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Internal Waves Study on a Narrow Steep Shelf of the Black Sea Using the Spatial Antenna of Line Temperature Sensors

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Internal Waves Study on a Narrow Steep Shelf of the Black Sea Using the Spatial Antenna of Line Temperature Sensors Journal of Marine Science and Engineering Article Internal Waves Study on a Narrow Steep Shelf of the Black Sea Using the Spatial Antenna of Line Temperature Sensors Andrey Serebryany 1,2,* , Elizaveta Khimchenko 1 , Oleg Popov 3, Dmitriy Denisov 2 and Genrikh Kenigsberger 4 1 Shirshov Institute of Oceanology, Russian Academy of Sciences, 117997 Moscow, Russia; [email protected] 2 Andreyev Acoustics Institute, 117036 Moscow, Russia; [email protected] 3 Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, 119017 Moscow, Russia; [email protected] 4 Institute of Ecology of the Academy of Sciences of Abkhazia, Sukhum 384900, Abkhazia; [email protected] * Correspondence: [email protected] Received: 4 August 2020; Accepted: 19 October 2020; Published: 22 October 2020 Abstract: The results of investigations into internal waves on a narrow steep shelf of the northeastern coast of the Black Sea are presented here. To measure the parameters of internal waves, the spatial antenna of three autonomous line temperature sensors were equipped in the depth range of 17 to 27 m. In observations that lasted for 10 days, near-inertial internal waves with a period close to 17 h and short-period internal waves with periods of 2–8 min, regularly approaching the coast, were revealed. The wave amplitudes were 4–8 m for inertial waves and 0.5–4 m for short-period internal waves. It was determined that most of the short-period internal waves approached from the southeast direction, from Cape Kodor. A large number of short waves reflected from the coast were also recorded. The intensification of short-period waves with inertial periodicity and the belonging of trains of short waves to crests of inertial waves were identified. In general, it was shown that the internal wave field at a narrow shelf significantly differs in its features from analogs of ordinary shelves of the Black Sea. Keywords: internal waves; inertial and short-period internal waves; space antenna; line temperature sensors; shelf; the Black Sea 1. Introduction The Black Sea is a unique phenomenon from many points of view, but here we will focus on only one aspect: internal waves. Internal waves are widespread all over the world’s oceans, including in marginal and inland seas. Internal waves propagate in a vertically stratified medium and perform a significant function as the main source of internal mixing (internal ventilation) in the ocean. The base source of the generation of internal waves is tidal forces. A large number of works have been devoted to field studies of internal waves in oceans and seas around the world [1–5]. Initially, investigations were carried out using contact methods. In recent decades, remote sensing methods have also been used [6,7]. Large-scale experiments have been conducted on ocean shelves, where the high intensification of internal waves are found [8–11]. As a result, there is information on the main characteristics of internal waves in various regions of the ocean, and regions of the world’s oceans where large-amplitude internal waves are known to exist. The features of internal waves, as nonlinear waves exhibiting the properties of solitons, have been described. In recent years, evidence was found of the existence of second mode internal waves in the ocean [12,13]. Various important effects associated with internal waves have J. Mar. Sci. Eng. 2020, 8, 833; doi:10.3390/jmse8110833 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 833 2 of 15 also been studied [14,15], leading to progress in the investigation of internal wave generation, among which, tidalJ. mechanisms Mar. Sci. Eng. 2020, 8 are, x FOR the PEER best REVIEW studied [16,17]. 2 of 16 In contrastnonlinear to the waves ocean exhibiting and open the properties seas, in of the solitons, enclosed have been Black described. Sea, tides In recent are years, small evidence [18] and largely do not affect thewas formation found of the of existence the field of second of internal mode internal waves. waves However, in the ocean internal [12,13]. waves Various do important exist here, playing an importanteffects role associated in vertical with mixing internal ofwaves the waterhave also column. been studied Observations [14,15], leading of to internal progresswaves in the in the Black investigation of internal wave generation, among which, tidal mechanisms are the best studied Sea cover a period[16,17]. of several decades [19–25]. In the specialized observations that have been carried out, not only haveIn contrast the parameters to the ocean and of internalopen seas, in waves the enclosed been Black measured, Sea, tides butare small also [18] the and mechanisms largely of their generation havedo not been affect identified the formation [26 of– 28the]. field The of internalinternal waves. wave However, field can internal be divided waves do into exist two here, ranges, one of playing an important role in vertical mixing of the water column. Observations of internal waves in which is longthe internal Black Sea wavescover a period with of an several inertial decades period. [19–25]. In In the the Black specialized Sea, observations this has a that range have of about 17 h. The range ofbeen short-period carried out, internalnot only have waves the hasparameters a frequency of internal close waves to thebeen buoyancy measured, frequency.but also the The periods of these wavesmechanisms are from of their minutes generation to several have been tens identified of minutes. [26–28]. The Most internal of the wave observations field can be divided of internal waves into two ranges, one of which is long internal waves with an inertial period. In the Black Sea, this has in the Black Seaa range have of beenabout carried17 h. The out range in of the short-period shelf zones. internal The waves properties has a frequency of the shelf close leadto the to the features of the observedbuoyancy field frequency. of internal The periods waves. of Fromthese waves this are perspective, from minutes observationsto several tens of onminutes. thesteep Most and narrow shelf in the northeasternof the observations part of internal of the waves Black in the Sea, Black where Sea have we been conducted carried out our in the investigations, shelf zones. The are of great properties of the shelf lead to the features of the observed field of internal waves. From this interest. Inperspective, the Black observations Sea, inertial on the internal steep and waves narrow perform shelf in the the northeastern role of thepart internalof the Black tides Sea, of the most energy-intensivewhere internalwe conducted waves. our investigations, As previous are of observations great interest. In have the Black shown, Sea, inertial inertial internal waves waves on a steep and narrow shelfperform have largerthe role amplitudesof the internal comparedtides of the most to waves energy-intensive on ordinary internal shelves waves. [As29 ].previous observations have shown, inertial waves on a steep and narrow shelf have larger amplitudes compared to waves on ordinary shelves [29]. 2. Study Area 2. Study Area In October–November 2019, we conducted a specialized experiment to study internal waves on a In October–November 2019, we conducted a specialized experiment to study internal waves on narrow shelfa narrow near theshelf Caucasian near the Caucasian coast coast of theof the Black Black Sea. Sea. The The measurements measurements were carried were out carriedin the out in the coastal zonecoastal near zone Cape near Sukhumsky Cape Sukhumsky (Figure (Figure1). 1). The The shelf shelf of ofthe thecoast coast of the cape of the is narrow cape isand narrow steep, and steep, with a sharpwith drop a sharp in depth. drop in The depth. average The average slope slope of the of bottomthe bottom at at the the measurement measurement site, site, near near the the platform, platform, is about 23° (Figure 2). The 100 m isobath lies about 200 m from the coast, while on the is about 23◦ ordinary(Figure shelf,2). The for example, 100 m isobathon the northeastern lies about shelf 200 near m fromGelendzhik, the coast, the 100 while m isobath on is the 5 km ordinary shelf, for example,from on the shore. northeastern shelf near Gelendzhik, the 100 m isobath is 5 km from the shore. J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 3 of 16 Figure 1.FigureThe 1. northeast The northeast part part of of thethe Black Black Sea Sea shoreline shoreline and position and positionof the study of site. the Map study showing site. Map showing mooring locations and bathymetry of the study region (below). Depth is in meters. The dots mark the mooring locations and bathymetry of the study region (below). Depth is in meters. The dots mark the position of moorings equipped with line temperature sensors (LTSs) and thermistor chain (TC). position of moorings equipped with line temperature sensors (LTSs) and thermistor chain (TC). Figure 2. Bottom relief at the measurement site, and the position of the measuring instruments. In the study area, north-west alongshore currents prevail. Often, there is an increase in the current velocity up to 0.5–1 m/s due to the rim current approaching the coast. This circumstance leads to an increase in inertial oscillations, even during calm weather. 3. Materials and Methods The originality of this experiment was in using the antenna of line temperature sensors (LTSs), which were employed along with other measuring instruments commonly used in our practice. The LTS is a well-known internal wave meter [30,31] in the form of an insulated wire, which is located vertically in the thermocline and measures the average temperature of the covered water layer, recording the oscillations of internal waves propagating along the thermocline.
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