Seasonal Pycnocline Research Articles

Dispersion in the Open Ocean Seasonal Pycnocline at Scales of 1–10 km and 1–6 days

AbstractResults are presented from two dye release experiments conducted in the seasonal thermocline of the Sargasso Sea, one in a region of low horizontal strain rate (~10−6 s−1), the second in a region of intermediate horizontal strain rate (~10−5 s−1). Both experiments lasted ~6 days, covering spatial scales of 1–10 and 1–50 km for the low and intermediate strain rate regimes, respectively. Diapycnal diffusivities estimated from the two experiments were κz = (2–5) × 10−6 m2 s−1, while isopycnal diffusivities were κH = (0.2–3) m2 s−1, with the range in κH being less a reflection of site-to-site variability, and more due to uncertainties in the background strain rate acting on the patch combined with uncertain time dependence. The Site I (low strain) experiment exhibited minimal stretching, elongating to approximately 10 km over 6 days while maintaining a width of ~5 km, and with a notable vertical tilt in the meridional direction. By contrast, the Site II (intermediate strain) experiment exhibited significant stretching, elongating to more than 50 km in length and advecting more than 150 km while still maintaining a width of order 3–5 km. Early surveys from both experiments showed patchy distributions indicative of small-scale stirring at scales of order a few hundred meters. Later surveys show relatively smooth, coherent distributions with only occasional patchiness, suggestive of a diffusive rather than stirring process at the scales of the now larger patches. Together the two experiments provide important clues as to the rates and underlying processes driving diapycnal and isopycnal mixing at these scales.

Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?

AbstractIn general, strong wind events can enhance ocean turbulent mixing, followed by episodic nutrient supply to the euphotic zone and phytoplankton blooms. However, it is unclear whether such responses to strong winds occur in the ice‐free Canada Basin, where the seasonal pycnocline is strong and the nutricline is deep. In the present study, we monitored a fixed‐point observation (FPO) station in the Canada Basin for about 3 weeks in the fall of 2014 to examine the oceanic and biological responses to strong winds. At the FPO site, oceanic microstructure measurements, hydrographic surveys, and water sampling were performed with high temporal resolution, recording internal wave propagation, eddy passage, and water mass changes. Strong winds and internal wave propagation significantly enhanced the mixing above and at the seasonal pycnocline, but their effects were diminished at the nutricline, which was much deeper than the seasonal pycnocline. Therefore, wind‐induced mixing did not increase the upward nutrient supply from the nutricline and did not impact phytoplankton (chlorophyll a) distribution in the surface layer of the FPO site. The temporal evolution of the chlorophyll a concentration was most closely related to water mass changes. We also observed prominent subsurface chlorophyll a maxima with abundant large‐sized phytoplankton that were likely carried by warm‐core eddies to the FPO site. Phytoplankton biomass may have been sustained by the high concentration of ammonium within the eddy and ammonium regeneration at the seasonal pycnocline, where particulate organic matter likely accumulated.

Numerical studies of the internal waves interaction with the Black Sea shelf topography at spring-summer stratification

This work investigates the regional specifics of the appearance and propagation of internal waves at the shelf of the Black Sea. The parameters of horizontal inhomogeneity associated with changes in the bottom relief and density of seawater that lead to instability and destruction of internal waves are studied. The numerical experiments for the spring and summer seasons on transects normal to the coast are conducted using data of temperature and salinity from the Bank of oceanographic data of the Marine Hydrophysical Institute of RAS. The article touches upon the actual theme of adaptation of satellite images obtained from open sources when solving oceanographic problems. The result of the processing of remote sensing data was information about the speeds and wavelengths for the observed phenomena. These values are used to recover the depth of the seasonal pycnocline within a two-layer approximation of the vertical density structure. For different bottom slopes and seasonal profiles of the buoyancy frequency, sites are determined where the transition from supercritical to critical or subcritical conditions is possible in the region of the Crimean coast.

Buoyancy Effect on the Winter South China Sea Western Boundary Current

AbstractThe month‐to‐month variation of the winter South China Sea (SCS) western boundary current (WBC) along the western slope is examined using drifter observations, satellite altimetry data, and an ocean reanalysis. The most surprising phenomenon is that the WBC velocity at the sea surface reaches the maxima in November–December, which cannot be explained by wind forcing and Kuroshio intrusion alone. Analysis results demonstrate that buoyancy effect should be considered to explain the month‐to‐month variation besides wind‐Kuroshio effects. In winter, cold‐and‐salty advection by the WBC from the north decreases/reverses the zonal density gradient in the seasonal pycnocline induced by wind forcing and Kuroshio intrusion and therefore weakens wind‐Kuroshio‐induced WBC. Buoyancy effect on the winter SCS WBC is opposite to wind‐Kuroshio effects. In addition, buoyancy effect reaches the maximum in January, which is concurrent with wind‐Kuroshio effects. As a result of their competition, the zonal density gradient in the seasonal pycnocline is maximum in November–December, resulting in the maximum surface velocity along the western slope occurring in November‐December. This study demonstrates the importance of buoyancy forcing to the winter SCS WBC.

Regional'nye osobennosti raspredeleniya chastoty Vyaysyalya – Brenta v moryakh Laptevykh i Vostochno-Sibirskom

Purpose. The aim of the paper is to study buoyancy frequency in the Laptev and East Siberian seas and to assess correlation relations between the intra-year variability of the Vaisal – Brent frequency maximum and the climatic indices reflecting the atmosphere and hydrosphere state. Methods and Results. Based on the World Ocean Atlas 2013 for 1955–2012, the features of spatial and temporal variability of the buoyancy frequency distribution and its maximum depth in the Laptev and East Siberian seas are considered. It is found that the highest values of the Vaisal – Brent frequency are observed in the estuaries of the rivers Lena, Khatanga, Kolyma and Indigirka in summer where they attain 70–86 cycle/hour. In the deep-water northern areas of the seas under study, the majority of the water area is covered with ice throughout the whole year. As a consequence the surface layer of water is heated slightly and stability of stratification is much lower. The buoyancy frequency values in the seasonal pycnocline attain 24–46 cycle/hour. As a result of advection of the transformed Pacific waters in the northern and southeastern regions of the East Siberian Sea, on the depths 25–55 m observed is the layer of constant pycnocline where the Vaisal – Brent frequency values can mount to 21 cycles/hour. Correlation analysis is performed for the relations between the intra-annual variability of the Vaisal – Brent frequency maximum and the climatic indices (North Atlantic Oscillation, Arctic Oscillation, Pacific Decadal Oscillation, Gulf Stream North Wall) reflecting the atmosphere and hydrosphere state. It is shown that relationship between the buoyancy frequency maximum and the two latter indices is the most pronounced. Conclusions. The results of the carried out investigations show that the features of spatial-temporal variability of the buoyancy frequency distribution and its maximum depth in the Laptev and East Siberian seas are conditioned by the river discharge and also by advection in the Arctic region of the Atlantic and transformed Pacific waters inflowing from the adjacent basins.

Collective sinking promotes selective cell pairing in planktonic pennate diatoms

Finding a partner in an inherently unsteady 3-dimensional system, such as the planktonic marine environment, is a difficult task for nonswimming organisms with poor control over their orientation. We experimentally investigate the process of cell pairing in pennate marine diatoms and present field evidence of its occurrence in the ocean. We describe the mechanism as a 3-step process in which pennate diatoms (i) vertically reorient while sinking from surface turbulent waters to a more stable environment (i.e., under the seasonal pycnocline), (ii) segregate from incompatible partners (e.g., dead or different sized cells), and (iii) pair with other partners as a result of the hydrodynamic instabilities generated by collective cell sinking. This is, eminently, a cell abundance-dependent process, therefore being more effective when population sinking is synchronized. We suggest that this selective process, enabling matching of size-compatible healthy partners, could be fundamental in understanding sexual reproduction in pennate diatoms.

Diagnosing the development of seasonal stratification using the potential energy anomaly in the North Pacific

Upper-ocean seasonal stratification (seasonal pycnocline and/or transition layer) is a ubiquitous feature and its vertical structure has large spatial variability. The density stratification regulates the stability of the upper ocean and thus can affect the oceanic response to atmospheric forcing and biogeochemical processes by modulating vertical mixing. In this study, we described the development of the seasonal stratification in terms of the stability of the water column, using the potential energy anomaly (PEA) as a metric based on Argo profiles. PEA budget analysis reveals that over most of the North Pacific, seasonal stratification develops under a vertical one-dimensional energy balance between an increase in PEA (i.e., a strengthening of the stratification) driven by atmospheric buoyancy forcing and a decrease in PEA associated with vertical mixing within the water column. Horizontal advection of PEA plays a significant role in the seasonal development of the stratification only in the regions of the western boundary current and equatorial current system south of 10°N. We find that, in addition to the total magnitude of the oceanic buoyancy gain, the balance between compositions of the atmospheric forcing (non-penetrating surface buoyancy forcing and penetrating radiative heating) is also important in explaining regional differences in the development of the seasonal stratification. The vertical diffusivity in the seasonal stratification estimated from the residual of the PEA budget is in the range from 5 × 10−5 m2 s−1 to 5 × 10−4 m2 s−1 and shows spatial and seasonal variability associated with local wind forcing.

Diel Vertical Variability of the Concentration of Chlorophyll a and Colored Dissolved Organic Matter in Possjet Bay, Sea of Japan

The vertical distribution of the concentration of chlorophyll a (Chl) and colored dissolved organic matter (CDOM), temperature (T), and salinity (S) were studied at two diel stations located at close coordinates and performed at a weekly interval and seven stations spaced 1 mi apart on hydrological section in the shelf zone of Possjet Bay (Sea of Japan) in the autumn of 2014. The Chl concentration was estimated from fluorescence intensity (FChl). In the surface layer of the diel stations, the Chl concentration varied within ± 10% of the mean value. In the water column, a negative correlation of FChl with T and a positive relationship of S with CDOM were established at both stations. At all stations, the maximum Chl concentration occurred under the seasonal pycnocline layer.

Sea Surface Signature of Internal Tides

AbstractThe upper ocean is energetic at scales below the Rossby deformation radius owing to the existence of internal waves and submesoscale fronts and vortices. These contribute significantly to energy dissipation, tracer mixing, and exchanges between the ocean surface layer, the ocean interior, and the atmospheric boundary layer. Internal waves and submesoscale motions both undergo strong spatial and seasonal variations, driven by different mechanisms. Here, we investigate the sea surface signature of internal waves and its seasonality using linear wave theory and a high‐resolution realistic simulation. In summer, internal waves are greatly amplified near the surface mostly due to a thin mixed layer bounded by a seasonal pycnocline. This surface amplification is well captured by linear theory, provided that the stratification at the base of the mixed layer is accurately represented. In winter, the superinertial motions are not fully explained by linear theory, reflecting impacts of the more energetic submesoscale motions.

Subtropical Mode Water and Permanent Pycnocline Properties in the World Ocean

AbstractA global reference state of the subtropical mode water and permanent pycnocline properties for the 2000–2015 period is presented. The climatology is obtained from a pattern recognition algorithm applied to stratification profiles from the Argo global array. The stratification features are identified as permanent upper ocean pycnostad and pycnocline even when the seasonal pycnocline is developed. The climatology shows that both Northern Hemisphere subtropical gyres have a qualitatively very similar stratification structure. The permanent pycnoclines in the North Atlantic and North Pacific show two deep centers colocated with thick subtropical and subpolar mode waters. These centers coincide with modes in the density and stratification space. These deep pycnocline centers are separated by a region with a shallower and thinner permanent pycnocline that is located downstream of Western Boundary Current Extensions and upstream of Eastern Subtropical Fronts. This feature creates a remarkable double‐bowl pattern at the basin scale. In the subtropical gyres of the South Atlantic and South Pacific Oceans, the mode water and permanent pycnocline structures are characterized by two modes in the density and stratification space that, unlike in the Northern Hemisphere, do not necessarily correspond to deep and thick permanent pycnocline regions. In the subtropical gyre of the South Indian Ocean, a single mode is found to correspond to a single center in the western part of the gyre. This study also shows that away from these deep centers where the pycnocline depth almost follows isopycnals, the permanent pycnocline experiences significant thermohaline gradients that are not density compensated.

Vertical turbulent exchange features in the layer of seasonal pycnocline in the northwestern part of the Black Sea

On the basis of experimental physical field’s microstructure data in the upper stratified layers of the Black Sea, the annual variability of the vertical turbulent exchange coefficient in the seasonal thermocline is investigated. The data were collected during eight expedition periods in 2007 – 2017, and they cover the north-western part of the Black Sea. The studies were conducted in different hydrological seasons from April to November. The measurements were carried out by the probing complex “Sigma-1”, the evaluation of turbulence energy dissipation was made by velocity pulsation gradients. The desired coefficient of vertical turbulent diffusion was calculated on the basis of the obtained experimental estimates of energy dissipation by known Osborn’s relationship. The analysis of these measurements makes it possible to calculate and use the vertical turbulent diffusion coefficient dependence on stratification to evaluate the vertical fluxes of heat, salt and other dissolved chemical and biological substances in the studied Black Sea region.

The role of internal waves in the late Quaternary evolution of the Israeli continental slope

Internal waves are widely present in world's oceans propagating in stratified ocean layers such as the seasonal pycnocline. At continental shelves and slopes, breaking of internal waves may create turbulence of sufficient energy to erode and transport sediments. In this study, we investigate the late Pleistocene to Recent impact of internal waves on the morphology of the Israeli Mediterranean continental slope. Based on mooring data, we show that the internal wave spectrum on the upper slope contains significant energy at near-inertial and semidiurnal tidal frequencies. Sediment subbottom profiler data show two fields of nearly contour parallel sediment waves located in water depths of 80 m–130 m and 190 m–350 m, respectively. A zone of flat seafloor morphology and remarkably constant slope inclination of 1.0°–1.2° separates both sediment wave fields. The entire stretch of seafloor between 80 m and 350 m corresponds to depths of major ocean density stratification, supporting the propagation of internal waves. In addition, this area is critically inclined with respect to semidiurnal internal tides and supercritically inclined with respect to near-inertial waves, two scenarios previously related to internal wave breaking. We propose that breaking internal waves control the inclination of the Israeli continental slope and contribute to sediment starvation within the zone of flat seafloor morphology. The formation of sediment waves is attributed to high-frequency trailing internal waves of similar wavelengths as sediment waves, observed within a seismic image of the water column. We correlate subbottom profiler data to a radiocarbon dated sediment core. No sediment waves formed from 16 cal ka–8 cal ka, indicating that the seafloor impact of internal waves ceased during a time of rapid sea level rise and fresh water input in the Eastern Mediterranean realm. The end of this period coincides with the formation of sapropel S1.

Vertical Structure of Currents in Northern Black Sea, October 2016

The paper presents the results of an analysis of hydrological data obtained on cruise 89 of the R/V Professor Vodyanitskii from September 30 through October 19, 2016, in the central area of the northern Black Sea. Currents were measured using a lowered acoustic Doppler current profiler (LADCP) to a depth of 350 m. Examples of the current velocity profiles and conditional density at individual stations are presented. The averaged characteristics of the vertical current structure in the seasonal and permanent pycnoclines are discussed. The isopycnally averaged kinetic energy (KE) demonstrates an almost linear dependence on the conditional density (σθ). In the seasonal pycnocline, EK = 0.2–0.013σθ; in the permanent pycnocline, EK = 0.12–0.007σθ. An interpretation is given for the variability of the kinetic energy with depth. It is shown that the maxima of the averaged vertical shear profile correspond to the occurrence depth of the seasonal and permanent pycnoclines. The mean Richardson number has a value of ~10 in the upper sea layer and decreases with depth, assuming a value of ~3 at the 300 m horizon. The estimate for the vertical turbulent diffusion coefficient from the simplified model shows its increase with depth.

SEASONAL CHANGES OF VERTICAL PROFILES OF CHLOROPHYLL A IN THE AMUR BAY (JAPAN SEA)

Seasonal changes of chlorophyll a profiles are traced over the Amur Bay (Peter the Great Bay, Japan Sea) in May-October, 2017 by means of oceanographic sonde-profiler equipped with fluorometer. Two principally different types of the vertical profiles are revealed, which were formed by different mechanisms of productivity: i) Chl a concentration had the maximum at the sea surface and decreased with the depth in the internal part of the bay occupied by the estuarine waters, and ii) Chl a concentration had the maximum below the seasonal pycnocline in the external part of the bay connected with the open sea. The highest Chl a concentration was observed in July-August for the estuarine type because of summer monsoon flood on the rivers, but in September for the marine type because of the coastal upwelling induced by monsoon winds change. Comparing these results with estimations of Chl a concentration made with the satellite data, insufficient correspondence is concluded for the external part of the bay, outside the estuarine zone, because the satellite data don’t reflect well the chlorophyll a in the subsurface layer and its seasonal variations. Thus, underestimation of real productivity and feeding ability of marine areas is available with the satellite data on chlorophyll a.

Assessment of the Effect of Increasing Temperature On the Ecology and Assemblage Structure of Modern Planktic Foraminifers in the Caribbean and Surrounding Seas

Abstract Habitat patterns of subtropical and tropical planktic foraminifers in the Caribbean Sea were obtained from plankton samples collected in spring 2009 and 2013. The spatial distribution in surface waters (3.5 m water depth) and depth habitat patterns (surface to 400 m) of 33 species were compared with prevailing water-mass conditions (temperature, salinity, and chlorophyll-a concentration) and planktic foraminiferal test assemblages in surface sediments. Distribution patterns indicate a significant relationship with seawater temperature and trophic conditions. A reduction in standing stocks was observed close to the Orinoco River plume and in the Gulf of Paria, associated with high turbidity and concomitant low surface-water salinity. In contrast, a transient mesoscale patch of high chlorophyll concentration in the eastern Caribbean Sea was associated with higher standing stocks in near surface waters, including high abundances of Globigerinita glutinata and Neogloboquadrina dutertrei. Globorotalia truncatulinoides mainly lives close to the seasonal pycnocline and can be linked to winter conditions indicated by lower sea-surface temperatures (SSTs) of ∼20°C. Globigerinoides sacculifer and Globoturborotalita rubescens were associated with oligotrophic conditions in the pelagic Caribbean Sea during early spring and showed a synodic lunar reproduction cycle. The live assemblages in the water column from 2009 and 2013 were similar to those reported in earlier studies from the 1960s and 1990s and to assemblages of tests in the surface sediments. Minor differences in faunal proportions were attributed to seasonal variability and environmental differences at the local scale. An exception was the low relative abundance of Globigerinoides ruber in the Caribbean Sea in 2009 compared to surface sediment samples and plankton net samples collected in the 1960s and 1990s. Decreasing abundance of Gs. ruber white in the Caribbean Sea may be associated with increasing SSTs over past decades and changes in nutrient flux and primary production.

Comparing a Multivariate Global Ocean State Estimate With High-Resolution in Situ Data: An Anticyclonic Intrathermocline Eddy Near the Canary Islands

The provision of high-resolution \textit{in situ} oceanographic data is key for the ongoing verification, validation and assessment of operational products, such as those provided by the Copernicus Marine Core Service (CMEMS). Here we analyze the ability of ARMOR3D - a multivariate global ocean state estimate that is available from CMEMS - to reconstruct a mesoscale anticyclonic intrathermocline eddy that was previously sampled with high-resolution independent \textit{in situ} observations. ARMOR3D is constructed by merging remote sensing observations with \textit{in situ} vertical profiles of temperature and salinity obtained primarily from the Argo network. \textit{In situ} data from CTDs and an Acoustic Doppler Current Profiler were obtained during an oceanographic cruise near the Canary Islands (Atlantic ocean). The analysis of the ARMOR3D product using the \textit{in situ} data is done over (i) a high-resolution meridional transect crossing the eddy center and (ii) a three-dimensional grid centered on the eddy center. An evaluation of the hydrographic eddy signature and derived dynamical variables, namely geostrophic velocity, vertical vorticity and quasi-geostrophic (QG) vertical velocity, demonstrates that the ARMOR3D product is able to reproduce the vertical hydrographic structure of the independently sampled eddy below the seasonal pycnocline, with the caveat that the flow is surface intensified and the seasonal pycnocline remains flat. Maps of ARMOR3D density show the signature of the eddy, and agreement with the elliptical eddy shape seen in the \textit{in situ} data. The major eddy axes are oriented NW-SE in both data sets. The estimated radius for the \textit{in situ} eddy is $\sim$ 46 km; the ARMOR3D radius is significantly larger at $\sim$ 92 km and is considered an overestimation that is inherited from an across-track altimetry sampling issue. The ARMOR3D geostrophic flow is underestimated by a factor of 2, with maxima of 0.11 (-0.19) m s$^{-1}$ at the surface, which implies an underestimation of the local Rossby number by a factor of 3. Both the \textit{in situ} and ARMOR3D eddies have decelerating flows at their northern edges. The ARMOR3D QG vertical velocity distribution has upwelling/downwelling cells located along the eddy periphery and similar magnitudes to the \textit{in situ}-derived QG vertical velocity.

Three‐Dimensional Dynamics of Baroclinic Tides Over a Seamount

AbstractThe Massachusetts Institute of Technology general circulation model is used for the analysis of baroclinic tides over Anton Dohrn Seamount (ADS), in the North Atlantic. The model output is validated against in situ data collected during the 136th cruise of the RRS “James Cook” in May–June 2016. The observational data set includes velocity time series recorded at two moorings as well as temperature, salinity, and velocity profiles collected at 22 hydrological stations. Synthesis of observational and model data enabled the reconstruction of the details of baroclinic tidal dynamics over ADS. It was found that the baroclinic tidal waves are generated in the form of tidal beams radiating from the ADS periphery to its center, focusing tidal energy in a surface layer over the seamount's summit. This energy focusing enhances subsurface water mixing and the local generation of internal waves. The tidal beams interacting with the seasonal pycnocline generate short‐scale internal waves radiating from the ADS center. An important ecological outcome from this study concerns the pattern of residual currents generated by tides. The rectified flows over ADS have the form of a pair of dipoles, cyclonic and anticyclonic eddies located at the seamount's periphery. These eddies are potentially an important factor in local larvae dispersion and their escape from ADS.

Assessment of thermal stratification, stability and characteristics of deep water zone of the southern Caspian Sea

Variations of seawater properties and stratification of water column in the southern deep water zone (470 m depth) of the Caspian Sea were evaluated based on field measured data. Variations of physical characteristics (temperature, salinity, density and sound speed) of seawater mainly were limited to the upper 100 m layers. Below this layer, seasonal changes of seawater parameters were minor. Vertical structure of temperature in the southern Caspian Sea waters was characterized by a significant seasonal thermocline approximately between 20 and 50 m depths in summer with a vertical gradient of around 16 °C. Destruction of the thermocline occurred with the general cooling of the sea surface water and deepening of the mixed layer during late of autumn and winter. Frequency of stability in water column often across the upper layers (surface mixed and thermocline) was more than deeper levels. Variations of salinity were around 12.4 PSU in the time of measurements. Structures of thermocline and pycnocline were characterized by a significant homogeneity especially in offshore stations during the year. According to high agreement between density and temperature in the Caspian Seawater, a strong seasonal pycnocline was observed in the position of the thermocline. Furthermore, structure of sound speed variations followed vertical variations of seawater temperature. Variations of chemical characteristics (dissolved oxygen and pH) of seawater mainly were continued from sea surface to near the sea bed in deep water zone.

Ageostrophic Secondary Circulation in a Subtropical Intrathermocline Eddy

AbstractVertical motions play a key role in the enhancement of primary production within mesoscale eddies through the introduction of nutrients into the euphotic layer. However, the details of the vertical velocity field w driving these enhancements remain under discussion. For the first time the mesoscale w associated with an intrathermocline eddy is computed and analyzed using in situ high-resolution three-dimensional (3D) fields of density and horizontal velocity by resolving a generalized omega equation valid for high Rossby numbers. In the seasonal pycnocline the diagnosed w reveals a multipolar structure with upwelling and downwelling cells located at the eddy periphery. In the main pycnocline w is characterized by a dipolar structure with downwelling velocities upstream of the propagation path and upwelling velocities downstream. Maximum values of w reach 6.4 m day−1. An observed enhancement of chlorophyll-a at the eddy periphery coincides with the location of the upwelling and downwelling cells. Analysis of the forcing terms of the generalized omega equation indicates that the mechanisms behind the dipolar structure of the w field are a combination of horizontal deformation and advection of vertical relative vorticity by ageostrophic vertical shear. The wind during the eddy sampling was rather constant and uniform with a speed of 5 m s−1. Diagnosed nonlinear Ekman pumping leads to a dipolar pattern that mirrors the inferred w. Horizontal ageostrophic secondary circulation is dominated by centripetal acceleration and closes the dipole w structure. Vertical fluxes act to maintain the intrathermocline eddy structure.

Turbulent Upper-Ocean Mixing Affected by Meltwater Layers during Arctic Summer

AbstractEvery summer, intense sea ice melt around the margins of the Arctic pack ice leads to a stratified surface layer, potentially without a traditional surface mixed layer. The associated strengthening of near-surface stratification has important consequences for the redistribution of near-inertial energy, ice–ocean heat fluxes, and vertical replenishment of nutrients required for biological growth. The authors describe the vertical structure of meltwater layers and quantify their seasonal evolution and their effect on turbulent mixing in the oceanic boundary layer by analyzing more than 450 vertical profiles of velocity microstructure in the seasonal ice zone north of Svalbard. The vertical structure of the density profiles can be summarized by an equivalent mixed layer depth hBD, which scales with the depth of the seasonal stratification. As the season progresses and melt rates increase, hBD shoals following a robust pattern, implying stronger vertical stratification, weaker vertical eddy diffusivity, and reduced vertical extent of the mixing layer, which is bounded by hBD. Through most of the seasonal pycnocline, the vertical eddy diffusivity scales inversely with buoyancy frequency (Kρ ∝ N−1). The presence of mobile sea ice alters the magnitude and vertical structure of turbulent mixing primarily through stronger and shallower stratification, and thus vertical eddy diffusivity is greatly reduced under sea ice. This study uses these results to develop a quantitative model of surface layer turbulent mixing during Arctic summer and discuss the impacts of a changing sea ice cover.