signatures are very clear over northeastern Asia even in
week 5 (29 July–4 August), and we noticed RWB signatures in PV data (figure not
shown). This clearly shows that the RWB splits the ASMA into two
anticyclones: one over the Tibetan region and another over the WP region. It
is clear that the equatorward penetration of extratropical forcing through
the subtropical westerly jet started in July and was further amplified by
the splitting of the ASMA into two during August.
It is well known that the RWB is an important mechanism for horizontal
transport between the extratropical lower stratosphere to the tropical UTLS
region. These RWBs can act as an agent for the transport of extratropical
stratospheric cold, dry, and O3-rich air into the ASMA during the
summer monsoon. Overall, it is concluded that the combination of the RWBs
and strong southward meandering of the subtropical westerly jet in 2015
causes significant dynamical and structural changes in the ASMA. These
changes in the ASMA dynamical structure in 2015 can influence the
concentrations of the different trace gases within the ASMA. Further, we
quantified the changes in O3, CO, and WV concentrations within the ASMA
during 2015 caused by the dynamical effects. The changes that occurred in
the O3, CO, and WV are discussed in the following sections.
Trace gases anomalies observed within the ASMA in 2015
It is well documented that the ASMA contains low (high) concentrations of
stratospheric tracers such as O3 (tropospheric tracers such as CO, WV,
etc.) and higher tropopause height compared to the region outside the
ASMA during boreal summer (Park et al., 2007; Randel et al., 2010; Santee et
al., 2017; Basha et al., 2020). Differences of the trace gases within and
outside of the ASMA are attributed to the strong winds and closed
streamlines associated with the ASMA, which act to isolate the air (Randel
and Park, 2006; Park et al., 2007). To see the changes in the trace gases
during 2015, we generated the background long-term mean of CO, O3, and
WV by using 10 years of MLS trace gas data from 2005 to 2014. Here the
results are discussed mainly based on the percentage changes relative to the
respective long-term monthly mean trace gases using Eq. (1).
Ozone relative percentage change in July 2015 with respect to
background climatological monthly mean observed at (a) 82 hPa, (b) 100 hPa,
and (c) 121 hPa. Subplots of (d)–(f) same as (a)–(c) but
for the month of August. The white (black) color contour represents 16.75 km
geopotential height at 100 hPa for the corresponding month in 2015 (mean of
2005–2014). The star symbols (black) shown in the figure represent the anomalies
greater than the ±2σ standard deviation of the long-term mean.
The results are obtained from MLS measurements.
Figure 7a to c (Fig. 7d–f) show the distribution of the relative percentage change
in the O3 concentrations within the ASMA at 82, 100, and 121 hPa
during July (August) 2015. The anomalies larger than ±2σ
standard deviation of the long-term mean are highlighted with star symbols in
the respective figures. The spatial distribution of changes in the O3
(Fig. 7) shows a clear increase in the O3 mixing ratios (> 40 %) within the ASMA in 2015. The observed increase within the ASMA is
quite distinct between July and August. In July, the O3 shows a
pronounced increase within the ASMA at all the pressure levels. Note that
the observed increase was statistically significant with a long-term mean with a standard deviation larger than 2σ (see the star symbols). This
increase is quite significant over the northeastern edges of the ASMA and
quite high at 100 hPa compared to 82 and 121 hPa. In August, the O3
shows quite different features compared to July (Fig. 7d–f). A strong
increase in the O3 is observed over the western and eastern edges of
the ASMA at all the pressure levels. The increase is quite significant at
100 hPa and even at 121 hPa. The increase in O3 is still appearing over
the northeastern edges of the ASMA in August as observed in July. Overall, a
significant enhancement of O3 within the ASMA is clear evidence in July
and August 2015.
The significant increase in O3 within the ASMA in 2015 might be due to
the transport from the midlatitudes through the subtropical westerly
jet (STJ) and also due to the
stratosphere to the troposphere transport. For example, the strong
enhancement of O3 within the ASMA at 100 hPa in July was strongly
matched with the observed high values of PV at a 350 K isentropic surface
(Fig. 6). This is further supported by the strong southward meandering of
STJ in July (Fig. 3). Thus, a clear transport of midlatitude
air with high PV and high O3 is evident during 2015. At the same time,
the enhancement of O3 was clearly observed at all the pressure levels
from 82 to 121 hPa, which is further support for the stratosphere to
the troposphere transport. Note that 82 hPa can represent the lower
stratosphere and 121 hPa the upper troposphere (Das et al., 2020). It
can be noticed that the ASMA is strongly associated with
troposphere–stratosphere transport, as well as stratosphere–troposphere
transport (Garny and Randel, 2016; Fan et al., 2017). Also, it is well
reported that the northern part of the ASMA is an active region for
stratosphere–troposphere transport processes (Sprenger et al., 2003;
Škerlak et al., 2014).
Similarly, significant lowering of O3, particularly at 100 and 82 hPa is clearly noticed over the tropics (Fig. 7). This is quite expected due
to the enhanced tropical upwelling (bringing O3-poor air from
troposphere) caused by the strong El Niño conditions in July and August 2015. As mentioned in the previous sections, strong El Niño conditions
are clearly evident in July and August 2015 (Fig. 1). The observed strongly
negative O3 anomalies over the tropics from the present study are well
matched with the previous studies (Randel et al., 2009; Diallo et al.,
2018). From the present results, it is very clear that there is a
significant decrease over the tropics and increase over the
midlatitudes in 2015. These changes observed in the O3 (decrease and
increase) are attributed to the strengthening of the tropical upwelling
and enhanced downwelling from the shallow branch of the Brewer–Dobson
circulation in the midlatitudes due to the strong El Niño conditions in
2015. Overall, it is concluded that initially, during July, the O3 is
transported into the anticyclone from the northeastern edges of the ASMA
region through the subtropical westerlies, and then it is isolated within
the ASMA region. This is further supported by the southward meandering of
the westerly jet and southward shift of the ASMA (negative GPH anomalies)
over the same region in July (Fig. 3). Also, a significant transport of
midlatitude dry air is clear from Fig. 6. Thus, it is clear from the
results that the stratosphere to troposphere transport and horizontal
advection along with the subtropical jet caused the strong enhancement of
the O3 within the ASMA in 2015.
Carbon monoxide relative percentage change during July 2015 with
respect to climatological monthly mean observed at (a) 100 hPa and (b) 146 hPa. Subplots of (c) and (d) same as (a) and (b) but for the month of
August. The white (black) color contour represents the 16.75 km geopotential
height at 100 hPa for the corresponding month in 2015 (mean of 2005–2014).
The star symbols (black) shown in the figure represent the anomalies greater
than the ±2σ standard deviation of the long-term mean. The
results are obtained from MLS measurements.
Figure 8a and b (Fig. 8c–d) show the spatial distribution of CO relative
percentage change at 100 and 146 hPa observed during July (August) 2015.
The white (black) color contour represents 16.75 km GPH at 100 hPa for the
corresponding month in 2015 (climatological mean). The observed changes in
the CO clearly exhibit quite distinct features between July and August as
observed in the O3. A significant decrease (∼ 30 %) is
noticed in the CO concentrations over most of the ASMA in July. The maximum
decrease in CO is noticed over the northeastern edges of the ASMA, located in the ∼ 30–45∘ N, 90–120∘ E region, whereas in August, the
decrease in CO is more concentrated over the eastern and western edges of the
ASMA at both the pressure levels. Overall, the MLS-observed CO was
∼ 30 % below average (percentage decrease) compared to the
climatological monthly mean within the ASMA in July and edges of the ASMA in
August 2015. It is noted that there is a considerable year-to-year
variability in the CO sources over the ASM region (Santee et al., 2017). The
major sources of the CO over the ASM region are from biomass burning and
industrial emissions. The observed decreased CO within the ASMA in 2015 might
be due to the year-to-year variability in the CO sources and the weaker
vertical transport due to the El Niño conditions in 2015.
Water vapor relative percentage change in July 2015 with respect
to background climatological monthly mean observed at (a) 82 hPa, (b) 100 hPa, and (c) 146 hPa. Subplots of (d)–(f) same as (a)–(c)
but for the month of August. The white (black) color contour represents the
16.75 km geopotential height at 100 hPa for the corresponding month in 2015
(mean of 2005–2014). The star symbols (black) shown in the figure represent the
anomalies greater than the ±2σ standard deviation of the
long-term mean. The results are obtained from MLS measurements.
Similarly, the WV relative percentage change at 82, 100, and 146 hPa
in July (August) 2015 are shown in Fig. 9a–c (Fig. 9d–f). The WV shows quite
different changes at all the pressure levels in July and August. At 146 hPa,
the WV exhibits a strong decrease (> 20 %) within the ASMA in
July, as well as in August. However, at 100 and 82 hPa, the WV shows
a relatively significant decrease within the ASMA in July compared to
August. From the WV observations, it is concluded that the WV is strongly
decreased at 146 hPa in both months, whereas at 100 and 82 hPa, the
decrease in WV is quite high in July compared to August. It is also observed
from Fig. 9 that there is a significant enhancement of WV over the
tropics at 146 hPa in both months. But the WV enhancement is quite
significant at 100 hPa, particularly during August compared to July. This
enhancement in the WV around the tropical tropopause region in August is
quite expected due to the El Niño conditions (Randel et al., 2009;
Konopka et al., 2016). Overall, the tropospheric tracers (CO and WV)
significantly decreased (∼ 30 % and 20 %) within the ASMA
during July and August 2015. These changes in the tropospheric tracers
might be due to the weaker vertical motions during the 2015 monsoon. A
weaker vertical transport from the boundary layer to the UTLS is generally
observed over the ASM region during El Niño periods (Fadnavis et al.,
2019). The El Niño conditions will suppress the monsoon convection and
cause weaker vertical transport during monsoons. It is also reported that the
summer monsoon in 2015 was weaker due to the strongest El Niño
conditions existing in 2015 (Tweedy et al., 2018; Yuan et al., 2019; Fadnavis
et al., 2019).
From these results, it is clear that the enhancement of O3 and lowering
of CO and WV is evident in July and August 2015 compared to the long-term
monthly mean. The observed high O3 and low WV within the ASMA from the
present study are consistent and well matched with the previous study
reported by Li et al. (2018). They demonstrated the importance of the
large-scale atmospheric dynamics and the stratospheric intrusions for high
O3 and low WV over Lhasa within the ASMA by using in situ balloon-borne
measurements. The O3 and WV changes strongly influence the background
temperature structure within the UTLS region (Venkat Ratnam et al., 2016;
Ravindra Babu et al., 2019b). Further, we investigated the tropopause
temperature changes within the ASMA by using COSMIC RO data. The results are
presented in the following section.
Tropopause temperature anomalies in 2015
It is well known that the tropopause plays a crucial role in the exchange of
WV, O3, and other chemical species between the troposphere and the
stratosphere. Most of these exchanges (WV to the lower stratosphere and
O3 to the upper troposphere) known as stratosphere–troposphere exchange
(STE) take place around the tropopause region (Fueglistaler et al., 2009;
Venkat Ratnam et al., 2016; Ravindra Babu et al., 2019b). It is well reported
that the tropopause within the ASMA is higher than the outside regions at
the same latitude (Randel et al., 2010; Santee et al., 2017). In the present
study, we mainly focused on changes in the cold-point tropopause temperature
(CPT) and lapse rate tropopause temperature (LRT) within the ASMA in July
and August 2015. The July and August 2015 monthly mean tropopause parameters
are removed from the respective climatological monthly mean which is
calculated by using COSMIC RO data from 2006 to 2014. One can note that we
have strictly restricted our analysis within the 40∘ N region for the cold-point tropopause. Figure 10a and b (Fig. 10c–d) show the CPT and LRT anomalies
observed in July (August) 2015. The tropopause temperature anomalies
(CPT and LRT) also exhibit a distinct pattern in July and August as observed in
O3 (Fig. 7). In July, the CPT and LRT show strong positive anomalies
(∼ 5 K) in most of the ASMA region. High positive CPT and LRT
anomalies are also noticed over the northwestern Pacific (NWP) region particularly below 20∘ N.
These CPT and LRT anomalies observed over the NWP region might be due to the El-Niño-induced changes in the Walker circulation and convective activity.
Previous studies also observed significant warm tropopause temperature
anomalies over the WP and maritime continent during the El Niño period
(Gettelman et al., 2001). In August, the strong positive CPT and LRT anomalies
(∼ 5 K) are concentrated over the northeastern edges of the
anticyclone where the WP mode of the anticyclone was separated from the
ASMA. The temperature anomalies at 1 km above and below the CPH also show
similar behavior as seen in the CPT and LRT during August 2015 (figures not
shown). Overall, the tropopause temperature anomalies in July and August 2015 within the ASMA are well correlated with the strong enhancement in the
O3 as shown in Fig. 7. However, the enhanced O3 anomalies (heating
due to the O3) themselves cannot explain the observed positive tropopause
temperature anomalies within the ASMA in 2015. This might be due to the El-Niño-induced changes in the convective activity and the circulation. It
is well known that the reversal of walker circulation and the shifting of
the convective activity (suppressed convective activity over the ASM region) are
generally observed during the warm phase of ENSO. It can be noticed that
apart from the convection, other factors such as stratospheric QBO and
atmospheric waves (gravity waves and Kelvin waves) also strongly influenced
the tropopause temperatures.
Spatial distribution of (a) lapse rate tropopause temperature
(LRT) and (b) cold-point tropopause temperature (CPT) anomalies during July 2015. Subplots of (c) and (d) same as (a) and (b) but for the month of
August 2015. The white (black) color contour represents the 16.75 km
geopotential height at 100 hPa for the corresponding month in 2015 (mean of
2005–2014). The star symbols (black) shown in the figure represent the anomalies
greater than the ±2σ standard deviation of the long-term mean.
Summary and conclusions
In this study, we investigated the detailed changes observed in the
structure, dynamics, and trace gas (ozone, water vapor, carbon monoxide)
variability within the ASMA in 2015 by using reanalysis products and
satellite observations. The tropopause temperature (CPT and LRT) on monthly
scales particularly during July and August 2015 was also discussed. To quantify
the changes that happened within the ASMA region, 11 years (2005–2015) of
O3, WV, and CO observations from the Aura-MLS data and 10 years
(2006–2015) of tropopause temperature data from the COSMIC RO temperature
profiles are used. The winds observed by NCEP-DOE Reanalysis 2 and GPH data
from 2005 to 2015 are also utilized. The results are obtained by comparing
the trace gas quantities in July and August 2015 with corresponding
long-term monthly mean quantities.
The trace gases within the ASMA exhibit substantial anomalous behavior in
July and August 2015. During July and August 2015, we observed an
enhancement of O3 and the lowering of CO and WV over most of the ASMA
region. The decrease in the tropospheric tracers (CO and WV) is quite
expected due to the weaker upward motions from the weak monsoon in 2015.
This is supported by a recent study reported by Fadnavis et al. (2019). It showed weaker upward motions and deficient rainfall in the 2015 monsoon due
to the strong El Niño conditions. However, the strong enhancement in the
stratospheric tracer (O3) within the ASMA particularly over the
northeastern edges of the ASMA during July is quite interesting. This
might be due to the stratospheric intrusions, as well as transport from the
midlatitudes. Based on Fishman and Seiler (1983), it was stated that the
positive correlation between CO and O3 indicates that the O3 is
produced in situ in the troposphere, whereas when the correlation is negative, it means the O3 originates from the stratosphere. We noticed a strong
negative correlation between CO and O3 in the present study with
increased O3 and decreased CO from the MLS measurements. This clearly
reveals that the observed increased O3 within the ASMA during 2015 is
of stratospheric origin. This is further supported by higher negative GPH
anomalies associated with a southward meandering of the subtropical westerly
jet over northeastern Asia in July (Figs. 3 and 4). Further, the increased
O3 at 100 and 121 hPa over western edges of the ASMA during August
clearly indicates the transport of the O3 towards outer regions through
the outflow of the ASMA (Fig. 7e–f). Interestingly, the tropopause
temperature obtained from the COSMIC RO data in July 2015 shows strong
positive temperature anomalies (∼ 5 K) over the entire ASMA
region. These warm tropopause temperatures again supported the increased
O3 within the ASMA during 2015. The major findings obtained from the
present study are summarized in the following.
The spatial extension of the ASMA region shows a mean higher than the long-term mean
except over northeastern Asia where it exhibits a strong southward shift in
July, whereas in August, the ASMA further separated into two anticyclones,
and the western Pacific mode anticyclone is clearly evident in August.
The combination of Rossby wave breaking and pronounced southward meandering
of subtropical westerlies plays a crucial role in the dynamical and
structural changes in the ASMA in 2015.
The strong enhancement in O3 at 100 hPa (> 40 %) is clearly
evident within the ASMA and particularly higher over the northeastern edges
of the ASMA in July. The enhanced O3 is strongly associated with a
dominant southward meandering of the subtropical westerlies. In August, the
increased O3 is significantly located over the western edges of the
ASMA. This clearly indicates the transport from the ASMA to the edges
through its outflow.
A significant lowering of CO and WV within the ASMA is noticed during summer
2015. The lowering of WV is higher at 146 hPa than 100 hPa.
Significantly positive tropopause temperature anomalies (∼ 5 K)
are observed in the entire ASMA region in July, whereas in August, the strong
positive anomalies are concentrated over the northeastern side of the ASMA.
The changes in the O3 concentrations (increase or decrease) within the
ASMA are one of the possible mechanisms strengthening or weakening the
ASMA (Braesicke et al., 2011). By using idealized climate model experiments,
Braesicke et al. (2011) clearly demonstrated that the strengthening
(weakening) of the ASMA occurred when the O3 is decreased (increased)
within the ASMA. The increased O3 within the ASMA warms the entire
anticyclone region and weakens the ASMA (Braesicke et al., 2011). Our
results from the present study are also in agreement with the results of
Braesicke et al. (2011). We also observed a pronounced increase in O3
within the ASMA associated with the significant warming of the tropopause, as well as
above and below the tropopause region, in 2015. By using precipitation index,
wind data, and stream functions, previous studies reported that the ASMA
circulation in 2015 was weaker than the normal (Tweedy et al., 2018; Yuan et
al., 2019). Based on our present results, the strongly enhanced O3
within the ASMA also might be one of the plausible reasons for weakening of
the ASMA in 2015.
Data availability
All the data used in the present study are available freely from the respective websites. MLS version 4.2 level 2 data can be downloaded from https://acdisc.gesdisc.eosdis.nasa.gov/data/Aura_MLS_Level2/ (NASA, 2021, last access: 15 January 2021). The COSMIC RO data can be downloaded from the CDAAC UCAR website https://cdaac-www.cosmic.ucar.edu/cdaac/products.html (CDAAC, 2021, last access: 15 January 2021). The NCEP Reanalysis 2 data were downloaded from NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their website http://www.cpc.ncep.noaa.gov/products/wesley/reanalysis2/kana/reanl2-1.html (NCEP-DOE, 2021, last access: 15 January 2021). ERA-Interim reanalysis data were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) from http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=pl/ (ECMWF, 2021, last access: 15 January). The monthly Niño 3.4 index data were downloaded from the Climate Prediction Center website http://www.cpc.ncep.noaa.gov (last access: 15 January 2021).
Author contributions
SRB designed the study and conducted research. SRB, GB and SKP performed data analysis. SRB wrote the first manuscript draft. MVR and NHL provided useful comments and revised the paper. SRB edited the final manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Aura MLS observations obtained from the GES DISC through their FTP site (https://mls.jpl.nasa.gov/index-eos-mls.php, last access: 15 January 2021) are duly acknowledged. We thank the COSMIC Data Analysis and Archive Center (CDAAC)
for providing RO data used in the present study through their FTP site
(http://cdaac-www.cosmic.ucar.edu/cdaac/products.html, last access: 15 January 2021). We also thank
NCEP/NCAR reanalysis for providing geopotential and wind data. We thank
ECMWF for providing ERA Interim reanalysis data.
Review statement
This paper was edited by Rolf Müller and reviewed by Paul Konopka, Dan Li, and one anonymous referee.
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