51 research outputs found
On the relationship between the Tropical Easterly Jet over West Africa and Sahel rainfall across various time scales
Der obertroposphĂ€rische tropische Oststrahlstrom ĂŒber Westafrika (Tropical Easterly Jet ĂŒber 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 ergrĂŒndet die ursĂ€chlichen Mechanismen sowie die Implikationen dieses statistischen Zusammenhangs und untersucht zudem, ob der WA-TEJ eine aktive Rolle fĂŒr die Niederschlagsentwicklung in der Sahelzone spielt, wie in frĂŒheren Studien vorgeschlagen. In dem ersten wissenschaftlichen Kapitel dieser Arbeit werden zeitlich hochauf- gelöste Beobachtungen und Reanalysen analysiert, um zu klĂ€ren, ob der WA-TEJ die Konvektion ĂŒber der Sahelzone durch seine Auswirkungen auf die obertroposphĂ€ri- sche Divergenz unterstĂŒtzt, wie in frĂŒheren Studien vorgeschlagen. Eine Analyse von 300 mesoskaligen konvektiven Systemen in der Sahel-Region zeigt, dass weder ihre Auslösung noch ihr Organisationsgrad mit signiïŹkanten WA-TEJ-Anomalien oder TEJ- induzierten Divergenzen in der oberen TroposphĂ€re verbunden sind. Auf der synop- tischen Zeitskala hinken WA-TEJ-Anomalien signiïŹkant Anomalien der konvektiven AktivitĂ€t um ein bis zwei Tage hinterher, was darauf hindeutet, dass Konvektionsan- omalien eher VerĂ€nderungen im WA-TEJ bewirken als umgekehrt. Die Ergebnisse des ersten Kapitels deuten somit darauf hin, dass der WA-TEJ keine aktive Rolle fĂŒr die SahelniederschlĂ€ge spielt, zumindest nicht ĂŒber seine Auswirkungen auf die obertro- posphĂ€rische Divergenz. WAM-Niederschlagsanomalien können die IntensitĂ€t des WA-TEJ signiïŹkant beein- ïŹussen, aber erklĂ€rt der lokale Antrieb des WA-TEJ durch die mit dem WAM-Nieder- schlag verbundene latente ErwĂ€rmung wirklich die beobachtete KovariabilitĂ€t zwi- schen WA-TEJ-IntensitĂ€t 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-VariabilitĂ€t untersucht. Dazu wird PUMA, ein atmosphĂ€risches Zirkulationsmodell basierend auf trockener Dyna- mik, durch ĂŒber die jeweilige Sommersaison gemittelte, dreidimensionalen Felder der diabatischen Heizung angetrieben ĂŒber den Zeitraum 1979-2017. Die Felder des diaba- tischen Antriebs werden in einen lokalen afrikanischen und einen entfernten âRest der Weltâ-Teil aufgeteilt. Es werden Simulationen durchgefĂŒhrt, bei denen sich die Jahr- zu-Jahr-VariabilitĂ€t der diabatische Heizung entweder auf die lokale oder abgelegene Regionen beschrĂ€nkt. Ich zeige, dass die interannuelle bis dekadische VariabilitĂ€t der WA-TEJ-IntensitĂ€t durch den EinïŹuss des entfernten diabatischen Antriebs dominiert wird, insbesondere durch die Wirkung von ENSO. Die WA-TEJ-VariabilitĂ€t, die aus- chlieĂlich auf die lokale diabatische Heizung zurĂŒckzufĂŒhren ist, ist zwar stark mit dem Sahelniederschlag korreliert aber schwach im Vergleich zur Wirkung der Fernein- ïŹĂŒsse. Letztendlich kann der entfernte diabatische Antrieb als ein wichtiger Treiber fĂŒr die beobachtete SahelniederschlagâWA-TEJ-Beziehung angesehen werden, da er nicht nur die WA-TEJ-VariabilitĂ€t dominiert, sondern auch oft Sahelniederschlagsanomalien mit gleichem Vorzeichen induziert. Ziel des dritten wissenschaftlichen Kapitels ist es, herauzuïŹnden, ob sich die Kova- riabilitĂ€t zwischen TEJ und Sahelniederschlag in einem anderen Klima grundlegend Ă€ndert. Als Fallstudie wĂ€hle ich das mittlere HolozĂ€n, eine Epoche, in der der WAM- Niederschlag viel intensiver war und seine VariabilitĂ€t weniger durch Telekonnektio- nen beeinïŹusst wurde. Im Vergleich zu einem vorindustriellen Kontrollklima simuliert das komplexe Erdsystemmodell MPI-ESM fĂŒr das mittlere HolozĂ€n eine schwĂ€chere KovariabilitĂ€t zwischen WAM-Niederschlag und WA-TEJ-IntensitĂ€t auf interannuellen bis dekadischen Zeitskalen. Um dies besser zu verstehen, werden die tropischen Trei- ber der WA-TEJ-VariabilitĂ€t im mittleren HolozĂ€n 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 gleichermaĂen die WA-TEJ-VariabilitĂ€t. Dieses unterschiedliche Verhalten im Vergleich zum gegen- wĂ€rtigen Klima scheint in erster Linie auf die abnehmende Bedeutung des entfernten diabatischen Antriebs zurĂŒckzufĂŒhren zu sein, dessen Amplitude vor allem durch die geringere interannuelle VariabilitĂ€t der MeeresberïŹĂ€chentemperaturen im tropischen PaziïŹk abgeschwĂ€cht wird. Die Ergebnisse deuten darauf hin, dass die verminderte Be- deutung des entfernten diabatischen Antriebs auch die verringerte KovariabilitĂ€t zwischen WA-TEJ und Sahelniederschlag wĂ€hrend des mittleren HolozĂ€ns erklĂ€rt.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 ïŹrst scientiïŹc chapter, temporally high-resolved observations and reanalyses are used to clarify whether the WA-TEJ fosters convection over the Sahel via its inïŹuence on the upper-level divergence, as suggested by former studies. I conduct an analysis of nearly 300 Sahelian mesoscale convective systems and ïŹnd that neither their initiation nor their degree of organization is signiïŹcantly associated with WA-TEJ anomalies or jet-induced upper-level divergence. On synoptic time scales, WA-TEJ anomalies also signiïŹcantly 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 ïŹrst chapter suggest that the WA-TEJ does not play an active role for Sahel rainfall, at least not via its eïŹect on upper-level divergence. WAM rainfall anomalies can signiïŹcantly aïŹect 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 â rainfall covariability on interrannual to decadal time scales? The second scientiïŹc 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 ïŹelds of the period of 1979-2017. The heating ïŹelds 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 inïŹuence of the remote forcing, especially by the eïŹect 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 inïŹuences. Ultimately, the variability of the remote diabatic heating â in particular over the tropical PaciïŹc â can likely be viewed as an important driver of the observed Sahel rainfallâWA-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 scientiïŹc chapter is to explore whether the TEJ â Sahel rain- fall covariability may fundamentally change in a diïŹerent 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 aïŹected 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 diïŹerence 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 PaciïŹc. I propose that the diminished importance of the remote diabatic forcing also mainly explains the reduced TEJârainfall covariability
The influence of DACCIWA radiosonde data on the quality of ECMWF analyses and forecasts over southern West Africa
During the DACCIWA (DynamicsâAerosolâChemistryâCloud Interactions in West Africa) field campaign âŒ900 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
Quantifying uncertainty in simulations of the West African monsoon with the use of surrogate models
Simulating the West African monsoon (WAM) system using numerical weather and climate models suffers from large uncertainties, which are difficult to assess due to nonlinear interactions between different components of the WAM. Here we present a fundamentally new approach to the problem by approximating the behavior of a numerical model â here the Icosahedral Nonhydrostatic (ICON) model â through a statistical surrogate model based on universal kriging, a general form of Gaussian process regression, which allows for a comprehensive global sensitivity analysis. The main steps of our analysis are as follows: (i) identify the most important uncertain model parameters and their probability density functions, for which we employ a new strategy dealing with non-uniformity in the kriging process. (ii) Define quantities of interest (QoIs) that represent general meteorological fields, such as temperature, pressure, cloud cover and precipitation, as well as the prominent WAM features, namely the tropical easterly jet, African easterly jet, Saharan heat low (SHL) and intertropical discontinuity. (iii) Apply a sampling strategy with regard to the kriging method to identify model parameter combinations which are used for numerical modeling experiments. (iv) Conduct ICON model runs for identified model parameter combinations over a nested limited-area domain from 28°âW to 34°âE and from 10°âS to 34°âN. The simulations are run for August in 4 different years (2016 to 2019) to capture the peak northward penetration of rainfall into West Africa, and QoIs are computed based on the mean response over the whole month in all years. (v) Quantify sensitivity of QoIs to uncertain model parameters in an integrated and a local analysis.
The results show that simple isolated relationships between single model parameters and WAM QoIs rarely exist. Changing individual parameters affects multiple QoIs simultaneously, reflecting the physical links between them and the complexity of the WAM system. The entrainment rate in the convection scheme and the terminal fall velocity of ice particles show the greatest effects on the QoIs. Larger values of these two parameters reduce cloud cover and precipitation and intensify the SHL. The entrainment rate primarily affects 2âm temperature and 2âm dew point temperature and causes latitudinal shifts, whereas the terminal fall velocity of ice mostly affects cloud cover. Furthermore, the parameter that controls the evaporative soil surface has a major effect on 2âm temperature, 2âm dew point temperature and cloud cover. The results highlight the usefulness of surrogate models for the analysis of model uncertainty and open up new opportunities to better constrain model parameters through a comparison of the model output with selected observations.</p
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 EuropeanâAfrican 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 â 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
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
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