A. Y. Erofeev

Declining Oxygen in the Northeast Pacific

AbstractClimate models predict a decrease in oceanic dissolved oxygen and a thickening of the oxygen minimum zone, associated with global warming. Comprehensive observational analyses of oxygen decline are challenging, given generally sparse historical data. The Newport hydrographic (NH) line off central Oregon is one of the few locations in the northeast Pacific with long oxygen records. Good quality data are available here primarily in two time blocks: 1960–71 and 1998–present. Standard sampling extends from midshelf (bottom depth of 58 m) to 157 km offshore (bottom depth of 2880 m). Shipboard measurements have been supplemented in recent years (2006–present) with data from autonomous underwater gliders. Oxygen declines significantly over this 50-yr period across the entire NH line. In addition to decrease in the vicinity of the oxygen minimum depth (~800 m), oxygen decreases across a range of density surfaces σθ = 26–27 within the thermocline, in the depth range 100–550 m. A core of decreasing oxygen (...

Buoyancy-Driven Coastal Currents off Oregon during Fall and Winter

During fall/winter off the Oregon coast, oceanographic surveys are relatively scarce because of rough weatherconditions.Thischallengehasbeenovercomebytheuseofautonomousunderwaterglidersdeployed along the Newport hydrographic line (NH-Line) nearly continuously since 2006. The discharge from the coastal rivers between northern California and the NH-Line reach several thousands of cubic meters per second, and the peaks are comparable to the discharge from the Columbia River. This freshwater input creates cross-shelf density gradients that together with the wind forcing and the large-scale Davidson Current results in strong northward velocities over the shelf. A persistent coastal current during fall/winter, which the authorscalltheOregonCoastalCurrent(OCC),hasbeenrevealed bythegliderdataset.Basedon atwo-layer model, the dominant forcing mechanismof the OCC is buoyancy,followed by the Davidson Current and then the wind stress, accounting for 61% (622.6%), 26% (618.6%), and 13% (611.7%) of the alongshore transports, respectively. The OCC average velocities vary from 0.1 to over 0.5ms 21 , and transports are on average 0.08 (60.07) Sverdrups (Sv; 1 Sv [ 10 6 m 3 s 21 ), with the maximum observed value of 0.49 Sv, comparable to the summertime upwelling jet off the Oregon coast. The OCC is a surface-trapped coastal current, and its geometryis highly affectedby the wind stress, consistentwith Ekmandynamics.The windstress has an overallsmall direct contribution to the alongshoretransport;however,it plays a primaryrole in modifyingthe OCC structure. The OCC is a persistent, key component of the fall/winter shelf dynamics and influences the ocean biogeochemistry off the Oregon coast.

Variations of thermohaline structure and turbulent mixing on the Black Sea shelf at the beginning of autumn cooling

Abstract Microstructure measurements taken prior and after some strong atmospheric events on the shallow Black Sea shelf allowed to track the formation and evolution of the thermohaline structure caused first by wind-induced mixing and local convection, later by a storm surge, and finally by intense heating during a short period of the ‘Indian summer’. It was found that even a day long mild surge can decrease the temperature by 2.5°C and increase salinity by 2 psu over the whole 20–25 m quasi-homogeneous upper layer, which was formed by previous intense vertical storm-induced mixing. During the following period of upwelling-favorable winds, the near-bottom temperature decreased by 8–9°C. The upwelling was accompanied by a series of thermohaline intrusions overlaying the inclined boundary of the sharp near-bottom thermocline. Restratification in the upper 10 m layer in the form of a series of quasi-homogeneous steps was successfully reproduced by a numerical model of wind-induced daytime mixing, followed by nighttime convection, using the measured sea-surface fluxes as background conditions. The vertical turbulence structure was depicted by the logarithm of the averaged kinetic energy dissipation, which showed a parabolic decrease from the boundary layers to the midpoint of internal weakly-stratified part of the water column. Intermittent turbulent patches were superimposed at this background profile, which closely coincided with mean structure of the vertical shear. A correlation between the averaged vertical profiles of the turbulent buoyancy Reynolds number Re b , mixing activity  G , and the gradient Richardson number Ri was found. The  G cannot be solely determined by Re b , it also strongly depends on Ri at all depths. Analytical relations betweeen these variables that may be used for mixing parameterization on the shelf are proposed.

MicroSoar: A New Instrument for Measuring Microscale Turbulence from Rapidly Moving Submerged Platforms

Abstract A new high-frequency turbulence measuring instrument, MicroSoar, has been developed, tested, and used to make scalar variance dissipation rate measurements. MicroSoar was mounted on the undercarriage of SeaSoar, a depth-programmable winged platform, and towed by a ship, at speeds up to 7 kt, in a depth range of the sea surface to 120 m. Sensors carried by MicroSoar were a fast thermistor, a pressure sensor, a microscale capillary conductivity sensor, and a three-axis accelerometer. With appropriate assumptions about the local T–S relation, measurements of microscale conductivity fluctuations can often be used to directly determine temperature variance dissipation rate (χT), the Cox number (Cx), and the scalar diathermal turbulent diffusivity (KT). Compared to conventional quasi-free-fall tethered vertically profiling instruments, MicroSoars major advantage lies in its ability to sample large fluid volumes and large geographic areas in a short time, and to provide, rapidly and simply, two-dimensi...

Anomalous Near-Surface Low-Salinity Pulses off the Central Oregon Coast

From mid-May to August 2011, extreme runoff in the Columbia River ranged from 14,000 to over 17,000 m3/s, more than two standard deviations above the mean for this period. The extreme runoff was the direct result of both melting of anomalously high snowpack and rainfall associated with the 2010–2011 La Niña. The effects of this increased freshwater discharge were observed off Newport, Oregon, 180 km south of the Columbia River mouth. Salinity values as low as 22, nine standard deviations below the climatological value for this period, were registered at the mid-shelf. Using a network of ocean observing sensors and platforms, it was possible to capture the onshore advection of the Columbia River plume from the mid-shelf, 20 km offshore, to the coast and eventually into Yaquina Bay (Newport) during a sustained wind reversal event. Increased freshwater delivery can influence coastal ocean ecosystems and delivery of offshore, river-influenced water may influence estuarine biogeochemistry.

Microstructure Measurements from a Towed Undulating Platform

MicroSoar, an undulating profiler capable of measuring turbulence parameters such as Thorpe scales and thermal dissipation rate while being towed at speeds of up t o4ms 21, offers the possibility of obtaining a closeto-synoptic image of mixing over large spatial areas. In this paper, the method of calculating Thorpe scales from the high-frequency MicroSoar data is developed, and results from data taken off the coast of Oregon during the summer of 2001 are presented. Large Thorpe scales and elevated measurements of the thermal dissipation rate are shown to be associated with shear at the edge of the coastal jet. It is further shown that using MicroSoar data rather than Sea-Bird conductivity‐temperature‐depth data extends the range of measurable overturns to smaller scales, particularly in regions of low stratification.

Mobile Autonomous Platforms for Passive-Acoustic Monitoring of High-frequency Cetaceans

Increased human activities in coastal and offshore waters, including renewable energy efforts such as the deployment and operation of wind, wave, and tidal energy converters, leads to potential negative impacts on marine ecosystems. Efficient monitoring of marine mammals in these areas using stationary passive-acoustic technologies is challenging. Many recreational and commercial activities (e.g., fishing) can hinder long-term operation of moored listening devices. Further, these waters are often utilized by cetaceans such as porpoise species which produce high-frequency echolocation clicks (peak frequency ~130 kHz) for navigation, communication, and prey detection. Because these ultrasonic signals are strongly absorbed during propagation, the acoustic detection range is limited to a few 100 m, and therefore the spatial coverage of stationary recorders is relatively limited. In contrast, mobile passive-acoustic platforms could potentially be used to survey areas of concern for high-frequency cetacean vocalizations and provide increased temporal coverage and spatial resolution. In a pilot study, a commercially available acoustic recorder featuring sampling rates of up to 384 kHz was customized and implemented on an autonomous underwater vehicle (AUV) and an unmanned surface vehicle (USV) and tested in the field. Preliminary results indicate that these systems (a) are effective at detecting the acoustic presence of high-frequency cetaceans such as porpoises, and (b) could be a valuable tool to monitor potential negative impacts of renewable energy and other anthropogenic disturbances in the marine environment.

Simultaneous operation of mobile acoustic recording systems off the Washington coast for cetacean studies

Acoustic monitoring of cetaceans was conducted using two buoyancy-driven AUVs in a deep-water canyon north of the Navys QUTR range off the Washington coast in April 2015. The two AUVs operated were the acoustically-equipped QUEphone, which is an APEXTM-based acoustic profiler float from Teledyne Webb, and the SeagliderTM from Kongsberg. A passive acoustic monitoring device, WISPR, from Embedded Ocean Systems (EOS) was installed on both AUVs. With one 512GB CF card and level-2 Free Lossless Audio Codec (FLAC), WISPR recorded sound continuously for 12 days at a 125 kHz sampling rate with 16-bit resolution. The Sealiders record showed high levels of flow noise below 100 Hz during ascent, 16 dB higher than during the descent. The Seagliders CTD generated 1-sec long line noise at 53.37 kHz at 5 to 10 sec intervals, while the Seagliders mass shifter generated 3-sec long band-limited noise below 10 kHz. The QUEphones acoustic record was generally quieter than the Seaglider primarily due to the fact that it is nearly stationary platform with less mechanical and electrical components to generate system noise. Despite higher system noise levels, the Seaglider detected twice as many calls/clicks resulting from its ability to actively stay in the target area where the population density of marine mammals was higher, while the QUEphone drifted away from the target area with the prevailing ocean currents.