Transport pathways of air originating in the upper-tropospheric Asian monsoon
anticyclone are investigated based on three-dimensional trajectories. The
Asian monsoon anticyclone emerges in response to persistent deep convection
over India and southeast Asia in northern summer, and this convection is
associated with rapid transport from the surface to the upper troposphere and possibly into the stratosphere. Here, we investigate the fate of air that
originates within the upper-tropospheric anticyclone from the outflow of deep
convection, using trajectories driven by ERA-interim reanalysis data.
Calculations include isentropic estimates, plus fully three-dimensional
results based on kinematic and diabatic transport calculations. Isentropic
calculations show that air parcels are typically confined within the
anticyclone for 10–20 days and spread over the tropical belt within
a month of their initialization. However, only few parcels (3 % at
360 K, 8 % at 380 K) reach the extratropical
stratosphere by isentropic transport. When considering vertical transport we
find that 31 % or 48 % of the trajectories reach the
stratosphere within 60 days when using vertical velocities or
diabatic heating rates to calculate vertical transport, respectively. In both
cases, most parcels that reach the stratosphere are transported upward within
the anticyclone and enter the stratosphere in the tropics, typically
10–20 days after their initialization at 360 K. This
suggests that trace gases, including pollutants, that are transported into
the stratosphere via the Asian monsoon system are in a position to enter the
tropical pipe and thus be transported into the deep stratosphere. Sensitivity
calculations with respect to the initial altitude of the trajectories showed
that air needs to be transported to levels of 360 K or above by deep
convection to likely (

Left panel: mean JJA 2006 divergence over 20–120

The atmospheric circulation associated with the Asian summer monsoon leads to
efficient vertical transport from the surface to the upper troposphere. The
upper-tropospheric monsoon circulation consists of a large anticyclone of a size similar to the northern stratospheric winter polar vortex. Distinct
tracer anomalies in the Asian monsoon anticyclone provide a signature of
strong upward transport from the surface to the upper troposphere

On the other hand, the anticyclone is strongly variable in its extent,
location and strength, and frequent eddy shedding indicates substantial flux
out of the anticyclonic circulation

The main goal of this study is to investigate the transport pathways and destinations of air originating within the upper-tropospheric Asian monsoon anticyclone. In particular, we study the efficiency, timescales and preferred pathways of transport from the anticyclone into the stratosphere. Possible pathways include quasi-horizontal mixing across the edge of the anticyclone and into the extratropical stratosphere and vertical transport into the tropical stratosphere, from where air can be transported into the deep stratosphere by the Brewer–Dobson circulation. We calculate trajectories on isentropic surfaces to study the confinement of the anticyclone and quasi-horizontal mixing and additionally investigate the full three-dimensional transport.

Previous studies have used back trajectories to evaluate the source regions
of air in the tropical lower stratosphere

One common problem of three-dimensional Lagrangian trajectory modeling is the
uncertainty and excessive noisiness that exists in vertical motion fields.
Two methods are commonly used: the kinematic approach uses vertical
velocities as provided by the reanalysis products, while the diabatic
approach uses diabatic heating rates as vertical velocities in a coordinate
system with potential temperature as vertical coordinate. The noisy character
of vertical velocities as provided by (re-)analysis data sets usually results
in strong dispersion of kinematic trajectories

Trajectories are calculated using a simple parcel trajectory model that was implemented for the purpose of this study. The model is a standard fourth-order Runge–Kutta trajectory calculation with a time step of 0.5 h, driven by winds and heating rates from reanalysis products. We calculate two-dimensional trajectories at isentropic levels as well as full three-dimensional trajectories. The three-dimensional trajectories are calculated using both vertical velocities as given by the reanalysis fields (“kinematic” trajectories) and using heating rates as vertical velocities (“diabatic” trajectories).

The reanalysis used for both isentropic and three-dimensional trajectories is
the ERA-Interim data set from the European Centre for Medium-range Weather Forecasts (ECMWF)

Trajectory calculations focus on the Asian summer monsoon season (June–August) for the year 2006. While using only 1 year for the analysis might limit the conclusions of our study, it was found in GR13 that 2006 is no outlier in terms of strength and variability of the monsoon anticyclone (see their Fig. 5). Furthermore, no major El Niño or La Niña event that might influence the anticyclone system occurred around this year. Therefore, we expect similar results for other years without unusual conditions.

The vertical distribution of deep convective outflow in the Asian monsoon
region is not well known. While convective up- and downdrafts are not resolved
in the wind field, their mean effect is reflected in the large-scale winds:
the anticyclone region is characterized by mean upward transport that
maximizes around 330–340 K (or 500 to 300 hPa).
Consequently, mean divergence is found above this region, representing mean
convective outflow (note, however, that the detailed vertical profile of
divergence is likely to be strongly influenced by the convective
parametrization used in the reanalysis forecast model).
Figure

Isentropic trajectories are initialized every day between 1 June and
31 August of the year 2006 within the center of the anticyclone at 360 and at
380 K. Following GR13, the center of the anticyclone is defined as
the grid points between 15–45

Three-dimensional trajectories are initialized at the 360 K level
within the anticyclone in regions of low PV as for the isentropic
calculations. Trajectories are initialized each day between 1 June and
31 August and are run forward 60 days. In Sect.

Example of isentropic trajectories on 360 K, released on 10 June.
Black dots highlight trajectory locations on 10, 12, 14 and 16 June, together
with PV (red:

Mean JJA distribution of isentropic trajectories (in percentage of total
number of trajectories) at the 380 K (top panels) and 360 K (bottom panels)
level on day 0, 10, 20 and 30 after release, overlaid with mean JJA PV
contours (at

As a first step, we study the confinement of air within the anticyclone at isentropic levels. An example of trajectories in the first 6 days
after their initialization within the anticyclone is shown in
Fig.

The mean horizontal distributions of trajectories after 10, 20 and
30 days of their initialization are shown in Fig.

Within the first 10 days, trajectories mostly remain within the
anticyclone both at 360 and 380 K. After
20 days, the likelihood of trajectories to be shed from the
anticyclone to the east or west and also to the south increases. While the
maximum of the distribution remains within the anticyclone after
30 days, trajectories are spread over the tropical belt, and a second
maximum in the distribution is found in the Southern Hemisphere near
20

At higher altitudes (380 K), the anticyclone becomes more confined:
the mean distributions of trajectories initialized within the anticyclone at the 380 K level after 10, 20 and 30 days are shown in
Fig.

The upper-tropospheric anticyclone has a distinct signature in tracers like
CO, which has near-surface sources (from combustion) and is transported
upward by deep convection associated with the monsoon. While tracers are
confined by the anticyclone, shedding events as shown in
Fig.

Mean JJA distribution of parcel locations averaged over day 20–30 (white: 0.02 to 0.07 %; black: 0.08 to 0.25 %) overlaid on mean JJA CO concentrations as measure by Microwave Limb Sounder (MLS) at 380 K (top panel) and 360 K (bottom panel).

Trajectories released within the anticyclone at 360 K on 10 June
after 20, 40 and 60 days for kinematic (top panels) and diabatic (bottom
panels) calculation of vertical transport. Green dots mark the initial
location. The blue line is the mean thermal tropopause at 20–120

Full three-dimensional transport of air parcels that originate in the upper-tropospheric anticyclone is investigated in the following. Trajectories are initialized at the 360 K level in the same fashion as the isentropic trajectories above (i.e., in regions of low PV). Trajectories are initialized each day from 1 June to 31 August and are run forward for 60 days.

An example of the temporal development of the latitude–height distribution of
trajectories over 60 days is shown in Fig.

The mean probability distribution functions (PDFs) averaged over all
trajectories released each day between 1 June to 31 August are shown in
Fig.

To understand the causes for the vertical spread in the trajectories,
Fig.

The transit time distributions for parcels traveling upward to the 380 and
the 400 K levels is shown in Fig.

To quantify the destination of parcels originating from the monsoon
anticyclone, we subdivide the atmosphere into nine regions, and the fraction of
trajectories located in each of those regions after 30 and 60 days of
integration is given in Table

The time series for the most important regions are shown in
Fig.

Fraction of trajectories located in different regions of the atmosphere after 30 and 60 days for kinematic/diabatic transport.

Mean distribution of three-dimensional kinematic trajectories (top panels) and
three-dimensional diabatic trajectories (bottom panels) released within the anticyclone at
360 K on day 20, 40 and 60 after their release given as fraction of total
number of trajectories (dark (5

Of those trajectories that do not travel upward and into the stratosphere, most descend to levels below 250 hPa (46 and 39 % after 60 days). For both diabatic and kinematic transport, most of those trajectories descend within the first 10 days.

Both the fraction of trajectories that travel to the stratosphere and those
that are transported downward are relatively constant after about
40–50 days (see Fig.

The destinations of trajectories that originate from the anticyclone that are discussed above give no information about the pathways the trajectories take to the respective region. Trajectories do not travel necessarily directly from the upper-tropospheric anticyclone to the destination region. For example, while 15 % of all trajectories end up in the northern extratropical stratosphere after 60 days, they are not necessarily transported there directly from the upper-tropospheric anticyclone via isentropic transport. Therefore, we discuss the pathways of trajectories from the upper-tropospheric anticyclone to the three most important destinations (tropical lower stratosphere (LS), northern extratropical LS and lower troposphere) in more detail in the following. While we discuss pathways for diabatic transport in the following, the relative importance of the different pathways for kinematic transport are qualitatively the same.

Mean distribution of trajectories with respect to potential temperature for kinematic (black) and diabatic (red) transport. The gray bar denotes the initial location.

Monthly mean vertical velocity (top panel, in pascal per second; note the
flipped color bar) and d

Transit time distribution from 360 to 380 K (top panel) and to 400 K (bottom panel) for diabatic (red) and kinematic (black) trajectories.

Fraction of trajectories located in the stratosphere (black), the tropical stratosphere only (blue), the northern extratropical stratosphere (green) and below 250 hPa (red) as a function of time for kinematic (top panel) and diabatic (middle panel) transport. Bottom panel: transit time distribution to the stratosphere.

Geographical distribution of locations of tropopause crossings (as fraction of all trajectories; top panel: kinematic; bottom panel: diabatic) and contours of thermal tropopause height (black; contours between 110 and 290 hPa with interval 20 hPa). In order to exclude short-term reversible events, results here only include parcels remaining in the troposphere 5 days before and in the stratosphere 5 days after crossing the tropopause.

To highlight the pathways to and from the tropical lower stratosphere, total
air parcel fluxes over 60 days to and from this region are shown in
Fig.

Total air parcel fluxes over 60 days (in percentage of total number of
trajectories) into and out of

A total of 15 % of all trajectories is located in the northern
extratropical lower stratosphere after 60 days. However, as revealed
by the budget for this region (Fig.

The distribution of trajectories that are transported downward to below
250 hPa has a maximum over the Middle East (Fig.

Mean JJA distribution of diabatic trajectories (in percentage of total
number of trajectories) that are transported downward (to

The different transport pathways from the upper-tropospheric anticyclone (at
360 K) to the stratosphere are summarized in Fig.

Schematic of the most prominent transport pathways of air
originating in the upper-tropospheric anticyclone around 360 K (gray box).
Numbers indicate fraction of trajectories (in percentage) that are located in the
respective regions after 60 days for diabatic transport. The width of the
arrows reflects the importance of the respective pathway. Contours show the
zonal mean wind (black solid: positive; black dashed: negative), the
tropopause (blue) and the 340, 360 and 380 K isentrope (red) averaged over
20–120

So far, we have analyzed air parcels that were initialized in the upper-tropospheric anticyclone at 360 K. However, deep convection might not
lift air parcels as high as 360 K. The profile of mean divergence
suggests maximum outflow at levels around 340 to 370 K
(Fig.

Left panel: distribution of diabatic trajectories in potential
temperature initiated between 340 to 380 K (

Figure

The mean profile of diabatic heating in the anticyclone region suggests that
air is lifted in the mean throughout the atmosphere, and that lifting is
strongest at 330–340 K (see Fig.

Fraction of diabatic trajectories that remain in the anticyclone region as a function of time after initialization for different initial potential temperature levels (see legend).

Overall, these results suggest that air parcels need to be lifted by deep
convection to levels around 360 K or above in order to be likely
transported further upward and into the stratosphere. It is well known that
parcels need to be lifted to a certain height threshold (commonly thought of
as the level of zero radiative heating) to travel further upward into the
stratosphere

Several sources of uncertainties affect the trajectory calculations presented
here. We performed sensitivity simulations to test the robustness of our
results. The sensitivity simulations are performed for a limited number of
initial days (10 days spaced evenly over the 3-month period June to
August). Results of the sensitivity calculations, described in detail below,
are shown in Fig.

Fraction of trajectories located in the indicated regions (

To test the sensitivity of our results to the exact setup of initial
trajectory positions, we performed additional calculations with trajectories
initialized in a box region from 15 to 45

Vertical velocities that are crucial to three-dimensional trajectory
modeling underlie two sources of uncertainty: biases of the data set used
and uncertainties in the calculation of vertical advection

Transport pathways of air from within the upper-tropospheric Asian monsoon
anticyclone have been investigated using trajectory calculations driven by
ERA-Interim reanalysis winds and diabatic heating fields. Efficient transport
from the surface to the upper-tropospheric anticyclone is indicated by observations of tracer anomalies in this region, with maxima in tracer
concentrations of tropospheric origin (like CO) and minima in those of stratospheric origin (like ozone)

Using three-dimensional trajectory calculations, we found that a considerable fraction of air parcels initially located within the upper-tropospheric anticyclone (at 360 K) reach the stratosphere within 60 days (31 % for kinematic and 48 % for diabatic trajectory calculations). The horizontal confinement over 10–20 days is sufficiently long to efficiently transport air upward and into the stratosphere: typical transit times from the anticyclone at 360 K to the tropopause are 10–15 days.

The most likely pathway from the upper-tropospheric anticyclone into the
stratosphere is ascent within the anticyclone region. In the mean, air
masses move upward and across the (sub-)tropical tropopause, as shown in
Fig.

The results presented here on the role of different transport pathways of air
from the upper-tropospheric anticyclone to the stratosphere were found to be
robust against the details of trajectory initialization. While results differ
when using kinematic versus diabatic vertical velocities, the overall
conclusions are valid for both methods. It is known that heating rates are
likely overestimated in the upper troposphere in ERA-Interim

Outflow from tropical deep convection is estimated by previous studies to be
found around levels of 340 to 370 K

The calculations and results shown here are relevant to the observations of
transport of volcanic gases and aerosols in the monsoon region associated
with the eruption of Mt. Nabro in June 2011

We thank S. Brinkop, M. Nützel and J. Bergman for comments on the manuscript and P. Jöckel for assistance with the ECMWF data. Three anonymous reviewers provided constructive comments which significantly improved the manuscript. We thank the ECMWF for providing the ERA-Interim data. This study was partially funded by the Deutsche Forschungsgemeinschaft (DFG) through the DFG research group SHARP (Stratospheric Change and its Role for Climate Prediction) and by the Helmholtz Association under grant number VH-NG-1014 (Helmholtz-Hochschul-Nachwuchsforschergruppe MACClim). This work was partially supported by the NASA Aura Science Program. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: G. Stiller