Conductivity, Temperature, Depth Data Research Articles

Analysis of internal wave in the Buru Island coastal waters, Banda Sea, Indonesia

The Indonesian throughflow (ITF) in the eastern part of Indonesia passes through the Maluku, Taliabu, Sula, and Buru Islands to the Banda Sea. The average depth of the Buru and Sula Islands is between 1000 m and 5000 m, with a narrow gap (sill) located to the north and south at a depth of 165 m on the sea map. This topography makes for interesting research on the western waters of Buru Island. Therefore, this study aimed to obtain salinity, temperature, and density values as water column stratification associated with the internal wave (IW), thermocline area, and current patterns at specific depths in the waters of Buru Island, Banda Sea. Data were collected from initial observations of Sentinel 1 satellite imagery of Buru Island to obtain an image of the waves in the water area. Furthermore, a single beam echosounder (SBES), conductivity temperature depth (CTD), and Copernicus Marine were analysed at the same position and time. The research found a non-linear IW Mode 2 at depths of 440–540 m with an estimated amplitude length is 40 km and an amplitude height of 100 m. Based on SBES echogram imaging, CTD data analysis, and modelling, there was an increase in IW density, a 35.25 PSU increase in salinity value at depths of 50∼200 m, a temperature decrease of 27.5 °C to 10 °C at depths of 50∼300 m, and a density stratification of 23.5 kg/m³ to 27 kg/m³ at depths of 50∼300 m.

CTD Sensors for Ocean Investigation Including State of Art and Commercially Available.

Over 70% of the earth's surface is covered by oceans; globally, oceans provides a huge source of wealth to humans. In the literature, several sensors have been developed to investigate oceans. Electrical conductivity temperature depth (CTD) sensors were used frequently and extensively. Long-term accurate CTD data is important for the study and utilization of oceans, e.g., for weather forecasting, ecological evolution, fishery, and shipping. Several kinds of CTD sensors based on electrics, optical, acoustic wave and radio waves have been developed. CTD sensors are often utilized by measuring electrical signals. The latest progress of CTD sensors will be presented in order of performance. The principles, structure, materials and properties of many CTD sensors were discussed in detail. The commercially available CTD sensors were involved and their respective performances were compared. Some possible development directions of CTD sensors for ocean investigation are proposed.

Seasonal Variation and Governing Dynamics of the Mixed Layer in the Indian Sector of the Southern Ocean

AbstractThe dynamics involved in the variability of temperature, salinity, and density distributions in the upper layer of the Indo‐Pacific sector of the Southern Ocean were studied using Acoustic Doppler Current Profiler and Conductivity Temperature Depth data from a mini triangle trans‐ocean buoy network mooring deployed at 60°S and 140°E between January 15th and 19th December 2012, together with hydrographic casts, reanalysis outputs, and satellite imagery. Spectral analysis confirmed that near‐inertial internal waves are a major factor transporting energy downward to the lower boundary of the mixed layer (ML), with 12.5 J m−3 or 45% of the mean surface energy being able to reach deeper than the ML depth during one of the analyzed events. During a period of net heat loss from the ocean surface to the atmosphere, most of the energy was concentrated below the maximum shear layer, at around 80 m depth. Internal wave events were found to be linked to scalar diffusivity increases of up to two orders of magnitude. A box model clarified that surface cooling plays the most significant role in mixing surface waters during winter, but it is not enough to explain variability below 100 m depth, where horizontal advection and eddy enhancement are also important in modulating warm water flux. Our results elucidate how wind energy might enhance mixing, even in ice‐covered regions, while influencing depths greater than previously thought.

Storstrømmen and L. Bistrup Bræ, North Greenland, Protected From Warm Atlantic Ocean Waters.

Storstrømmen and L. Bistrup Bræ are 20‐ and 10‐km wide, surge type glaciers in North Greenland in quiescent phase that terminate in the southernmost floating ice tongue in East Greenland. Novel multi‐beam echo sounding data collected in August 2020 indicate a seabed at 350–400 m depth along a relatively uniform ice shelf front, 100 m deeper than expected, but surrounded by shallower terrain (<100 m) over a 30‐km wide region that blocks the access of warm, salty, subsurface Atlantic Intermediate Water (AIW) at +1.6°C. Conductivity temperature depth data reveal waters in front of the glaciers at −1.8°C not connected to AIW in the outer fjord, Dove Bugt. The recent grounding line retreat of the glaciers is attributed to glacier thinning at its ablation rate, with little influence of ocean waters, which illustrates the fundamental importance of knowing the bathymetry of glacial fjords.

Retreat of Humboldt Gletscher, North Greenland, Driven by Undercutting From a Warmer Ocean.

Humboldt Gletscher is a 100‐km wide, slow‐moving glacier in north Greenland which holds a 19‐cm global sea level equivalent. Humboldt has been the fourth largest contributor to sea level rise since 1972 but the cause of its mass loss has not been elucidated. Multi‐beam echo sounding data collected in 2019 indicate a seabed 200 m deeper than previously known. Conductivity temperature depth data reveal the presence of warm water of Atlantic origin at 0°C at the glacier front and a warming of the ocean waters by 0.9 ± 0.1°C since 1962. Using an ocean model, we reconstruct grounded ice undercutting by the ocean, combine it with calculated retreat caused by ice thinning to floatation, and are able to fully explain the observed retreat. Two thirds of the retreat are caused by undercutting of grounded ice, which is a physical process not included in most ice sheet models.

Long-Term Changes in the Water Mass Properties in the Balearic Channels Over the Period 1996–2019

The analysis of a 24-year time series of Conductivity-Temperature-Depth (CTD) casts collected in the Balearic Channels (1996–2019) has allowed detecting and quantifying long-term changes in water mass properties in the Western Mediterranean. For the complete period, the intermediate waters have experienced warming and salting at rates of 1.4°C/100yr and 0.3–0.6/100yr for the Western Intermediate Water, and 1°C/100yr and 0.3–0.4/100yr for the Levantine Intermediate Water. The density of these two water masses has not changed. The deep waters, defined as those denser than 29.1 kg/m3, showed positive trends in temperature, salinity, and density (0.8°C/100yr, 0.2/100yr, and 0.02 kg.m–3/100yr, respectively). The high temporal variability of the upper layer makes the detection of long-term changes more difficult. Nevertheless, combining CTD data with temperature data from the oceanographic station at L’Estartit and simulated data from the NCEP/NCAR reanalysis, it can be established that the Atlantic Water increased its temperature at a rate of 2.1–2.8°C/100yr and likely its salinity at a rate of 0.6/100yr. The water column absorbed heat at a rate equivalent to 1–1.2 W/m2. All these trends are much higher than those reported in previous works (more than double in some cases). The warming of the water column produced an increase in the thermosteric component of sea level. However, this increase was compensated by the decrease in the halosteric component. Besides these changes, other alterations related to the Western Mediterranean Transition have been observed over shorter periods. The temperature and salinity of the intermediate waters increased before the winter of 2004/2005 and then the temperature and salinity of the deep waters increased dramatically in 2005. The density of the deep water reached values unprecedented before 2005. Deep and intermediate waters were uplifted by the presence of such dense deep waters. The arrival of warmer and saltier intermediate waters from the Eastern Mediterranean is also observed, mainly after 2010.

Assimilating In Situ and Remote Sensing Observations in a Highly Variable Estuary–Shelf Model

AbstractTo improve the forecasting performance in dynamically active coastal waters forced by winds, tides, and river discharges in a coupled estuary–shelf model off Hong Kong, a multivariable data assimilation (DA) system using the ensemble optimal interpolation method has been developed and implemented. The system assimilates the conductivity–temperature–depth (CTD) profilers, time series buoy measurement, and remote sensing sea surface temperature (SST) data into a high-resolution estuary–shelf ocean model around Hong Kong. We found that the time window selection associated with the local dynamics and the number of observation samples are two key factors in improving assimilation in the unique estuary–shelf system. DA with a varied assimilation time window that is based on the intratidal variation in the local dynamics can reduce the errors in the estimation of the innovation vector caused by the model–observation mismatch at the analysis time and improve simulation greatly in both the estuary and coastal regions. Statistically, the overall root-mean-square error (RMSE) between the DA forecasts and not-yet-assimilated observations for temperature and salinity has been reduced by 33.0% and 31.9% in the experiment period, respectively. By assimilating higher-resolution remote sensing SST data instead of lower-resolution satellite SST, the RMSE of SST is improved by ~18%. Besides, by assimilating real-time buoy mooring data, the model bias can be continuously corrected both around the buoy location and beyond. The assimilation of the combined buoy, CTD, and SST data can provide an overall improvement of the simulated three-dimensional solution. A dynamics-oriented assimilation scheme is essential for the improvement of model forecasting in the estuary–shelf system under multiple forcings.

Inversion of Oceanic Parameters Represented by CTD Utilizing Seismic Multi-Attributes Based on Convolutional Neural Network

In Recent years, seismic data have been widely used in seismic oceanography for the inversion of oceanic parameters represented by conductivity temperature depth (CTD). Using this technique, researchers can identify the water structure with high horizontal resolution, which compensates for the deficiencies of CTD data. However, conventional inversion methods are model-driven, such as constrained sparse spike inversion (CSSI) and full waveform inversion (FWI), and typically require prior deterministic mapping operators. In this paper, we propose a novel inversion method based on a convolutional neural network (CNN), which is purely data-driven. To solve the problem of multiple solutions, we use stepwise regression to select the optimal attributes and their combination and take two-dimensional images of the selected attributes as input data. To prevent vanishing gradients, we use the rectified linear unit (ReLU) function as the activation function of the hidden layer. Moreover, the Adam and mini-batch algorithms are combined to improve stability and efficiency. The inversion results of field data indicate that the proposed method is a robust tool for accurately predicting oceanic parameters.

Symmetrical and Asymmetrical Rectifications Employed for Deeper Ocean Extrapolations of In Situ CTD Data and Subsequent Sound Speed Profiles

The multinational Argo program, which was initiated in 1999, has completed its global requirement of 3000 floats deployed by 2007. This program has revolutionized ocean observations with the provision of varying data in the upper half of the ocean. However, various studies have reiterated the requirement for deep ocean coverage, since the ocean below 2000 meters (m) is warming. In this regard, full-depth studies are mandatory in order to estimate the rising sea level due to thermal expansion and analyze critical parameters of deep ocean circulation sub 2000 m; further, data below 2000 m are mandatory for multifarious model simulations. As a landmark initiative, in mid-2015, the “Deep Argo Implementation Workshop” was held in Hobart. An array comprising 1228 floats was suggested by G. C. Johnson, rendering coverage of 5° latitude × 5° longitude × 15-day cycles. This was conclusively agreed to be an affordable solution for varying scientific needs for assessing data in abyssal oceans. Thence, Deep New profilINg float of JApan (NINJA) and Deep Arvor floats were developed by Japan and France, respectively, to cover depths of 0–4000 m. Similarly, Deep Autonomous Profiling Explorer (APEX) and Deep Sounding Oceanographic Lagrangian Observer (SOLO) by the United States were designed to cover 0–6000 m. The data offered by this underdeveloped deep pilot array are scarce on both temporal and spatial scales. This particular study offers an ingenious and novel approach to extrapolating conductivity–temperature–depth (CTD) profiles, as well as sound speed profiles (SSPs), in abyssal oceans below 2000 m. The primitive results of this method exhibited certain discrepancies which were subsequently rectified by modifying the aforementioned method both symmetrically and asymmetrically in an innovative way. The final outcomes of this method are almost identical to the in situ values obtained from Deep Argo floats, and in this way, offer a way to compute deep ocean calculations both spatially and temporally since Deep Argo floats are aimed at relatively sparse deployments and require a longer duration to provide data (5° latitude × 5° longitude × 15-day cycles) as compared to Core Argo data (3° latitude × 3° longitude × 10-day cycles). The SSP computations were conducted by employing multiple equations such as Chen and Millero, Del Grosso, and UNESCO (United Nations Educational, Scientific, and Cultural Organization) algorithms. The study concludes by offering transmission loss rectifications by employing the aforementioned method as a future course of action.

An exclusive in-situ dataset on physicochemical parameters in the gappy northern Bay of Bengal.

In-situ measurement of physical characteristics of seawater (temperature, salinity, and density) collected with a single fire module CTD (Conductivity, Temperature, Depth) in the northern Bay of Bengal (NBoB), as a part of the Indian Ocean, are given in this article. Due to the scanty of data in the NBoB, such in-situ measurements will certainly play a crucial role in understanding regional oceanic processes. The presented data were collected onboard during two cruises covering 12 stations with high resolution (0.2 m depth) up to 50-meter contour depth from 20.5778 N, 92.3578 E to 21.287 N, 89.696 E. We present raw CTD data as ASCII (.dat) and Comma Separated Value (.csv), and processed CTD data as netCDF (.nc) format in this article. Additionally, the calculated Mixed Layer Depth that was derived from the netCDF (.nc) file was included in the tabular format. This processed in-situ data would be most useful as a baseline for further studies in coastal and offshore physicochemical properties in the NBoB.

Monitoring of Discharged Water from Groundwater and Land-based Aquaculture Effluent using CTD Data in Shinyang Bangdu Bay, Jeju Island, Korea

The Sinyang Bangdu Bay in Jeju Island, a semi-inclosed sea, is introduced by the discharged water of the groundwater and land-based aquaculture effluent (LAE) from the surrounding land. To investigate the distribution of the discharged water into the Sinyang Bangdu Bay, 370 Conductivity-Temperature-Depth (CTD) casting data (temperature and salinity) were collected on the nine sampling points from April to September 2019. To understand the spatial and temporal variation of the influent water with changing tide, CTD observation was continuously performed 1-5 times at a 1-hour interval in the daytime. CTD hydrographic data were used the downcast data controlled by constant speed. CTD casting data, water temperature and salinity, were averaged at 0.1 m intervals using the filter mean. The groundwater and LAE in the study area show relatively lower salinity than surrounding waters, and this feature was shown on the west coast in the study area. The both influent waters into the study area had a different spatial influence with changing tidal current. During the flood tide, the west coast of the study area was influenced by the LAE because the discharged water moved to northward along the west coastline of the bay. During the ebb tide, the west coast of the study area was influenced by the groundwater because the discharged water moved southward along the west coastline of the bay. The vertical variation of the discharged water sharply changed within 1 m, and the feature was not observed at more than 1 m wa ter depth.

The Value of Sustained Ocean Observations for Sea Ice Predictions in the Barents Sea

ABSTRACT Dynamical prediction systems have shown potential to meet the emerging need for seasonal forecasts of regional Arctic sea ice. Observationally constrained initial conditions are a key source of skill for these predictions, but the direct influence of different observation types on prediction skill has not yet been systematically investigated. In this work, we perform a hierarchy of observing system experiments with a coupled global data assimilation and prediction system to assess the value of different classes of oceanic and atmospheric observations for seasonal sea ice predictions in the Barents Sea. We find notable skill improvements due to the inclusion of both sea surface temperature (SST) satellite observations and subsurface conductivity–temperature–depth (CTD) measurements. The SST data are found to provide the crucial source of interannual variability, whereas the CTD data primarily provide climatological and trend improvements. Analysis of the Barents Sea ocean heat budget suggests that ocean heat content anomalies in this region are driven by surface heat fluxes on seasonal time scales.

Global thermosteric sea level change contributed by the deep ocean below 2000 m estimated by Argo and CTD data

Due to an insufficient amount of observational data, little is known about the temperature variation in the global abyssal ocean and its contribution to sea level change. In this study, we estimated the monthly seawater temperature below 2000 m from 2005–2015 by combining Array for Real-time Geostrophic Oceanography (Argo) and conductivity-temperature-depth (CTD) data. For this purpose, we divided the global ocean into 38 regions according to the CTD temperature profile properties. For each region, we used CTD data to construct a mathematical model to describe the relation of temperatures between 1000–2000 m and those below 2000 m. We then used this mathematical model and the Argo data to reconstruct the temperature profile below 2000 m and evaluate its thermostatic contribution to sea level change. We discovered that the deep ocean below 2000 m contributed 0.12 ± 0.03 mm/yr to the mean global thermosteric sea level rise from 2005–2015. Our result was confirmed by geodetic observations, i.e., the rate of thermosteric sea level rise calculated using altimeter, Gravity Recovery and Climate Experiment (GRACE), and Argo data was approximately 0.15 ± 0.41 mm/yr. The contribution of each ocean layer generally decreased with depth, and there was an obvious fluctuation between approximately 4000–5000 m, interpreted as being caused by geothermal heating of the ocean floor.

Variability of the Pacific‐Derived Arctic Water Over the Southeastern Wandel Sea Shelf (Northeast Greenland) in 2015–2016

AbstractA portion of the freshwater transport through Fram Strait consists of low‐salinity Pacific‐derived Arctic water flowing southward along the east coast of Greenland. The pathways of this water are currently unclear. An Ice Tethered Profiler deployed over the southeastern Wandel Sea shelf (northeast Greenland) in May 2015 collected a profile every 3 hr for a year recording conductivity‐temperature‐depth (CTD) and Colored Dissolved Organic Matter (CDOM) fluorescence. This was accompanied by velocity observations. The CTD data revealed that the subsurface water (~15–85 m depth) characterized by high CDOM resembles the “cold Halostad” in the Canada Basin formed by the injection of Pacific water. A coastal branch of the Pacific water outflow from the Arctic Ocean supplies the Wandel Sea halostad, which shows a clear seasonal pattern. From July to October–November, the halostad is shallow, more saline, warmer, and with less CDOM. Conversely, from November to April, the halostad deepens, cools, freshens and CDOM increases, likely indicating a higher fraction of Pacific winter water. The CTD surveys, wind and current data, and numerical simulations show that the seasonal variation of wind over the continental slope likely controls seasonal changes of this intermediate water layer. Over northeast Greenland, winter winds have a northerly component from November to April, favoring Ekman transport of the Pacific‐derived water to the Wandel Sea shelf. In contrast, the prevailing southerly summer winds result in retreat of the Pacific‐derived water off the shelf. The landfast ice off‐slope extension modifies wind‐forcing disrupting seasonal patterns.

From pole to pole: 33 years of physical oceanography onboard R/V &amp;lt;i&amp;gt;Polarstern&amp;lt;/i&amp;gt;

Abstract. Measuring temperature and salinity profiles in the world's oceans is crucial to understanding ocean dynamics and its influence on the heat budget, the water cycle, the marine environment and on our climate. Since 1983 the German research vessel and icebreaker Polarstern has been the platform of numerous CTD (conductivity, temperature, depth instrument) deployments in the Arctic and the Antarctic. We report on a unique data collection spanning 33 years of polar CTD data. In total 131 data sets (1 data set per cruise leg) containing data from 10 063 CTD casts are now freely available at doi:10.1594/PANGAEA.860066. During this long period five CTD types with different characteristics and accuracies have been used. Therefore the instruments and processing procedures (sensor calibration, data validation, etc.) are described in detail. This compilation is special not only with regard to the quantity but also the quality of the data – the latter indicated for each data set using defined quality codes. The complete data collection includes a number of repeated sections for which the quality code can be used to investigate and evaluate long-term changes. Beginning with 2010, the salinity measurements presented here are of the highest quality possible in this field owing to the introduction of the OPTIMARE Precision Salinometer.

Tidal currents and Kuroshio transport variations in the Tokara Strait estimated from ferryboat ADCP data

AbstractFrom 2003 to 2011, current surveys, using an acoustic Doppler current profiler (ADCP) mounted on the Ferry Naminoue, were conducted across the Tokara Strait (TkS). Resulting velocity sections (1234) were used to estimate major tidal current constituents in the TkS. The semidiurnal M2 tidal current (maximum amplitude 27 cm s−1) was dominant among all the tidal constituents, and the diurnal K1 tidal current (maximum amplitude 21 cm s−1) was the largest among all the diurnal tidal constituents. Over the section, the ratios, relative to M2, of averaged amplitudes of M2, S2, N2, K2, K1, O1, P1, and Q1 tidal currents were 1.00:0.44:0.21:0.12:0.56:0.33:0.14:0.10. Tidal currents estimated from the ship‐mounted ADCP data were in good agreement with those from the mooring ADCP data. Their root‐mean‐square difference for the M2 tidal current amplitude was 2.0 cm s−1. After removing the tidal currents, the annual‐mean of the net volume transport (NVT) through the TkS ± its standard derivation was 23.03 ± 3.31 Sv (Sv = 106 m3 s−1). The maximum (minimum) monthly mean NVT occurred in July (November) with 24.60 (21.47) Sv. NVT values from the ship‐mounted ADCP were in good agreement with previous geostrophic volume transports calculated from conductivity temperature depth data, but the former showed much finer temporal structure than those from the geostrophic calculation.

Coastal ocean forecasting with an unstructured grid model in the southern Adriatic and northern Ionian seas

Abstract. SANIFS (Southern Adriatic Northern Ionian coastal Forecasting System) is a coastal-ocean operational system based on the unstructured grid finite-element three-dimensional hydrodynamic SHYFEM model, providing short-term forecasts. The operational chain is based on a downscaling approach starting from the large-scale system for the entire Mediterranean Basin (MFS, Mediterranean Forecasting System), which provides initial and boundary condition fields to the nested system. The model is configured to provide hydrodynamics and active tracer forecasts both in open ocean and coastal waters of southeastern Italy using a variable horizontal resolution from the open sea (3–4 km) to coastal areas (50–500 m). Given that the coastal fields are driven by a combination of both local (also known as coastal) and deep-ocean forcings propagating along the shelf, the performance of SANIFS was verified both in forecast and simulation mode, first (i) on the large and shelf-coastal scales by comparing with a large-scale survey CTD (conductivity–temperature–depth) in the Gulf of Taranto and then (ii) on the coastal-harbour scale (Mar Grande of Taranto) by comparison with CTD, ADCP (acoustic doppler current profiler) and tide gauge data. Sensitivity tests were performed on initialization conditions (mainly focused on spin-up procedures) and on surface boundary conditions by assessing the reliability of two alternative datasets at different horizontal resolution (12.5 and 6.5 km). The SANIFS forecasts at a lead time of 1 day were compared with the MFS forecasts, highlighting that SANIFS is able to retain the large-scale dynamics of MFS. The large-scale dynamics of MFS are correctly propagated to the shelf-coastal scale, improving the forecast accuracy (+17 % for temperature and +6 % for salinity compared to MFS). Moreover, the added value of SANIFS was assessed on the coastal-harbour scale, which is not covered by the coarse resolution of MFS, where the fields forecasted by SANIFS reproduced the observations well (temperature RMSE equal to 0.11 °C). Furthermore, SANIFS simulations were compared with hourly time series of temperature, sea level and velocity measured on the coastal-harbour scale, showing a good agreement. Simulations in the Gulf of Taranto described a circulation mainly characterized by an anticyclonic gyre with the presence of cyclonic vortexes in shelf-coastal areas. A surface water inflow from the open sea to Mar Grande characterizes the coastal-harbour scale.

Acoustic reflections in the water column of Krishna-Godavari offshore basin, Bay of Bengal.

Seismic oceanographic studies from various oceans worldwide have indicated that the acoustic reflections are mostly observed along thermal boundaries within the water column. However, the authors present a case study of seismic data from Krishna-Godavari Basin which shows that salinity variations also play an important role in the occurrence of water column reflections. The observed reflection is modeled using the reflectivity series derived from the salinity and temperature profiles from a nearby Conductivity-Temperature-Depth (CTD) location. Sensitivity analysis of temperature and salinity on soundspeed shows that the effect of salinity cannot be ignored for modeling acoustic reflections. The synthetic seismogram matches well with the observed reflection seismic data. Remarkable similarities between the reflection seismic and the salinity profile in the upper thermocline suggest the importance of salinity variations on the water column reflection. Furthermore, impedance inversion of the reflectivity data reveals several thermohaline structures in the water column. The origin of these thermohaline structures is largely unaddressed and may be attributed to the fresh water influx coming from Himalayan and Peninsular rivers or due to the presence of different water masses in the Indian Ocean which warrants a detailed study using concurrent seismic and CTD data.

Basin-Wavelength Equatorial Deep Jet Signals across Three Oceans

AbstractEquatorial deep jets (EDJs) are equatorially trapped, stacked, zonal currents that reverse direction every few hundred meters in depth throughout much of the water column. This study evaluates their structure observationally in all three oceans using new high-vertical-resolution Argo float conductivity–temperature–depth (CTD) instrument profiles from 2010 to 2014 augmented with historical shipboard CTD data from 1972 to 2014 and lower-vertical-resolution Argo float profiles from 2007 to 2014. The vertical strain of density is calculated from the profiles and analyzed in a stretched vertical coordinate system determined from the mean vertical density structure. The power spectra of vertical strain in each basin are analyzed using wavelet decomposition. In the Indian and Pacific Oceans, there are two distinct peaks in the power spectra, one Kelvin wave–like and the other entirely consistent with the dispersion relation of a linear, first meridional mode, equatorial Rossby wave. In the Atlantic Ocean, the first meridional mode Rossby wave signature is very strong and dominates. In all three ocean basins, Rossby wave–like signatures are coherent across the basin width and appear to have wavelengths the scale of the basin width, with periods of about 5 yr in the Indian and Atlantic Oceans and about 12 yr in the Pacific Ocean. Their observed meridional scales are about 1.5 times the linear theoretical values. Their phase propagation is downward with time, implying upward energy propagation if linear wave dynamics hold.

Tomographic mapping of a coastal upwelling and the associated diurnal internal tides in Hiroshima Bay, Japan

AbstractTemperature variations caused by a typhoon were measured in the northern part of Hiroshima Bay by four coastal acoustic tomography (CAT) systems. The horizontal distributions of depth‐averaged temperature from 0 to 8 m were mapped at 10 min intervals between the 11 and 25 September 2013. The horizontal distributions of a coastal upwelling and the associated diurnal internal tides were reconstructed well by regularized inversion based on the grid segmented method, using one‐way travel time data along five successful sound transmission lines. Station‐to‐station ranges were corrected in such a way that sound speed (determined from one‐way travel time data) was equal to sound speed calculated from a couple of CTD (conductivity‐temperature‐depth) data sets on each transmission line. In addition, all station positions were adjusted to make focal points at the geographical positions of the transducers. The corrections increased the accuracy of temperature measurements to make temperature errors as small as 0.073–0.079°C. The high accuracy made it possible to map the temperature structure with a variation range of less than 0.5°C. An upwelling grew from 16 to 17 September, due to a typhoon‐derived northerly wind. The diurnal internal tide resonated with the semidiurnal external tide, which was pronounced after the upwelling decayed (18 September), around the time the spring tide occurred. The upwelling and mixing fractions were formulated. These fractions increased continuously as the upwelling grew. Complete mixing was observed during the upwelling's mature phase.