Accurate depictions of the tropopause and its changes are important for
studies on stratosphere–troposphere exchange and climate change. Here,
the fidelity of primary lapse-rate tropopause altitudes and double tropopause
frequencies in four modern reanalyses (ERA-Interim, JRA-55, MERRA-2, and
CFSR) is examined using global radiosonde observations. In addition,
long-term trends (1981–2015) in these tropopause properties are diagnosed in
both the reanalyses and radiosondes. It is found that reanalyses reproduce
observed tropopause altitudes with little bias (typically less than ±150 m) and error comparable to the model vertical resolution. All reanalyses
underestimate the double tropopause frequency (up to 30 % lower than
observed), with the largest biases found in JRA-55 and the smallest in CFSR.
The underestimates in double tropopause frequency are primarily attributable
to the coarse vertical resolution of the reanalyses. Significant increasing
trends in both tropopause altitude (40–120 m per decade) and double
tropopause frequency (≥3 % per decade) were found in both the
radiosonde observations and the reanalyses over the 35-year analysis period
(1981–2015). ERA-Interim, JRA-55, and MERRA-2 broadly reproduce the patterns
and signs of observed significant trends, while CFSR is inconsistent with the
remaining datasets. Trends were diagnosed in both the native Eulerian
coordinate system of the reanalyses (fixed longitude and latitude) and in a
coordinate system where latitude is defined relative to the mean latitude of
the tropopause break (the discontinuity in tropopause altitude between the
tropics and extratropics) in each hemisphere. The coordinate relative to the tropopause break
facilitates the evaluation of tropopause behavior within the
tropical and extratropical reservoirs and revealed significant differences in
trend estimates compared to the traditional Eulerian analysis. Notably,
increasing tropopause altitude trends were found to be of greater magnitude
in coordinates relative to the tropopause break, and increasing double tropopause
frequency trends were found to occur primarily poleward of the tropopause
break in each hemisphere.
Introduction
The tropopause – the boundary between the often unstable, convectively
dominated troposphere and stably stratified stratosphere – is an important
boundary for many studies in the atmospheric sciences. For example, long-term
changes in tropopause altitude are considered to be an indicator of climate
change e.g.,. Based on radiosonde and satellite observations, previous
studies show a significant global rising trend in the tropopause altitude
during the last several decades
e.g.,.
employed output from climate model
simulations to assess tropopause trends and attributed the long-term increase
to increased greenhouse gases and stratospheric ozone depletion, which leads
to a warming of the troposphere and a cooling of the stratosphere. These
changes in atmospheric composition enhance the meridional temperature
gradient in the upper troposphere and lower stratosphere (UTLS), which in
turn strengthens the subtropical jets (to maintain thermal wind balance) and
accelerates the Brewer–Dobson circulation (BDC)
e.g.,.
The structure and variability of the tropopause also play a key role in
stratosphere–troposphere exchange (STE) studies. The fidelity of the
tropopause altitude directly impacts the quantification of STE because the
troposphere–stratosphere boundary itself is fundamental to identifying an
exchange event and its impact on UTLS composition
e.g.,.
Moreover, the temperature of the tropopause, especially in the tropics,
regulates the transport of water vapor (a powerful greenhouse gas) from the
troposphere to the stratosphere .
Several tropopause criteria have been used in prior studies and are based on
thermal, dynamical, and chemical characteristics of the atmosphere
. The original tropopause definition is based on
the temperature lapse rate (the negative of the vertical temperature
gradient) according to criteria put forth by the World Meteorological
Organization . This lapse-rate tropopause is globally reliable
and found to commonly coincide with the sharpest stability and chemical
transitions between the troposphere and stratosphere
. The only known
exceptions to this reliability of the lapse-rate tropopause occur within
complex UTLS stability environments e.g., layered UT and LS air near
the subtropical jet during Rossby wave breaking
events; and over the Antarctic during
austral winter, where there exists an erroneously high lapse-rate tropopause
altitude due to weakened stability in the lower stratosphere
. The issue over the Antarctic is confined to latitudes
poleward of 60∘ S for about 3 months out of the year. The WMO
definition also allows for identification of more than one tropopause if low-stability layers are observed for a substantial depth above the primary
tropopause. Double lapse-rate tropopauses have been the focus of many studies
during the past 2 decades and have been found to be largely related to
Rossby wave breaking events and associated STE above the subtropical jets
e.g.,.
Tropopause definitions based on either the maximum static stability gradient
in the vertical dimension or curve fitting to the static stability profile,
which is typically characterized as a step function from uniformly low
stability in the troposphere and high stability in the stratosphere, have
been increasingly used in global tropopause studies
. These
approaches often provide unique information on tropopause structure, such as
its sharpness (the depth of the troposphere–stratosphere stability
transition). However, static stability definitions frequently fail in the
subtropics where the stability profile is often layered (e.g., double
lapse-rate tropopauses), with maxima at multiple altitudes or at altitudes
well removed from the most prominent transition, or the transition from
troposphere to stratosphere occurs over a deep (≥3 km) layer.
Alternative definitions to the lapse-rate tropopause or one based on static
stability are often applied in specific locations, separated mostly by
latitude. In the tropics, the UTLS temperature minimum, known as the cold
point, is often used to define the tropopause. This cold point tropopause is
typically employed in studies that examine troposphere-to-stratosphere
transport of water vapor
. While easily defined,
cold point tropopause altitudes are only reliable within the deep tropics
(between 20∘ S and 20∘ N) because outside of this region the
coldest temperature in a vertical profile is not always associated with the
transition between tropospheric and stratospheric air (indicated by stability
or composition). Profiles containing multiple lapse-rate tropopauses are a
good example of when the cold point tropopause fails. Potential vorticity
(PV), which is conserved in an adiabatic and frictionless flow, is commonly
used for transport studies in the extratropics and often used to define a
dynamical tropopause
. The PV threshold used ranges from
±1–4 PVU (where 1 PVU =10-6 km2 kg-1 s-1) in previous
studies, with most using ±2 PVU e.g.,. The PV value that best coincides with
the lapse-rate tropopause varies with latitude and season and, if this
variability is not accounted for, it can result in large differences in
quantitative transport studies . For
example, found that changing the PV iso-surface from
±2 to ±4 PVU resulted in a reversal of the net STE between the
tropics and extratropics from Rossby wave breaking. The dynamical tropopause
is only reliable in close proximity to and poleward of the subtropical jets
since PV approaches a value of 0 near the Equator and iso-surfaces diverge
from the lapse-rate and cold point tropopauses in the deep tropics.
One final type of tropopause definition that has been used in many studies is
that based on chemical composition. Multiple studies have used ozone (O3)
profiles to define the tropopause, where the O3 tropopause is defined
using absolute thresholds for O3 concentration and vertical gradients of
O3. Unique limitations exist for
O3 tropopause altitudes due to the seasonality and location-dependent
variability of the ideal O3 concentration threshold value
. Another chemical tropopause definition
exists that uses multiple coincident trace gas concentrations to define the
UTLS chemical transition layer, often leveraging O3 and carbon monoxide
. However,
such coincident chemical observations are uncommon, making the method
impractical for climatological analysis. Artificial tracers have become
increasingly used in numerical models to allow for a chemical tropopause
definition, with the 90-day lifetime tracer (known as e90) being a widely
used choice in the chemistry climate model community .
Primary limitations of this approach are that it cannot be applied to
observations and it is not incorporated into reanalysis models, which are
commonly used for climatological analyses.
In summary, multiple tropopause definitions exist in the community and are
often chosen based on the goals of the study. The primary goal of this study
is to evaluate long-term changes in tropopause characteristics globally.
Since the lapse-rate tropopause is a global definition that can be easily
applied to both conventional observations and model output, agrees with the
sharp stability and chemical transitions between the troposphere and
stratosphere, and enables the unique opportunity to study multiple tropopause
structures, we employ this definition for analysis in this study.
Radiosondes have been the traditional source of thermodynamic profiles of the
atmosphere from the near surface up to 30 km since the 1950s, and have been
widely used for studying long-term changes in the tropopause
e.g.,. A
primary shortcoming of radiosonde observations is the limited spatial
coverage since they are mostly launched from land masses. Another limitation
is that not all radiosonde flights from a given location are successful,
leading to discontinuities in the data record. Moreover, despite the fact
that the number of sites providing operational radiosonde observations has
increased over time, the number of locations with long-term records suitable
for trend studies is relatively small. More recently, modern high-resolution
reanalysis models have been used to evaluate tropopause characteristics
globally since they provide data that are spatially and temporally continuous
. Reanalyses assimilate
global quality-controlled observations to provide best estimates of past
three-dimensional atmospheric states and are used to develop an understanding
of a wide range of atmospheric processes, which is often not possible using
observations alone . Several reanalyses are publicly
available and cover historical periods of 30 years or longer, with modern
reanalyses such as MERRA and MERRA-2 (the National Aeronautics and Space
Administration Modern-Era Retrospective analysis for Research and
Applications, Versions 1 and 2), ERA-Interim (the European Centre for
Medium-Range Weather Forecasts interim reanalysis), JRA-55 (the Japanese
Meteorological Association 55-year reanalysis), and CFSR (the National
Centers for Environmental Prediction Climate Forecast System Reanalysis)
being widely used for research today.
While tropopause altitudes can be similarly evaluated in observations and
reanalyses, the reanalyses can provide us with a broader understanding of
tropopause behavior given the spatial and temporal limitations of the
observations outlined above. For example, compared the
frequency of double tropopauses in five modern reanalyses during 1980–2014
and highlighted the sensitivity of the double tropopause identifications to
model vertical resolution. further argued that
differences in STE estimates based on reanalysis output are partly due to the
differences in vertical grid spacing. Moreover, reanalyses are highly
dependent on the underlying global forecast models, data input sources, and
assimilation systems . For instance, although both
ERA-Interim and MERRA-2 assimilate O3 profiles from the Aura Microwave
Limb Sounder, they use climatological and prognostic O3 fields for
radiation calculations
. This, in turn, may have a
significant impact on assimilated ozone and stratospheric temperatures during
winter and spring, which could impact tropopause calculations.
Here, we investigate the accuracy of primary tropopause altitudes and double
tropopause frequency in four modern reanalyses (ERA-Interim, JRA-55, MERRA-2,
and CFSR) and diagnose long-term (1981–2015) trends in tropopause altitude
and double tropopause frequency using both radiosonde observations and
reanalyses. In particular, we address the following three research questions. (1) How well
do modern reanalyses represent the lapse-rate tropopause? (2) What are the
recent trends in tropopause altitude and double tropopause frequency and how
do they vary spatially? (3) How sensitive are tropopause altitude trends
to the geographic coordinate system used? This research provides a unique
evaluation of model performance and of climate variability and change and a
physical perspective of UTLS dynamics, including STE that is commonly
associated with double tropopause events.
Data and methodsReanalysis output
ERA-Interim output is available from 1979 to the present on an approximately
80 km horizontal grid and with 750–1250 m vertical resolution in the UTLS
, where the UTLS is defined here as the 8–18 km altitude
layer. JRA-55 is available from 1958 to the present on a ∼60 km
horizontal grid . Similar to ERA-Interim, JRA-55 has
750–1250 m vertical grid spacing in the UTLS. MERRA-2 is available from 1979
to the present on a 0.5∘×0.625∘ longitude–latitude grid
and at ∼1100 m vertical resolution throughout the UTLS
. CFSR is available from 1979 to 2010 on a
0.5∘×0.5∘ longitude–latitude grid and at 700–900 m
vertical resolution in the UTLS. CFSR output is extended to the year 2015 in
this study using analyses from the Climate Forecast System version 2 (CFSv2)
model. For a more detailed discussion of these reanalyses and their
differences (including profiles of vertical resolution), see
. All tropopause analyses are performed using daily 00:00 UTC fields from each reanalysis (though not shown, analyses using alternative
synoptic times for shorter time periods are consistent). Geopotential height
was computed for each reanalysis model-level output using the
moisture-included hypsometric equation. Meteorological parameters are
interpolated linearly to a regular 1∘×1∘
longitude–latitude grid in the horizontal for analysis to enable 1-to-1
comparison (this choice has a negligible impact on the reported results).
Temperature profiles are linearly interpolated to a regular 200 m vertical
resolution prior to tropopause identification.
Global radiosonde data
Radiosonde data used in this study were obtained from the Integrated Global
Radiosonde Archive (IGRA) Version 2 . The IGRA database
provides historical radiosondes from locations around the world that have
been comprehensively quality controlled and corrected for gross errors. These
data are the best long-term historical record available for studies of the
vertical structure of the UTLS (global satellite-based observations, such as
radio occultation, are available for only the last ∼18 years). IGRA
radiosonde observations are mainly available twice daily at 00:00 and 12:00 UTC. Given that a small number of stations launch radiosondes at nonstandard
times, we included launches that occurred between 21:00 and 03:00 UTC in the
00:00 UTC analysis and those between 09:00 and 15:00 UTC in the 12:00 UTC
analysis. The IGRA radiosondes are provided at mandatory (conventional
pressure levels) and significant levels, which preserves any substantial
lapse-rate changes in the original data (observations are typically taken at
6 s intervals, resulting in ≤50 m vertical resolution), but can result in
coarse vertical resolution in the UTLS (>1 km). Such coarse resolution of
the profile can prevent successful application of the lapse-rate tropopause
by limiting the number of observations used to satisfy the second criterion
of the WMO definition and the criterion for identifying multiple tropopauses
(see Sect. below), as is true for the
reanalysis output. Thus, in order to enable thorough evaluation of the WMO
criteria and reliable tropopause identification, radiosonde data are also
linearly interpolated to a 200 m regular vertical grid prior to tropopause
identification. Only soundings that have valid observations between 5 and
22 km in altitude are used to identify the tropopause. The original high-vertical-resolution (∼5 m) profiles from select National Weather Service
sites in the United States were also retrieved for illustration purposes only
(see Fig. ).
Lapse-rate tropopause identification
As outlined in the Introduction, we employ the WMO lapse-rate tropopause
definition for analysis in this study. The WMO definition defines the first
tropopause in a profile as “the lowest level at which the lapse rate falls
to 2 ∘C km-1 or less, provided the average lapse rate
between this level and all higher levels within 2 km also does not exceed 2 ∘C km-1” . The WMO definition allows for a
secondary tropopause “if above the first tropopause the average lapse rate
between any level and all higher levels within 1 km exceeds 3 ∘C km-1, then a second tropopause is defined by the same criterion.” To
avoid boundary layer inversions and false tropopause identification, as well
as secondary tropopauses above the altitude of that typically observed for
the primary tropopause in the tropics, the algorithm is applied only to
altitudes ranging from 5 to 22 km. Lapse rates are calculated for each
profile using a forward (upward) difference scheme in the form Γ(zi)=-∂T/∂z≈-(Ti+1-Ti)/(zi+1-zi), where
Γ is the lapse rate in degrees Celsius per kilometer, T is temperature in
degrees Celsius, and z is altitude in kilometers. Tropopause altitudes from reanalysis
are geopotential altitudes.
Example temperature profiles from the National Weather Service radiosonde site in Corpus Christi, Texas,
at 00:00 UTC on (a) 30 January and (b) 12 May 2008. Black lines show the original, full high-resolution radiosonde
profile and gray dots show the reduced levels saved in the IGRA data. Coincident temperature profiles from ERA-Interim
are shown in blue, JRA-55 in purple, MERRA-2 in red, and CFSR in green, with circles along these lines denoting each
native model level. Colored arrows denote the locations of primary and secondary lapse-rate tropopause altitudes calculated using each temperature profile.
Example WMO lapse-rate tropopause altitudes calculated for two randomly
selected radiosonde profiles launched at the Corpus Christi, Texas, National
Weather Service office (97.5∘ W longitude, 27.78∘ N latitude)
in the United States are shown in Fig. . Both of these profiles
have two WMO tropopauses. Here, we show both the full-resolution temperature
profile obtained by each radiosonde and the reduced-resolution profile from
the IGRA archive to demonstrate common differences in the level of detail
between native data and those reported at mandatory and significant levels. As
demonstrated in both profiles, the IGRA data preserve all significant lapse-rate transitions and result in nearly equivalent tropopause altitude
definitions to those computed using the full-resolution profile (differences
are ≤100 m). Consistent results are found when selecting profiles
randomly from other stations and time periods (not shown). Thus, we are
confident that mandatory and significant levels included in the IGRA data are
suitable for tropopause analyses following the methods employed here.
Coincident temperature profiles and tropopause altitudes from each reanalysis
model are superimposed in Fig. and show much less detail than
the higher-resolution observations and a general inability to capture
multiple tropopause altitudes. These example profiles have shallow inversion
layers above the primary lapse-rate tropopause altitude, which are often not
well captured in the coarse-resolution reanalysis model profiles. As a
result, the secondary tropopause is often erroneously classified as the
primary tropopause altitude in model output e.g., see also Figs. 4
and 6 and discussion from. Extensive comparisons of tropopause
altitudes and multiple tropopause frequencies between the radiosondes and
reanalyses are provided in Sect. .
Trend analyses
Trends over the 35-year analysis period (1981–2015) of this study are
calculated using monthly mean primary tropopause altitudes and the monthly
fraction of profiles with double tropopauses. For radiosondes, we require a
station to have at least 20 days of suitable profiles (all from either 00:00
or 12:00 UTC and including mandatory and significant levels up to an altitude of
22 km or higher) to compute a monthly mean tropopause altitude or double
tropopause fraction. Trends are only calculated for radiosonde stations that
have sufficient observations for at least half of the total number of months
in the 35-year period and a roughly even distribution of data points
throughout the period (data gaps <5 years and no missing data near the
beginning and end of the time series) for adequate trend analysis. Monthly
tropopause time series from both radiosondes and reanalyses are then
deseasonalized using a high-pass filter that removes variability at timescales less than or equal to 1 year. Linear regression is used on the
filtered time series to measure trends over the 35-year period. Trends are
deemed significant if they exceed the 3σ uncertainty of the measured
slope, which is analogous to statistical significance at the 99 %
confidence level for a Gaussian distribution.
For reanalyses, trends are also calculated in an alternative coordinate
system. A sharp discontinuity in the primary lapse-rate tropopause altitude
is found near the subtropical jet and known as the “tropopause break”
e.g.,. Tropopause altitudes are
uniformly high (≥15 km) in the tropics and uniformly low (mostly 8–12 km
altitude) in the extratropics. In fact, as shown by ,
, and others, global and hemispheric frequency
distributions of tropopause altitude are bimodal. As a result, the tropopause
break is easily defined in model output as globe circling contours of a
threshold tropopause altitude or pressure coinciding with the frequency
minimum between the tropical and extratropical modes typically
∼14 km or ∼150 hPa; e.g., see. Because the
location (latitude) of the subtropical jets and tropopause breaks varies
considerably in space and time, trend analyses in the vicinity of the
tropopause breaks can be adversely impacted by their variability and
potential long-term changes in latitude. Thus, to remove this variability and
evaluate trends within the tropical and extratropical reservoirs separately,
we also analyze trends in the reanalyses using a
latitude coordinate relative to the tropopause break. Tropopause break latitudes are identified at each 00:00 UTC
analysis using contours of the tropopause altitude that coincides with the
frequency minimum between tropical and extratropical modes in each
hemisphere. Monthly means are then calculated by averaging the instantaneous
tropopause fields on the relative latitude grid in each hemisphere. For
plotting, analyses relative to the tropopause break are mapped using the long-term
mean break latitudes and any data extending beyond the Equator and pole are
trimmed from each hemisphere.
Note that time series used for trend analysis were not adjusted for potential
discontinuities and/or biases owing to changes in instrumentation or other
factors e.g., see. Thus, some
elements of the trend analyses (especially for the radiosonde observations)
may be impacted by these artifacts. However, a substantial number of time
series with large, significant trends (see Sect. ) were manually evaluated and no
discontinuities were found (not shown). Thus, we expect such factors to have
a minimal impact on the results outlined below.
Bias and root-mean-square differences between reanalysis tropopause identifications and radiosonde tropopause
identifications. Bias is defined as reanalysis minus radiosonde, with the frequencies of positive and negative bias in
parentheses. The number of radiosonde profiles used for primary tropopause altitudes and double tropopause frequencies is
99 023 and 45 181, respectively.
The values given are root-mean-square differences and mean
differences between reanalyses and radiosonde observations.
ResultsTropopause validation
To evaluate the fidelity of tropopause altitudes in the reanalyses, we first
compare tropopause altitudes from the gridded reanalysis output with the
radiosonde data at 00:00 UTC only. Instantaneous tropopause altitudes computed
on the 1∘×1∘ longitude–latitude grids are interpolated
linearly in space to the locations of the radiosondes for comparison. Results
of these comparisons are shown for individual months during each season:
January, April, July, and October between 1981 and 2015. In total, 317
radiosonde stations and approximately 9.9×104 profiles are used for
this validation, with their geographic locations shown as circles and squares in Fig. . Biases and errors (rms differences) in primary reanalysis
tropopause altitudes from this comparison are listed in Table .
More than half of the primary tropopause altitudes in MERRA-2 are found to
have a positive bias, whereas the majority of biases in the remaining
reanalyses (ERA-Interim, JRA-55, and CFSR) are negative. Errors in the
tropopause altitudes range from 950 to 1200 m, which is comparable to the
vertical grid spacing of the reanalyses.
All reanalyses produce too few double tropopauses compared to the radiosonde
observations. Double tropopause frequency in the reanalyses is significantly
underestimated, with 67 %–85 % of the sample having a negative bias. Biases
are largest in JRA-55, with over 22 percentage points fewer double
tropopauses than observed, and biases are smallest in CFSR, which has 13 percentage
points fewer double tropopauses than observed. Errors in double tropopause
frequency show similar differences among the reanalyses. The lower double
tropopause frequency in reanalyses is likely due to the coarse vertical
resolution of the models. As outlined in Sect.
and illustrated in Fig. , accurate tropopause identification
requires vertical resolution of ≤1 km to detect shallow low- and high-stability layers that are often responsible for the occurrence of multiple
tropopauses. CFSR, which has the lowest bias and error, also has the finest
vertical grid resolution in the UTLS, while MERRA-2 typically has finer
resolution than ERA-Interim and JRA-55 at the altitude of the secondary
tropopause. The differences in bias and rms error between models with
approximately equivalent vertical grids, such as ERA-Interim and JRA-55,
suggest that the representation of atmospheric processes and/or the data
assimilation system used could also be responsible for some of the
underrepresentation of double tropopause events.
To better evaluate the role of model vertical resolution as a source for the
observed tropopause differences, we repeated the validation with degraded
radiosonde observations. In particular, each IGRA radiosonde profile was
linearly interpolated to the fixed model-level grid of each reanalysis in
order to limit the detail of the observed temperature profile to that
available from each model. These degraded radiosonde profiles were then used
to calculate unique observation-based tropopause altitudes to compare with
each reanalysis and determine the resulting bias and error, which is expected
to be reduced (especially for double tropopauses). Table shows
these evaluations using the degraded radiosonde profiles and confirms that
the biases and errors in both primary tropopause altitude and double
tropopause frequency largely decrease. However, some bias and error remain
(and for MERRA-2 tropopause altitudes, increases), which suggests that
alternative sources of error (e.g., data assimilation, model
physics and dynamics) are significant.
(a) Average bias and (b) root-mean-square differences in instantaneous primary tropopause
altitudes between radiosonde and reanalyses within five latitude bands.
Open and closed symbols show the statistics as a function of season, with January (July) given as closed (open) circles,
April (October) given as closed (open) squares, and the annual values given as closed stars. Results for ERA-Interim are shown
in blue, JRA-55 in purple, MERRA-2 in red, and CFSR in green. Latitude values along the x axis and vertical lines denote
the boundaries of each latitude band. The number of radiosonde stations (profiles) used for each band are as follows:
113 (3.5×104) in the Northern Hemisphere extratropics (45–90∘ N), 152 (5.0×104) in the
Northern Hemisphere subtropics (20–45∘ N), 25 (7000) in the deep tropics (20∘ S–20∘ N), 20
(5000) in the Southern Hemisphere subtropics (20–45∘ S), and 7 (2000) in the Southern Hemisphere extratropics
(45–90∘ S). Results using the complete IGRA radiosonde observations are shown on the left and results following
the degradation of IGRA profiles to the native vertical grid of each reanalysis prior to tropopause calculation are shown on the right.
In addition to overall performance of the reanalyses, there are seasonal and
regional variations in the tropopause biases and errors. In order to better
understand the source of these variations, we group the comparisons by
latitude to reduce potential sources of uncertainty from the nonuniform
global distribution of the radiosonde locations. Figures and show the bias and rms error for primary tropopause altitudes
and double tropopause frequency, respectively, within five latitude bands:
two in the extratropics of each hemisphere (45–90∘ N and
45–90∘ S), two in the subtropics of each hemisphere (20–45∘ N
and 20–45∘ S), and one in the deep tropics (20∘ S–20∘ N).
The Southern Hemisphere subtropics and extratropics tend to have larger
errors than their counterparts in the Northern Hemisphere, likely due to (in
part) fewer data sources for assimilation and fewer profiles used for
analysis. Regionally, the largest primary tropopause errors are found within
the subtropics of each hemisphere – commonly associated with the location of
the subtropical jet and tropopause break. The complicated stability structure
within the subtropics is known to lead to large errors in tropopause
altitudes within models and is primarily the result of inadequate
representation of multiple tropopauses and the precise location of the
tropopause break . The largest differences within the
subtropics occur during the winter season of each hemisphere, which is
consistent with the time period during which the subtropical jet reaches its
maximum intensity and double tropopauses are more
frequent e.g.,. The tropopause
break is also sharpest in the vicinity of the subtropical jets at this time
due to thermal wind balance (i.e., the latitudinal temperature gradients in
the vicinity of the jet are largest). Biases and errors in double tropopause
frequency are largest in the subtropics and extratropics of each hemisphere
and small within the tropics, where multiple tropopauses are generally
infrequent. Biases and errors are also largest in the winter of each
hemisphere and smallest during the summer.
As in Fig. , but for the monthly double tropopause
frequency.
Eulerian mean tropopause altitude trends
The long-term trend in the tropopause altitude is considered to be an
indicator of climate change. Here, we evaluate trends in Eulerian monthly
mean primary tropopause altitudes using both radiosondes and reanalyses
(Fig. ). Statistically significant trends in tropopause altitude
are found in the radiosonde observations for many locations across the globe.
Most trends point to an increase in tropopause altitude over time, with the
largest increases (100–200 m per decade) found over western China, the
contiguous United States, eastern Europe, and Indonesia. A small region of
significant decreasing primary tropopause altitudes is found over Siberia and
the subtropical Pacific.
Eulerian mean trends of the primary tropopause altitude from 1981 to 2015 for IGRA radiosonde observations
and the reanalyses. For IGRA trends, circles denote 00:00 UTC trends and squares denote 12:00 UTC trends, with filled
symbols denoting statistical significance. Colored areas of the reanalysis maps are statistically significant,
while line-filled regions are not. Thick black lines in each reanalysis map show the 35-year mean tropopause break latitudes.
Significant tropopause altitude trends in the reanalyses are in broad
agreement with those identified from the radiosonde observations. In
particular, regions with dense radiosonde coverage and large, positive trends
are well represented in each reanalysis. The small, negative trend identified
over Siberia is also reproduced in each reanalysis and agrees with previous
studies , but this
behavior may be an artifact of changes in instrumentation and quality control
of the radiosonde data over time . Trends in the
reanalyses are generally larger and more variable outside of the regions with
dense radiosonde coverage. Some notable features are the upward/downward
tropopause altitude trend dipoles found over the eastern subtropical Pacific
in ERA-Interim, JRA-55, and MERRA-2. This dipole is not found in CFSR and the
magnitudes of the trends and spatial extent of the dipole vary considerably
in the remaining reanalyses. Moreover, the dipole is consistent with
significant narrowing of the tropics over the eastern Pacific that has been
identified in the reanalyses via subtropical jet and tropopause break
analysis .
Another notable difference among the reanalyses is the depiction of
tropopause trends across the tropics, where MERRA-2 and CFSR show
considerably larger upward trends than ERA-Interim and JRA-55. ERA-Interim
also depicts a downward trend over the central Pacific, which is not observed
in the remaining reanalyses. Trends in JRA-55 and MERRA-2 appear to be more
consistent with the limited number of radiosondes available in the tropics,
especially over Indonesia, the central Pacific, and northern Australia.
Finally, the tropopause altitude trends over Antarctica are found to be
inconsistent among the reanalyses. Namely, a rising trend is found over
Antarctica in ERA-Interim and MERRA-2, while a less extensive decreasing
trend is found in JRA-55 and CFSR.
Break-relative tropopause altitude trends
As discussed briefly in the previous section, recent studies have identified
significant regional changes in the width of the tropics that introduce some
uncertainty to the precise nature of tropopause changes when diagnosing
trends in an Eulerian framework. Therefore, in order to mitigate the effects
of a meandering tropopause break and focus on tropopause changes in the
tropical and extratropical reservoirs alone, we employ a coordinate relative to the tropopause
break here. Figure shows the geographic
distribution of primary tropopause altitude trends in a latitude coordinate relative to the tropopause
break for each reanalysis from 1981 to 2015.
Considerable differences are found in the diagnosed trends here compared to
those using the Eulerian monthly mean fields. In particular, significant
trends are larger in magnitude in the coordinate relative to the tropopause break
and are more consistent amongst the reanalyses, especially over the eastern
subtropical Pacific. ERA-Interim, JRA-55, and MERRA-2 are broadly consistent
with one another, with large upward trends in tropopause altitude in the
extratropics over the Pacific and similar patterns and signs of significant
trends elsewhere.
As in Fig. , tropopause altitudes in relative latitude to the tropopause break from reanalyses only.
MERRA-2 shows substantially different trends throughout the tropics relative
to ERA-Interim and JRA-55, with both larger upward magnitudes and unique
patterns over the western Pacific and Indian oceans. CFSR shows some
consistency with MERRA-2 in the tropics in terms of the trend magnitude, but
is inconsistent in pattern (especially over the Americas). In ERA-Interim,
JRA-55, and MERRA-2, trends in the tropics are largest immediately
equatorward of the tropopause break latitude in each hemisphere, while CFSR
shows the greatest trends in the deep tropics. However, the Eulerian mean
trends in Fig. show more consistent behavior amongst the
reanalyses in the depiction of these poleward maxima in altitude trends
within the tropics, which is in agreement with previous analyses
e.g.,. Outside of the tropics, CFSR is in broad
disagreement with the remaining reanalyses and suggests largely decreasing
trends in tropopause altitude.
Double tropopause climatology and trends
As outlined in Sect. , double tropopause
frequencies often have large bias and error in the reanalyses, which is
mainly attributed to their coarse vertical grid resolution. To better
understand the nature of these errors, maps of annual-mean double tropopause
frequency are presented for the radiosondes and reanalyses in Fig. . These maps are consistent with similar analyses of radiosondes,
satellite observations, and reanalyses in prior studies
e.g.,, and show
that there are belts of high double tropopause frequency in the northern
subtropics and midlatitudes of each hemisphere, largely near and poleward of
the subtropical jets and tropopause breaks. The patterns and spatial extent
of these belts are consistent between the radiosonde observations and
reanalyses, but there are considerable differences in the frequency values.
Radiosondes show that these high-frequency belts are characterized by values
≥40 %, while the reanalyses are at least 10 %–20 % lower. The
magnitudes of the differences between the radiosonde and reanalysis
frequencies within the high-frequency belts are consistent with those found in
the overall bias and error evaluation (Fig. ). Outside of the
high-frequency belts, there is a unique feature found within the tropics in
CFSR. Namely, a narrow band of double tropopause frequency between 10 % and 30 % is seen along the Equator stretching from central Africa to eastern
Indonesia. This feature does not exist in the remaining analyses and is
poorly sampled by the radiosonde network. However, the small number of
stations available in western Indonesia do show consistent double tropopause
frequencies. These double tropopauses may be driven (in part) by shallow,
lateral transport of extratropical lower stratospheric air into the tropical
upper troposphere on the eastern edge of the Asian monsoon anticyclone
e.g.,, but the lack of continuity of this feature
between the midlatitude high-frequency belt and the enhanced frequency belt
along the Equator suggests that the dynamics of the monsoon anticyclone may
also be important to their formation.
Average double tropopause frequency from 1981 to 2015 for IGRA radiosonde observations and the reanalyses.
For IGRA, circles denote 00:00 UTC frequencies and squares denote 12:00 UTC frequencies. Thick black lines in each
reanalysis map show the 35-year mean tropopause break latitudes.
Eulerian mean trend maps for double tropopause frequency during 1981–2015 are
shown for the radiosondes and reanalyses in Fig. . Trends in the
double tropopause frequency are found to be statistically significant at
almost all of the radiosonde locations and show substantial increases in
frequency (≥2 % per decade) nearly everywhere. The largest increasing
trends (≥3 % per decade) for double tropopause frequency are found in
the midlatitudes in each hemisphere and poleward of the high-frequency belts
in the long-term climatology, with some small (mostly <1 % per decade)
decreasing trends over Siberia, southern China, and the Caribbean (locations
with climatologically low double tropopause frequency). The midlatitude
increasing trends are comparable to those diagnosed in
between 1970 and 2006 for the
30–60∘ N and 30–60∘ S latitude belts, which were 3.3 %
and 6.6 % per decade, respectively. Taken together with the
climatological double tropopause distribution, these trends imply that the
area of frequent double tropopause environments is increasing in each
hemisphere, mostly indicating a northward expansion of the high-frequency
belts.
As in Fig. , but for trends in double tropopause
frequency.
As in Fig. , but for reanalyses only in a coordinate relative to the tropopause
break.
Areas of significant increasing trends in double tropopause frequency are
largely consistent between the radiosondes and reanalyses, with CFSR being
the only exception. In particular, ERA-Interim, JRA-55, and MERRA-2 all show
large increases in double tropopause frequency along and mostly north of the
tropopause break. CFSR shows mostly decreasing trends in double tropopause
frequency across the globe and is broadly inconsistent with the radiosonde
observations. Where areas of significant trends agree in pattern and sign
between the reanalyses and radiosondes, the magnitudes are largely consistent
near the tropopause break and inconsistent poleward of the break.
ERA-Interim, JRA-55, and MERRA-2 do not reproduce the poleward extent of
the significant increasing trends in observations well, especially over North
America. A unique increasing trend is found along the Equator in MERRA-2,
which is consistent with the location of the narrow band of moderate double
tropopause frequency found in the CFSR climatology. While none of the
remaining reanalyses show this feature, it is consistent with trends observed
in the small number of radiosonde stations over western Indonesia and the
western Pacific. Finally, the double tropopause frequency trends over
Antarctica are decreasing in all reanalyses, but the area and magnitude of
the trend vary considerably. The most consistent element of this feature is
found over western Antarctica, but there are no radiosonde observations in
this region to validate such a trend and the climatological frequencies in
this region are small.
Maps of annual-mean double tropopause frequency and frequency trends in
coordinates relative to the
tropopause break from the reanalyses are shown in
Figs. and , respectively. In general, there is less
variation in the patterns of both high-frequency double tropopause regions
and increasing double tropopause frequency trends between the Eulerian and
analyses relative to the
tropopause break. Annual-mean frequencies are higher in coordinates relative
to the
tropopause break and maximize poleward of the mean
tropopause break latitude throughout each hemisphere. Significant increasing
double tropopause frequency trends are also found primarily poleward of the
tropopause breaks in each hemisphere, but are otherwise mostly consistent
with the Eulerian analysis. Two notable exceptions are the areas of
significant increasing trends over the northern and southern east Pacific and
east Atlantic in ERA-Interim, JRA-55, and MERRA-2, where the largest
increasing trends were found to be mostly equatorward of the mean tropopause
break latitude in the Eulerian analysis. In coordinates relative to the tropopause break, the areas of greatest increasing trends are generally centered
on or poleward of the tropopause break in the Northern Hemisphere and
poleward of the tropopause break in the Southern Hemisphere.
As in Fig. , but for reanalyses only in a coordinate relative to the tropopause
break.
Observed 35-year (1981–2015) annual mean surface temperature changes from the NASA GISS surface temperature analysis.
Conclusions and discussion
In this study, we examined the fidelity of primary tropopause altitudes and
double tropopause frequency in four modern reanalyses (ERA-Interim, JRA-55,
MERRA-2, and CFSR) using the WMO lapse-rate tropopause definition. Long-term
trends in the primary tropopause altitude and double tropopause frequency
over a 35-year period (1981–2015) were also examined using both radiosonde
observations and reanalyses. All reanalyses were found to reproduce observed
primary tropopause altitudes with little bias and error comparable to the
vertical grid resolution of the models, which is consistent with previous
model tropopause evaluations e.g.,.
Bias and errors in the primary tropopause altitude were found to vary
regionally, with the largest magnitudes of both routinely found in the
subtropics of each hemisphere (Fig. ). Double tropopause
frequencies are broadly underrepresented in the reanalyses, with biases of up
to 30 percentage points lower than observed. JRA-55 consistently showed the
largest double tropopause bias, while CFSR consistently showed the lowest.
The majority of error in double tropopause frequency was found in the
subtropics and high latitudes of each hemisphere, where double tropopause
environments are most common. Based on the differences in vertical grid
resolution of the models and the necessary conditions for multiple tropopause
identification using the WMO definition, the underestimates in double
tropopause frequency in the reanalyses are argued to primarily be the result
of too coarse vertical grid spacing. When the radiosonde data are degraded to
the vertical grid resolution of each reanalysis, the biases and errors in
double tropopause frequency are greatly reduced, while the biases and errors
in primary tropopause altitude show little sensitivity to this change (Table ; Figs. and ).
Trends in primary tropopause altitudes were found to be significant and
increasing (i.e., upward) across most of the globe in the radiosonde
observations, largely ranging from 40 to 120 m per decade (Fig. ). Some similar but significant decreasing altitude trends were
found for a few radiosonde stations in Siberia. The reanalyses broadly
reproduce the patterns and signs of significant trends in the radiosonde
observations, with some disagreement in the areas of significant upward
trends over China, Australia, and northern Antarctica. Outside of the regions
with dense radiosonde coverage, there are significant upward and downward
primary tropopause altitude trend dipoles over the central and eastern
subtropical Pacific in ERA-Interim, JRA-55, and MERRA-2. Trend patterns in
CFSR in this region are not consistent with the remaining reanalyses. To
limit the impact of frequent meandering of the tropopause break and long-term
trends in its location to the diagnosed primary tropopause altitude trends
within the tropical and extratropical reservoirs, we also computed trends in
a latitude coordinate relative to the tropopause break (Fig. ). The
analysis relative to the
break revealed larger trends (≥120 m per decade) in
both the tropics and extratropics, which were increasing nearly everywhere
and greatest within the tropics immediately equatorward of the tropopause break
latitudes in each hemisphere and in the extratropical reservoir over the
eastern Pacific. As found in the Eulerian tropopause trend analysis,
ERA-Interim, JRA-55, and MERRA-2 showed consistent patterns and magnitudes of
significant trends, except for some locations within the tropics. CFSR was
once again broadly inconsistent with the remaining reanalyses and showed
decreasing tropopause altitude trends throughout most of the extratropical
reservoir.
Depending on the reference frame (Eulerian or relative to the tropopause break), the
maxima in tropopause altitude trends in the tropics immediately equatorward
of the mean tropopause break latitudes may indicate widening of the tropics
and/or changes in the strength of the subtropical jets. Changes in jet speed
can impact tropopause altitudes through changes in the magnitude of the
associated vertical ageostrophic circulations around the jets. These
circulations advect tropical upper troposphere air poleward above the jet
altitude and extratropical lower stratosphere air downward and equatorward
below in regions where the jet speed is increasing from west to east. Thus,
changes in tropopause altitude near the tropopause break latitudes can be
dynamically forced, with lower extratropical tropopause altitudes poleward of
the jet and higher tropopause altitudes equatorward of the jet in regions
where the west-to-east gradient in jet wind speed is increasing and vice
versa in regions where the west-to-east gradient is decreasing.
find decreasing trends in subtropical jet wind speeds in
the eastern Pacific within the Northern Hemisphere and increasing trends
elsewhere and increasing trends in subtropical jet wind speeds in the
eastern Pacific and decreasing trends over the Indian Ocean within the
Southern Hemisphere (e.g., see their Fig. 8). Thus, dynamically driven
changes in tropopause altitude near the subtropical jet are expected to be
upward within the tropics from the eastern Pacific across North America and
decreasing from Asia across the central Pacific in the Northern Hemisphere,
while dynamically driven trends are expected to be smaller in the Southern
Hemisphere. There are some patterns in tropopause altitude trends that are
consistent with this expectation, but it does not appear to be a major source
of the diagnosed trends.
Patterns in tropopause altitude are also expected to be driven (in part) by
the geographic distribution of observed surface (and tropospheric) warming
during the 1981–2015 period. Figure shows changes in global
surface temperatures during the 35-year analysis period from the NASA Goddard
Institute for Space Studies (GISS) surface temperature analysis
. Patterns of long-term surface warming are
consistent with the diagnosed trends in tropopause altitude here,
particularly within the midlatitudes. For example, the two prominent regions
of tropopause altitude increases found in the eastern midlatitude Pacific
within each hemisphere coincide with locally enhanced surface warming during
this time period. In addition, the patterns of increasing tropopause altitude
trends over North America and Greenland also closely resemble patterns in
surface warming there. Decreasing trends in the midlatitudes to high latitudes of the
Southern Hemisphere found in the reanalyses are also consistent with
decreasing trends in surface temperatures. These similarities imply that
surface (and tropospheric) warming and cooling may be a significant source of
diagnosed tropopause altitude trends in the extratropics. Sources of the
upward trends within the tropics and their patterns are less clear.
Primary tropopause altitude trends over Antarctica were found to vary
considerably among the reanalyses, with increasing trends in ERA-Interim and
MERRA-2 and decreasing trends in JRA-55 and CFSR. These conflicting trends
over Antarctica may be a result of different ozone input sources and
assimilation as well as the dynamical responses to ozone concentration
changes
e.g.,.
For example, ERA-Interim and MERRA-2 assimilate ozone retrievals (both
profiles and total column ozone, TCO) from SBUV and SBUV/2, as well as TCO
from OMI and profiles from Aura MLS, while JRA-55 only assimilates TCO and
CFSR assimilates TCO and relatively coarse-vertical-resolution profiles from
SBUV and SBUV/2. In particular, the rising Antarctic tropopause for
ERA-Interim and MERRA-2 may be associated with a strengthening of the
stratospheric polar vortex, which results in an elevated tropopause altitude
due to the anomalous residual meridional upwelling in the polar latitudes and
downwelling in the midlatitudes . The decreasing
tropopause altitudes in JRA-55 and CFSR may be associated with stratospheric
ozone recovery and an acceleration of the
BDC. Future work is needed to better elucidate the contributions from these
known processes to the diagnosed long-term tropopause altitude trends. If
possible, additional research on tropopause characteristics and variability
using observations in this region would also be helpful.
Significant increasing trends in double tropopause frequency were found
nearly everywhere in the radiosonde observations, with the largest trends
near and poleward of the tropopause break (≥3 % per decade; Fig. ). Considering the long-term climatology of double tropopauses
(Fig. ), the observed trends imply that the high-frequency double
tropopause belt in the subtropics and midlatitudes of each hemisphere is
expanding poleward over time. The ERA-Interim, JRA-55, and MERRA-2 reanalyses
showed consistent regions of long-term increasing double tropopause frequency
trends, but underestimated the poleward extent of these trends compared to
the observations. As found in the primary tropopause altitude trend analyses,
CFSR showed trends in double tropopause frequency that were largely
inconsistent with the remaining reanalyses and observations.
Given the relationship between double tropopauses, Rossby wave breaking, and
STE above the subtropical jets, the increasing frequency of double
tropopauses over time implies that similar increases in Rossby wave breaking
and STE have occurred during this time period and are mostly poleward. The trend
analysis relative to the
tropopause break (Fig. ) further confirmed
that these increasing trends are almost entirely on the poleward side of the
tropopause break in each hemisphere. Consistent increases in Rossby wave
breaking and transport have been found in recent studies. In particular,
increases in Rossby wave breaking frequency using MERRA-2 output at an
altitude between the primary and secondary tropopauses, where one would
expect the closest relationship between Rossby wave breaking and the
occurrence of double tropopauses, have been documented in
. Modeling studies suggest that transport of air
from the tropics into the extratropical lower stratosphere has also increased
in both hemispheres, which has been related to recently observed decreases in
lower stratospheric ozone in the extratropics
. In comparison, the spatially limited
increasing trends for double tropopause frequency found equatorward of the
tropopause break in this study may indicate an increase in equatorward
transport of stratospheric air from the extratropics into the tropical upper
troposphere , but more work is needed to better
understand the impact of these tropopause changes.
Recognizing the lack of long-term radiosonde observations over the oceans and
throughout much of the Southern Hemisphere, it is not surprising that
reanalysis tropopause altitude errors and altitude trends differ the most in
these regions. In addition to impacts directly related to data assimilation,
differences in long-term trends between the reanalyses are likely the result
of (1) differences in vertical grids, (2) differences in the representation
of physical and dynamical processes that impact both short- and long-term
tropopause change, and/or (3) differences in the accuracy of multiple
tropopause identification. It is not clear which of these factors is the most
significant contributor to the observed differences, but differences in
multiple tropopauses are likely responsible for much of the disagreement
within the subtropics and high-frequency double tropopause belts in each
hemisphere. In particular, since failing to identify a multiple tropopause is
most often the result of misidentifying the secondary tropopause as the
primary tropopause, long-term trends may be enhanced or reduced as a result
of these errors (especially if they have a time dependence). As suggested by
, the primary tropopause inversion layer depth has
been changing over time, increasing in some regions and decreasing in others.
Such changes will limit accurate identification of primary lapse-rate
tropopause altitudes in the regions where it is getting shallower and improve
identification in the regions where it is getting deeper. These changes may
induce false trends in primary tropopause altitude and double tropopause
frequency in the reanalyses, which we have not attempted to diagnose here.
Future studies should investigate the factors responsible for differences in
reanalysis trends in further detail.
In summary, this work has shown that global tropopause altitudes and the
frequency of double tropopauses have largely increased between 1981 and 2015.
These changes are relevant to climate and UTLS composition since increases in
primary tropopause altitude are believed to be associated with a warming
climate and double tropopause events often provide a physical indication of
STE between the tropical upper troposphere and extratropical lower
stratosphere. Broad agreement between three out of four of the modern
reanalyses included in this study provides some confidence in their
depictions of UTLS change. In addition, the consistency between reanalysis
tropopause identifications and those from available radiosonde observations
suggests that the tropopause and its behavior are well represented in modern
reanalyses. Future work is needed to examine long-term variability and trends
in tropopause characteristics using additional observations and models,
including existing model output from future climate projections. Longer time
periods and a greater number of potential solutions from available models may
provide increased confidence in the sign, magnitudes, and locations of the
trends diagnosed in this study and those projected to occur in the future.
Data availability
The IGRA radiosondes and most reanalysis data were obtained
from and are available at archives maintained by either the developers of the
reanalyses or a related third party: IGRA 10.7289/V5X63K0Q, ERA-Interim 10.5065/D6CR5RD9,
JRA-55 10.5065/D6HH6H41, and MERRA-2 10.5067/WWQSXQ8IVFW8, GISTEMP 10.7289/V5W9574V.
Model-level data from CFSR are
available upon request from Karen H. Rosenlof (karen.h.rosenlof@noaa.gov).
Author contributions
TX and CH designed the analysis and TX carried it out. Both authors prepared the paper.
Competing interests
The authors declare that they have no conflict of
interest.
Special issue statement
This article is part of the special
issue “The SPARC Reanalysis Intercomparison Project (S-RIP) (ACP/ESSD
inter-journal SI)”. It is not associated with a conference.
Acknowledgements
This work is funded by the National Natural Science Foundation of China grant
41505033 and China Scholarship Council. The authors thank the agencies that
provided reanalysis data used in this study: ERA-Interim from the European
Centre for Medium Range Weather Forecasts (ECMWF) and JRA-55 from the Japan
Meteorological Agency (JMA), both obtained from Reanalysis Data Archive (RDA)
managed by Computational and Information Systems Laboratory (CISL) at the
National Center for Atmospheric Research (NCAR), CFSR/CFSv2 from the National
Centers for Environmental Prediction (NCEP), and MERRA-2 from the National
Aeronautics and Space Administration (NASA) Global Modeling and Assimilation
Office (GMAO). We also thank the National Oceanic and Atmospheric
Administration (NOAA) National Centers for Environmental Information (NCEI)
for providing IGRA radiosonde data and the full-resolution radiosonde data
used for comparison in Fig. and the GISTEMP Team for providing GISS
Surface Temperature Analysis.
Review statement
This paper was edited by Mathias Palm and reviewed by two anonymous referees.
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