On the relationship between the Tropical Easterly Jet over West Africa and Sahel rainfall across various time scales

Der obertroposphärische tropische Oststrahlstrom über Westafrika (Tropical Easterly Jet über West Africa, WA-TEJ), ein integraler Bestandteil der westafrikanischen Monsunzirkulation (WAM), ist stark mit Schwankungen des Sahelniederschlags auf interannuallen bis multidekadischen Zeitskalen korreliert. Diese Arbeit ergründet die ursächlichen Mechanismen sowie die Implikationen dieses statistischen Zusammenhangs und untersucht zudem, ob der WA-TEJ eine aktive Rolle für die Niederschlagsentwicklung in der Sahelzone spielt, wie in früheren Studien vorgeschlagen. In dem ersten wissenschaftlichen Kapitel dieser Arbeit werden zeitlich hochauf- gelöste Beobachtungen und Reanalysen analysiert, um zu klären, ob der WA-TEJ die Konvektion über der Sahelzone durch seine Auswirkungen auf die obertroposphäri- sche Divergenz unterstützt, wie in früheren Studien vorgeschlagen. Eine Analyse von 300 mesoskaligen konvektiven Systemen in der Sahel-Region zeigt, dass weder ihre Auslösung noch ihr Organisationsgrad mit signifikanten WA-TEJ-Anomalien oder TEJ- induzierten Divergenzen in der oberen Troposphäre verbunden sind. Auf der synop- tischen Zeitskala hinken WA-TEJ-Anomalien signifikant Anomalien der konvektiven Aktivität um ein bis zwei Tage hinterher, was darauf hindeutet, dass Konvektionsan- omalien eher Veränderungen im WA-TEJ bewirken als umgekehrt. Die Ergebnisse des ersten Kapitels deuten somit darauf hin, dass der WA-TEJ keine aktive Rolle für die Sahelniederschläge spielt, zumindest nicht über seine Auswirkungen auf die obertro- posphärische Divergenz. WAM-Niederschlagsanomalien können die Intensität des WA-TEJ signifikant beein- flussen, aber erklärt der lokale Antrieb des WA-TEJ durch die mit dem WAM-Nieder- schlag verbundene latente Erwärmung wirklich die beobachtete Kovariabilität zwi- schen WA-TEJ-Intensität und Sahelniederschlag auf interannualen bis dekadischen Zeitskalen? Mein zweites wissenschaftliches Kapitel befasst sich mit dieser Frage, in- dem es die wichtigsten tropischen Treiber der WA-TEJ-Variabilität untersucht. Dazu wird PUMA, ein atmosphärisches Zirkulationsmodell basierend auf trockener Dyna- mik, durch über die jeweilige Sommersaison gemittelte, dreidimensionalen Felder der diabatischen Heizung angetrieben über den Zeitraum 1979-2017. Die Felder des diaba- tischen Antriebs werden in einen lokalen afrikanischen und einen entfernten „Rest der Welt“-Teil aufgeteilt. Es werden Simulationen durchgeführt, bei denen sich die Jahr- zu-Jahr-Variabilität der diabatische Heizung entweder auf die lokale oder abgelegene Regionen beschränkt. Ich zeige, dass die interannuelle bis dekadische Variabilität der WA-TEJ-Intensität durch den Einfluss des entfernten diabatischen Antriebs dominiert wird, insbesondere durch die Wirkung von ENSO. Die WA-TEJ-Variabilität, die aus- chließlich auf die lokale diabatische Heizung zurückzuführen ist, ist zwar stark mit dem Sahelniederschlag korreliert aber schwach im Vergleich zur Wirkung der Fernein- flüsse. Letztendlich kann der entfernte diabatische Antrieb als ein wichtiger Treiber für die beobachtete Sahelniederschlag–WA-TEJ-Beziehung angesehen werden, da er nicht nur die WA-TEJ-Variabilität dominiert, sondern auch oft Sahelniederschlagsanomalien mit gleichem Vorzeichen induziert. Ziel des dritten wissenschaftlichen Kapitels ist es, herauzufinden, ob sich die Kova- riabilität zwischen TEJ und Sahelniederschlag in einem anderen Klima grundlegend ändert. Als Fallstudie wähle ich das mittlere Holozän, eine Epoche, in der der WAM- Niederschlag viel intensiver war und seine Variabilität weniger durch Telekonnektio- nen beeinflusst wurde. Im Vergleich zu einem vorindustriellen Kontrollklima simuliert das komplexe Erdsystemmodell MPI-ESM für das mittlere Holozän eine schwächere Kovariabilität zwischen WAM-Niederschlag und WA-TEJ-Intensität auf interannuellen bis dekadischen Zeitskalen. Um dies besser zu verstehen, werden die tropischen Trei- ber der WA-TEJ-Variabilität im mittleren Holozän mittels PUMA-Experimenten mit der selben Methodik wie im zweiten Kapitel untersucht. Im Gegensatz zum heutigen Kli- ma bestimmen nun der lokale und der entfernte diabatische Antrieb gleichermaßen die WA-TEJ-Variabilität. Dieses unterschiedliche Verhalten im Vergleich zum gegen- wärtigen Klima scheint in erster Linie auf die abnehmende Bedeutung des entfernten diabatischen Antriebs zurückzuführen zu sein, dessen Amplitude vor allem durch die geringere interannuelle Variabilität der Meeresberflächentemperaturen im tropischen Pazifik abgeschwächt wird. Die Ergebnisse deuten darauf hin, dass die verminderte Be- deutung des entfernten diabatischen Antriebs auch die verringerte Kovariabilität zwischen WA-TEJ und Sahelniederschlag während des mittleren Holozäns erklärt.The upper-tropospheric Tropical Easterly Jet over West Africa (WA-TEJ) is an integral part of the West African Monsoon (WAM) circulation and is strongly correlated with Sahel rainfall changes on interannual to multi-decadal time scales. This thesis aims to understand the underlying mechanisms and implications of this statistical relation and explores whether the WA-TEJ might play an active role for Sahel rainfall as proposed by previous studies. In the first scientific chapter, temporally high-resolved observations and reanalyses are used to clarify whether the WA-TEJ fosters convection over the Sahel via its influence on the upper-level divergence, as suggested by former studies. I conduct an analysis of nearly 300 Sahelian mesoscale convective systems and find that neither their initiation nor their degree of organization is significantly associated with WA-TEJ anomalies or jet-induced upper-level divergence. On synoptic time scales, WA-TEJ anomalies also significantly lag anomalies in convective activity by one or two days, indicating that convection anomalies are more likely to drive changes in the WA-TEJ than vice versa. In synopsis, the results of the first chapter suggest that the WA-TEJ does not play an active role for Sahel rainfall, at least not via its effect on upper-level divergence. WAM rainfall anomalies can significantly affect the WA-TEJ intensity, but does the local forcing of the WA-TEJ by WAM rainfall-related latent heating really explain the observed WA-TEJ – rainfall covariability on interrannual to decadal time scales? The second scientific chapter tackles this question by investigating the main tropical drivers of WA-TEJ variability. For this purpose, PUMA, an AGCM based on dry dynamics, is driven by summer mean 3D diabatic heating fields of the period of 1979-2017. The heating fields are split up into a local African and remote rest of world part. Simula- tions are conducted in which the year-to-year variability of the diabatic heating is ei- ther restricted to the local or remote regions. It is found that the interannual to decadal variability of WA-TEJ intensity is dominated by the influence of the remote forcing, especially by the effect of ENSO. The solely local forcing-induced WA-TEJ variability, while highly correlated with Sahel rainfall, is too weak to stand out against the remote influences. Ultimately, the variability of the remote diabatic heating – in particular over the tropical Pacific – can likely be viewed as an important driver of the observed Sahel rainfall–WA-TEJ relationship as it not only dominates the WA-TEJ variability, but also tends to induce same-sign changes in Sahel rainfall. The objective of the third scientific chapter is to explore whether the TEJ – Sahel rain- fall covariability may fundamentally change in a different climate. As a suitable case study, I select the mid-Holocene, a climatic period when the WAM was substantially stronger and its variability was less affected by teleconnections. Compared to a pre- industrial control climate, the state-of-the-art Earth system model MPI-ESM simulates for the mid-Holocene a weaker covariability between WAM rainfall and WA-TEJ inten- sity on interannual to decadal time scales. To better understand this, the tropical drivers of the mid-Holocene WA-TEJ variability are investigated via PUMA experiments, us- ing the same methods as in chapter two. In contrast to today, both the local forcing through WAM rainfall anomalies and the remote diabatic forcing play an equal role for the WA-TEJ variability during the mid-Holocene. This difference to the present-day state appears to be primarily caused by the diminished importance of the remote di- abatic forcing, whose amplitude is attenuated mainly due to the reduced interannual variability of SSTs in the tropical Pacific. I propose that the diminished importance of the remote diabatic forcing also mainly explains the reduced TEJ–rainfall covariability

The influence of DACCIWA radiosonde data on the quality of ECMWF analyses and forecasts over southern West Africa

During the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) field campaign ∼900 radiosondes were launched from 12 stations in southern West Africa from 15 June to 31 July 2016. Subsequently, data-denial experiments were conducted using the Integrated Forecasting System of the European Centre for Medium-range Weather Forecasts (ECMWF) to assess the radiosondes\u27 impact on the quality of analyses and forecasts. As observational reference, satellite-based estimates of rainfall and outgoing long-wave radiation (OLR) as well as the radiosonde measurements themselves are used. With regard to the analyses, the additional observations show positive impacts onwinds throughout the troposphere and lower stratosphere, while large lower-tropospheric cold and dry biases are hardly reduced. Nonetheless, downstream, that is farther inland from the radiosonde stations,we find a significant increase (decrease) in low-level night-time temperatures (monsoon winds) when incorporating the DACCIWA observations, suggesting a possible linkage via weaker cold air advection fromthe Gulf of Guinea. The associated lower relative humidity leads to reduced cloud cover in the DACCIWA analysis. Closer to the coast and over Benin and Togo, DACCIWA observations increase low-level specific humidity and precipitable water, possibly due to changes in advection and vertical mixing. During daytime, differences between the two analyses are generally smaller at low levels. With regard to the forecasts, the impact of the additional observations is lost after a day or less. Moderate improvements occur in low-level wind and temperature but also in rainfall over the downstream Sahel, while impacts on OLR are ambiguous. The changes in precipitation appear to also affect high-level cloud cover and the tropical easterly jet. The overall rather small observation impact suggests that model and data assimilation deficits are the main limiting factors for better forecasts inWest Africa. The new observations and physical understanding from DACCIWA can hopefully contribute to reducing these issues

The climate of a retrograde rotating Earth

To enhance understanding of Earth's climate, numerical experiments are performed contrasting a retrograde and prograde rotating Earth using the Max Planck Institute Earth system model. The experiments show that the sense of rotation has relatively little impact on the globally and zonally averaged energy budgets but leads to large shifts in continental climates, patterns of precipitation, and regions of deep water formation.Changes in the zonal asymmetries of the continental climates are expected given ideas developed more than a hundred years ago. Unexpected was, however, the switch in the character of the European–African climate with that of the Americas, with a drying of the former and a greening of the latter. Also unexpected was a shift in the storm track activity from the oceans to the land in the Northern Hemisphere. The different patterns of storms and changes in the direction of the trades influence fresh water transport, which may underpin the change of the role of the North Atlantic and the Pacific in terms of deep water formation, overturning and northward oceanic heat transport. These changes greatly influence northern hemispheric climate and atmospheric heat transport by eddies in ways that appear energetically consistent with a southward shift of the zonally and annually averaged tropical rain bands. Differences between the zonally averaged energy budget and the rain band shifts leave the door open, however, for an important role for stationary eddies in determining the position of tropical rains. Changes in ocean biogeochemistry largely follow shifts in ocean circulation, but the emergence of a super oxygen minimum zone in the Indian Ocean is not expected. The upwelling of phosphate-enriched and nitrate-depleted water provokes a dominance of cyanobacteria over bulk phytoplankton over vast areas – a phenomenon not observed in the prograde model.What would the climate of Earth look like if it would rotate in the reversed (retrograde) direction? Which of the characteristic climate patterns in the ocean, atmosphere, or land that are observed in a present-day climate are the result of the direction of Earth's rotation? Is, for example, the structure of the oceanic meridional overturning circulation (MOC) a consequence of the interplay of basin location and rotation direction? In experiments with the Max Planck Institute Earth system model (MPI-ESM), we investigate the effects of a retrograde rotation in all aspects of the climate system.The expected consequences of a retrograde rotation are reversals of the zonal wind and ocean circulation patterns. These changes are associated with major shifts in the temperature and precipitation patterns. For example, the temperature gradient between Europe and eastern Siberia is reversed, and the Sahara greens, while large parts of the Americas become deserts. Interestingly, the Intertropical Convergence Zone (ITCZ) shifts southward and the modeled double ITCZ in the Pacific changes to a single ITCZ, a result of zonal asymmetries in the structure of the tropical circulation.One of the most prominent non-trivial effects of a retrograde rotation is a collapse of the Atlantic MOC, while a strong overturning cell emerges in the Pacific. This clearly shows that the position of the MOC is not controlled by the sizes of the basins or by mountain chains splitting the continents in unequal runoff basins but by the location of the basins relative to the dominant wind directions. As a consequence of the changes in the ocean circulation, a super oxygen minimum zone develops in the Indian Ocean leading to upwelling of phosphate-enriched and nitrate-depleted water. These conditions provoke a dominance of cyanobacteria over bulk phytoplankton over vast areas, a phenomenon not observed in the prograde model.</p

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Rapid elimination of CO through the lungs: coming full circle 100 years on

At the start of the 20th century, CO poisoning was treated by administering a combination of CO2 and O2 (carbogen) to stimulate ventilation. This treatment was reported to be highly effective, even reversing the deep coma of severe CO poisoning before patients arrived at the hospital. The efficacy of carbogen in treating CO poisoning was initially attributed to the absorption of CO2; however, it was eventually realized that the increase in pulmonary ventilation was the predominant factor accelerating clearance of CO from the blood. The inhaled CO2 in the carbogen stimulated ventilation but prevented hypocapnia and the resulting reductions in cerebral blood flow. By then, however, carbogen treatment for CO poisoning had been abandoned in favour of hyperbaric O2. Now, a half-century later, there is accumulating evidence that hyperbaric O2 is not efficacious, most probably because of delays in initiating treatment. We now also know that increases in pulmonary ventilation with O2-enriched gas can clear CO from the blood as fast, or very nearly as fast, as hyperbaric O2. Compared with hyperbaric O2, the technology for accelerating pulmonary clearance of CO with hyperoxic gas is not only portable and inexpensive, but also may be far more effective because treatment can be initiated sooner. In addition, the technology can be distributed more widely, especially in developing countries where the prevalence of CO poisoning is highest. Finally, early pulmonary CO clearance does not delay or preclude any other treatment, including subsequent treatment with hyperbaric O2

Sahel rainfall - tropical easterly jet relationship on synoptic to intraseasonal time scales

The Tropical Easterly Jet (TEJ) is a characteristic upper-level feature of the West African Monsoon (WAM) circulation. Moreover, the TEJ over West Africa is significantly correlated with summer Sahel rainfall on interannual and decadal time scales. In contrast, the relationship between Sahel rainfall and the regional TEJ on synoptic to intraseasonal time scales is unclear. Therefore, this relationship is investigated by means of multiple statistical analyses using temporally highly resolved measurement and reanalysis data. It is shown that average correlations between convective activity and regional TEJ intensity remain below 0.3 for all synoptic to intraseasonal time scales. Especially on the synoptic time scale, the TEJ significantly lags anomalies in convective activity by one or two days which indicates that convection anomalies are more likely to drive changes in the regional TEJ than vice versa. To further shed light on the role of the TEJ for rainfall over West Africa, a previously proposed effect of TEJ-induced upper-level divergence on the development of mesoscale convective systems (MCS) is examined more closely. An analysis of nearly 300 Sahelian MCSs shows that their initiation is generally not associated with significant TEJ anomalies or jet-induced upper-level divergence. Furthermore, no statistically significant evidence is found that preexisting TEJ-related upper-level divergence anomalies affect intensity, size and lifetime of MCSs. A limiting factor of this study is the focus on TEJinduced upper-level divergence. Therefore, a possible effect of the TEJ on Sahel rainfall via other mechanisms cannot be ruled out and should be subject to future studies