The Tropical Tropopause Layer – Detailed Thermal Structure, Decadal Variability and Recent Trends

The tropical tropopause layer (TTL) is a key region for troposphere-stratosphere exchange and acts as a “gate” for trace gases entering the stratosphere. In particular, tropical tropopause temperatures (TPTs) control the content of stratospheric water vapour, which influences stratospheric chemistry, radiation and circulation and is also an important driver of surface climate. Decadal variability or even long-term trends in TPTs and stratospheric water vapour are of great interest but are still not well understood. A comprehensive analysis of the TTL, including its detailed thermal structure, recent variability and dominant processes spanning time scales of years to decades, is conducted in this thesis using the recently available decade of high accuracy and high vertical resolution Global Positioning System Radio Occultation (GPS-RO) data, the Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis data, and a series of model simulations with NCAR's fully-coupled CESM model, which employs the chemistry climate model WACCM as its atmospheric component. The GPS-RO data measures a significant warming of TPTs and a weakening of the strength of the tropopause inversion layer (TIL) since 2001. Based on a series of model simulations, which switch on/off the corresponding factors, this recent warming in the TTL is mainly due to internal variability, i.e. a decrease in sea surface temperatures (SSTs) and a strengthening in Quasi-Biennial Oscillation (QBO) associated westerlies. A version of WACCM with higher vertical resolution (~ 300 m) reproduces this recent temperature variability better than with the standard vertical resolution (~ 1 km). This thesis provides the first evidence for a connection between TPTs and the Pacific Decadal Oscillation (PDO), from both observations and model simulations. The phase of the PDO, and in particular the change from positive to negative phases around the year 2000, can very well explain the recently observed TPT (multi-) decadal variability. This connection between SSTs and TPTs has consequences for stratospheric water vapour and may provide an important feedback on the Earth's global surface temperatures. Additionally, the hotly debated (multi-) decadal variability in lower stratospheric (LS) water vapour between 1979 and 2014, can be well understood with the 11-year solar cycle, the decadal El-Niño Southern Oscillation (ENSO) and the PDO. LS water vapour lags the solar cycle by 2-3 years and can be explained using a link between the solar cycle, decadal ENSO variations and tropopause temperature variability. This thesis highlights the importance of a fine vertical resolution for climate models and improves the understanding of the TTL temperature and LS water vapour variability over the recent decades. In particular it opens up a debate of the connection between stratospheric decadal to multidecadal variability and modes of SST variability, such as the PDO

Quantifying contributions to the recent temperature variability in the tropical tropopause layer

The recently observed variability in the tropical tropopause layer (TTL), which features a warming of 0.9 K over the past decade (2001–2011), is investigated with a number of sensitivity experiments from simulations with NCAR's CESM-WACCM chemistry–climate model. The experiments have been designed to specifically quantify the contributions from natural as well as anthropogenic factors, such as solar variability (Solar), sea surface temperatures (SSTs), the quasi-biennial oscillation (QBO), stratospheric aerosols (Aerosol), greenhouse gases (GHGs) and the dependence on the vertical resolution in the model. The results show that, in the TTL from 2001 through 2011, a cooling in tropical SSTs leads to a weakening of tropical upwelling around the tropical tropopause and hence relative downwelling and adiabatic warming of 0.3 K decade-1; stronger QBO westerlies result in a 0.2 K decade-1 warming; increasing aerosols in the lower stratosphere lead to a 0.2 K decade-1 warming; a prolonged solar minimum contributes about 0.2 K decade-1 to a cooling; and increased GHGs have no significant influence. Considering all the factors mentioned above, we compute a net 0.5 K decade-1 warming, which is less than the observed 0.9 K decade-1 warming over the past decade in the TTL. Two simulations with different vertical resolution show that, with higher vertical resolution, an extra 0.8 K decade-1 warming can be simulated through the last decade compared with results from the "standard" low vertical resolution simulation. Model results indicate that the recent warming in the TTL is partly caused by stratospheric aerosols and mainly due to internal variability, i.e. the QBO and tropical SSTs. The vertical resolution can also strongly influence the TTL temperature response in addition to variability in the QBO and SSTs

The influence of natural and anthropogenic factors on major stratospheric sudden warmings

Major stratospheric sudden warmings are prominent disturbances of the Northern Hemisphere polar winter stratosphere. Understanding the factors controlling major warmings is required, since the associated circulation changes can propagate down into the troposphere and affect the surface climate, suggesting enhanced prediction skill when these processes are accurately represented in models. In this study we investigate how different natural and anthropogenic factors, namely, the quasi-biennial oscillation (QBO), sea surface temperatures (SSTs), anthropogenic greenhouse gases, and ozone-depleting substances, influence the frequency, variability, and life cycle of major warmings. This is done using sensitivity experiments performed with the National Center for Atmospheric Research's Community Earth System Model (CESM). CESM is able to simulate the life cycle of major warmings realistically. The QBO strengthens the climatological stratospheric polar night jet (PNJ) and significantly reduces the frequency of major warmings through reduction of planetary wave propagation into the PNJ region. Variability in SSTs weakens the PNJ and significantly increases the major warming frequency due to enhanced wave forcing. Even extreme climate change conditions (RCP8.5 scenario) do not influence the total frequency but determine the prewarming phase of major warmings. The amplitude and duration of major warmings seem to be mainly determined by internal stratospheric variability. We also suggest that SST variability, two-way ocean/atmosphere coupling, and hence the memory of the ocean are needed to reproduce the observed tropospheric negative Northern Annular Mode pattern after major warmings

Decadal variability of the tropical tropopause temperature and its relation to the Pacific Decadal Oscillation

Tropopause temperatures (TPTs) control the amount of stratospheric water vapour, which influences chemistry, radiation and circulation in the stratosphere, and is also an important driver of surface climate. Decadal variability and long-term trends in tropical TPTs as well as stratospheric water vapour are largely unknown. Here, we present for the first time evidence, from reanalysis and state-of-the-art climate model simulations, of a link between decadal variability in tropical TPTs and the Pacific Decadal Oscillation (PDO). The negative phase of the PDO is associated with anomalously cold sea surface temperatures (SSTs) in the tropical east and central Pacific, which enhance the zonal SST gradient across the equatorial Pacific. The latter drives a stronger Walker Circulation and a weaker Hadley Circulation, which leads to less convection and subsequently a warmer tropopause over the central equatorial Pacific. Over the North Pacific, positive sea level pressure anomalies occur, which damp vertical wave propagation into the stratosphere. This in turn slows the Brewer-Dobson circulation, and hence warms the tropical tropopause, enabling more water vapour to enter the stratosphere. The reverse chain of events holds for the positive phase of the PDO. Such ocean-troposphere-stratosphere interactions may provide an important feedback on the Earth’s global surface temperature

Stratospheric ozone depletion from future nitrous oxide increases

We have investigated the impact of assumed nitrous oxide (N2O) increases on stratospheric chemistry and dynamics by a series of idealized simulations. In a future cooler stratosphere the net yield of NOy from a changed N2O is known to decrease, but NOy can still be significantly increased by the increase of N2O. Results with a coupled chemistry-climate model (CCM) show that increases in N2O of 50%/100% between 2001 and 2050 result in more ozone destruction, causing a reduction in ozone mixing ratios of maximally 6%/10% in the middle stratosphere at around 10 hPa. This enhanced destruction could cause an ozone decline in the second half of this century in the middle stratosphere. However, the total ozone column still shows an increase in future decades, though the increase of 50%/100% in N2O caused a 2%/6% decrease in TCO compared with the reference simulation. N2O increases have significant effects on ozone trends at 20–10 hPa in the tropics and at northern high latitude, but have no significant effect on ozone trends in the Antarctic stratosphere. The ozone depletion potential for N2O in a future climate depends both on stratospheric temperature changes and tropospheric N2O changes, which have reversed effects on ozone in the middle and upper stratosphere. A 50% CO2 increase in conjunction with a 50% N2O increase cause significant ozone depletion in the middle stratosphere and lead to an increase of ozone in the upper stratosphere. Based on the multiple linear regression analysis and a series of sensitivity simulations, we find that the chemical effect of N2O increases dominates the ozone changes in the stratosphere while the dynamical and radiative effects of N2O increases are insignificant on average. However, the dynamical effect of N2O increases may cause large local changes in ozone mixing ratios, particularly, in the Southern Hemisphere lower stratosphere

Solar impacts on decadal variability of tropopause temperature and lower stratospheric (LS) water vapour: a mechanism through ocean–atmosphere coupling

Solar signals in the atmosphere and the ocean, especially in tropopause temperatures and lower stratospheric water vapour are investigated using recent observational and reanalyses data sets for the period from 1958 through 2013. Previous observational and modeling studies demonstrated solar influences in the lower stratosphere resembling a positive Northern Annular Mode due to the top-down mechanism involving enhanced solar UV radiation in the stratosphere during solar maxima and dynamical amplification mechanisms in the atmosphere. We found that these stratospheric changes might propagate down to the troposphere and become zonally asymmetric with characteristic pressure and wind pattern over the North Atlantic and North Pacific. Such changes in tropospheric circulation are related to anomalous positive SST anomalies in the central Pacific which resemble an El Niño Modoki event. We show for the first time with ocean reanalysis data that these SST anomalies are amplified by a positive feedback through oceanic subsurface currents and heat transport in the equatorial Pacific. Anomalous warm SSTs in the equatorial central Pacific change the zonal SST gradient and lead to anomalous westerly winds and currents in the western Pacific and easterly winds and currents in the eastern Pacific. This indicates a convergence and less upwelling and therefore enhances the positive SST anomalies in the equatorial central Pacific. Such a positive feedback results in a peak of El Niño Modoki events about 2 years after the solar maximum. These solar-induced signals in the ocean in turn modify the circulation and convection in the troposphere, resulting in lagged solar signals of anomalous high tropopause heights and negative anomalies in tropopause temperatures as well as in lower stratospheric water vapour over the equatorial Pacific which are in agreement with a time evolving solar-induced El Niño Modoki-like SST pattern. We demonstrate a solar modulation of intrinsic decadal climate variability over the Pacific which is amplified by positive feedbacks between the ocean and the atmosphere

Recent variability of the tropical tropopause inversion layer

The recent variability of the tropopause temperature and the tropopause inversion layer (TIL) are investigated with Global Positioning System Radio Occultation data and simulations with the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM). Over the past decade (2001–2011) the data show an increase of 0.8 K in the tropopause temperature and a decrease of 0.4 K in the strength of the tropopause inversion layer in the tropics, meaning that the vertical temperature gradient has declined, and therefore that the stability above the tropopause has weakened. WACCM simulations with finer vertical resolution show a more realistic TIL structure and variability. Model simulations show that the increased tropopause temperature and the weaker tropopause inversion layer are related to weakened upwelling in the tropics. Such changes in the thermal structure of the upper troposphere and lower stratosphere may have important implications for climate, such as a possible rise in water vapor in the lower stratosphere

Direct and indirect effects of solar variations on stratospheric ozone and temperature

We have used a fully coupled chemistry-climate model (WACCM) to investigate the relative importance of the direct and indirect effects of 11a solar variations on stratospheric temperature and ozone. Although the model does not contain a quasi-biennial oscillation (QBO) and uses fixed sea surface temperature (SST), it is able to produce a second maximum solar response in tropical lower stratospheric (TLS) temperature and ozone of approximately 0.5 K and 3%, respectively. In the TLS, the solar spectral variations in the chemistry scheme play a more important role than solar spectral variations in the radiation scheme in generating temperature and ozone responses. The chemistry effect of solar variations causes significant changes in the Brewer-Dobson (BD) circulation resulting in ozone anomalies in the TLS. The model simulations also show a negative feedback in the upper stratosphere between the temperature and ozone responses. A wavelet analysis of the modeled ozone and temperature time series reveals that the maximum solar responses in ozone and temperature caused by both chemical and radiative effects occur at different altitudes in the upper stratosphere. The analysis also confirms that both the direct radiative and indirect ozone feedback effects are important in generating a solar response in the upper stratospheric temperatures, although the solar spectral variations in the chemistry scheme give the largest solar cycle power in the upper stratospheric temperature

Detaillierte thermische Struktur, dekadische Variabilität und aktuelle Trends

The tropical tropopause layer (TTL) is a key region for troposphere- stratosphere exchange and acts as a “gate” for trace gases entering the stratosphere. In particular, tropical tropopause temperatures (TPTs) control the content of stratospheric water vapour, which influences stratospheric chemistry, radiation and circulation and is also an important driver of surface climate. Decadal variability or even long-term trends in TPTs and stratospheric water vapour are of great interest but are still not well understood. A comprehensive analysis of the TTL, including its detailed thermal structure, recent variability and dominant processes spanning time scales of years to decades, is conducted in this thesis using the recently available decade of high accuracy and high vertical resolution Global Positioning System Radio Occultation (GPS-RO) data, the Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis data, and a series of model simulations with NCAR's fully-coupled CESM model, which employs the chemistry climate model WACCM as its atmospheric component. The GPS-RO data measures a significant warming of TPTs and a weakening of the strength of the tropopause inversion layer (TIL) since 2001. Based on a series of model simulations, which switch on/off the corresponding factors, this recent warming in the TTL is mainly due to internal variability, i.e. a decrease in sea surface temperatures (SSTs) and a strengthening in Quasi- Biennial Oscillation (QBO) associated westerlies. A version of WACCM with higher vertical resolution (\texttildelow 300 m) reproduces this recent temperature variability better than with the standard vertical resolution (\texttildelow 1 km). This thesis provides the first evidence for a connection between TPTs and the Pacific Decadal Oscillation (PDO), from both observations and model simulations. The phase of the PDO, and in particular the change from positive to negative phases around the year 2000, can very well explain the recently observed TPT (multi-) decadal variability. This connection between SSTs and TPTs has consequences for stratospheric water vapour and may provide an important feedback on the Earth's global surface temperatures. Additionally, the hotly debated (multi-) decadal variability in lower stratospheric (LS) water vapour between 1979 and 2014, can be well understood with the 11-year solar cycle, the decadal El-Ni{\~n}o Southern Oscillation (ENSO) and the PDO. LS water vapour lags the solar cycle by 2-3 years and can be explained using a link between the solar cycle, decadal ENSO variations and tropopause temperature variability. This thesis highlights the importance of a fine vertical resolution for climate models and improves the understanding of the TTL temperature and LS water vapour variability over the recent decades. In particular it opens up a debate of the connection between stratospheric decadal to multidecadal variability and modes of SST variability, such as the PDO.Zusammenfassung Die Schicht um die tropische Tropopause (tropical tropopause layer - TTL) ist eine Schlüsselregion für den Austausch zwischen Tropo- und Stratosphäre und Haupteintragsregion von Spurengasen in die Stratosphäre. Tropische Tropopausentemperaturen bestimmen die Menge des stratosphärischen Wasserdampfes, der sowohl die stratosphärische Chemie, als auch die Strahlung und Zirkulation beeinflusst und ein wichtiger Treiber des Klimas an der Erdoberfläche ist. Dekadische Variabilität oder sogar langfristige Trends in den Tropopausentemperaturen und im stratosphärischen Wasserdampf sind daher von großem Interesse, jedoch bisher nicht gut verstanden. In dieser Arbeit wird eine umfassende Analyse der TTL, einschließlich ihrer detaillierten thermischen Struktur und ihrer aktuellen Variabilität auf Zeitskalen von Jahren bis Jahrzehnten durchgeführt. Dazu werden die nun für eine Dekade verfügbaren, sehr genauen und vertikal hoch aufgelösten Global Positioning System Radio Occultation (GPS-RO) Daten, die MERRA (Modern Era Retrospective- analysis for Research and Applications) Reanalysedaten, sowie eine Reihe von Modellsimulationen mit einem voll gekoppelten Klima-Chemiemodell vom NCAR (CESM-WACCM), welches bis in die Thermosphäre reicht, verwendet. Die GPS-RO Daten zeigen eine signifikante Erwärmung der Tropopausentemperaturen und eine Abschwächung der Stärke der Inversionsschicht oberhalb der Tropopause (Tropopause Inversion Layer – TIL) seit 2001. Basierend auf einer Reihe von Modellsimulationen, in welchen die entsprechenden natürlichen und anthropogenen Faktoren ein- bzw. ausgeschaltet werden, kann diese Erwärmung in der TTL vor allem auf interne Variabilität zurückgeführt werden. Dafür verantwortlich sind insbesondere eine Abnahme der Meeresoberflächentemperaturen und eine Verstärkung der Westphase der stratosphärischen Quasi-Biennial Oscillation (QBO). Eine vertikal höher aufgelöste Modellversion (~ 300 m in der TTL) reproduziert diese Temperaturvariabilität besser als die Standardauflösung (~ 1 km). Im Rahmen dieser Arbeit wird erstmalig ein Zusammenhang zwischen den Tropopausentemperaturen und der PDO (Pacific Decadal Oscillation) sowohl in Beobachtungs- als auch Modelldaten hergestellt und ein Mechanismus vorgeschlagen. Die Phase der PDO, und insbesondere die Änderungen von einer positiven zu einer negativen Phase um das Jahr 2000, können die beobachtete (multi-)dekadische Variabilität der Tropopausentemperaturen gut erklären. Die Verbindung zwischen Meeresoberflächen- und Tropopausentemperaturen beeinflusst wiederum den stratosphärischen Wasserdampf und könnte eine wichtige Wechselwirkung zur globalen Erdbodentemperatur darstellen. Die momentan stark diskutierte (multi-)dekadische Variabilität im Wasserdampf der unteren Stratosphäre zwischen 1979 und 2014 kann mit dem 11-jährigen Sonnenfleckenzyklus, der dekadischen El-NiÑo Southern Oscillation (ENSO) und der PDO Variabilität erklärt werden. Das Wasserdampfsignal ist zwei bis drei Jahre nach einem Sonnenfleckenmaximum am stärksten und kann mit der Verbindung zwischen dekadischer ENSO Variabilität und Tropopausentemperaturen verstanden werden. Diese Arbeit unterstreicht die Bedeutung einer feinen vertikalen Auflösung für Klimamodelle im Bereich der TTL und verbessert das Verständnis der Temperatur- und Wasserdampfvariabilität in der unteren Stratosphäre in den letzten Jahrzehnten. Insbesondere eröffnet sie eine Diskussion über den Zusammenhang zwischen dekadischer bis multidekadischer stratosphärischer Variabilität und Variabilitätsmoden im Ozean wie zum Beispiel der PDO