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Double diffusion (DD) structures, in two types of diffusive convection (DC) and salt-fingering (SF), occur due to vertical temperature and salinity gradients with different diffusion coefficients. Areas such as the Strait of Hormuz, which has a thermohaline exchange between the Persian Gulf and the Sea of Oman, are suitable places for the formation of DD structures. Based on the results, the formation of DC structure is significant compared to other processes in the west of the Strait of Hormuz in December. Fluctuations in temperature and the growth of DC structure cause mixing and changing the depth of the boundary layer of temperature and salinity in the water. So that in the presence of the DC structure, the decrease in the value of the sound speed (1552 m/s) extends to a depth of 40 m, but the sound speed increases to 1555 m/s at the place where the warm water rises. The results show that the sound transmission loss increases by 5-15 dB in the place of strong DC structure. When the sound frequency increases, the transmission loss caused by the DC structure decreases. But for rays with a small propagation angle, a significant expansion is created in the propagation steps and wavelength of the rays, which increases up to 2 times with the increase in the depth of the sound source. • Over time, the use of acoustic instruments in the seas has increased in the field of marine exploration or communication, and it is very important to understand the marine environment and the processes that affect the use of these instruments. The effect of various processes on the propagation of sound waves has been investigated. But, many of these studies have focused on horizontal structures such as eddies, and investigating the change caused by DC in the sound signal has been neglected. Therefore, according to the formation of DC structure in the Strait of Hormuz and its practical importance, the changes in the sound signal at the place of the formation of this structure were investigated and the results were presented in this paper. [ABSTRACT FROM AUTHOR]

Submesoscale features, characterized by a low salinity layer originating from river discharges, enhance water column stability in a New Zealand shelf sea. Using a combination of data from multiple ocean glider surveys and regional modeling, we show that low salinity submesoscale features (LSMFs) can cause increased stratification on the order of 10−4s−2. Modeled oceanographic conditions compared well to observations, especially in austral spring. Stably stratified LSMFs can replace the previously well-mixed layer in the water column up to a distance of 100 km offshore before getting entrained by the regional barotropic current in Greater Cook Strait. LSMFs observed from glider surveys and reproduced from modeled results generate strong vertical and horizontal salinity differences of ΔS ~ 0.45 psu. These salinity differences define density fronts and stratification in the upper ~ 30 m. Temperature differences of up to ΔT ~ 1.4°C associated with LSMFs were not large enough to entirely cancel the density effect of salinity. The offshore advection reach of LSMFs is partly constrained by the variability of the barotropic d'Urville Current. Its presence and strong winds inhibit the propagation of LSMFs offshore in Greater Cook Strait, enhancing mixing and deepening the mixed layer depth. In contrast, moderate winds and weak current enable the propagation of LSMFs furthest offshore in Greater Cook Strait, where the water column becomes stably stratified. A stably stratified regime allows increased exposure of upper layers to light availability, encouraging phytoplankton growth, which may lead to enhanced primary production in the region.

The Southern Ocean plays an important role in mediating oceanic uptake of CO2 and heat due to a strong meridional overturning circulation. Gridded Argo float data for 2004–2017 were used to evaluate subsurface processes at the mixed layer depth (MLD) that occur in a narrow deep mixing band. Shifts in the value of the Turner Angle at the MLD indicate that early in the season the MLD deepens slowly as it encounters and is stabilized by a subsurface salt maximum. By September mixing has penetrated this salinity feature and the rate of deepening is faster once the MLD is deeper than the depth where the maximum salinity occurs (~150–200 m). This distinctive salinity layer is the result of surface Ekman transport of fresh water from the south and subsurface advection of high‐salinity water from the north. Two configurations of the Community Earth System Model (CESM) ocean‐ice forced hindcast experiments—one with 1° and the other with 0.1° horizontal resolution (Parallel Ocean Program low and high resolutions [POP‐LR and POP‐HR], respectively)—are compared with the Argo data for 2005–2009. POP‐LR has a shallow MLD bias common to many Fifth Coupled Models Intercomparison Project (CMIP5) models, while POP‐HR has a mix of deep and shallow MLD biases. While both models were able to replicate the large‐scale processes leading to formation of a high‐salinity layer, the salinity feature in POP‐HR is too strong and deep. Neither model was able to replicate the vertical mixing processes leading to penetration of the subsurface salt maximum. Plain Language Summary: The Southern Ocean that circles the Antarctic continent is important in controlling the amount of global heat and carbon exchanges with the atmosphere because during the winter the surface waters mix deeply down into the ocean. Argo floats drift throughout the world oceans collecting temperature and salinity data from the surface to about 2,000‐m depth. This study uses gridded Argo float data for 2004–2017 to understand the vertical mixing. We identify the importance of a subsurface high‐salinity feature in the Southern Ocean that stabilizes the column and impacts the depth of vertical mixing, and we investigate the origin of this salinity feature. We also analyze the representation of the salinity feature in the Community Earth System Model (CESM) at both a coarse and a fine resolution for 2005–2009. We find that both model configurations capture the main processes leading to the formation of the high‐salinity layer. However, neither model configuration was unable to replicate the vertical mixing processes leading to penetration of the subsurface salt maximum. Key Points: Gridded Argo data indicate that a subsurface salinity maximum mediates the rate of mixed layer deepeningTurner angle at the mixed layer depth provides a useful metric to evaluate the impact of salinity on stabilityNeither coarse nor fine‐resolution models can consistently replicate the processes necessary to penetrate the salinity feature [ABSTRACT FROM AUTHOR]

We use three transatlantic cruises (1957, 1993 and 2010) along 7.5°N to analyse inter-decadal variations of the neutral buoyancy frequency (with neutral density replacing potential density) and Turner angle. We also use Argo data from the 2003-2011 period to show that, within central and intermediate waters, the changes between the three sections are slightly greater than the seasonal and inter-annual variations, so they may be interpreted as actual inter-decadal variations. The results point to a generalized sinking of isoneutrals between 1957 and 2010, with maximum zonally-averaged values of about 100 m in the central and upper deep layers. They also reveal the occurrence of substantial changes in the intermediate and neighbouring water strata, with differential vertical sinking of isoneutrals and the transformation of their thermohaline characteristics. The neutral buoyancy frequency increased in the lower central and upper intermediate layers and decreased in the lower intermediate and upper deep layers. The distributions of Turner angle highlight a predominance of salt-fingering in the North Atlantic, except for a gravitationally doubly-stable layer located immediately below the intermediate water core; this stable layer thinned substantially between 1957 and 2010 because of the sinking of the lower intermediate isoneutrals acting together with actual water transformations from Antarctic Intermediate Waters into North Atlantic Deep Waters. We conclude that a significant portion of the water column underwent both increased vertical stratification and enhanced salt-fingering, two mechanisms with opposite effects on the effective vertical diffusion.

Profiles of salinity and temperature were measured in the strait of Hormuz (SH) during the winter of 2012, spring and summer of 2013. To investigate the double diffusion (DD) processes, Turner (TU) angle values are calculated in all the stations in the SH. Different TU angle values correspond to salt fingering (SF), diffusive convection (DC) and stable stratification. The distributions of the two forms of DD were plotted vertically along transects in the eastern, central and western part of the SH, and corresponding DD processes were described. The results show that both SF and DC occurred in most part of the study area. Two different water masses (the Indian Ocean surface water and the Persian Gulf water) were evident at the SH, and SF and DC were evident at the interface of two water masses. Due to evaporation, SF occurred in the surface layer of most Stations throughout the year. In the eastern part of the SH, occurrences of DC were more feasible in wintertime. SF was the main phenomenon at the end of hot season. For central part, SF occurred throughout the year in water column. In the western part, water column was stable in summer and DC happened in most part of water column in winter. [ABSTRACT FROM AUTHOR]

The physical reanalysis component of the Mediterranean Forecasting System is used to construct a high-resolution three-dimensional atlas of the Turner Angle. An assessment of the model quality shows a maximum degree of agreement with observations in the water column between 150 and 1000 m depth. The mean state of the favourable conditions for double diffusion processes is evaluated and the recent decadal variability is studied in terms of changes in the water mass properties. The results show that approximately 50% of the Mediterranean Sea is favourable to double diffusion processes, from which around 47% is associated with salt fingering. The Tyrrhenian, Ionian and southwestern Mediterranean are the most vulnerable basins to salt fingering, and the strongest processes can occur in the Tyrrhenian deep waters. Diffusive convection is most likely to occur in the Ionian, Aegean and eastern Mediterranean at vertical levels deeper than 1000 m. The observed gradual warming and salinification of the Mediterranean after 1997 decreased and increased the possibilities of the occurrence of salt fingers and double diffusive convections, respectively. The climatological atlas that is presented in this paper provides a three-dimensional picture of the regions that are either doubly stable or favourable to double diffusion instability and allows for the characterization of the diffusive properties of the water masses. [ABSTRACT FROM AUTHOR]

Structures caused by temperature and salinity gradients and their resulting mixing have a significant effect on sound propagation in shallow and deep waters. One of the vertical structures is salt-fingering (SF), which is formed by the vertical gradient of temperature and salinity, when warm and salty water is located above cold and fresh water. Quantification of double diffusion (DD) structures is done by the Turner (Tu) angle theory. Warm surface water from the Oman Sea, evaporation and salty surface water of the Persian Gulf provides the initial conditions for the formation of SF in the Strait of Hormuz. In depth less than 5 m, weak SF (45 < Tu ≤ 67.5) are formed and by dominating water buoyancy, salt fingers gradually grow and strong SF (67.5≤Tu < 90) extend from the surface to 25 m deep. This process causes mixing and changing the depth of the boundary layer. The diffusion structure in the SF plays a significant role in the deflection and energy loss of the sound signal. By passing through the SF structure on the surface, the sound signal is scattered, and its transmission is lost up to 80 dB (TLS), and the propagation of the sound along the channel is weakened. With increasing frequency and propagation angle, the deflection occurs from the location of the SF towards the cold and fresh water. But by passing through the place of the strong SF, the sound signal is propagated with a delay and 10–20 dB increase in TLS. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper describes the thermohaline characteristics of the Arctic Front to the west of Spitsbergen, in terms of the double-diffusive environment and the potential for densification through cabbeling. The front separates the warm, saline Atlantic water in the West Spitsbergen Current from the cooler, fresher water on the West Spitsbergen Shelf. We have investigated processes at the front that can contribute to, or enhance, mixing and water mass modification in relation to heat transport to the Arctic. Hydrographic data were collected along a cross-shelf section in September 2005. The double-diffusive properties along the section were determined by calculating Turner angles, and cabbeling was investigated with a simple linear mixing scheme. Double diffusion, in the form of diffusive layering, was found to be active within the Arctic Front, with considerable interleaving between water masses. Furthermore, mixing of water masses across the front was found to generate a potential increase in density of more than 0.03 kg m−3 through cabbeling, which would then promote sinking and convergence. Our analyses indicate that the two processes of double diffusion and cabbeling are active at the Arctic Front. We discuss their potential contribution to maintaining the density-compensated nature of the front, and conclude that they will promote isopycnal mixing and subsequent modification of the core water of the West Spitsbergen Current. [ABSTRACT FROM AUTHOR]

The World Ocean Atlas 1998 is used to determine the global field of the meridional density ratio Rρhy=αΔT/βΔS, where temperature and salinity changes ΔT and ΔS are evaluated along meridians, in and below the mixed layer. The focus of the analysis is the identification of regions where the Rρhy field matches the values Rρ=2 sometimes suggested as the commonly perceived state of the ocean and Rρ=1, the condition of density compensation. Results are presented through fields of the meridional Turner angle Tuhy=arctan(Rhyρ) and through histograms of Tuhy for the Pacific, Atlantic and Indian Oceans at the ocean surface and at 300 m depth. At the 300-m depth level, which in the subtropics is representative of conditions in the permanent thermocline, the most frequently encountered values of the meridional density ratio are Rρhy=3.2 in the North and South Pacific, Rρhy=2.0 in the South Atlantic and Indian and Rρhy=1.6 in the North Atlantic Ocean. Conditions in the mixed layer are more variable and show seasonal differences, but Rρhy=2.0 occurs prominently in all ocean regions during winter and in all regions but the Atlantic during summer. Summer values for the Atlantic Ocean are Rρhy=3.2 in the Northern Hemisphere and Rρhy=2.4 in the Southern Hemisphere. Detailed analysis of Rρhy across the Subtropical Front (STF) confirms the most frequently observed values but shows zonal variation along the front in some oceans. Nearly complete density compensation (Rρhy=1) in the mixed layer is encountered in the STF of the eastern North Pacific, the eastern South Pacific and the eastern Indian Ocean. The eastern Indian Ocean south of Australia is also the only region where complete density compensation in the STF occurs below the mixed layer. [ABSTRACT FROM AUTHOR]

Abstract: The warm and saline deep inflow into the Eastern Gotland Basin (EGB), which lasted from the end of November 1997 until the beginning of May 1998, was the first of a series of warm deep water inflows. Temporal fluctuations of the deep circulation were monitored at 170 m depth by two current meter moorings deployed at the north-eastern and south-western rim. The three-dimensional structure of stratification parameters was inferred from the comparison of two high-resolution hydrographic surveys carried out before and during the end of the 1997/1998 inflow event. The results indicate that near-bottom isopycnals were lifted more than 50 m during the inflow up to a depth of 170–180 m. Since the basin is enclosed at 170 m depth this implies a complete deep water renewal. Detected warm-water intrusions are shown to enhance the temperature variability on isopycnal surfaces at intermediate depth, creating strong inverse temperature gradients below the halocline in an environment with stable stratification due to salinity. Corresponding Turner angles suggest that diffusive convection may significantly contribute to diapycnal mixing in this region. In contrast, for the deepest near-bottom layers only marginally affected by intrusions, basin-scale budgets of heat and salinity suggest comparable diffusivities (κ s =1.3–2.1×10−5 m2/s for salt and κ s =1.6–2.8×10−5 m2/s for potential temperature), implying that double-diffusion does not play a major role in mixing there. For the intermediate layer between 90 and 170 m, where the budget method could not be adopted, we applied an idealized model for the decay of temperature variance associated with the intrusions. The results indicated that the intrusions decay within a few weeks, corresponding to vertical diffusivities of a few times 10−5 m2/s. [Copyright &y& Elsevier]