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Abstract Distributed acoustic sensing (DAS) is an emerging oceanographic technique in which an interrogator continuously records nanoscale strain of a fiber-optic cable, such as a telecommunication cable, with meter-scale measurement spacing over tens of kilometers. Empirical methods have recently been established for calculating pressure spectra to measure ocean surface gravity wave statistics from DAS strain. Here, we compile data from six submarine DAS experiments to provide a comparison between studies and establish recommendations for using DAS to measure ocean waves. Data were collected from Alaska, Hawaii, Massachusetts, North Carolina, and Oregon, United States, with different interrogators on different cable types in 0–60 m of water with 0–4 m of burial. Ground-truth measurements of ocean waves were provided by standard near-bed or sea surface instruments. The raw strain recorded in each experiment varied over four orders of magnitude, which could not be explained by water depth, wave conditions, or interrogator settings and suggests that cable characteristics and burial depth are important factors controlling strain magnitude and measurement quality. Strain spectra were converted to near-bed pressure spectra using a frequency-dependent, location-specific empirical correction factor, and DAS-derived pressure spectra were used to calculate wave statistics. The correction factors varied over 10 orders of magnitude between sites yet provided accurate calculations of wave height and period (root-mean-square error of 0.2–0.6 m forH_sand 0.2–1.6 s forT_eandT_p). The volume of data necessary for calibration is discussed. This meta-analysis highlights future oceanographic applications of DAS. Significance StatementDistributed acoustic sensing (DAS) is an emerging technology for measuring ocean waves on seafloor fiber-optic cables, such as telecom cables. The advantage of DAS is that it can record thousands of measurements per second at meter-scale spacing over tens of kilometers. We compare six datasets to characterize DAS-derived strain for measuring ocean waves. The differences in strain magnitude observed between datasets were not explained by water depth, wave height or period, or instrument settings. It is likely that cable composition and depth of burial control the magnitude of the recorded strain. Despite these differences, each dataset was empirically calibrated to produce accurate measurements of wave statistics. DAS is a promising new oceanographic technology, and new applications should be explored. 

Abstract B.F.J. Schonland, advised and encouraged by C.T.R. Wilson, made two unsuccessful searches for runaway electrons from thunderstorms in the 1930s. These findings stand in marked contrast with research results over the last decade and ironically set this field of research back many decades. Schonland's lack of success is traced to gamma ray attenuation in the atmosphere above Johannesburg (1,780 m MSL) and to his restriction to nine thunderstorms. 

Abstract In lightning research, there is a growing interest in measuring the extremely low frequency (ELF, 3 Hz–3 kHz) electromagnetic (EM) radiation of lightning, as this frequency band can be used to infer various characteristics of lightning discharges that are currently not available from state‐of‐the‐art lightning detection networks. One of these characteristics is the presence of a continuing current (CC), which can last for hundreds of milliseconds and therefore poses an increased risk of physical lightning damage. In this paper, we investigate the modeling capability of the global EM resonance field excited by lightning with a CC using a modified version of a well‐known analytical model describing Schumann resonances (SRs) and a full FDTD model. Since analytical models are much faster and require significantly less memory than full numerical models, they are widely used to interpret ELF data. On the other hand, the flexibility of a full numerical model allows the simulation of model configurations that cannot be described by analytical models. To use the two models confidently, it is important to check their consistency for similar configurations. Here, we demonstrate that, for a uniform Earth‐ionosphere cavity, the theoretical ELF spectra provided by the analytical and full numerical models show good agreement ∼7(±5) % for both the impulse‐like (describing SRs) and exponentially decaying (describing the presence of a CC) current sources. Our results confirm that the analytical model is well suited to interpret ELF measurements for the purpose of studying global lightning activity or individual lightning discharges. 

The Atlantic meridional overturning circulation (AMOC) is a large-scale circulation pattern responsible for northward heat transport in the Atlantic and is associated with climate variations on a wide range of time scales. Observing the time-varying AMOC has fundamentally changed our understanding of the large-scale ocean circulation and its interaction with the climate system, as well as identified shortcomings in numerical simulations. With a wide range of gains already achieved, some now ask whether AMOC observations should continue. A measured approach is required for a future observing system that addresses identified gaps in understanding, accounts for shortcomings in observing methods and maximizes the potential to guide improvements in ocean and climate models. Here, we outline a perspective on future AMOC observing and steps that the community should consider to move forward. This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’. 

In recent years, many researchers have proposed new models for synaptic plasticity in the brain based on principles of machine learning. The central motivation has been the development of learning algorithms that are able to learn difficult tasks while qualifying as "biologically plausible". However, the concept of a biologically plausible learning algorithm is only heuristically defined as an algorithm that is potentially implementable by biological neural networks. Further, claims that neural circuits could implement any given algorithm typically rest on an amorphous concept of "locality" (both in space and time). As a result, it is unclear what many proposed local learning algorithms actually predict biologically, and which of these are consequently good candidates for experimental investigation. Here, we address this lack of clarity by proposing formal and operational definitions of locality. Specifically, we define different classes of locality, each of which makes clear what quantities cannot be included in a learning rule if an algorithm is to qualify as local with respect to a given (biological) constraint. We subsequently use this framework to distill testable predictions from various classes of biologically plausible synaptic plasticity models that are robust to arbitrary choices about neural network architecture. Therefore, our framework can be used to guide claims of biological plausibility and to identify potential means of experimentally falsifying a proposed learning algorithm for the brain. 

Abstract Although typically used to measure dynamic strain from seismic and acoustic waves, Rayleigh‐based distributed acoustic sensing (DAS) is also sensitive to temperature, offering longer range and higher sensitivity to small temperature perturbations than conventional Raman‐based distributed temperature sensing. Here, we demonstrate that ocean‐bottom DAS can be employed to study internal wave and tide dynamics in the bottom boundary layer, a region of enhanced ocean mixing but scarce observations. First, we show temperature transients up to about 4 K from a power cable in the Strait of Gibraltar south of Spain, associated with passing trains of internal solitary waves in water depth <200 m. Second, we show the propagation of thermal fronts associated with the nonlinear internal tide on the near‐critical slope of the island of Gran Canaria, off the coast of West Africa, with perturbations up to about 2 K at 1‐km depth and 0.2 K at 2.5‐km depth. With spatial averaging, we also recover a signal proportional to the barotropic tidal pressure, including the lunar fortnightly variation. In addition to applications in observational physical oceanography, our results suggest that contemporary chirped‐pulse DAS possesses sufficient long‐period sensitivity for seafloor geodesy and tsunami monitoring if ocean temperature variations can be separated. 

Abstract The electromagnetic waves in the Schumann resonance (SR) frequency range (<100 Hz) radiated by natural “lightning antennas” excite the Earth‐ionosphere cavity confined between the Earth's surface and the lower ionosphere. The peak frequencies of SR are known to vary with source‐observer distance (SOD), while the daily frequency range (DFR:f_max − f_min) is also indicative of the average size of thunderstorm regions. This paper provides observational evidence for these relationships based on SR frequency observations of the vertical electric (E_Z) field component at Nagycenk (NCK), Hungary in Central Europe from the period 1994–2015. Variations of the peak frequencies are considered on the annual, seasonal and diurnal time scales as well as during a specific event when squall‐line formation of lightning activity in South America moves toward NCK. DFR is studied in relation to the El Niño Southern Oscillation (ENSO). Increasing area of lightning activity in mid‐high Northern hemisphere latitudes has been identified by DFR variations during the transition from warm to cold episodes of the ENSO in 1998 and 2010. The extension of the lightning area is considered as a consequence of energy released in the tropics and exported to higher latitudes with some months of delay from the end of the El Niño episodes. The frequency variations are interpreted via model calculations and supported with satellite‐based optical lightning observations (Optical Transient Detector, Geostationary Lightning Mapper). The described variations of SR peak frequencies and DFR yield information on the global/regional lightning dynamics and on this basis they have important application to climate issues as well.