Air-sea interactions and water mass transformation during a katabatic storm in the Irminger Sea

We use a global 5-km resolution model to analyse the air-sea interactions during a katabatic storm in the Irminger Sea originating from the Ammassalik valleys. Katabatic storms have not yet been resolved in global climate models, raising the question of whether and how they modify water masses in the Irminger Sea. Our results show that dense water forms along the boundary current and on the shelf during the katabatic storm due to the heat loss caused by the high wind speeds and the strong temperature contrast. The dense water contributes to the North Atlantic Deep Water and thus to the Atlantic Meridional Overturning Circulation (AMOC). The katabatic storm triggers a polar low, which in turn amplifies the near-surface wind speed in a positive feedback, in addition to acceleration from a breaking mountain wave. Resolving katabatic storms in global models is therefore important for the formation of dense water in the Irminger Sea, which is relevant to the AMOC, and for the large-scale atmospheric circulation by triggering polar low

Profiles of C- and N-trace gas production in N-saturated forest soils

International audienceThis study provides for the first time data on the stratification of NO and N2O production with soil depth under aerobic and anaerobic incubation conditions for different temperate forest sites in Germany (spruce, beech, clear-cut) and the Netherlands (Douglas fir). Results show that the NO and N2O production activity is highest in the forest floor and decreases exponentially with increasing soil depth. Under anaerobic incubation conditions NO and N2O production was in all soil layers up to 2-3 orders of magnitude higher then under aerobic incubation conditions. Furthermore, significant differences between sites could be demonstrated with respect to the magnitude or predominance of NO and N2O production. These were driven by stand properties (beech or spruce) or management (clear-cut versus control). With regard to CH4 the most striking result was the lack of CH4 uptake activity in soil samples taken from the Dutch Douglas fir site at Speulderbos, which is most likely a consequence of chronically high rates of atmospheric N deposition. In addition, we could also demonstrate that CH4 fluxes at the soil surface are obviously the result of simultaneously occurring uptake and production processes, since even under aerobic conditions a net production of CH4 in forest floor samples was found. The provided dataset will be very useful for the development and testing of process oriented models, since for the first time activity data stratified for several soil layers for N2O, NO, and CH4 production/oxidation activity for forest soils are provided

Response of a tropical cyclone to a subsurface ocean eddy and the role of boundary layer dynamics

We analyse a tropical cyclone simulated for a realistic ocean-eddy field using the global, nonhydrostatic, fully coupled atmosphere–ocean ICOsahedral Nonhydrostatic (ICON) model. After intensifying rapidly, the tropical cyclone decays following its interaction with a cold wake and subsequently reintensifies as it encounters a subsurface, warm-core eddy. To understand the change in the azimuthal-mean structure and intensity of the tropical cyclone, we invoke a conceptual framework, which recognises the importance of both boundary-layer dynamics and air–sea interactions. Crucially, the framework recognises that the change in the mean radius of updraught at the boundary-layer top is regulated by the expanding outer tangential wind field through boundary-layer dynamics. The decrease in the average equivalent potential temperature of the boundary-layer updraught during the early decay phase is related to an increase in the mean radius of the updraught rather than air–sea interactions. However, later in the decay phase, air–sea interactions contribute to the decrease, which is accompanied by a decrease in the vertical mass flux in the eyewall updraught and, ultimately, a more pronounced spin-down of the tropical cyclone. Air–sea interactions are also important during reintensification, where the tendencies are reversed, that is, the mean radius of the boundary-layer updraught decreases along with an increase in its average equivalent potential temperature and vertical mass flux. The importance of boundary-layer dynamics to the change in the azimuthal-mean structure is underscored by the ability of a steady-state slab boundary-layer model to predict an increasing and, to a lesser extent, decreasing radius of forced ascent for periods of decay and reintensification, respectively. Finally, our simulation highlights the importance of the ocean-eddy field for tropical cyclone intensity forecasts, since the simulated warm-core eddy does not display any sea-surface temperature (SST) signal until it is encountered by the tropical cyclone. © 2021 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley Sons Ltd on behalf of the Royal Meteorological Society

Coherent Control of the Exciton Dynamics in the FMO Protein

We have achieved first steps toward coherent control of excitonic energy migration in the FMO pigment-protein complex, by combining femtosecond pulse shaping with a feedback loop using an evolutionary algorithm. The experimental conditions achieved, with a rotating sample, a cryostat, and a pulse shaper, are sufficient for closed loop optimizations.</p

Biogenic volatile organic compound and respiratory CO₂ emissions after ¹³C-labeling: online tracing of C translocation dynamics in poplar plants

Background: Globally plants are the primary sink of atmospheric CO2, but are also the major contributor of a large spectrum of atmospheric reactive hydrocarbons such as terpenes (e.g. isoprene) and other biogenic volatile organic compounds (BVOC). The prediction of plant carbon (C) uptake and atmospheric oxidation capacity are crucial to define the trajectory and consequences of global environmental changes. To achieve this, the biosynthesis of BVOC and the dynamics of C allocation and translocation in both plants and ecosystems are important. Methodology: We combined tunable diode laser absorption spectrometry (TDLAS) and proton transfer reaction mass spectrometry (PTR-MS) for studying isoprene biosynthesis and following C fluxes within grey poplar (Populus x canescens) saplings. This was achieved by feeding either 13CO2 to leaves or 13C-glucose to shoots via xylem uptake. The translocation of 13CO2 from the source to other plant parts could be traced by 13C-labeled isoprene and respiratory 13CO2 emission. Principal Finding: In intact plants, assimilated 13CO2 was rapidly translocated via the phloem to the roots within 1 hour, with an average phloem transport velocity of 20.3±2.5 cm h21. 13C label was stored in the roots and partially reallocated to the plants’ apical part one day after labeling, particularly in the absence of photosynthesis. The daily C loss as BVOC ranged between 1.6% in mature leaves and 7.0% in young leaves. Non-isoprene BVOC accounted under light conditions for half of the BVOC C loss in young leaves and one-third in mature leaves. The C loss as isoprene originated mainly (76–78%) from recently fixed CO2, to a minor extent from xylem-transported sugars (7–11%) and from photosynthetic intermediates with slower turnover rates (8–11%). Conclusion: We quantified the plants’ C loss as respiratory CO2 and BVOC emissions, allowing in tandem with metabolic analysis to deepen our understanding of ecosystem C flux