We present subannual observations (2009–2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered ...a few decades ago by increased ocean‐induced thinning, which may have initiated marine ice sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by < 4%, with flow recovering as the ocean warmed to prior temperatures. During this cold‐ocean period, the evolving glacier‐bed/ice shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin‐wide thinning signal did not change. Thus, as predicted by theory, once marine ice sheet instability is underway, a single transient high‐amplitude ocean cooling has only a relatively minor effect on ice flow. The long‐term effects of ocean temperature variability on ice flow, however, are not yet known.
Key Points
Pine Island Glacier speed is correlated with ocean temperature
Grounded ice speed slowed by only ~1% despite ~60% drop in ocean heat content
Ice speed recovered after the cold‐ocean anomaly ended
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 30
cm/s with peak time-averaged velocities of 40–60
cm/s depending on wave and tidal conditions. Drifters that episodically exited the surf zone were transported approximately 2 surf zone widths offshore at ∼
20
cm/s.
The echo statistics of a randomly rough, randomly oriented prolate spheroid that is randomly located in the beampattern are investigated from physics-based principles both analytically and ...numerically. This is a direct-path geometry in which reflections from neighboring boundaries are not a factor. The statistics and, in particular, the tails of the probability density function (pdf) and probability of false alarm (PFA) are shown to be strongly non-Rayleigh and a strong function of shape of scatterer. The tails are shown to increase above that associated with a Rayleigh distribution with increasing degree of elongation (aspect ratio) of the scatterer and when roughness effects are introduced. And, as also shown in previous studies, the effects associated with the scatterer being randomly located in the beam contribute to the non-Rayleigh nature of the echo.
Surf zone eddies coupled with rip current morphology MacMahan, Jamie H.; Reniers, Ad J. H. M.; Thornton, Ed B. ...
Journal of Geophysical Research - Oceans,
July 2004, Letnik:
109, Številka:
C7
Journal Article
Recenzirano
Odprti dostop
Energetic very low frequency (VLF; frequencies <0.004 Hz) surf zone eddies (SZEs) were observed on a beach composed of shore‐connected shoals with quasi‐periodic (∼125 m) incised rip channels at Sand ...City, Monterey Bay, California. Incident waves consisted of predominantly shore‐normal narrow‐banded swell waves. SZEs were located outside the gravity region in alongshore wave number, ky, spaced within the VLF band, and did not appear to exist in higher‐frequency bands. The SZEs were significant (Urms,VLF ∼ 0.25 m/s) and constant in intensity within the surf zone (shore‐connected shoals and rip channels) and rapidly decreased offshore. The alongshore and cross‐shore SZE velocity variances were similar in magnitude. VLF SZE velocities were not forced by VLF surface elevations and were not well correlated with rip current flows (r2 = 0.18). There is an indication that the SZEs were related to wave forcing, with the SZEs statistically correlated with incoming sea‐swell wave height (r2 = 0.49). F‐ky spectral estimates illustrate a strong relationship between rip channel spacing and SZE cross‐shore velocities (ky = ±0.008 m−1) and minimal SZE alongshore velocity variation (ky = 0 m−1). Data analysis suggests that the SZEs are not simply instabilities of an unstable rip current jet. A simple conceptual model suggests that SZE f‐ky spectra can be explained by the entire rip current circulation cells oscillating predominantly in the cross shore and slightly in the alongshore.
Infragravity rip current pulsations MacMahan, Jamie H.; Reniers, Ad J. H. M.; Thornton, Edward B. ...
Journal of Geophysical Research - Oceans,
January 2004, Letnik:
109, Številka:
C1
Journal Article
Recenzirano
Odprti dostop
The origins of rip current pulsations within the infragravity frequency band (0.004–0.04 Hz) are determined from measurements made with arrays of colocated pressure and velocity sensors deployed on a ...beach with persistent rip current channels. The observations indicate significant energy in cross‐shore infragravity velocities that varies spatially in the alongshore due to the presence of rip channels. Infragravity velocities are smaller within the rip channel than on the shore‐connected shoal owing to differences in water depth. Rip current pulsations at infragravity frequencies are linked to standing infragravity motions but not to the ponding and subsequent release of water by wave group pumping.
Rip current kinematics and beach morphodynamics were measured for 44 days at Sand City, Monterey Bay, CA using 15 instruments composed of co-located velocity and pressure sensors, acoustic Doppler ...current profilers, and kinematic GPS surveys. The morphology consisted of a low-tide terrace with incised quasi-periodic rip channels, representative of transverse bars. Offshore (17 m depth) significant wave height and peak period ranged 0.20–3.0 m and 5–20 s. The mean wave direction was consistently near 0° resulting in rip channel morphology, which evolved in response to the changing wave characteristics. An inverse relationship between sediment accreting on the transverse bar and eroding in the rip channel was found. The spatial distribution of sediment is reflected in the background rip current flow field. The mean velocity magnitudes within the rip channel (transverse bars) increased offshore (onshore) with decreasing tidal elevations and increased with increasing sea-swell energy. Eulerian averaged flows were predominantly shoreward on the transverse bars and seaward within the rip channel throughout the experiment, resulting in a persistent cellular circulation, except during low wave energy. The rip current spacing to the rip channel width was less than or equal to two, which suggests that the rip currents are influenced by each other and that no two-dimensional bar return flow should be present. The vertical velocity profile on the bar indicated that the flow was predominantly shoreward. The flow field within the surf zone was depth uniform, except for significant shear occurring near the surface, owing to Stokes drift. The wave-induced transport hypothesis is evaluated.
An
in situ iron-enrichment experiment near the Galapagos Islands was performed in October 1993. Here we report the theoretical and practical considerations of creating such a patch of iron-enriched ...surface water, as well as the strategies employed for the detection of the patch and the biological and chemical signals which developed, in an area dominated by advective processes. Physical and chemical models were used to predict the speciation, solubility, and the final concentration of iron in surface waters injected with acidic iron sulfate. A trial injection off the California coast in which 800
L of a 0.5
M FeSO
4 were introduced into the ship’s wake over a 1.5
km
2 area, was used to test these predictions. Iron concentrations were determined continually onboard during the initial experiment as the ship steamed in transects through the enriched patch. The results indicate excellent spatial agreement with model predictions and final concentrations that were consistent with the chemical model. However, the use of a Cartesian coordinate system during injection resulted in an extremely compressed, heterogeneous patch. Results from this preliminary experiment were then applied towards the development and implementation of the first open ocean iron enrichment experiment (IronEx I) near the Galapagos Islands in October 1993. The development and results of these methodologies are presented.
In the IronEx I equatorial experiment, a Lagrangian coordinate system was established using a drogued buoy (equipped with GPS and packet radio) and the iron-enriched area (64
km
2 containing 443
kg of Fe) was tagged with the inert chemical tracer sulfurhexafluoride (SF
6). This strategy resulted in a fairly rectangular, homogeneous enriched patch initially detectable by both Fe and SF
6 determination. Shipboard analysis and airborne observations confirmed good spatial agreement between the Lagrangian drifter and the biological and chemical signatures in the patch.
Biological and chemical sampling of the enriched area showed an increase in chlorophyll, primary production, biomass and photosynthetic energy conversion efficiency relative to waters outside the patch, supporting the hypothesis that iron limits phytoplankton growth and biomass in a ‘bottom up’ manner in this area. The ability to create a coherent patch and track it over time led to this first open-ocean test of the iron hypothesis.
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.
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.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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.
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