ALBERT

Description: The Pan-African belts of Malawi contain a largely unexplored endowment of gem bearing pegmatites. We present U–Pb in zircon (LA-ICPMS) and Rb–Sr mineral isochron geochronological and isotope data from pegmatites across Malawi. The pegmatites contain tourmaline, beryl, aquamarine, zircon, amethyst and sunstone as gemstone species. Two zircon bearing pegmatites in southern Malawi intruded early in the Pan-African orogenic cycle at 719 ± 5 Ma and 729 ± 4 Ma and are associated with the emplacement of alkaline rocks that formed during an intra-continental rifting episode in the eastern part of former Rodinia. One further zircon pegmatite containing inherited zircon of a similar age (746 ± 44 Ma) was emplaced at 598 ± 15 Ma, after the assembly of Western and Eastern Gondwana and the formation of the East African Orogen (EAO). The majority of the analysed pegmatites, however, are significantly younger. The ∼550 Ma pegmatites were emplaced during the Kuunga Orogeny, correlating with the collision of northern and southern Gondwana cratonic entities. During a prolonged post-collisional period, possibly related to crustal collapse and extension, further gem-mineralised pegmatites formed at ∼520 ± 6 Ma and ∼500–485 Ma. The youngest pegmatite intruded in the southern Malawian Ntcheu area in the Middle Ordovician at ∼460 Ma. A large spread in 87Sr/86Sr initial isotopic ratios between 0.70556 and 0.79018 suggests a variety of magma sources for the Kuunga-related pegmatites with a variably strong crustal affinity.

Description: One proposed climate intervention scheme would produce a sulfate aerosol layer in the stratosphere, which would not only reduce total insolation at the surface, but also increase the diffuse fraction of sunlight. An increase in diffuse radiation leads to the diffuse radiation fertilization effect, which could boost plant productivity, influence atmosphere-biosphere interactions, and modify the terrestrial carbon, energy, and water budgets. To effectively simulate stratospheric aerosol intervention and to understand its impact, it is critical to also accurately represent potential changes in diffuse radiation. However, current climate models have significant discrepancies when simulating diffuse radiation compared to observations. Here we analyze diffuse radiation from various stratospheric aerosol intervention scenarios (G3, G4, G6sulfur, and ARISE-SAI) simulated by multiple earth system models. Despite having the same radiative forcing or temperature targets, these earth system models show different changes in diffuse radiation. Under G6sulfur scenario, CESM2 and UKESM1-0-LL show similar increases in diffuse radiation, while CNRM-ESM2-1 simulates decreasing diffuse radiation. Moreover, in CNRM-ESM2-1, there is no scattering effect from injected aerosols, which is difficult to justify conceptually. We further quantify the impacts of these differences on surface climate change under the stratospheric aerosol climate intervention. Our results suggest that more work is needed to better understand this understudied consequence of stratospheric aerosol intervention.

Description: The rainfall characteristics of four Monsoon Deep Depressions (MDD) originating over the Bay of Bengal basin have been investigated in the present study using a coupled ocean-atmospheric model (COAWST) and a stand-alone atmospheric (WRF) model with a lead time of up to 72h. It is found that though the tracks of the MDDs have been reasonably simulated, the intensity was overestimated in both sets of simulations compared to India Meteorological Department best estimates. Decomposition of the contributors to the rainrate for the composite of the storms in the deep depression (DD) phase showed that the moisture sources/sinks play a more important role than the cloud sources/sinks in modulating the rainfall processes. Further analysis of the moisture sources/sinks showed that the vertical and horizontal advection are the major drivers in modulating the contribution of the moisture sources/sinks. The validation of rainfall using CMORPH datasets suggested that the coupled simulations had a higher skill in rainfall prediction. Furthermore, the composite of different components of moisture sources/sinks was found to be more realistically simulated in COAWST compared to CNTL upon validation with MERRA datasets. Analysis of the composite energetics showed that scarcity of bulk kinetic energy in the later hours of the DD phase in COAWST led to the dissipation of the storm core, which led to better prediction of rainfall. On the other hand, a re-intensification of the storm core by means of condensational heating led to an overestimation of rainfall in WRF, which finally resulted in lower skill in rainfall prediction.