We analyse the influence of the El Niño Southern Oscillation (ENSO) on
the atmospheric circulation and the mean ozone distribution in the tropical
and subtropical UTLS region. In particular, we focus on the impact of ENSO on
the onset of the Asian summer monsoon (ASM) anticyclone. Using the
Multivariate ENSO Index (MEI), we define climatologies (composites) of
atmospheric circulation and composition in the months following El Niño
and La Niña (boreal) winters and investigate how ENSO-related flow
anomalies propagate into spring and summer. To quantify differences in the
divergent and non-divergent parts of the flow, the velocity potential (VP)
and the stream function (SF) are respectively calculated from the ERA-Interim
reanalysis in the vicinity of the tropical tropopause at potential
temperature level θ=380 K. While VP quantifies the well-known ENSO
anomalies of the Walker circulation, SF can be used to study the impact of
ENSO on the formation of the ASM anticyclone, which turns out to be slightly
weaker after El Niño winters than after La Niña winters. In addition,
stratospheric intrusions around the eastern flank of the anticyclone into the
tropical tropopause layer (TTL) are weaker in the months after strong El
Niño events due to more zonally symmetric subtropical jets than after La
Niña winters. By using satellite (MLS) and in situ (SHADOZ) observations
and model simulations (CLaMS) of ozone, we discuss ENSO-induced differences
around the tropical tropopause. Ozone composites show more zonally symmetric
features with less in-mixed ozone from the stratosphere into the TTL during
and after strong El Niño events and even during the formation of the ASM
anticyclone. These isentropic anomalies are overlaid with the well-known
anomalies of the faster (slower) Hadley and Brewer–Dobson circulations after
El Niño (La Niña) winter. The duration and intensity of El Niño-related anomalies may
be reinforced through late summer and autumn if the El Niño conditions
last until the following winter.
Introduction
El Niño and La Niña are opposite phases of the El Niño Southern Oscillation (ENSO), which
originates from the coupled interaction between the tropical Pacific and the overlying atmosphere e.g..
ENSO is widely recognized as a dominant mode of the Earth's climate variability .
In the troposphere, ENSO manifests in the anomalies of the zonal distribution of convection
which are triggered by positive (El Niño) and negative (La Niña) sea surface temperature (SST)
anomalies in the central and eastern Pacific .
The SST anomalies typically peak during the Northern Hemisphere (NH) winter
(hereafter, seasons refer to the NH), but prolonged events
may last for months or years .
Strong El Niño events disrupt the Walker circulation and lead to its
breakdown during the warm-ocean phases . Strong El Niño
events also propagate upwards above the tropopause by accelerating the Brewer–Dobson (BD) circulation and moistening the stratosphere
. Using satellite observations and
model simulations of water vapour and mean age of air,
have recently shown that wet (dry) and young (old) tape recorder anomalies
propagate upwards in the tropical lower stratosphere in the months following
El Niño (La Niña). They found that these anomalies are around +0.3
(-0.2) ppmv and -4 (+4) months for water vapour and age of air.
The Asian summer monsoon (ASM) anticyclone is a dominant feature of the
circulation in the upper troposphere lower stratosphere (UTLS) during summer
. This nearly stationary anticyclone
extends well into the lower stratosphere up to about 18 km (or
θ=420 K) and effectively isolates the air masses of tropospheric
origin inside from the much older, mainly stratospheric air outside this
anticyclone . This anticyclone has been
repeatedly identified as a key pathway for stratosphere–troposphere exchange
(STE) in summer and autumn, both quasi-isentropically into the lowermost
stratosphere and into the upper branch of the BD circulation, especially for
water vapour and pollutants entering the global stratosphere
.
Generally, enhanced isentropic STE between the extratropics and tropics is
caused by the monsoon systems, in particular by the ASM during NH summer
. showed that, indeed, the
subtropical jet acting as a transport barrier between the extratropics and
tropics weakens during NH summer. Consequently, enhanced isentropic transport
occurs in both directions, out of the tropics and from the extratropics into
the tropics (termed in-mixing, in the following). Related stratospheric
signatures can be found in the tropical tropopause layer (TTL) as diagnosed
from NASA Aura Microwave Limb Sounder (MLS) observations of HCl and ozone
. This in-mixed ozone contributes to more than
half of the annual cycle of ozone in the upper part of the TTL
. Enhanced quasi-isentropic transport from the
tropics to the midlatitude lowermost stratosphere driven by the ASM is also
clearly observed both for tracers and water vapour
.
A regionally resolved view of the processes coupling ENSO with the stratosphere,
mainly during the winter and spring, has been adopted in several previous studies
.
However, there are only a few publications investigating the impact of ENSO on the ASM anticyclone
and on the related STE . This is in contrast with a large number of investigations connecting
ENSO with the tropospheric variability of the ASM, such as weather patterns and precipitation,
which have a long tradition starting with the pioneering studies of and .
In this study, we investigate how the ENSO winter signal propagates into the
following seasons. In particular, we characterize the impact of ENSO on the
upper branches of the Walker and Hadley circulation in the UTLS. We focus on
the ASM anticyclone, its strength as well as its efficiency for in-mixing of
stratospheric ozone into the TTL. We investigate how long through the year
ENSO-related differences can last in the TTL, both in the meteorological
reanalysis as well as in long-term satellite, Lagrangian model and in situ
ozone data. Section 2 discusses data and methods for our analysis. Section 3
describes the seasonal propagation of ENSO anomalies. Section 4 quantifies
the influence of ENSO anomalies on the seasonality of ozone in the TTL.
Section 5 provides the discussion. The last section gives the conclusions.
Data and methods
Multivariate ENSO Index (MEI) from the NOAA Climate Diagnostic Center,
http://www.esrl.noaa.gov/psd/enso/mei (last access: 6 June 2018) .
The red lines denote the threshold values (±0.9) defining the El Niño (positive) and
La Niña (negative) composites as used in this paper. Grey shading shows winter seasons (December–February, DJF).
Figure modified from .
There are several indices that indicate the phase of ENSO, and they are highly correlated .
Here, the Multivariate ENSO Index (MEI, Fig. )
from the NOAA (National Oceanic and Atmospheric Administration) Climate
Diagnostic Center,
http://www.esrl.noaa.gov/psd/enso/mei (last access: 6 June 2018), is used to quantify the ENSO variability .
MEI is calculated based on sea surface pressure, zonal and meridional
components of the surface wind, SST and total cloudiness fraction of the sky
over the tropical Pacific. The two phases of ENSO typically show pronounced
features in late autumn, winter and early spring
. Correspondingly, MEI shows peak values during
this period. Negative and positive values of MEI quantify La Niña and El
Niño events, respectively.
Hereafter, we define two winter composites (December-February, DJF) of ENSO
events using the condition MEI <-0.9 for La Niña and MEI > 0.9
for El Niño (red lines in Fig. ) as discussed in
. The winter months defining these two composites (17
months for 6 La Niña events and 28 months for 12 El Niño) are listed
in Table . The quasi-biennial oscillation (QBO) phase during the
considered months is also listed
(http://www.cpc.ncep.noaa.gov/data/indices/qbo.u50.index, last access: 6 June 2018) and shows that our composites are only weakly
biased by the westerly phase.
El Niño episodes which last over the whole of the following year, are selected
as the special long-lasting El Niño cases and are in bold in
Table in black (like during 1987 and 1992). The
exceptional El Niño in 1982, which starts in spring 1982 and lasts until
autumn 1983, is also considered to be a long-lasting El Niño case (in bold in Table ). These three cases, as well as the
influence of QBO on the results, will be separately discussed in
Sect. .
List of all relevant La Niña and El Niño winter months
during the period of 1979–2015. In total there are 17 and 28 months for the
La Niña and El Niño composites, respectively, which are listed above
(DJF for December, January and February). Within the La Niña composite
there are 7 months in the easterly phase (E) and 10 months in the westerly
phase (W) of the QBO (defined by 30-day smoothed equatorial wind at 50 hPa).
For El Niño composites 11 months are in the easterly phase and 17 months
are in the westerly phase. The years in bold mark the long-lasting El Niño
episodes (for details, see text). Table modified from .
La Niña El Niño YearMonthsQBOYearMonthsQBO1988–1989DJFW1979–1980DE1998–1999DJFE1982–1983DJFW1999–2000DJFW1986–1987DJFE2007–2008DJFE1987–1988DJW2010–2011DJFW1991–1992DJFE2011–2012DJW–E1992–1993FW1994–1995DJFE–W1997–1998DJFW2002–2003DJFW2006–2007DJW2009–2010DJFE2015–2016DW
To study the effect of strong ENSO winters on the UTLS in the following
months, we also consider climatologies of “shifted” composites for
different seasons (DJF, JFM, FMA, MAM, AMJ, MJJ and JJA); e.g. AMJ
represents 4 months after ENSO winters (DJF). The mean value of a composite
is defined from the averaged monthly means of its elements. A Monte Carlo
significance test is used to investigate whether the La Niña and El
Niño composites are statistically different or not. Monte Carlo
significance test procedures consist of comparing the observed data
with random samples generated in accordance with the hypothesis being tested
. We call two (La Niña and El Niño) composites
statistically different when the significance of the Monte Carlo test for their
difference is passed at a 95 % confidence level after at least 1000
iteration steps.
Climatologies (composites) of the stream function (SF, in 106 m2 s-1) at θ=380 K calculated from
ERA-Interim (1979–2015) for months following La Niña (left) and El Niño (right) winters until
summer (from top to bottom). The arrows represent the rotational horizontal wind. Magenta isolines indicate
the strong convection regions based on OLR (thick and thin lines represent 210 and 220 W m-2 contours).
The blue rectangles mark the locations of strong anticyclone in NH (for details, see text).
Hereafter, the star in the figure marks the location of the SHADOZ station (Hilo, Hawaii)
where long-term ozonesonde observations are available (see Sect. 4.3).
To quantify ENSO anomalies in the climatological flow patterns, stream
function (SF) ψ and velocity potential (VP) χ are calculated
using meteorological data from ERA-Interim reanalysis
during 1979–2015 . According to the Helmholtz theorem, an
arbitrary 2-D horizontal flow u=(u,v) can be separated into a
non-divergent (i.e. rotational) part ua with ∇⋅ua=0 and
a divergent (i.e. irrotational) part ub with ∇×ub=0,
i.e.
u=ua+ub=k×∇ψ+∇χ,
where both parts can also be expressed in terms of the potentials ψ and
χ. Here, k denotes the unit vector perpendicular to the considered
2-D surface. SF and VP are scalar quantities which are easy to plot and
widely applied in meteorology and oceanography to represent large-scale flow
fields see e.g.. SF quantifies the position
and strengths of the cyclones and anticyclones. Following ,
we use VP to represent the Walker circulation and the zonal mean of VP to
quantify the Hadley circulation. SF and VP will be divided into El Niño
and La Niña composites as described above.
Ozone distributions are used to validate our diagnostic of the flow and to
understand the effect of ENSO on the atmospheric composition in the UTLS
region. MLS ozone data (version 4.2) and the Hilo (Hawaii) ozonesonde data
from Southern Hemisphere ADditional OZonesondes (SHADOZ, )
are used (see http://croc.gsfc.nasa.gov/shadoz, last access: 6 June 2018) as references. MLS measurements provide 8 and 6 months of
data for the 3 La Niña and 3 El Niño episodes from 2004 to 2015.
Respectively there are 14 and 11 months of data for the 5 La Niña and 5 El
Niño events from SHADOZ ozondesondes covering the period 1998–2015.
Chemical Lagrangian Model of the Stratosphere (CLaMS) simulations
driven by the ERA-Interim
reanalysis are used to obtain robust statistical composites of ozone (with
the same number of La Niña/El Niño months as for SF and VP). Outgoing
long-wave radiation (OLR) monthly data from NOAA during 1979–2015 complete
our analysis as a proxy for deep convection (see
https://www.esrl.noaa.gov/psd/data/gridded/data.interp_OLR.html, last access: 6 June 2018).
ENSO anomalies at the tropical tropopause from winter to summer
In this section, we use the composites of the SF and the VP introduced above to illustrate some ENSO-related differences
in the mean flow properties around the tropical tropopause.
Cyclones and anticyclones
Mean zonal wind in the tropics over south-eastern Asia
(5∘–20∘ N, 40∘–120∘ E)
following La Niña, El Niño and long-lasting El Niño winters at 200 hPa. The transition from
positive to negative values marks the onset of the Asian summer monsoon (ASM). The zero mark on the x axis denotes the middle of the
DJF season (i.e. 15 January).
Seasonal variations in SF after strong La Niña and El Niño winters are shown in Fig. .
Here, respective climatologies are plotted at the potential temperature level
θ=380 K, which roughly marks the tropopause in the tropics and in the
extratropics separates the overworld from the lowermost stratosphere
. The panels in Fig. start from
the winter (top, DJF) and end with the summer distribution (bottom, JJA).
Because the divergent part of the flow at θ=380 K is very small
compared to its rotational part, isolines of SF approximate the
climatological streamlines, whereas the strongest horizontal gradients of SF
describe the highest flow velocities. The anticyclones are represented by
positive and negative SF values in the NH and Southern Hemisphere (SH), with
highest and lowest values corresponding to their centres, respectively.
During DJF, the flow in the tropical UTLS between 60∘ E and
120∘ W is dominated by two equatorially symmetric anticyclones
resembling the well-known (symmetric) Matsuno–Gill solution with the heat
source from convection located symmetrically over the equator
.
The climatological sources of heat can be approximated by the lowest values
of the OLR. The analogous composites for OLR (magenta contours in
Fig. ) as for the SF are built with respect to La Niña and El
Niño conditions. Thus, following the symmetric Matsuno–Gill solution as a
proxy, the relevant latent heat sources for the anticyclones originate mainly
in the western Pacific, especially during La Niña, and these sources are
partially shifted to the east during El Niño events.
Over the course of the following 6 months, as the intertropical convergence
zone (ITCZ) moves northwards, these two anticyclones shift to the north-west
roughly following the position of convection . The
anticyclone in the NH intensifies, starting in May and June, and forms the
well-known Asian summer monsoon (ASM) anticyclone during NH summer. In
addition a weaker anticyclone in the SH can also be diagnosed. Thus, the
summer configuration resembles more a superposition of a symmetric and
antisymmetric Matsuno–Gill solution .
Now we discuss the differences in the large-scale flow in the UTLS caused by
ENSO (i.e. differences between the left and the right column of
Fig. ). The most striking difference in DJF is a much weaker
meridional disruption of the subtropical jets during El Niño than during
La Niña winters, mainly in the NH subtropics between 170∘ E and
70∘ W. At the lower levels (not shown), stratospheric
intrusions coincide with regions of the so-called “westerly ducts”, which
are much weaker during El Niño .
Furthermore, the equatorially symmetric anticyclones are more pronounced for
the La Niña composites due to stronger and more localized convection in
the western Pacific. These differences are also present during FMA, become
smaller during AMJ and disappear during JJA mainly because forcing of the
summer dynamics, especially of the ASM, is only weakly related to the winter
forcing.
The average value of the stream function (a) in the domain of 0∘–35∘ N, 0∘–160∘ W
and velocity potential (b) in the domain of 30∘ S–40∘ N, 90∘ E–140∘ W for La Niña
(solid line) and El Niño (dotted line) composites at θ=380 K. The grey shading region denotes the period with
statistically significant differences between the two composites.
The mean climatological anticyclone in AMJ (Fig. ) is at the very
beginning phase of the ASM anticyclone after El Niño winters, while it
develops quickly and strengthens after La Niña winters. We use the
transition of the upper-level (at 200 hPa) flow from westerly to
easterly winds
over south-eastern Asia (5∘–20∘ N, 40∘–120∘ E) to characterize the onset of the monsoon as discussed in
. By using the shifted composites as in the previous section,
it turns out that the onset of the ASM after La Niña is about half a
month earlier than after El Niño (Fig. ). The difference in
SF between La Niña and El Niño composites lasts from winter (DJF) to
early summer (AMJ) and are insignificant in summer (JJA) as noted earlier.
Same as Fig. but for the velocity potential (VP; in 105 m2 s-1) at θ=380 K with arrows
denoting the divergent part of the horizontal wind.
To prove the statistical significance of the ENSO anomalies in the SF
composites, we compare their mean values averaged over a representative
region shown as a blue rectangle in Fig. . The domain, defined as
0∘–35∘ N, 0∘–160∘ W, contains the NH
anticyclone from winter to summer. The results are shown in the left panel of
Fig. . The period with statistically different composites is
shaded grey. Thus, the NH anticyclone in La Niña years is significantly
stronger than in El Niño years within the first 5 months of the year,
i.e. until MJJ. This statistical analysis indicates that the influence of
ENSO on the anticyclone propagates from winter until early summer. The mean
SF difference between La Niña and El Niño composites from winter to
early summer is ∼6×106 m2 s-1.
Walker circulation
Zonal mean of the velocity potential at θ=360 K defining the Hadley circulation and calculated
for La Niña (a) and El Niño composites (b). The difference between La Niña and El Niño
composites (c). The average intensity of the Hadley circulation calculated for the domain of 20∘ S–20∘ N (d).
Complementary to SF, the divergent part of the horizontal flow can be described by the VP
and is shown in Fig. .
Note that VP is a factor of 10 smaller than SF, which is consistent with the
fact that the non-divergent rather than divergent part dominates the flow at
θ=380 K. Following , the positive peak of VP
indicates the intensity of the Walker circulation and the zonal mean of VP
(VP‾) quantifies the Hadley circulation (see below). The
positive values of VP represent the divergence or, using the continuity equation,
the strength of the upwelling, while the negative values are related to
convergence or downwelling. In this way, the upper branch of the Walker
circulation can be diagnosed in Fig. . The intensities of the
Walker circulation are similar to the results from .
The positive peak values of VP lie in the western and central tropical
Pacific for La Niña and El Niño DJF climatologies, respectively. They
correspond to the locations of rising motion. The mean upwelling
(downwelling) activity in La Niña winters is much stronger than in El
Niño winters, in agreement with the well-known weakening of the Walker
circulation after El Niño events . In spring (FMA) the
differences between the two composites are smaller than in winter. At the
beginning of summer (AMJ), the centres of the divergence start to shift from
the tropics to the extratropics and the differences become even smaller. In
JJA, these centres reach the China Sea. The strengths and positions of the
convergence/divergence centres in the La Niña composite are comparable to
those of El Niño in that season.
As was done for SF, the statistical significance of the ENSO anomalies in the
VP composites is diagnosed in the right panel of Fig. . The
blue rectangle in Fig. , defined as 30∘ S–40∘ N, 90∘ E–140∘ W, represents the region of
the ascending branch of the Walker circulation. The mean positive values over
this blue rectangle are calculated. The domain allows quantification of the
average upwelling of the Walker circulation. The divergence in the La
Niña composite is significantly higher than in the El Niño composite
within the first 5 months of the year. The mean VP difference between La
Niña and El Niño composites from winter to early summer is ∼22×105 m2 s-1.
Hadley circulation
Seasonal ozone climatology derived from MLS observations (2004–2015, version 4.2) at θ=380 K
for La Niña and El Niño composites from winter to summer months (from top to bottom).
Regions with statistically significant differences are marked by the black dots. The black isolines represent ozone of 185 ppbv, which mark the tropopause (see text).
The zonal mean of VP (VP‾) is used to represent the Hadley
circulation (Fig. a and b).
Note that the peak values of VP are more than three times larger than
VP‾. In winter, VP‾ is positive in SH
and negative in NH. The positive peaks represent the locations of rising air
and correspond to the ITCZ. The negative peaks represent the locations of
sinking air. The rising and sinking motions form the mean meridional Hadley
circulation. This circulation is weaker after La Niña than after El
Niño episodes, and the differences between the La Niña and El
Niño composites decrease in summer.
The latitudes of positive peaks show that the rising motion is shifted
southwards after El Niño winters compared to La Niña winters.
Correspondingly, the ITCZ is located around 4 and 6∘ S for
the La Niña and El Niño composites. Figure c
shows the difference between La Niña and El Niño composites. The
upwelling and downwelling after El Niño are much stronger than after La
Niña from DJF to MAM. The difference is smaller after AMJ. To check
the statistical significance of such differences, the average rising
intensity of the Hadley circulation, which is located in the tropics (from
20∘ S to 20∘ N), is calculated (Fig. d). The
values after El Niño winters are higher than after La Niña winters,
especially from DJF to MAM as noted before. The mean difference is about
2×105 m2 s-1, and is insignificant starting from April.
Impacts on ozone distribution
So far we have investigated the influence of ENSO anomalies on the
atmospheric circulation, especially on the mean horizontal flow quantified in
terms of the SF (Fig. ) and VP
(Fig. ). Such changes in the atmospheric circulation will also
affect the distribution of atmospheric constituents . Ozone is a sensitive indicator of transport properties in the
UTLS region due to its strong vertical and horizontal gradients and its
relatively long chemical lifetime. Furthermore, in the sub- and extratropics
around the subtropical jet, the ozone distribution is mainly determined by
transport rather than by chemistry. In this section, we quantify the impact
of ENSO anomalies on the mean ozone distribution based on MLS satellite data,
CLaMS simulations and SHADOZ ozonesonde data.
In particular, we now investigate the influence of ENSO on the isentropic
in-mixing of high stratospheric ozone values into the TTL
. In the following, the ozone isoline at the tropopause is
used to quantify the effect of isentropic in-mixing at θ=380 K.
estimated the monthly mean climatological ozone
concentration at the tropopause based on MOZAIC measurements. They found a
maximum value in May (120 ppbv) and a minimum value in November (65 ppbv).
Here, the isoline of 120 ppbv is used as the ozone boundary for CLaMS
composites to obtain a conservative estimate of stratospheric influence. MLS
ozone has a high bias of ∼ 40 % at 100 hPa in the tropics
and even by as much as ∼ 70 % inside the ASM
anticyclone . Therefore, the isoline of 185 ppbv is used as a
proxy for the tropopause in the MLS composites.
MLS composites
(a, b) Isolines of MLS ozone (185 ppbv, black lines in Fig. ) approximating the
tropopause at θ=380 K for different seasons
following La Niña (a) and El Niño (b) winters from DJF (red) to JJA (black). (c)
The mean concentration of ozone from the blue domain in the top panel (0∘–25∘ N, 60∘ E–120∘ W)
marking the region of strongest ENSO-related differences in in-mixing .
Figure 7 shows MLS ozone mixing ratio distributions at θ=380 K from winter to
summer after La Niña and El Niño winters. The ozone isoline at the
tropopause is represented by the black solid line. During DJF and FMA, the El
Niño composite is more zonally symmetric compared to La Niña. This is
consistent with the less disturbed subtropical jets after El Niño winters
as discussed in the last section. The region of enhanced in-mixing can be
recognized as a tongue of high ozone which emerges around 120∘ W,
30∘ N during DJF and is shifted in the following months to the west
until the ASM anticyclone forms.
During AMJ, this feature of in-mixing is much more pronounced for the La
Niña than for the El Niño composite. This may be related to the
differences in the developing process of the ASM anticyclone between La
Niña and El Niño shown in Fig. . The mean anticyclone in
AMJ is in the very first phase after El Niño, while the ASM anticyclone
develops more quickly after La Niña and the ozone distribution is
affected by a stronger ASM anticyclone during this period. The largest pattern
difference between La Niña and El Niño ozone composites occurs during
this period, while the SF shows the largest pattern difference in winter
(Fig. , top). Ozone in-mixing anomalies seem to be delayed compared
to the distribution of SF.
The black dots in Fig. provide information about regions
with statistically significant differences between La Niña and El
Niño composites. We can see that the differences exist almost everywhere,
especially in the regions of strong in-mixing described above. During the
mature phase of the ASM anticyclone (JJA), the number of black dots decrease
strongly, but there is still a region of significant in-mixing differences on
the ozone tongue as well as on the extratropical side of the tropopause. We will
return to this point later. Ozone values in the centre of the ASM anticyclone
are lower after La Niña than after El Niño in JJA, which is consistent
with the similar differences in the SF (cf. Fig. ).
Zonally averaged (10∘–130∘ E) time series of MLS ozone at θ=380 K (version 4.2; for more
details see ) over the course of these 3 representative years (a–c) 2008 (after La Niña winter),
2009 (a normal year) and 2010 (after El Niño winter).
The isolines of ozone representing the tropopause
are combined together in Fig. a and b to illustrate the pattern
of the seasonality of the ENSO-related differences in in-mixing. To quantify
such differences, the mean concentration inside the blue domain 0∘–25∘ N,
60∘ E–120∘ W is calculated and shown in
Fig. c. The grey shading highlights the seasons with
statistically significant differences between La Niña and El Niño
composites, which are from DJF to AMJ. The average results inside the
in-mixed region attest that ozone concentration after El Niño is about
16 ppbv lower than after La Niña from winter (DJF) to early summer
(AMJ). The difference is a manifestation of the influence of stronger
Hadley and BD circulations and weaker in-mixing after El Niño than after La
Niña on the horizontal distribution of ozone around the tropopause
. Starting from summer, the
difference in ozone distribution between El Niño and La Niña is
statistically insignificant. Starting in JJA, the concentration of in-mixed
ozone after El Niño years is even higher than after La Niña years.
(a, b) Same as Fig. but for CLaMS ozone with the isoline value of 120 ppbv. (c)
Same as Fig. c but also including the results for ENSO subcomposites with the QBO westerly phase
(dotted line), QBO easterly phase (dashed line) and long-lasting El Niño events (cyan line).
To better understand such statistical differences, now we investigate the MLS
observations in more detail for three example years which are representative
of typical El Niño, La Niña and neutral conditions. Following the
method described in , in Fig. we plot the
time series of the zonally averaged ozone (10∘–130∘ E) at
380 K during 2008 (i.e. after La Niña), 2009 (i.e. during a neutral
year) and 2010 (i.e. after El Niño). Over the course of these 3
representative years, the differences in ozone between the equator and
∼ 30∘ N mainly result from different intensities of in-mixing
and the BD circulation. Specifically, the ozone mixing ratios after El
Niño winter (2010) are much lower than after La Niña winter (2008) or
even during a normal year (2009), with a negative anomaly persisting from
January to June, supporting our statistical results in Figs.
and . The isentropic intrusions transport less ozone from high
latitudes to the tropics following El Niño winters.
However, there is more in-mixed ozone in 2010 than in 2008 and 2009 from June
to September. This could be a consequence of the differences in the
BD circulation (stronger after El Niño than after La Niña winters),
which may cause higher ozone values in the northern extratropics and,
consequently, stronger isentropic gradients of ozone after El Niño
winters. It means that under El Niño conditions, transport of ozone-rich
air from the extratropics to the tropics is inhibited during winter and
spring by the strong subtropical jet, but transport to the tropics may
occur later in summer when the subtropical jet is weaker. We will come back
to this point in Sect. .
In-mixing from CLaMS
As discussed in Fig. 5, CLaMS reproduces
the ENSO anomalies in ozone observed by MLS fairly well. However, at the time of writing
the MLS composites cover only 11 years with very few strong El Niño and La
Niña events. Using CLaMS ozone, we are able to extend our period to 37
years from 1979 to 2015 and obtain statistically more robust results.
Composites of the ozonesonde measurements from SHADOZ in Hilo, Hawaii 19.43∘ N, 155.04∘ W during 1998–2015.
Black and red lines represent the seasonal mean profiles for La Niña and El Niño composites.
The shading indicates the standard deviation of the mean. The dotted and dashed lines represent the results for subcomposites
defined by the westerly and easterly phases of the QBO.
Figure 10 (top) shows the same type of distribution as Fig. (top) but
for 37 years of CLaMS ozone simulations and with the tropopause defined by
the ozone isoline with 120 ppbv. The ozone concentrations from CLaMS
simulations are about 50 ppbv lower than MLS measurements at
θ=380 K, in part because of the zero ozone boundary condition at the
ground, but they show similar patterns to MLS ozone. The CLaMS ozone
distributions also show in-mixing activity over the eastern and central Pacific
in subsequent months following La Niña winters, with more zonally
symmetric features during months following El Niño. The signatures of
in-mixing over the tropical Pacific are much stronger after the onset of
the ASM anticyclone (AMJ) for both composites and extend deeper into the
tropics after La Niña than after El Niño winters. The differences
disappear in JJA.
The largest difference between the ENSO composites exists around the eastern
flank of the ASM anticyclone. To quantify this difference from CLaMS
simulations, the mean concentrations in the blue domain are calculated
(i.e. in the same way as for MLS) and are shown as solid black and red lines
in Fig. c for La Niña and El Niño composites
(the results for the long-lasting El Niño years and for the
subcomposites related to the different QBO phases are also shown and will be
discussed in Sect. ). As for the MLS composites, the CLaMS results
show a similar pattern with less in-mixed ozone from El Niño winters
to early summer and more in-mixed ozone in the late summer and autumn,
although statistically significant differences can only be found until AMJ
(grey shading). The ozone concentration after El Niño is about 12 ppbv
lower than after La Niña. This difference obtained from CLaMS simulations
for the time period 1979–2015 is slightly smaller than from MLS measurements
for the time period 2004–2015.
In-mixing from SHADOZ
MLS measurements and CLaMS simulations as described above provide the
ENSO-related differences in the horizontal distribution of ozone. The
vertical influence of ENSO anomalies on the ozone distribution near the
tropopause can also be inferred from the ozonesonde data obtained at the
SHADOZ station in Hilo, Hawaii 19.43∘ N, 155.04∘ W
(marked with a star in Figs. , , and
) from 1998 to 2015. Hilo is located in the central Pacific at
the edge of the climatological position of the anticyclone in winter (see
Fig. ). The air over Hilo is strongly affected by the meridional
disruption of the subtropical jet from winter (DJF) to early summer (AMJ)
following La Niña winters, while it is within the tropics following El
Niño winters.
The resolution of the SHADOZ ozone profiles is not the same for the whole
period, so the data are degraded to the vertical resolution of 200 m for all
years to calculate the ENSO composites introduced in Sect. .
The anomalies of zonally averaged ozone in the western and central Pacific (120∘ E,
120∘ W)
from DJF to JJA based on CLaMS simulations covering 1979–2015. The solid and dashed lines are the zonal means of the
westerlies (10, 17, 24 and 30 m s-1) and easterlies (-5, -10 and -20 m s-1).
Red and white lines represent potential temperature (K) and geopotential height (km).
Figure shows the ENSO-related seasonal variation in ozone with altitude over Hilo (red and black solid profiles), as well as
their variability due to the QBO phase (dotted and dashed lines), which will be discussed in the next section.
The mean ozone profiles during and after El Niño show a characteristic
S-shaped structure for all the seasons, with the lowest value near the
surface, a maximum near 6 km, a minimum near 12–13 km, and a subsequent
increase toward stratospheric values. The minimum ozone concentrations at
∼ 12–13 km are located at the level of main convective outflow and
are therefore caused by uplift of tropospheric air
. On the other hand, the ozone profiles from
La Niña winters do not show such a minimum. On average, the ozone
concentration for La Niña is about 44 ppbv higher than for El Niño
from 9 to 18 km in DJF (top left). The ozone concentration differences
between La Niña and El Niño during FMA and AMJ (top right and bottom
left) are smaller, with mean values around 38 and 20 ppbv.
Finally, there is no clear difference between these two composites during JJA
(bottom right).
The results from SHADOZ indicate that the air masses are more affected by
in-mixing following La Niña years, and that this effect is not only
confined to the region around 380 K (≈ 15 km) but can be diagnosed
throughout the whole UTLS region. Especially in winter, ENSO-related
anomalies in the ozone profile are quite large (from 9 to 21 km) compared
to other seasons. The influence lasts from winter (DJF) to early summer
(AMJ) but vanishes during JJA. Interestingly, the ENSO anomaly of in-mixing
changes polarity in the middle troposphere below 9 km. We discuss this point in
the following section.
Discussion
The ENSO anomaly induces two types of variability in the global ozone
distribution: on the one hand, the stronger Hadley/BD circulation during and
after El Niño winters transports less ozone into the TTL and more ozone
in the extratropical lower stratosphere and, consequently, stronger
latitudinal gradients of ozone on all isentropes in the UTLS region have to
be expected . On the other hand, a
less disturbed subtropical jet after El Niño more effectively suppresses
the isentropic in-mixing of ozone into the tropics during winter and spring
(this effect was extensively shown in this paper), while during late summer
and autumn higher ozone values, although less frequently, can be in-mixed into
the TTL.
The latter effect can be seen in the MLS observations at θ=380 K
(Fig. ) and are mainly caused by isentropic in-mixing around the
eastern flank of the ASM anticyclone. This effect can also be inferred from
our statistical analysis of the enhanced mean ozone values in the blue region
discussed in Fig. . The values shown in Figs.
and for MLS and CLaMS suggest that during late
summer and autumn the in-mixed ozone is higher after El Niño than after La
Niña winters, although we cannot prove the statistical robustness of this
result. In addition, all the SHADOZ mean profiles around 3–9 km
(Fig. ) show higher ozone for El Niño than for La Niña
composites.
To discuss this point in more detail, Fig.
shows from top to bottom the seasonal results of the zonal mean
(120∘ E–120∘ W) ozone anomalies after La Niña and El
Niño from the surface to 70 hPa as derived from the respective CLaMS
composites. The depicted ozone anomalies are mainly due to changes in the
Hadley/BD circulation, with the largest negative (positive) changes in the
TTL and positive (negative) changes in the lower extratropical stratosphere
mainly in the NH following El Niño (La Niña) winters. Although the
largest ozone anomalies can be found in DJF and FMA, their absolute values
weaken in the following months, especially in the tropics. In addition, the
positive anomaly in the north of the subtropical jets (black lines in
Fig. ) under El Niño conditions propagates downwards
into the middle troposphere, mainly in the NH.
We conclude that enhanced tropical upwelling in DJF and FMA following El
Niño transports ozone-poor air from the surface to the TTL. Likewise, the
enhanced downwelling poleward of the subtropical jets following El Niño
transports ozone-rich air from the stratosphere to the sub- and extratropical
middle troposphere. The higher ozone as observed by MLS at θ=380 K
during late summer 2010 (Fig. ) as well as the higher ozone
in the middle troposphere below 9 km in Hilo during DJF and FMA following El
Niño (Fig. ) may be partially related to the isentropic
transport of ozone-rich air from the stratosphere. While in the first case,
the isentropic transport happens above the jet, mainly on the eastern flank
of the ASM anticyclone, in the second case the isentropic pathway of
transport is related to the isentropes below the jet, i.e. to the θ
surfaces between 320 and 340 K
.
Inspired by the work of showing decreasing Indian summer
monsoon rainfall after long-lasting El Niño events, episodes which last
until the autumn or over the whole year following the El Niño winters are
now selected (i.e. the years 1982, 1987 and 1992 listed in
Table ). Here, we investigate whether their mean influence on the
atmospheric circulation and on the ozone distribution, although not
statistically significant, will increase the El Niño-related effects
derived in the previous sections. Table
shows the peak values of SF, VP and the Hadley circulation found inside the blue
domains in Figs. , and .
List of the maximum strength of the NH anticyclone (SF in 106 m2 s-1), Walker circulation (VP in 105 m2 s-1)
and Hadley circulation (HC in 105 m2 s-1) after La Niña, El Niño and long-lasting El Niño
found inside the blue domains in Figs. , and .
Number of monthsLa Niña El Niño Long-lasting El Niño after ENSO winterSFVPHCSFVPHCSFVPHC0341272425832625135291341152125732225111242261121617551518782132610815655168412341412017593212582952314718141312388231629148162714520239527
Indeed, SF, VP and the Hadley circulation averaged over these 3 years show
the strongest anomalies if compared to all El Niño years. In particular, the
ASM anticyclone is weaker and the Hadley circulation is stronger for most
considered months following the long-lasting El Niño winters. The onset
date of the ASM after long-lasting El Niños is even slightly later than
after the other El Niño winters (Fig. ). Accordingly, the
ozone concentrations in the tropics are less disturbed by isentropic
intrusions from the subtropics. Consequently, the lowest ozone concentrations are
detected in the blue domain in Fig. until the end of summer,
at which time ozone following long-lasting El Niños switches to having
the highest values in early autumn (cyan line in Fig. ). This
indicates that if El Niño does not decay until the following summer, its
influence on the ASM anticyclone and ozone will last longer.
found that the superposition of El Niño and easterly QBO
phase increases ozone flux from the stratosphere into the troposphere,
resulting in enhanced tropospheric ozone values in midlatitudes. The
opposite effect occurs for the combination of La Niña and westerly QBO
phase. Motivated by this study, we investigate how the QBO phase affects our
results. Table shows that La Niña winters are almost equally
affected by westerly and easterly QBO phases, while during El Niño the
westerly QBO phase occurs more often. To quantify the potential influence of
the QBO phase, we compare the difference between La Niña and El Niño
subcomposites defined by the westerly and easterly phases. The CLaMS results
in the blue rectangle at 380 K (Fig. c) show that the ozone
concentration after La Niña events is higher than after El Niño
events during both phases of the QBO, but their difference is larger during
the easterly than during the westerly QBO phase. Similarly, the SHADOZ ozone
data (Fig. ) show that the ozone concentration after La
Niña events is higher (lower) than after El Niño events in the UTLS
(middle troposphere) during both phases of the QBO, while the respective
subcomposites show larger differences during the easterly than during the
westerly phase. This indicates that our results on the ENSO effects are
robust, but the difference will be enhanced (weakened) during the easterly
(westerly) phase of the QBO.
Conclusions
ENSO typically shows the strongest signal in boreal winter, but
it can affect the atmospheric circulation and constituent distributions until
the next autumn. To quantify the influence of ENSO on the atmosphere from a
dynamical perspective, the stream function (SF) and the velocity potential
(VP) are introduced. SF and VP represent the divergence-free and the
rotation-free parts of the horizontal wind field, respectively. The results
show that the subtropical jets after El Niño winters are more zonally
symmetric than after La Niña winters. Furthermore, the meridional
disruption of the subtropical jets during El Niño are weaker compared to
La Niña winters. The anticyclonic circulation in the tropics following El
Niño is weaker than following La Niña. The strength of the ASM
anticyclone after El Niño is slightly weaker than after La Niña in
early boreal summer, and the onset date in El Niño years is about half a
month later than in La Niña years. VP after El Niño is weaker than
after La Niña from winter to early summer because of the weaker Walker
circulation in El Niño years. The Hadley circulation after El Niño is
much stronger than after La Niña from winter to spring.
The anomalies in the atmospheric circulation caused by ENSO also affect the
distribution of atmospheric composition. MLS satellite measurements
(2004–2015) and CLaMS simulations (1979–2015) are used to analyse the
influence of ENSO on the ozone distribution in the vicinity of the tropopause
(380 K). The results from CLaMS simulations show similar patterns to the MLS
measurements. In both, ozone patterns after La Niña winters and springs
show in-mixing over the eastern and central Pacific, while the ozone patterns
after El Niño winters and springs are more zonally symmetric. The
in-mixing difference between La Niña and El Niño is striking during
the onset of the ASM anticyclone (AMJ). Intrusions from the high-latitude
stratosphere reach much deeper into the tropics after La Niña winters
than after El Niño winters. This indicates that the ozone anomaly lags
behind
the atmospheric circulation anomaly in El Niño and La Niña winters by
about 4 months. Based on the ozonesonde data from SHADOZ (1998–2015) in
Hilo, Hawaii, the vertical impact of ENSO on the ozone distribution is
investigated. The well-known vertical S-shaped structure only exists
in the ozone profiles following El Niño but not La Niña from winter
to early summer. The ozone concentration in the UTLS after El Niño is
lower than after La Niña from DJF to AMJ. Our results demonstrate that
the air masses over Hilo following La Niña encounter stronger (weaker)
in-mixing in the UTLS (middle troposphere) compared to El Niño.
Weaker in-mixing and stronger Hadley circulation due to El Niño cause
lower ozone mixing ratios in the tropical UTLS compared to La Niña from
winter to early summer. However, the in-mixed ozone following El Niño
winters may become higher in the subtropical middle troposphere as well as in
the TTL in late summer and autumn. This effect is related to a stronger
Hadley/BD circulation after El Niño compared to La Niña, which may
cause higher ozone values in the extratropics and, consequently, stronger
isentropic and meridional gradients of ozone after El Niño winters. The
duration and intensity of the El Niño-related anomalies are amplified only if
the long-lasting episodes are considered. The ENSO-related anomalies are
enhanced (weakened) during the easterly (westerly) phase of the QBO.
The stream function, velocity potential and CLaMS model
data may be requested from the authors (x.yan@fz-juelich.de or
p.konopka@fz-juelich.de). The ENSO MEI index data can be obtained from the
website http://www.esrl.noaa.gov/psd/enso/mei/ (last access: 6 June
2018). The QBO data were freely downloaded from
http://www.cpc.ncep.noaa.gov/data/indices/qbo.u50.index (last access: 6
June 2018). The OLR data are available at the website
https://www.esrl.noaa.gov/psd/data/gridded/data.interp_OLR.html (last
access: 6 June 2018). The MLS version 4.2 data can be obtained from the MLS
website https://mls.jpl.nasa.gov. The SHADOZ ozonesonde data are
available at the website http://croc.gsfc.nasa.gov/shadoz (last access:
6 June 2018).
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by the Strategic Priority Research Program of the
Chinese Academy of Sciences, grant no. XDA2006010203, the National Natural
Science Foundation of China, grant no. 91337214, 41675040 and the
International Postdoctoral Exchange Fellowship Program 2015 under grant no.
20151011. The European Centre for Medium-Range Weather Forecasts (ECMWF)
provided meteorological analysis for this study. OLR and ENSO MEI index data
are provided by NOAA. Ozonesonde data are provided through the SHADOZ
database. Work at the Jet Propulsion Laboratory, California Institute of
Technology, was carried out under a contract with the National Aeronautics
and Space Administration. We would like to thank Suvarna Fadnavis for some
discussions which motivated us to do this work. The stream function and
velocity potential are calculated based on the method from Hiroshi L. Tanaka.
Excellent programming support was provided by Nicole Thomas.The article processing charges for this open-access
publication were covered by a Research Centre
of the Helmholtz Association. Edited by:
Martin Dameris
Reviewed by: three anonymous referees
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