Season

Summary

A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical cultures whose number of seasons varies. The Northern Hemisphere experiences most direct sunlight during May, June, and July, as the hemisphere faces the Sun. The same is true of the Southern Hemisphere in November, December, and January. It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer months, which increases the solar flux. However, due to seasonal lag,

Official source

https://en.wikipedia.org/wiki/Season

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Statistical methods for accounting and understanding ozone trends derived from observations

Eirini Boleti

Emissions of ozone precursors have been regulated in Europe since around 1990 with air quality control measures, which resulted in reductions of nitrogen oxides and volatile organic compounds concentrations. In order to understand how these measures have affected tropospheric ozone, it is important to investigate its long-term temporal evolution in different types of environments and various geographic regions. Uncertainties in ozone long-term trends are associated to variations originating from meteorological influence on ozone. Also, ozone temporal evolution can vary significantly among different regions and types of environment. In this PhD thesis we used sophisticated statistical tools, and developed robust statistical approaches to study long-term trends of tropospheric ozone that reflect emissions reductions. First, we focus on a meteorological adjustment of ozone observations in order to derive long-term trends with lower uncertainties compared to common practices. In addition, a classification scheme for stations in Europe is needed to interpret response of ozone in site groups with similar spatio-temporal characteristics. A detailed long-term trend analysis was performed based on decomposition of the mean ozone observations in the time domain. The different time-dependent variations of ozone were extracted, namely the long-term trend, seasonal and short-term variability. This allows subtraction of the meteorologically driven seasonal variation from the observations and estimation of long-term trends on de-seasonalized concentrations. In addition, ozone peak concentrations were investigated using a localized regression approach, which corrects for meteorological influence. The meteorological adjustment of the mean and peak ozone allows estimation of long-term trends with lower uncertainty. A site grouping in Europe was developed using the long-term and seasonal variations of ozone. The implemented clustering approach based on the long-term variation resulted in a site type classification while a geographical classification was achieved on the seasonal variation. We observed that, despite the implementation of regulations, mean ozone has been increasing in most of the sites until mid-2000s, although, afterwards, a decline or a leveling off was observed. The point when the trend changes from increasing to decreasing depends on the site type; the closer a site locates to emission sources the later the change occurred. Also, it was concluded that with time urban and rural environments become more similar in terms of ozone concentrations. On the other hand, peak ozone has been reducing in most stations, while in sites close to emissions it increased until mid-2000s when it started to level off. The influence of air pollutants hemispheric transport is depicted in remote sites, where ozone increased until beginning of 2000s and decreased afterwards. A two-dimensional classification scheme, reflecting site type and region, has showed that mainly the site type influences ozone long-term trends, while the location is more important for temporal changes in ozone inter-annual cycle and relationship to temperature.

EPFL2020

Extreme summer drought events are predicted to increase in intensity and frequency within the current century. The resulting water scarcity could dramatically impact human activities related to agriculture, due to the strong relationship between precipitations and plant biomass production. However, little is known about the interactions between timing of drought and vegetation dynamics along the growing season. In Switzerland, were grasslands are at the core of the agriculture, droughts could have severe economical consequences. Thus, farmers should be provided with insights on how to adapt and optimize their management to the changing climate. In order to further our understanding on the response of grasslands to different timing of drought and its interaction with other factors such as management intensity, productivity or soil characteristics, we developed the GRASSALT project -which was the context of this PhD- as a joint venture between AGROSCOPE, EPFL and WSL, with the aim to investigate this challenge through a multidisciplinary perspective at the crossroad between applied and fundamental research. The setup of a realistic drought experiment in three semi-natural grasslands in the Swiss Jura Mountains (even though this thesis is focusing mainly on two) where 70% of the precipitations were removed during two months at two different (during the peak of vegetation growing season vs after this peak later in the season) and two management intensities were applied, enabled us to investigate how grassland communities responded to drought at different timings and sites and investigate its interacted with management. To perform this investigation we adopted a combined agronomic, functional and ecophysiological approach that leads us to analyse the effect of drought from the plant community level to the physiological level and to the analysis of the aboveground-belowground interaction. The main findings of this thesis were the following: (i) a drought event occurring later in the growing season leads to stronger reduction in aboveground productivity than a drought event occurring during the peak of growing season. (ii) The decrease in aboveground productivity is mediated by an intraspecific variability in the functional traits of the most abundant plant species. (iii) the effect of drought is more deleterious under higher mowing intensity. (iv)The effect of drought on plant aboveground and belowground productivity is dependent on soil types and modulated through the effects on microbial communities in response to climate. Altogether the presented work has shown that timing of drought plays a big role on the impact of drought on the productivity of grasslands per se or by interacting with otherfactorsand that this requires further in-depth studies to offer efficient management strategies to farmers.

EPFL2019

Mixing processes and their ecological implications: From vertical to lateral variability in stratified lakes

Oscar Rodrigo Sepúlveda Steiner

The physical environment of natural waters influences biogeochemical processes to generate specific ecological niches, promoting biophysical interactions. Bacteria and phytoplankton communities can form spatial structures, such as layers and patches. The physical characteristics of these structures in lakes, particularly their vertical and horizontal variability are the focus of this PhD thesis. Using temperature microstructure measurements, we aim to characterize turbulent mixing within biological formations and their surroundings in lakes. We start in Lake Cadagno, where unusual bio-convectively-driven mixing takes place. Then we move to Lake Zurich where a thin layer of cyanobacteria persists throughout the stratified season. Finally, we study the thermocline of Lake Geneva, a large and more energetic system, where basin-scale processes are expected to induce lateral variability of algae. While the first two interactions concern vertical structures, we use an autonomous underwater glider, equipped with a turbulence package, to explore lateral variability in this large lake. Bioconvection observed in Lake Cadagno offered a unique environment to analyze the role of microorganism in shaping the water column they inhabit. Within the stratified water column, a highly concentrated layer of heavy, motile and photoautotrophic sulfur bacteria Chromatium okenii migrates upward to form a subsurface convective mixed layer. Field measurements revealed that the mixed layer persists throughout the diel cycle, maintaining a virtually unchanged structure. Direct estimates of turbulent diffusion indicate that without active convective turbulence, the mixed layer would be smoothed in â2.5 hours. As this time-scale is much shorter than a night and in principle C. okenii need light, the nighttime mixed layer is not expected. Using intensive and high-resolution measurements throughout two diel cycles, we provide proof that bioconvection occurs also at night and is responsible for the mixed layer persistence. The second interaction was the thin layer of cyanobacteria Planktothrix rubescens forming every spring in the thermocline of Lake Zurich. In this zone, our measurements revealed only tiny overturns, resulting in negligible vertical exchange. This strong stratification inhibits mixing and provides a remarkably stable environment for the P. rubescens thin layer, explaining its persistence. Finally, in Lake Geneva, we first concentrated efforts in the validation of glider-based turbulence estimations, possibly the first of their kind in a large-lake. Although weak turbulence and strong stratification hinder the applicability of state of the art procedures, we demonstrate that our measurements capture the expected variance and spectral shape. We explore the data from repeated transects to assess lateral variability of chlorophyll-a patches. This thesis documents in an exemplary way how vertical turbulent processes interact with bacteria and phytoplankton layers in lakes. Regarding lateral variability, the results presented are a first step for future in-situ studies of phytoplankton patches affected by turbulence and transport processes.

EPFL2020

Mixing processes and their ecological implications: From vertical to lateral variability in stratified lakes

Oscar Rodrigo Sepúlveda Steiner

EPFL2020

Statistical methods for accounting and understanding ozone trends derived from observations

Eirini Boleti

EPFL2020

EPFL2019