Introduction
Sudden stratospheric warming (SSW) is a high latitude wintertime phenomenon
which is characterized by a sudden enhancement in temperature by several
kelvins at 10 hPa and above, followed by a deceleration and even sometimes,
reversal of the zonal westerly wind to easterly. Scherhag (1952) was the
first who detected a SSW by a radiosounding over Berlin. The SSWs occur in
winter when planetary waves interact with the background flow (Matsuno,
1971). Breaking and dissipation of westward propagating planetary waves at
stratospheric altitudes decelerate or even reverse the prevailing eastward
flow of the polar winter time stratosphere. The SSWs are often accompanied
by either a displacement of the polar vortex toward midlatitudes or an
elongation which results in splitting of the vortex into daughter vortices.
Though the SSW is a high latitude phenomenon, it influences the thermal
structure, circulation and distribution of minor constituents globally.
First observational case studies regarding stratospheric chemical
composition changes during the SSW were performed using in situ (Dutsch and
Braun, 1980) and SAGE I satellite measurements (Wanget et al., 1983). Due to
the availability of several satellite observations in the recent decades,
there have been many reports on chemical composition changes at
high-latitudes during the major SSW of 2004, 2006 and 2009 (Manney et al., 2008a, b, 2009; Randel et al., 2006, 2009). Using Sounding of Atmosphere by
Broadband Emission radiometry (SABER) on board
Thermosphere-Ionosphere-Mesosphere Energetics and Coupling (TIMED) satellite
data sets, Siskind et al. (2007) observed elevation of stratopause and
a downward mesospheric transport of NO during the 2006 SSW event. Manney et al. (2009) investigated stratospheric chemistry of polar vortex during the
2009 and 2006 SSW events and they observed downward descent of CO, CH4
and H2O from mesosphere during the decay of vortex fragments.
Similarly, Salmi et al. (2011) also reported about the dowward transport of
NOx from mesosphere to upper stratosphere, though the NOx
involved catalytic ozone production/depletion cycle was shown to be
ineffective in upper stratosphere. Using Global Ozone Monitoring by
Occultation of Stars (GOMOS) data sets, Sofieva et al. (2012) observed
strong horizontal mixing of NO2 with outside-vortex air and large
NO3 enhancement correlating with the temperature enhancment. Damiani
et al. (2014) also investigated a few chemical constituents (O3,
ClONO2, HCl, ClO, CO, NOx, CH4 etc.) over polar region for
a few SSW events. They found increment in ozone which could increase ClO and
hence ClONO2 along with a decrease in HCl. In case of methane
(CH4), they observed temporal evolution after the SSW events which they
attributed to ingression of mid-latitude air. Using ground based microwave
radiometer observations from 20 to 70 km over Bern (Switzerland), Flury
et al. (2009) observed decrease of ozone and increase of H2O during the
vortex breakdown period due to mixing of humid air from subtropical
mesosphere. Tschanz and Kampfer (2015) also observed increase of water vapor
over Sodankylä, Finland (67.4∘ N, 26.6∘ E) derived
from the middle atmospheric water vapor radiometer (MIAWARA) measurements.
There are only few studies over the equatorial region and low-latitudes on
chemical composition changes during the SSW events, although enhancement of
ozone mixing ratio has consistently been observed (Sridharan et al., 2012;
Nath et al., 2015). Sridharan et al. (2012) suggested that the enhancement
of the equatorial and low-latitude ozone during the SSW event could be due
to weakening of Brewer–Dobson circulation (BDC) responsible for transporting
ozone to higher latitudes due to decrease in planetary wave activity. In the
present study, changes in chemical composition in the upper stratosphere
over the equator during a few SSW events are investigated and an attempt is
made to give a feasible chemical interpretation about the observed
variations.
Data sets used
MLS data for the equatorial O<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:math></inline-formula>,
H<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>O VMR and temperature
The Earth Observing System (EOS) Microwave Limb Sounder (MLS) is one of the
four instruments on the NASA's EOS Aura satellite, which is part of NASA's
A-train group of Earth observing satellites, launched on 15 July 2004
(Manney et al., 2005). The MLS makes measurements of atmospheric
composition, temperature, humidity and cloud ice globally both day and
night. It observes thermal microwave emission from Earth's “limb” (the edge
of the atmosphere) viewing forward along the Aura spacecraft flight
direction, scanning its view from the ground to ∼90 km every
∼25 s. Aura is in a near-polar 705 km altitude orbit. As
Earth rotates underneath it, the Aura orbit stays fixed relative to the sun
to give daily global coverage with ∼13 orbits per day. MLS is
giving data since August 2004. In this present study we used MLS Version 3
VMR data of ozone (O3) and temperature (T) for 2009 and 2012 SSWs and
water vapor (H2O) during the SSWs of 2004, 2009 and 2012 to study their
variation in upper stratosphere. The MLS useful data range for O3 VMR,
H2O VMR and T are as follows for the pressure level range 261–0.02,
316–0.002 and 261–0.001 hPa respectively. The data accuracy in the
height range considered for observation is 4–9 %.
MIPAS-ENVISAT data for CH<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msub></mml:math></inline-formula> VMR
MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is
a limb-scanning Fourier infrared spectrometer on board the European
Environmental Satellite (ENVISAT); more detailed characteristics regarding
the measurements bythis spaceborne instrument are given by Carli et al. (2004) and Raspollini et al. (2006). The sunsynchronous polar orbit provides
a global coverage with nearly 14 orbits per day at a horizontal resolution of
approximately 500 km. More than 20 trace constituents are observed in the
upper troposphere and in the stratosphere. Currently, the MIPAS level-2
operational products are provided by the European Space Agency (ESA). These
products include the temperature, and the concentrations of
H2O,CH4, N2O, O3, HNO3 and NO2. In its
original nominal measurement mode, MIPAS scanned the Earth limb at 17
tangent altitudes of 6, 9, …, 39, 42, 47, 52, 60, and 68 km. The vertical
resolution is 3 km for the 13 lower most tangent altitudes and increases to
8 km at the upper end of the limb scan. MIPAS CH4 VMR data has been
compared with satellite instruments including ACE-FTS, HALOE, SOFIE and
SCIAMACHY (Laeng et al., 2015). Above 50 km altitude, MIPAS methane mixing
ratios agree almost perfectly with ACE, and above 40–45 km with SOFIE.
Between 30 and 40 km excellent agreement with SCIAMACHY has been found. In
the lower stratosphere (below about 25–30 km) MIPAS CH4 is biased high,
and the most likely esimate of this bias is 0.2 ppmv. MIPAS data are
available for the years 2002–2012. In this study, methane volume mixing
ratio data have been taken from MIPAS-ENVISAT data source for the SSWs
occurred in 2004, 2009 and 2012.
ECMWF ERA-Interim data for zonal wind and temperature
ERA-Interim is the latest European Centre for Medium-Range Weather Forecasts
(ECMWF) global atmospheric reanalysis of the period 1979 to present
(Berrisford, 2009). This follows on from the ERA-15 and ERA-40 re-analysis
projects. The datasets include data on surface, PV, potential temperature
and pressure surfaces. The ECMWF has in the past produced three major
reanalyses: FGGE, ERA-15 and ERA-40. The last of these consisted of a set of
global analyses describing the state of the atmosphere and land and
ocean-wave conditions from mid-1957 to mid-2002. The ERA-Interim is an
“interim” reanalysis of the period 1989-present in preparation for the
next-generation extended reanalysis to replace ERA-40.
In this present study, ERA-Interim data have been used for the observation
of zonal wind as well as temperature (grid 1.5∘×1.5∘)
variations for the pressure levels 100–1 hPa during the SSWs of 2004, 2009
and 2012.
Results
2011/12 winter and equatorial temperature and ozone
Figure 1a and b shows the zonal mean of ERA- Interim temperature difference
between 90 and 60∘ N and of ERA- Interim zonal wind
at 60∘ N respectively for the pressure levels 100–1 hPa and for
the duration 1 December 2011 to 18 February 2012. The SSW is normally
identified from the positive temperature between 90 and
60∘ N latitudes at 10 hPa. Before day number 43, the temperature
difference is -10 to -20 K, which means that the temperature at
60∘ N is more than that at 90∘ N indicating that the
normal cold winter is prevailing. A clear sudden enhancement in temperature
can be observed from day number 43 in the height range 10–1 hPa. When the
warming starts the temperature difference becomes positive, as the polar
stratospheric temperature is increased by several kelvins. Here the
temperature difference between 90 and 60∘ N rises
upto 16 K (on day number 55) and it remains positive during day numbers
43–64, which indicates the duration of the SSW. When the SSW occurs, the
polar westerly wind gets decelerated up to 5 ms-1 and as it does not get
reversed to easterly, this event is considered only as a minor SSW. Figure 1c and e show the altitude profiles of MLS zonal mean of ozone (O3) VMR
and temperature over the equator for the height region 20–60 km for the
duration of 1 December 2011 to 18 February 2012. As the onset of SSW takes
place, O3 VMR shows enhancement, whereas temperature and water vapor
decrease in the middle and upper stratosphere from day number 49. Though the
onset of SSW is marked on day number 43 at 10 hPa, it may be noted that the
polar temperature starts to increase from day number 29 at higher heights
(>10 hPa). Time mean removed zonal mean O3 VMR and
temperature have been plotted in Fig. 1d and Fig. 1f respectively. It can be
observed from the figures that there is an increase in the time mean removed
ozone VMR by about 0.6 ppmv during day number 29–35, whereas the temperature
decreases by a value ∼4 K from day number 29.
2008/09 winter and equatorial temperature and ozone
Figure 2a and b shows the zonal mean of ERA-Interim temperature difference
between 90 and 60∘ N and of ERA-Interim zonal wind at
60∘ N respectively for the pressure levels 100–1 hPa for the time
period 1 December 2008 to 18 February 2009. The temperature
difference is around -10 to -15 K before the day number 52 and it becomes
positive from day number 52 in the height range 10–1 hPa indicating the
onset of the SSW event. It reaches a maximum of 25.8 K on day number 54 and
the positive temperature difference (the SSW event) persists during the day
numbers 52–57. The zonal mean wind at 60∘ N becomes westward on
day number 55 at and above 10 hPa revealing that the SSW event is a major
one. Figure 2c and e show the altitude profiles of zonal mean of ozone (O3)
VMR and temperature over the equator obtained from MLS for the heights 20–60 km for the duration of 1 December 2008–18 February 2009. For more clear
interpretation, Fig. 2d and f are plotted which show time mean removed zonal
mean ozone VMR and temperature. Day number 52 onwards, the time mean removed
O3 VMR increases, reaches a high value of around 0.8 ppmv and gradually
reduces after day number 60. The time mean removed temperature decreases
more to a maximum of -9 K during day numbers 50–55. After day number 60, it
starts to become positive again.
Equatorial water vapor and methane during the 2008/09 SSW
Figure 3a shows zonal mean temperature difference between 90 and
60∘ N for the pressure levels 10–1 hPa for the time period 1 December 2008 to 18 February 2009 for reference. As shown earlier, the major
SSW event occurs during the day numbers 52–57. Figure 3b shows time mean
removed zonal mean H2O VMR over the equator for the height range 20–60 km. Above 40 km, day number 35 onwards the H2O VMR starts to decrease
by small amount (∼0.1 ppmv). As the day of onset approaches,
the time removed H2O VMR decreases by an amount 0.5 ppmv and it
persists up to day number 62. The source of water vapor in this height
region (upper stratosphere) is oxidation of methane (CH4) (Dessler,
2000; Seinfield and Pandis, 2006). For investigating the CH4 variation
during the time of observation, MIPAS onboard ENVISAT data are being
considered to show the time mean removed zonal mean CH4 VMR in Fig. 3c.
Above 3 hPa, time removed CH4 VMR starts to increase from day number
35, reaches a value around 0.15 ppmv on day number 53. After day number 70,
the CH4 VMR slowly reduces.
Equatorial water vapor and methane during the 2011/12 SSW
In the case of 2011/12 winter, the time mean removed zonal mean H2O and
CH4 VMR are shown in Fig. 4 along with the state of zonal mean
temperature and zonal mean wind as reference. During this winter, a minor
SSW event occurs during the day numbers 43–64. However, SSW begins at higher
heights from day number 29 (Fig. 4a). Above 40 km, the time mean removed
H2O VMR starts to decrease from day number 29 and reaches a value
∼0.48 ppmv around day number 45. After day number 70, the
value reduces. Time mean removed CH4 VMR begins to increase from
a value of 0.018 ppmv around day number 30 to a value of 0.15 ppmv around day
number 45. After day number 70, the value reduces. From these two cases, it
is apparent that the minor constituents respond to the temperature
variations above 10 hPa rather than at 10 hPa, which has been traditionally
used to identify the occurrence of SSW. The variations of H2O and
CH4 are not simply seasonal in nature, as they are quite different in
the two winters and their maximum or minimum VMR coincide with the SSW
event.
Equatorial water vapor and methane during the 2003/04 SSW
To verify the consistencies of H2O and CH4VMR variations
observed during the two SSW events of 2009 (major) and 2012 (minor), the
variations of the minor constituents, during the 2004 major SSW event are
also presented. In Fig. 5a zonal mean temperature difference is shown
between 90 and 60∘ N for the duration 1 December 2003–18 February 2004 for the identification of the SSW event. The onset of
the warming can be observed on day number 36 and the event persists up to
day number 44. Figure 5b and c show time mean removed zonal mean H2O and
CH4 VMR over the equator for the same time period. The time mean
removed H2O VMR shows a decrease from day number 32, reaches a value
∼0.7 ppmv around day number 40 and begins to decrease from
day number 60. The time mean removed CH4 VMR is also observed to
increase from day number 30 above 3 hPa and attain a maximum of
∼0.17 ppmv on day number 41. Large values of CH4 VMR
persist until day number 60. In this case study the H2O and CH4 VMRs
show consistent decrease and increase respectively in association with all
the SSW events considered here.
Discussion and conclusions
In the present study, changes in the VMRs of ozone (O3), water vapor
(H2O) obtained from the MLS and VMR of methane (CH4) obtained from
MIPAS over the equator during the SSW events of 2004, 2009 and 2012 are
studied. The zonal mean temperature difference between 90 and
60∘ N and zonal wind are used to identify the occurrences of the
SSW events. The ozone and CH4 VMRs show enhancement as the onset of SSW
occurs whereas H2O VMR shows a decrease. The observed variation of
H2O and CH4 VMR during the 2009 and 2012 SSW was verified with the
case study of the SSW occured in 2004 which shows similar type of results as
observed in case of 2009 and 2012. In the equatorial stratosphere ozone
production is due to photolysis of molecular oxygen. This ozone get
transported poleward due to BDC (Andrews et al., 1987). During normal time,
the O3 concentration is more in higher latitudes compared to that over
the equatorial and lower latitudes. During the SSW, the OVMR is found to
increase because of the weakening of BDC due to reduction in planetary wave
activity with the onset of warming. Sridharan et al. (2012) observed
enhancement of ozone during the major SSW of years 2006 and 2009 and
suggested as a reason for the enhancement of mesospheric semi-diurnal tide
which has been consistenly observed during the major SSW. Nath et al. (2015)
also observed increasing OVMR during the 2013 major SSW event. The H2O
VMR decreases in association with the warming at heights greater than 10 hPa. In the upper stratosphere, the main source of water vapor is the
oxidation of methane. Oxidation of methane can occur via two ways (Brasseur
and Solomon, 1995):
CH4+O(1D)⟶CH3+OHCH4+OH⟶CH3+H2O
Atomic oxygen (O) production results in from the photolysis of O3.
O3+hν⟶O2+O
O produced in the above reaction can be in one of two electronic states:
3P (“triplet P”) or 1D (“singlet D”). O (1D) is rapidly
converted to O (3P) through collisions with molecules such as O2
and CO2. So the abundance of more reactive O (1D) is much less
compared to that of less reactive O (3P) in stratosphere (Dessler,
2000). The ozone destruction rate and hence the ozone mixing ratio, are
strongly dependent on the mixing ratio of atomic oxygen O. The O mixing
ratio is determined by the O production (by O3 photolysis, insensitive
to temperature) and O destruction. The oxygen destruction occurs by the
reaction
O+O2+M⟶O3+M
The rate of the reaction is k0 [O]*[O2]*[M] ([] sign
stands for concentration) where rate constant k0=(6×10-34)⋅(T/300)-2.3cm6molecules-2s-1. This
three-body reaction is inversely dependent on temperature (Seinfield
and Pandis, 2006). A temperature increase as observed during the SSW
over higher latitudes results in a decrease of the reaction rate
constant of reaction (1) by a factor of approximately 2 which leads to
a significantly higher mixing ratio of atomic oxygen (O) and a smaller
O3 VMR (Flury et al., 2009). The decrease of temperature over
the equatorial latitudes during the SSW events probably shifts the
O/O3 partitioning towards O3, resulting in higher
O3 VMR and lower atomic oxygen abundance. In the present study
variations of hydroxyl (OH) VMR (Fig. 6) over the equator observed by
MLS are shown only for the 2009 SSW event, as the MLS data are not
available for 2004 and 2012 SSW events. The time mean removed OH
VMR
shows a very little insignificant decrease of about
0.0001 ppmv during day numbers 48–60 (Fig. 6b). Hence it can
be concluded that most probably the reduction of atomic oxygen
abundance affects the methane oxidation during the occurrence of SSW
events resulting in the decrease in water vapor mixing ratio.