The tropopause marks the transition between the well-mixed troposphere and the stably stratified stratosphere. Besides the change in stratification, fundamental changes in the composition of chemical constituents, such as water vapor or ozone, take place in this region . The tropopause generally acts as a dynamic barrier for cross-tropopause transport. In the tropics, exchange between the troposphere and the stratosphere is mainly determined by the upwelling branch of the Brewer–Dobson circulation (BDC) and by deep convection. Tropical cross-tropopause transport is the main source of water vapor in the stratosphere and plays an important role in stratospheric chemistry and its radiative budget . In the extra-tropics, stratosphere-troposphere exchange takes place via quasi-horizontal transport at the edge of subtropical and polar jets, affecting tropospheric ozone concentrations and, hence, tropospheric and surface climate .

Tropopause properties not only contain information about possible troposphere-stratosphere exchange, but they can also be associated with the state of certain atmospheric characteristics. For example, due to their simple latitudinal structure, tropopause pressure or tropopause altitude characteristics can serve as a measure for the width of the tropical belt .

Tropopause characteristics react to both tropospheric and stratospheric temperature changes. Studies about tropopause altitude changes as an indicator of climate change have been conducted by , , and , to name a few. All these studies have consistently found evidence of a decrease in global tropopause temperature and pressure (increase in tropopause altitude) due to anthropogenic tropospheric warming and lower stratospheric cooling e.g., .

Over the last decades, radiosonde data have been the most important observational data source to study tropopause parameters and their characteristics . With their multiple-decade data record and high vertical resolution, these measurements are very valuable for monitoring inter-annual and intra-annual variations of tropopause parameters. On the other hand, coverage in the Southern Hemisphere (SH) and above oceans is very poor, which makes it hard to catch spatial characteristics and changes. Thus, analysis and reanalysis products have been additionally used to investigate tropopause characteristics globally see, e.g.,.

However, to investigate tropopause properties based only on observational data, the relatively new global positioning system (GPS) radio occultation (RO) technique delivers well-suited data. The RO method provides near-vertical profiles of atmospheric thermodynamic variables with high vertical resolution (better than 1 km) and global coverage. Other features of RO measurements include all-weather capability, high accuracy, high precision, and long-term stability . A number of studies confirmed the feasibility and excellent eligibility of RO measurements for monitoring the atmosphere and for climate change detection .

The combination of the excellent RO profile quality in the upper troposphere and lower stratosphere and data availability above ocean and land (including polar regions) make these data highly suited for tropopause parameter evaluation. The first studies using GPS RO data for tropopause determination were conducted by , , and for the tropical region. As RO data proved to provide accurate information on tropopause characteristics, these data have been increasingly used in recent years . Recently, a new method for tropopause determination from RO profiles based on bending angle information has been introduced by . This technique can be applied earlier in the RO data retrieval, avoiding additional processing. compared tropopause data resulting from this algorithm to conventional lapse rate tropopause data and found generally good agreement.

In this work, we take advantage of having more than 12 years of RO observational data available. We use the dense field of RO profiles to compute multi-year statistics of tropopause altitude and temperature and to analyze El Niño–Southern Oscillation (ENSO)- and quasi–biennial oscillation (QBO)-related variability on a small scale. Inter-annual anomalies of tropopause altitude and temperature depict the strong stratospheric influence at high latitudes. Our results confirm those of previous studies, but show climatological tropopause characteristics for the longer RO record available now.

The structure of this paper is as follows: in Sect. , a description of the RO measurement principle, the tropopause algorithm, and the binning and statistical methods is given. In Sect. , latitudinal and longitudinal characteristics are discussed. Section  describes the annual cycle of averaged tropopause data for different latitudinal bands, and, in Sect. , inter-annual variability is discussed. Section  provides a summary.

Figure  shows the mean annual cycle for the mean and median of HT and TT for 10∘ latitudinal bands. Generally, HT follows the cycle of incoming radiation, with maximum altitudes in summer, a decrease in fall, minimum altitudes in winter, and an increase in spring. The mean annual cycle is very pronounced in the subtropics and at mid latitudes, with amplitudes of more than 2 km. The amplitude decreases towards the tropics and high latitudes. In the tropics, the annual cycle is weak as HT changes are only about 1 km. In the northern hemispheric tropics, between the Equator and 20∘ N, HT shows an annual cycle with maximum altitudes in winter and minimum altitudes in summer due to the strong influence of the BDC on HT. The BDC northern hemispheric branch has a strong annual cycle, with maximum tropical upwelling in winter. As a result, tropical tropopause altitudes increase in winter, following the strong increase of equatorial upwelling .

At high latitudes, the mean annual cycle of HT fundamentally behaves differently for the NH and the SH. In the NH, the annual cycle is a combination of two waves. It consists of a single wave pattern over subpolar eastern Siberia and North America with maximum HT in boreal summer (minimum HT in boreal winter), and a double wave pattern over northern Europe, western Siberia, and high Arctic latitudes with maximum HT in summer and winter in the NH (minimum HT in spring and fall in the NH) . In the SH, a reversed mean annual cycle occurs over Antarctica. It can be explained by the gradual decrease of stratospheric temperatures during austral winter due to the lack of incoming radiation. Minimum stratospheric temperatures and thus highest HT are observed in August. Due to the shift in phase from mid latitudes to high latitudes, there is no pronounced cycle at all for the latitudinal band 50 to 60∘ S.

The comparison of mean and median shows good agreement, except for the latitudinal bands 20 to 30∘ N/S. Differences between mean and median in these regions have already been found for January and July, as shown in Fig. . However, the complete annual cycle gives further insight in the development of tropopause distribution. In the NH, the median has a very weak annual cycle that follows that of the tropical tropopauses. Due to strong equatorial upwelling in winter in the NH, most tropopauses are pushed to high altitudes even within the 20 to 30∘ N latitudinal band. The mean, on the other hand, is strongly affected by the very low tropopauses occurring in this region (see Fig. ). In boreal summer, such low tropopauses do not occur due to the smooth decrease of HT from low to high latitudes. Therefore, the mean has a pronounced annual cycle, following that of mid latitudes. As a result, mean and median agree very well during summer in the NH, but differences between mean and median increase in fall and maximize (1 km) in winter.

In the SH, the situation is different, as the annual cycle of HT has the same phase in the tropics and subtropics. The median is always higher than the mean, as more tropopauses show tropical characteristics. The resulting offset is 0.5 km in summer and 1 km in winter in the SH.

The shift from tropical to extra-tropical tropopause characteristics also causes minor differences between mean and median at latitudes from 30 to 50∘ N/S.

The mean annual cycle of TT is shown in Fig.  (right panel). Again, high HT corresponds to low TT, and vice versa. A weak annual cycle is found in the tropics with amplitudes of less than 3 K. It increases towards mid latitudes to about 5 K.

In the NH, there is hardly any annual cycle for the 40 to 50∘ latitudinal band, and a shift in phase towards high latitudes. Similar to , we also find a single wave pattern of TT with maximum temperatures in polar NH summer and minimum temperatures in winter in the NH, rather than a mixed wave pattern as found for HT. The amplitude of this TT annual cycle ranges from 2.5 to 5 K, increasing towards higher latitudes.

In the SH, the annual cycle of TT inversely follows that of HT for all latitudes, including the 6-month shift of its phase from approximately 50∘ S polewards. In this region, the amplitude strongly increases with latitude to more than 12 K at polar latitudes. TT is much lower in austral winter than in boreal winter. This difference is caused by the extremely cold polar vortex in the SH, on the one hand, and the more frequent occurrence of sudden stratospheric warming (SSW) events in the NH on the other hand. During SSW events, stratospheric temperatures can increase by up to 50 K within a couple of days, which affects TT.

Differences in the mean and median of TT correspond to differences in the mean and median of HT.

As the tropopause is influenced by both tropospheric and stratospheric conditions, inter-annual anomalies in tropopause properties can be caused by events in the troposphere, such as ENSO cold and warm phases, or in the stratosphere, such as SSW events or QBO.

ENSO has an impact on weather and climate on a global scale . ENSO warm phases cause tropospheric warming and stratospheric cooling . For zonal mean temperatures at low and mid latitudes, the ENSO signal shows a transition from warming to cooling near the tropopause. However, the transition of the local temperature response occurs well below the tropopause , yielding strong longitudinal variations of tropopause characteristics .

There was strong ENSO activity during the time range from 2007 to 2013. Major warm phases occurred during 2006/2007 and 2009/2010, and major cold phases during 2007/2008, 2008/2009, 2010/2011, and 2011/2012.

Figure  depicts ENSO and QBO regression coefficients for the regression of tropopause altitude and temperature. The strongest ENSO signal occurs above the tropical central Pacific (120 to 180∘ W), where HT correlates positively with ENSO warm phases and TT correlates inversely with ENSO warm phases (please note that the color bars of the ENSO and QBO coefficients are reversed for the regression of tropopause altitude and temperature for easier comparison). Another ENSO signal occurs for TT above the Maritime Continent (100 to 160∘ E), where TT correlates positively with ENSO warm phases. These results agree well with the findings of . QBO generally influences tropopause characteristics on a smaller scale and shows the out-of-phase character of the tropics and mid to high latitudes . TT and QBO anomalies correlate positively between 20∘ S and 20∘ N and are statistically significant for most bins. There are also some statistically significant areas at southern hemispheric mid and high latitudes, which agree well with results of .

Furthermore, we analyze the zonal mean response of tropopause parameters to inter-annual atmospheric variability. Anomalies of the Hmean (red) and Tmean (blue) for the different latitudinal bands are shown in Fig.  (NH) and Fig.  (SH) for September 2001 to December 2013.

In the tropics, variations are generally small with altitude (temperature) anomalies being smaller than 750 m (3 K) for the observed time range. These variations are caused by a combination of QBO and ENSO (see above). Anomalies at latitudes between 20 and 50∘ show some variability, but no distinct pattern. At high latitudes, however, stratospheric conditions have a strong impact on polar HT and TT, especially in winter and spring.

SSW events have a relatively strong influence on tropopause characteristics. During SSW events, stratospheric temperatures can increase by up to 50 K within a couple of days and cause low HT and high TT.

In the NH, relatively strong SSW events occurred in the winter of 2003/2004, 2005/2006, 2008/2009, 2009/2010, and 2012/2013 during the observed time range. Figure  shows lower HT and higher TT for these months. In January/February 2005, 2007, and 2011, when no or only a very weak SSW event occurred, tropopause anomalies were significantly higher and colder (up to 1.5 km/7 K) at high latitudes in the NH. Generally, the signal appears not only at polar latitudes, but between 60 and 90∘ N.

During the observed time range, only one SSW event took place in the SH. In Fig. , HT anomalies of -2 km (TT anomalies of 10 K) can be seen late on in the year 2002 for the 80 to 90∘ S latitudinal band. This event was so strong that it even affected tropopause characteristics in the 50 to 60∘ S latitudinal band. Stratospheric conditions were also exceptionally warm for winter and spring 2004

http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/

RO observations featuring high vertical resolution, global availability, and high accuracy, were used to investigate global characteristics of the lapse rate tropopause. Climatological tropopause characteristics for the RO record from 2001 to 2013 extend previous studies on the tropopause structure and its temporal variability. Latitudinal and longitudinal variations, as well as the annual cycle and inter-annual variability, were analyzed for tropopause altitude (HT) and temperature (TT).

Tropopause properties were analyzed using individual measurements, as well as averaged data. To obtain information about the latitudinal distribution of HT and TT, the mean and its standard deviation, as well as median, quartiles, and extreme values (box-and-whisker plots) were shown for 5∘ latitudinal bands for multi-year monthly averages.

HT and TT showed the well-known latitudinal structure with high and cold tropical tropopauses (16 to 17 km/190 to 195 K) and lower and warmer extra-tropical tropopauses (8 to 12 km/up to 230 K). Double tropopauses were found at the Equator, close to the subtropical jets, and at winter hemispheric mid and high latitudes.

Variability in the subtropical tropopause break leads to a large spread in the HT and TT distribution in the 20 to 30∘ latitudinal band during winter. Since the major part of tropopauses is located at high altitudes, but some tropopauses also show extra-tropical characteristics, the median (Hmed) is systematically higher than the mean (Hmean). Differences exceed 1 km in both the NH and SH. Accordingly, Tmean is up to 4 K higher than Tmed.

For second tropopauses, the mean and median of HT and TT generally showed small differences. The distribution showed weak latitudinal variations, with most second tropopauses between 14 and 20 km in the extra-tropical winter hemisphere and between 18 and 20 km in the tropics.

Longitudinal variations of HT and TT occur due to land–sea coverage and orography. Zonal asymmetries at northern hemispheric mid latitudes (40 to 60∘ N) are caused by large-scale Rossby wave activity, most pronounced in boreal winter . In boreal summer, strong convective activity in the Asian monsoon region leads to exceptionally high and cold tropopauses .

The mean annual cycles of HT and TT vary strongly in amplitude with latitude. While smallest amplitudes were found in the tropics, amplitudes were largest at mid and southern hemispheric high latitudes. At northern hemispheric low latitudes, tropopause parameters follow the annual cycle of the BDC with maximum tropical upwelling in winter, leading to higher/colder tropopauses in winter than in summer. Therefore, the mean annual cycle is in phase within 20∘ S and 20∘ N. A 6-month phase shift of the annual cycle was found over Antarctica. Due to the lack of incoming radiation in polar winter in the SH and the strong polar vortex, very low stratospheric temperatures lead to highest HT (lowest TT) during that time of the year.

The mean annual cycle of Hmean and Hmed (Tmean and Tmed) agreed well for all latitudinal bands except 20 to 30∘ N/S. In this region, the mean was systematically lower than the median throughout the year. Furthermore, we showed that the 20 to 30∘ N median followed the tropical tropopause annual cycle as most tropopauses are located at high altitudes. The mean, however, is affected by some very low tropopauses that mainly occur during boreal winter and therefore followed the extra-tropical tropopause annual cycle.

Inter-annual variability of tropopause characteristics can be caused by both tropospheric and stratospheric events. An OLS regression analysis showed a strong ENSO signal above the tropical central Pacific and a QBO signal in the tropics, confirming previous studies on the influence of ENSO and QBO on tropopause altitude and temperature.

Furthermore, anomalies of averaged HT and TT showed signatures of both ENSO and QBO at low latitudes. At high latitudes, large anomalies could be attributed to strong variability of polar vortex strength, including SSW events.

The ability to detect reliable trends in the relatively short RO record is limited by the large atmospheric variability during the last couple of years (e.g., a strong El Niño event in 2009/2010 and two strong La Niña events in 2010/2011 and 2011/2012). Even for OLS regression analyses (including ENSO and QBO), tropopause trends were inconclusive for different periods (2001 to 2010; 2001 to 2012). Therefore, we did not include a discussion of tropopause parameter trends in this paper.