The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC, 2019--2020), a year-long drift with the Arctic sea ice, has provided the scientific community with an unprecedented, multidisciplinary dataset from the Eurasian Arctic Ocean, covering high atmosphere to deep ocean across all seasons. However, the heterogeneity of data and the superposition of spatial and temporal variability, intrinsic to a drift campaign, complicate the interpretation of observations. In this study, we have compiled a quality-controlled physical hydrographic dataset with best spatio-temporal coverage and derived core parameters, including the mixed layer depth, heat fluxes over key layers, and friction velocity. We provide a comprehensive and accessible overview of the ocean conditions encountered along the MOSAiC drift, discuss their interdisciplinary implications, and compare common ocean climatologies to these new data. Our results indicate that, for the most part, ocean variability was dominated by regional rather than seasonal signals, carrying potentially strong implications for ocean biogeochemistry, ecology, sea ice, and even atmospheric conditions. Near-surface ocean properties were strongly influenced by the relative position of sampling, within or outside the river-water influenced Transpolar Drift, and seasonal warming and meltwater input. Ventilation down to the Atlantic Water layer in the Nansen Basin allowed for a stronger connectivity between subsurface heat and the sea ice and surface ocean via elevated upward heat fluxes. The Yermak Plateau and Fram Strait regions were characterized by heterogeneous water mass distributions, energetic ocean currents, and stronger lateral gradients in surface water properties in frontal regions. Together with the presented results and core parameters, we offer context for interdisciplinary research, fostering an improved understanding of the complex, coupled Arctic System.

Central Arctic properties and processes are important to the regional and global coupled climate system. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) Distributed Network (DN) of autonomous ice-tethered systems aimed to bridge gaps in our understanding of temporal and spatial scales, in particular with respect to the resolution of Earth system models. By characterizing variability around local measurements made at a Central Observatory, the DN covers both the coupled system interactions involving the ocean-ice-atmosphere interfaces as well as three-dimensional processes in the ocean, sea ice, and atmosphere. The more than 200 autonomous instruments (“buoys”) were of varying complexity and set up at different sites mostly within 50 km of the Central Observatory. During an exemplary midwinter month, the DN observations captured the spatial variability of atmospheric processes on sub-monthly time scales, but less so for monthly means. They show significant variability in snow depth and ice thickness, and provide a temporally and spatially resolved characterization of ice motion and deformation, showing coherency at the DN scale but less at smaller spatial scales. Ocean data show the background gradient across the DN as well as spatially dependent time variability due to local mixed layer sub-mesoscale and mesoscale processes, influenced by a variable ice cover. The second case (May–June 2020) illustrates the utility of the DN during the absence of manually obtained data by providing continuity of physical and biological observations during this key transitional period. We show examples of synergies between the extensive MOSAiC remote sensing observations and numerical modeling, such as estimating the skill of ice drift forecasts and evaluating coupled system modeling. The MOSAiC DN has been proven to enable analysis of local to mesoscale processes in the coupled atmosphere-ice-ocean system and has the potential to improve model parameterizations of important, unresolved processes in the future.

26 The Multidisciplinary drifting Observatory for the Study of the Arctic Climate 27 (MOSAiC, 2019/2020), a year-long drift with the Arctic sea ice, has provided 28 the scientific community with an unprecedented, multidisciplinary dataset from 29 the Eurasian Arctic Ocean, covering high atmosphere to deep ocean across all 30

In September 2019, the research icebreaker Polarstern started the largest multidisciplinary Arctic expedition to date, the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) drift experiment. Being moored to an ice floe for a whole year, thus including the winter season, the declared goal of the expedition is to better understand and quantify relevant processes within the atmosphere–ice–ocean system that impact the sea ice mass and energy budget, ultimately leading to much improved climate models. Satellite observations, atmospheric reanalysis data, and readings from a nearby meteorological station indicate that the interplay of high ice export in late winter and exceptionally high air temperatures resulted in the longest ice-free summer period since reliable instrumental records began. We show, using a Lagrangian tracking tool and a thermodynamic sea ice model, that the MOSAiC floe carrying the Central Observatory (CO) formed in a polynya event north of the New Siberian Islands at the beginning of December 2018. The results further indicate that sea ice in the vicinity of the CO (<40 km distance) was younger and 36 % thinner than the surrounding ice with potential consequences for ice dynamics and momentum and heat transfer between ocean and atmosphere. Sea ice surveys carried out on various reference floes in autumn 2019 verify this gradient in ice thickness, and sediments discovered in ice cores (so-called dirty sea ice) around the CO confirm contact with shallow waters in an early phase of growth, consistent with the tracking analysis. Since less and less ice from the Siberian shelves survives its first summer (Krumpen et al., 2019), the MOSAiC experiment provides the unique opportunity to study the role of sea ice as a transport medium for gases, macronutrients, iron, organic matter, sediments and pollutants from shelf areas to the central Arctic Ocean and beyond. Compared to data for the past 26 years, the sea ice encountered at the end of September 2019 can already be classified as exceptionally thin, and further predicted changes towards a seasonally ice-free ocean will likely cut off the long-range transport of ice-rafted materials by the Transpolar Drift in the future. A reduced long-range transport of sea ice would have strong implications for the redistribution of biogeochemical matter in the central Arctic Ocean, with consequences for the balance of climate-relevant trace gases, primary production and biodiversity in the Arctic Ocean.

Rip currents are fascinating, natural, surf zone phenomena that occur daily on many beaches throughout the world. My colleagues, students, advisors, and I have been studying rip currents for more than 10 years and have performed more than 10 comprehensive field experiments on various beaches throughout the world using different observational techniques and model simulations to improve our understanding and prediction of rip currents. We have written a series of scientific articles describing the intricacies and complexities of rip current behavior using statistical and mathematical equations. These manuscripts are typically published in professional journals, which often do not communicate our results to those who would benefit from the information-the beachgoing public and ocean swimmers. Herein, we summarize our findings to help people of all ages gain a better understanding of currents at the coast.

The accepted view of rip currents is that they are an efficient mechanism for transporting material out of the surf zone. Previous rip current campaigns on natural beaches have focused on Eulerian measurements with sparse in situ pressure and current meter arrays. Here, for the first time, spatially synoptic estimates of rip current flow patterns, vorticity, and Lagrangian transport behavior are measured in the field using a fleet of 30 position-tracking surfzone drifters during multiple rip current occurrences on an open coast beach in Monterey, CA. Contrary to the classic view (Shepard et al., 1941), the rip current flow field consisted of semi-enclosed, large-scale vortices that retained the drifters and resulted in a high number of Lagrangian observations that are temporally and spatially repeated. Approximately 19% of the drifters deployed in the rip currents exited the surf zone per hour, on average during the experiments. The observed surf zone retention of drifters is consistent with measurements from different open coast beach rip current systems (14% at meso-macrotidal Truc Vert, France and 16% at macrotidal Perranporth, United Kingdom). The three-hour-average cross-shore rip current velocity at Monterey was 30cm/s with peak time-averaged velocities of 40–60cm/s depending on wave and tidal conditions. Drifters that episodically exited the surf zone were transported approximately 2 surf zone widths offshore at ∼20cm/s.

We show for the first time that low-energy waves can induce a rip current system over subtle alongshore bathymetric variations. Comprehensive field measurements across a rip current that morphologically migrated (similar to 12 m/day) through a coherent cross- and alongshore array of co-located pressure and velocity sensors were obtained. The rip current is associated with a small bathymetric surfzone non-uniformity (1 in 300 alongshore variation). The circulation was kinematically non-uniform for similar to 5% of the time over the course of the 20 day experiment and was present at low tides associated with increases in rip current activity. The presence of the rip Current and mild-sloped rip channel induce statistically significant alongshore wave direction, directional spreading, infragravity waves, and very low frequency motions. variations in H-rms, wave direction, directional spreading, infragravity waves, and very low frequency motions. Changes in the directional spreading are correlated with the presence of very low frequency motions influenced by the presence of the rip current. Published by Elsevier B.V.

Scale relations are necessary to choose proper sediment for physical models. The use of lightweight artificial sediment makes it possible to preserve the Reynolds number, Shields number and relative settling velocity altogether. There is also the possibility to preserve the Shields number in combination with the Dean number which would be more suitable in the case of significant wave breaking. Both cases have significant scale effects. A physical model test showed that the scale relations are useful under restricted conditions.