The Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition took place between October 2019 and September 2020 giving the rare opportunity to monitor sea-ice properties over a full annual cycle. Here we present 24 high-resolution orthomosaics and 14 photogrammetric digital elevation models of the sea-ice surface around the icebreaker RV Polarstern between March and September 2020. The dataset is based on >34.000 images acquired by a helicopter-borne optical camera system with survey flights covering areas between 1.8 and 96.5 km2 around the vessel. Depending on the flight pattern and altitude of the helicopter, ground resolutions of the orthomosaics range between 0.03 and 0.5 m. By combining the photogrammetric products with contemporaneously acquired airborne laser scanner reflectance measurements selected orthomosaics could be corrected for cloud shadows which facilitates their usage for sea-ice and melt pond classification algorithms. The presented dataset is a valuable data source for the interdisciplinary MOSAiC community building a temporal and spatially resolved baseline to accompany various remote sensing and in situ research projects.

Abstract. The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the troposphere. The influence of the MJO on the middle atmosphere (MA) and particularly on its temperature is of interest for both the understanding of MJO-induced teleconnections and research on the variability of the middle atmosphere. However, only few studies dealing with the influence of the MJO on MA temperature are available. We analyze statistically the connection of the MJO and the MA zonal mean temperature based on observations by the MLS satellite instrument. We consider all eight MJO phases, different seasons and the state of the quasi-biennial oscillation (QBO). We show that the MA temperature is influenced by the state of MJO in large areas of the MA and under roughly all considered atmospheric conditions. The zonal mean temperature response is characterized by a particular spatial pattern, which we link to the interhemispheric coupling (IHC) mechanism, a known dynamical feature of the MA. The strongest temperature deviations are on the order of ± 10 K and are found in the polar winter MA during boreal winter when the QBO is in the easterly phase. Other atmospheric conditions also show temperature responses with the characteristic spatial pattern, but weaker and more noisy. The QBO turns out to have a relatively big influence during boreal winter but only a small influence during austral winter. We also discuss the role of sudden stratospheric warmings (SSWs), which have an ambivalent influence on our interpretation, because they introduce strong temperature variability in the polar winter MA themselves. In addition, SSWs are one possibility to explain the QBO influence during boreal winter. Furthermore, we also analyze the change of the temperature response pattern while the MJO progresses from one phase to the next. We find a largely systematic reaction of the MA to the phase changes, particularly a gradual altitude shift of the MA temperature response pattern, which can be seen more or less clearly depending on the atmospheric conditions. Overall, a major outcome of the present study is the finding that the tropospheric MJO can trigger the IHC mechanism, which affects many areas of the MA. It is therefore a noteworthy example for the complex couplings across different atmospheric layers and geographical regions in the atmosphere. Additionally, it highlights close linkages of known dynamical features of the atmosphere, particularly the MJO, the IHC, the QBO, and SSWs. Because of the wide coverage of atmospheric regions and included dynamical features, the results might help to further constrain the underlying dynamical mechanisms and could be used as a benchmark for the representation of atmospheric couplings on the intraseasonal timescale in atmospheric models.

<p>In recent years, the existence of firn aquifers in the Antarctic Peninsula (AP) has been confirmed by in situ observations. Due to their importance for understanding the hydrology of the Antarctic ice sheet, a more spatially comprehensive assessment of AP firn aquifers is desirable. The purpose of this study is to map firn aquifers in the AP from space using C-band Synthetic Aperture Radar imagery from ESA's Sentinel-1 mission. This product enables the detection of firn aquifers at 1 km<sup>2</sup> resolution for the period 2017 to 2020. The method is based on quantifying the characteristic shape of the backscatter curve over time during the (partial) refreezing of the liquid water in the firn layer after each peak melt season. In this context, both seasonal aquifers and perennial aquifers are detected together, acknowledging that their backscatter signature in any given year is indistinguishable with the given method. With the new method, seasonal firn aquifers are being detected in the north and northwest of the AP, as well as on the Wilkins Ice Shelf and the George VI Ice Shelf. Imposing the aquifers to occur during all available years, as a proxy for perennial firn aquifers, limits their extent to the north and northwest AP. Both distributions agree well with model simulations. Further in situ and modelling studies and longer time series of satellite observations are needed to validate the results of this study.</p>

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