ALBERT

Description: The factors regulating phytoplankton community composition play a crucial role in structuring aquatic food webs. However, consensus is still lacking about the mechanisms underlying the observed biogeographical differences in cell size composition of phytoplankton communities. Here we use a trait-based model to disentangle these mechanisms in two contrasting regions of the Atlantic Ocean. In our model, the phytoplankton community can self-assemble based on a trade-off emerging from relationships between cell size and (1) nutrient uptake, (2) zooplankton grazing and (3) phytoplankton sinking. Grazing ‘pushes’ the community towards larger cell sizes, whereas nutrient uptake and sinking ‘pull’ the community towards smaller cell sizes. We find that the stable environmental conditions of the tropics strongly balance these forces leading to persistently small cell sizes and reduced size diversity. In contrast, the seasonality of the temperate region causes the community to regularly reorganize via shifts in species composition and to exhibit, on average, bigger cell sizes and higher size diversity than in the tropics. Our results raise the importance of environmental variability as a key structuring mechanism of plankton communities in the ocean and call for a reassessment of the current understanding of phytoplankton diversity patterns across latitudinal gradients.

Description: The surface temperature at high latitudes — especially in the Arctic — has warmed faster than the global mean temperature since the 1990s. This process, called Polar amplification, is expected to continue this century. Concurrent with Polar amplification, upper tropospheric Tropical warming is projected in the 21st century. Low-level Polar and upper-level Tropical warming influence the equator-to-pole temperature gradient in opposite ways and hence the mid-latitude jet stream. However, the effect of modified equator-to-pole temperature gradients is not fully understood. Earlier studies argued that low-level Polar warming causes a weaker and wavier jet stream. Here, we study the influence of Polar warming on the jet stream speed, position and its waviness by increased SSTs. We performed four idealised aquaplanet simulations with the Open Integrated Forecast System. First, we compared the jet stream intensity and position of the control simulation with the warmed simulations. Second, we studied the influence on jet stream waviness quantified by a modified Sinuosity Index: we adjusted the original metric to better capture the jet on an aquaplanet. Despite strong Polar and uniform warming, the mean of the Sinuosity Index distributions of the warmed simulations changed negligibly. However, the most extreme waviness events decreased with Polar warming and increased with uniform warming. Our results contradict findings in other studies on the consequences of low-level Polar warming and its influence on jet stream waviness. We conclude that a weaker jet stream does not have to become wavier with reduced temperature gradient and discuss the implications.

Description: In February 2022, a cluster of severe extratropical cyclones hit North-Western Europe within one week and caused widespread damage by wind gusts and accumulated precipitation. In that week, extratropical cyclones Dudley, Eunice and Franklin developed over the North-Atlantic within a baroclinic environment with strong jet streams accompanied with atmospheric rivers. We hypothesise that diabatic heating through latent heat release within the extratropical cyclones created a baroclinic environment favourable for secondary cyclogenesis. We presume that latent heating on the trailing cold-front of the primary cyclone was essential for the secondary cyclogenesis. To identify the influence of latent heat release on the cyclogenesis of these storms, we performed idealised simulations with the Open Integrated Forecast System (OpenIFS). Latent heat of vaporisation was doubled, switched off and reduced with a factor of hundred. The control run captured the individual cyclones, their intensities and path reasonably well; OpenIFS is suitable to fulfil our objectives. First results showed that reduced latent heating negatively impacted the cyclogenesis of secondary cyclone Eunice — the severest extratropical cyclone during that week. Overall, the baroclinicity was strongly weakened over the North-Atlantic. Furthermore, we plan experiments that examine the importance of latent heat release along the cold-front of the primary cyclone by only reducing latent heat in a box along the trailing cold-front. With this case-study we explore diabatic heating as a pathway for cyclone clustering.

Description: Benthic oxygen dynamics and the exchange of oxygen and other solutes across the sedimentwater interface play a key role for the oxygen budget of many limnic and shallow marine systems. The sediment-water fluxes are largely determined by two factors: sediment biogeochemistry and the thickness of the diffusive boundary layer that is determined by near-bottom turbulence. Here, we present a fully coupled benthic-pelagic modeling system that takes these processes and their interaction into account, focusing especially on the modulation of the sediment-water fluxes by the effects of near-bottom turbulence and stratification. We discuss the special numerical methods required to guarantee positivity and mass conservation across the sediment-water interface in the presence of rapid element transformation, and apply this modeling system to a number of idealized scenarios. Our process-oriented simulations show that near-bottom turbulence provides a crucial control on the sediment-water fluxes, the oxygen penetration depth, and the re-oxidation of reduced compounds diffusing upward from the deeper benthic layers especially on time scales of a few days, characterizing oceanic tides,internal seiching motions in lakes, and mesoscale atmospheric variability. Our results also show that the response of benthic-pelagic fluxes to rapid changes in the forcing conditions (e.g., storm events) can only be understood with a fully coupled modeling approach.