Physical Oceanographers Research Articles

“Mud acoustics” and the ocean acoustics program

Shallow water acoustics is one of the major thrusts of the Office of Naval Research Ocean Acoustics Program and has been an important area of interest to Navy programs since at least the work of Pekeris, Ewing, and Worzel in the 1940’s. In the 1950’s and early 1960’s, there was a flurry of activity both in the US and UK focused on propagation within and reflection from the seabed and the acoustical characteristics marine sediments. This work formed the basis of a research program carried out by the applied ocean acoustics branch of the acoustics division at the Naval Research Laboratory in the 1970’s. In April 1991, with the Cold War coming to an end, and a shift in interest from blue water to the littorals, ONR sponsored a workshop on shallow water acoustics at the Woods Hole Oceanographic Institution. This meeting, including underwater acousticians, geologists, geophysicists, and physical oceanographers, largely set the stage for the next 30 years of shallow water acoustics research supported by ONR. Prominent efforts include the ONR Geoclutter Program and the Shallow Water 2006 experiments, both focused on regions dominated by sandy bottoms. This talk describes more recent efforts in regions characterized by muddy bottoms.

Variations of acoustic ducting in the presence of gulf stream warm core rings at the New England shelfbreak

The physical oceanographic (PO) conditions at the New England shelfbreak have become more dynamic with increased warming and meandering of the gulf stream. Consequently, underwater sound propagation in the area has also become more variable. The most profound PO feature that breaks off from Gulf Stream instabilities is warm core rings, which have complex interactions with bathymetry and seasonal shelf water masses. Warm core rings can cause shelf water streamers to extend offshore, resulting in surface and/or subsurface acoustic ducting conditions. This study observed acoustic surface ducting and breakdown conditions induced by a shelf water streamer at the New England shelfbreak. Ducting conditions were variable when a subsequent warm core ring intruded the acoustic source and receiver network, mixing with the shelf water streamer and influencing the sound propagation during and after the acoustic duct breakdown period. The field observations were also compared with a broadband numerical model. With increasing gulf stream warm core ring formations, streamer induced surface ducts are becoming more relevant in shelf break acoustics. [Work supported by the Office of Naval Research.]

Influence of stratified sub-bottom sediment layers on acoustic simulations in the New England shelf break environment

During the 2021 New England Shelf Break Acoustics (NESBA) Experiment, a large number of acoustic signal transmissions along and across the continental shelf were compared with combined physical oceanographic (PO) and ocean acoustic (OA) models. This experiment aimed to examine the influence of environmental changes in the water column on acoustic propagation in the shelf break environment. Sub-bottom surveys carried out by a combination of autonomous underwater vehicles (AUVs) and ship-board instruments revealed distinctly stratified sediment layers. Accounting for the properties of sub-bottom sediment layers is an integral component of diagnosing mismatch between experimental acoustic data and simulations, as these properties can influence the attenuation and scattering of received signals. A comparison of homogeneous half-space bottom models calculated at sea in May of 2021 and improved models which incorporate the results of sub-bottom surveys is presented, and the importance of this modeling consideration is examined for the purposes of data and model comparisons. [This work was supported by the Office of Naval Research.]

Overview of the New England shelf break acoustics (NESBA) experiment

A joint ocean acoustics experiment was conducting at the southern edge of the New England Shelf in spring 2021 to investigate the sound propagation and scattering effects of physical oceanographic (PO) processes, marine geological (GEO) features, and biology (BIO) in the shelfbreak area. The experiment consisted of a network of sound sources, hydrophone arrays, and physical oceanography, and biological survey equipment was established. The environmental processes of particular interest include shelfbreak fronts, thermohaline intrusions, shelf water streamers, and internal waves, along with seafloor depressions and slopes, submarine canyons, and variable seabed properties (GEO), as well as fish schooling and shoaling (BIO). With fixed propagation paths, acoustic fluctuations can be correlated with environmental variations within the observation network with a temporal and spatial scales ranging from minutes to weeks and up to 30 km. One of the primary experiment objectives was on adaptive sampling and tracking of acoustic sensitivity “hot spots” to improve the real-time joint ocean acoustics and circulation modeling that was conducted on a research vessel in the field. This talk will review the design concept of this ocean acoustic network and provide an overview the experiment results. [Work supported by the Office of Naval Research.]

Vorticity in the Wake of Palau from Lagrangian Surface Drifters

Abstract Wake eddies are important to physical oceanographers because they tend to dominate current variability in the lee of islands. However, their generation and evolution has been difficult to study due to their intermittency. In this study, 2 years of observations from Surface Velocity Program (SVP) drifters are used to calculate relative vorticity (ζ) and diffusivity (κ) in the wake generated by westward flow past the archipelago of Palau. Over 2 years, 19 clusters of five SVP drifters ∼5 km in scale were released from the north end of the archipelago. Out of these, 15 were entrained in the wake. We compare estimates of ζ from both velocity spatial gradients (least squares fitting) and velocity time series (wavelet analysis). Drifters in the wake were entrained in either energetic submesoscale eddies with initial ζ up to 6f, or island-scale recirculation and large-scale lateral shear with ζ ∼ 0.1f. Here f is the local Coriolis frequency. Mean wake vorticity is initially 1.5f but decreases inversely with time (t), while mean cluster scale (L) increases as L ∝ t. Kinetic energy measured by the drifters is comparatively constant. This suggests ζ is predominantly a function of scale, confirmed by binning enstrophy (ζ2) by inverse scale. We find κ ∝ L4/3 and upper and lower bounds for L(t) are given by t3/2 and t1/2, respectively. These trends are predicted by a model of dispersion due to lateral shear. We argue the observed time dependence of cluster scale and vorticity suggest island-scale shear controls eddy growth in the wake of Palau.

Hydrodynamic and TS Structure Dataset of the São Francisco and Parnaiba Brazilian Rivers

Running a numerical simulation with the ROMS model in the regions of the mouths and plumes of the São Francisco and Parnaiba rivers, this dataset was obtained. It is the result of two scenarios, one of them taking into account the tides and the other not. It consists of 50 files (in NetCDF format) divided into 25 for each of the mentioned scenarios. Among the 25 files, 1 corresponds to the spatial discretization parameters of the input; 12 corresponds to the numer-ical simulation in the Parnaiba River and the remaining 12 to the São Francisco River. These files contain the daily average of all input and output parameters of the simulations. The ROM-STOOLS scripting package was used to prepare the input data. The ROMS model input was the surface forcings from the COADS dataset; the WOA09 and SODA datasets were used as initial and lateral boundary conditions, respectively. The bathymetry was taken from ETOPO2 and the tide was acquired from the TPXO7 satellite product. The hydrodynamics and TS structure data contained in this dataset are useful for physical oceanographers to study the thermodynamics of the waters and other physical processes occurring in the mouth and plume region of the São Francisco and Parnaiba Rivers. It is also useful for chemical and biological oceanographers to study the behavior of nutrients, chlorophyll and primary productivity.

Prediction of the ocean water sound speeds via attribute-guided seismic waveform inversion

Oceanic temperature and salinity are important for studying ocean dynamics and marine ecosystems, and for developing climate prediction models. Physical oceanographers measure these properties using expendable instruments, providing accurate measurements at high vertical resolution at sparse lateral locations. Accuracy of these temperature and salinity data when laterally interpolated and used for ocean dynamics, marine biology, and weather-related studies is questionable. Marine seismic data indicate visible reflections within the water columns, which, in turn, are primarily controlled by the vertical and lateral variations of the ocean water sound speeds. Because these sound-speed variations are functions of temperature and salinity, water-column seismic reflection data have been inverted for sound speeds and used to estimate temperature and salinity. These seismically derived temperature and salinity data have lower vertical, but much finer lateral resolution than those measured by expendable instruments, and therefore are useful for physical oceanography, marine biology, and climate research. However, quality of the seismic inversion relies on a good initial model, which requires generating models at sparse locations over 2D or 3D seismic data volumes and interpolating them over the horizon, manually picked from the stacked seismic data. Because horizon picking is subject to human error and bias, replacing it with an automated process is desirable. We outline a new attribute-guided methodology for initial model generation where no horizon picking is necessary. Starting with this initial model, we run prestack waveform inversion based on genetic algorithm optimization and demonstrate that our method can predict reliable sound-speed profiles. We conclude that our method is a good way for estimating ocean water sound speeds and relating them to the temperature and salinity.

On properties of seawater defined by temperature, salinity, and pressure

Hydrographic station data, consisting principally of temperature and salinity determinations, have been used by physical oceanographers to develop a climatological picture of the distribution of these quantities in the oceans of the world. Density as determined by Knudsen's formula, taken together with hydrostatic and geostrophic dynamics, also provides a crude picture of oceanic flow. However, the data probably contain substantially more information than has been derived from them in the past.The quantity that is orthogonal to potential-density curves in the S plane is suggested as a useful variable to complement the information contained in potential density. The derivation of this quantity, denoted by τ in this paper, is straightforward. A polynomial expression for τ that is suitable for computer calculations of τ from hydrographic station data is given. Shown are examples of hydrographic station data from the Atlantic plotted on the τσ diagram. The information contained in the τσ diagram shows many of the features exhibited in the TS plane. Vertical sections of τ appear to provide information about mixing in different parts of the Atlantic. The distribution of τ for abyssal waters at selected stations in the oceans of the world resembles the distribution of abyssal density as plotted by Lynn and Reid (1968). From the data presented, it appears that τ may serve as a good tracer for abyssal water movements.Since τ is defined to be orthogonal to σ, the expectation is that τ is a dynamically passive variable. However, since σ does not correlate with abyssal densities, it appears to lose dynamical significance at great depth, and τ assumes dynamical significance because of its orthogonality to σ. This unexpected feature leads to an exploration of the dynamical significance of σ. A natural starting point is the question of stability of abyssal water.A distinction is made between stability as determined by in situ determinations and as determined by the potential-density (σ) distribution. Simple examples are presented to show that analysis based on σ alone can lead to incorrect conclusions about gravitational stability of the water in the abyssal ocean. The reason is that seawater is a multicomponent thermodynamic system, and the thermodynamic coefficients are functions of pressure, salinity, and temperature. This functional dependence leads to adjustments in density as a water particle moves adiabatically in the vertical direction so that a layer of water that appears to be unstable near the surface may be stable (as determined by in situ determination) at great depth. A local potential density, which is simply the vertical integral of the in situ stability, is derived. This quantity gives a precise picture of gravitational stability in the vertical direction. Some distributions of local potential density are shown.Originally published May 15, 1972, in the Journal of Marine Research 30(2), 227???255.

ПУТЬ ДЛИНОЮ В 90 ЛЕТ. К 90-ЛЕТИЮ УЧАСТНИКА ЭКСПЕРИМЕНТА ПОЛИГОН–70 ПРОФЕССОРА М.Н. КОШЛЯКОВА

The article commemorates the 90th anniversary of Prof. M.N. Koshlyakov – prominent scientific figure of the Russian Federation, physical oceanographer, recipient of the Makarov’s Prize, Doctor of Physical and Mathematical Sciences, Honored Professor of the Moscow Institute of Physics and Technology. He is Chief Scientist, the founder and the first Head of the Laboratory of Marine Currents at the P.P. Shirshov Institute of Oceanology of the Russian Academy of Sciences. Prof. Koshlyakov made major contributions to studies of temporal and spatial variability of oceanographic variables over a broad range of scales. Over his career, he participated and led over two dozen scientific expeditions, including such large ocean projects as Polygon–67, Polygon–70, POLYMODE, MEGAPOLYGON, ATLANTEX–90. His expeditions extended to regions of the western boundary currents of the Atlantic, Indian, and Pacific Oceans, to observe new, previously unknown components of the circulation of the World Ocean waters. Prof. Koshlyakov is the lead author of the discovery of open ocean mesoscale eddies made by a team of oceanographers at the Institute of Oceanology at the turn of the 1960–1970s. This discovery became a milestone in oceanography of the 20th century that dramatically changed our understanding of the processes governing the Ocean and their influence on the Earth’s climate. From mid-1960s to early 1990s, Prof. Koshlyakov was a direct participant and subsequently the leader of the main domestic and international experiments on the study of ocean eddies.

Editorial: Biographical Memoirs, Volume 69

Editorial: Biographical Memoirs, Volume 69

Empirical orthogonal function regression: Linking population biology to spatial varying environmental conditions using climate projections

Ecologists and oceanographers inform population and ecosystem management by identifying the physical drivers of ecological dynamics. However, different research communities use different analytical tools where, for example, physical oceanographers often apply rank-reduction techniques (a.k.a. empirical orthogonal functions [EOF]) to identify indicators that represent dominant modes of physical variability, whereas population ecologists use dynamical models that incorporate physical indicators as covariates. Simultaneously modeling physical and biological processes would have several benefits, including improved communication across sub-fields; more efficient use of limited data; and the ability to compare importance of physical and biological drivers for population dynamics. Here, we develop a new statistical technique, EOF regression, which jointly models population-scale dynamics and spatially distributed physical dynamics. EOF regression is fitted using maximum-likelihood techniques and applies a generalized EOF analysis to environmental measurements, estimates one or more time series representing modes of environmental variability, and simultaneously estimates the association of this time series with biological measurements. By doing so, it identifies a spatial map of environmental conditions that are best correlated with annual variability in the biological process. We demonstrate this method using a linear (Ricker) model for early-life survival ("recruitment") of three groundfish species in the eastern Bering Sea from 1982 to 2016, combined with measurements and end-of-century projections for bottom and sea surface temperature. Results suggest that (a) we can forecast biological dynamics while applying delta-correction and statistical downscaling to calibrate measurements and projected physical variables, (b) physical drivers are statistically significant for Pacific cod and walleye pollock recruitment, (c) separately analyzing physical and biological variables fails to identify the significant association for walleye pollock, and (d) cod and pollock will likely have reduced recruitment given forecasted temperatures over future decades.

William Scoresby as an Arctic physical oceanographer

Encouraged by naturalists Robert Jameson and Joseph Banks, whaler William Scoresby became an expert on the natural and physical processes at work in the European Arctic. Original letters between Scoresby and these naturalists, housed in the archive of the Whitby Literary and Philosophical Society (Yorkshire, England), document in the language of the times his biological observations and experiments in physical oceanography. Scoresby's researches resulted in An Account of the Arctic Regions, with a History and Description of the Northern Whale-fishery in 1820, which became a seminal work in Arctic science. Among the prescient observations in An Account of the Arctic Regions was a description of deep strata of water, under currents moving in different directions from the surface. A copy of An Account of the Arctic Regions was given as a gift to Norwegian scientist-explorer Fridtjof Nansen in 1897 upon the completion of the Fram expedition (1893–1896) and still resides in his personal library in Norway. In it is an underlined passage, suggesting that Nansen had read the whaler's book, perhaps in preparation for writing his own volumes on Arctic science, The Norwegian North Polar Expedition, 1893–1896 (1900–1906). Then, by inference, Nansen had been familiar with Scoresby's description of the under currents. In The Norwegian North Polar Expedition Nansen wrote that he had observed similar patterns of deep-water movements during the Fram expedition. This phenomenon must have perplexed him, because he posed the problem to the Swedish mathematician-oceanographer Vagn Walfrid Ekman, who mathematically described the water movement. Ekman's resulting model, a spiral staircase of descending deep-water currents, became known as the Ekman Spiral.

Sound propagation in the surface mixed layer and upper ocean: An overview of relevant ocean dynamics and mode scattering theory

The mixed layer and upper ocean are a region of immense interest to physical oceanographers, meteorologists, and climate scientists because this is the boundary through which energy, momentum, buoyancy, and gasses are exchanged between the ocean and the atmosphere. The upper ocean is also a rich ecosystem for a vast array of ocean organisms and marine wildlife as well as a region of great Naval tactical importance. Given these factors, it is rather surprising that ocean acoustics has paid little attention to this significant region, aside from very high-frequency studies of bubble properties and gas entrainment. From the standpoint of transmission loss, the major work on the problem seems to go back to the Acoustic, Meteorological, Oceanographic Survey (AMOS) of the mid-50s. Here, a review is presented of relevant ocean processes that may be important for acoustic propagation at frequencies ranging from several hundreds of hertz to several kilohertz, where the foundation of the analysis is normal mode and transport theory. Processes of interest are surface gravity waves including subsurface currents, internal waves and tides, wind driven inertial oscillations and Langmuir circulations, eddies and submesoscale processes, turbulence and spice, bubbles, and fishes.

Christopher N. K. Mooers (1935–2018)

This passionate physical oceanographer, a leader and catalyst in coastal studies, explored many facets of nearshore circulation and advanced the modeling and forecasting of coastal dynamics.

Evaluation of wind induced currents modeling along the Southern Caspian Sea

To improve our understanding of the Caspian Sea hydrodynamics, its circulation is simulated with special focus on wind-driven currents of its southern basin. The hydrodynamic models are forced with a newly developed fine resolution wind field to increase the accuracy of current modeling. A 2D shallow water equation model and a 3D baroclinic model are applied separately to examine the performance of each model for specific applications in the Caspian Sea. The model results are validated against recent field measurements including AWAC and temperature observations in the southern continental shelf region. Results show that the 2D model is able to well predict the depth-averaged current speed in storm conditions in narrow area of southern coasts. This finding suggests physical oceanographers apply 2D modeling as a more affordable method for extreme current speed analysis at the continental shelf region. On the other hand the 3D model demonstrates a better performance in reproducing monthly mean circulation and hence is preferable for surface circulation of Caspian Sea. Monthly sea surface circulation fields of the southern basin reveal a dipole cyclonic-anticyclonic pattern, a dominant eastward current along the southern coasts which intensifies from May to November and a dominant southward current along the eastern coasts in all months except February when the flow is northward. Monthly mean wind fields exhibit two main patterns including a north-south pattern occurring at warm months and collision of two wind fronts especially in the cold months. This collision occurs on a narrow region at the southern continental shelf regions. Due to wind field complexities, it leads to a major source of uncertainty in predicting the wind-driven currents. However, this source of uncertainty is significantly alleviated by applying a fine resolution wind field.

South China Sea: the gateway to China's deep-sea ambitions

Abstract For most parts of its history, China has largely turned its back on ocean exploration. Even after it started oceanographic research in the 1950s, the focus was mostly on coastal and offshore waters. But this changed a decade ago when the country began to invest heavily on deep-sea research—resulting in the launch in 2011 of its first multi-disciplinary deep-sea research programme called the South China Sea (SCS)-Deep to probe the mystery of this marginal sea. Covering an area of 3.5 million square kilometres and with a maximum depth of 5500 metres, SCS occupies a scientifically interesting position between the world's highest mountains, the Himalayas, and the deepest point on Earth, the Mariana Trench in the western Pacific Ocean. In a forum organized by National Science Review at the Annual Conference of the South China Sea-Deep Programme held in January in Shanghai, a panel of scientists explained what China's deep-sea ambitions are, why SCS is a fantastic natural laboratory, the importance of international collaboration, what China needs to do to develop cutting-edge marine technologies and how SCS could be an ideal platform for regional cooperation. Nianzhi Jiao an ecologist at Xiamen University, Xiamen, China Dongxiao Wang a physical oceanographer at South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China Jian Lin a marine geophysicist at Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA, and South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China Pinxian Wang a paleoceanographer at Tongji University, Shanghai, China Jiwei Tian a physical oceanographer at Ocean University of China, Qingdao, China Aiqun Zhang chief engineer at the Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya, China

Ocean currents and marine life

Ocean currents have many profound impacts on marine life, moving not only animals and plants around the ocean but also redistributing heat and nutrients. While some of these impacts have been well known for many decades, there have been major recent developments in this area. Biologists are increasingly collaborating with physical oceanographers. At the same time, methods to accurately predict ocean currents and their variability have improved over a broad range of spatial and temporal scales. Emerging from these initiatives is an understanding of how currents impact the connectivity of marine populations, how they influence the migration of strong swimmers, including whales and turtles, and how changing currents, as part of global climate change, may re-shape entire communities.

Marine Mammals Exploring the Oceans Pole to Pole: A Review of the MEOP Consortium

Polar oceans are poorly monitored despite the important role they play in regulating Earth’s climate system. Marine mammals equipped with biologging devices are now being used to fill the data gaps in these logistically difficult to sample regions. Since 2002, instrumented animals have been generating exceptionally large data sets of oceanographic CTD casts (>500,000 profiles), which are now freely available to the scientific community through the MEOP data portal (http://meop.net). MEOP (Marine Mammals Exploring the Oceans Pole to Pole) is a consortium of international researchers dedicated to sharing animal-derived data and knowledge about the polar oceans. Collectively, MEOP demonstrates the power and cost-effectiveness of using marine mammals as data-collection platforms that can dramatically improve the ocean observing system for biological and physical oceanographers. Here, we review the MEOP program and database to bring it to the attention of the international community.

Recent development and application of inverted echo sounders in observational physical oceanography

Since the first experiment carried out by Rossby in Bermuda, inverted echo sounders have been used in physical oceanography observations for half a century. The inverted echo sounder measures the round-trip acoustic travel time from the sea floor to the sea surface, thus acquiring vertically integrated information on the thermal structure, from which the first baroclinic mode of thermocline motion can be inferred. Arrays of inverted echo sounders have been deployed almost all over the global ocean to observe internal waves, mesoscale eddies, western boundary currents, etc., providing valuable targeted data for physical oceanographers. Acoustic aspects of the inverted echo sounder performance have been recently examined. Sources of error affecting instrument performance include tidal effects, barotropic adjustment, ambient acoustic noise and sea surface roughness. The latter two effects are explored with a simulation that includes surface wave reconstruction, acoustic scattering based on the Kirchhoff approximation, wind generated noise, sound propagation, and the instrument's signal processing circuitry. Not only does the analysis enhance our understanding of the acoustic travel time data but also suggests new approaches to extend the application of inverted echo sounders, for example, for sensing wind over the sea. New deployments of inverted echo sounders and recent developments in hardware system and auxiliary accessories are also introduced.

Operationalizing integrated ecosystem assessments within a multidisciplinary team: lessons learned from a worked example

Abstract Between 2014 and 2016, an interdisciplinary team of researchers including physical oceanographers, biologists, economists and anthropologists developed a working example of an Integrated Ecosystem Assessment (IEA) for three ecologically distinct regions of the Northwest Atlantic; Georges Bank, the Gulf of Maine and the Grand Banks, as part of the International Council for the Exploration of the Sea (ICES) Working Group on the Northwest Atlantic Regional Sea (WGNARS). In this paper, we review the transdisciplinary and collaborative process by which the IEA was developed, with a particular focus on the decision points arising from the IEA construct itself. The aim is to identify key issues faced in developing any IEA, practical decisions made to address these issues within the working group and lessons learned from the process.