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Abstract During glacial terminations, massive iceberg discharges and meltwater pulses in the North Atlantic triggered a shutdown of the Atlantic Meridional Overturning Circulation (AMOC). Speleothem calcium carbonate oxygen isotope records (δ 18 O Cc ) indicate that the collapse of the AMOC caused dramatic changes in the distribution and variability of the East Asian and Indian monsoon rainfall. However, the mechanisms linking changes in the intensity of the AMOC and Asian monsoon δ 18 O Cc are not fully understood. Part of the challenge arises from the fact that speleothem δ 18 O Cc depends on not only the δ 18 O of precipitation but also temperature and kinetic isotope effects. Here we quantitatively deconvolve these parameters affecting δ 18 O Cc by applying three geochemical techniques in speleothems covering the penultimate glacial termination. Our data suggest that the weakening of the AMOC during meltwater pulse 2A caused substantial cooling in East Asia and a shortening of the summer monsoon season, whereas the collapse of the AMOC during meltwater pulse 2B (133,000 years ago) also caused a dramatic decrease in the intensity of the Indian summer monsoon. These results reveal that the different modes of the AMOC produced distinct impacts on the monsoon system. 

Madagascar and the Mascarene Islands of Mauritius and Rodrigues underwent catastrophic ecological and landscape transformations, which virtually eliminated their entire endemic vertebrate megafauna during the past millennium. These ecosystem changes have been alternately attributed to either human activities, climate change, or both, but parsing their relative importance, particularly in the case of Madagascar, has proven difficult. Here, we present a multimillennial (approximately the past 8000 years) reconstruction of the southwest Indian Ocean hydroclimate variability using speleothems from the island of Rodrigues, located ∼1600 km east of Madagascar. The record shows a recurring pattern of hydroclimate variability characterized by submillennial-scale drying trends, which were punctuated by decadal-to-multidecadal megadroughts, including during the late Holocene. Our data imply that the megafauna of the Mascarenes and Madagascar were resilient, enduring repeated past episodes of severe climate stress, but collapsed when a major increase in human activity occurred in the context of a prominent drying trend. 

We report limb measurements of the oxygen dayglow emission at 297.2 nm performed during four Martian dust storms. The emission peak provides a good remote sensing tool to probe changes of the altitude of the 39 mPa pressure level for the first time during dust storms. We illustrate the time variation of these changes and compare them with the infrared opacity in the lower atmosphere. We find that the 39 mPa level rises in response to the increase in dust opacity. It reaches a plateau, and additional dust load does not significantly increase its altitude. Numerical simulations with the LMD global circulation model shows a similar response, except for the event observed during MY33 regional storm when the model fails to reproduce the observed variations. Observations collected during the onset of the global dust storm in June 2018 show that the upper atmosphere rapidly responds within two Martian days to the increased amount of tropospheric dust.

 

We propose a mechanism for the formation of the horse‐collar auroral configuration during periods of strongly northward interplanetary magnetic field (IMF), invoking the action of dual‐lobe reconnection (DLR). Auroral observations are provided by the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite and spacecraft of the Defense Meteorological Satellite Program (DMSP). We also use ionospheric flow measurements from DMSP and polar maps of field‐aligned currents (FACs) derived from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Sunward convection is observed within the dark polar cap, with antisunward flows within the horse‐collar auroral region, together with the NBZ FAC distribution expected to be associated with DLR. We suggest that newly closed flux is transported antisunward and to dawn and dusk within the reverse lobe cell convection pattern associated with DLR, causing the polar cap to acquire a teardrop shape and weak auroras to form at high latitudes. Horse‐collar auroras are a common feature of the quiet magnetosphere, and this model provides a first understanding of their formation, resolving several outstanding questions regarding the nature of DLR and the magnetospheric structure and dynamics during northward IMF. The model can also provide insights into the trapping of solar wind plasma by the magnetosphere and the formation of a low‐latitude boundary layer and cold, dense plasma sheet. We speculate that prolonged DLR could lead to a fully closed magnetosphere, with the formation of horse‐collar auroras being an intermediate step.