Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and ...the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Low-mode internal tides are generated at tall submarine ridges, propagate across the open ocean with little attenuation, and reach distant continental slopes. A semidiurnal internal tide ...beam, identified in previous altimetric observations and modeling, emanates from the Macquarie Ridge, crosses the Tasman Sea, and impinges on the Tasmanian slope. Spatial surveys covering within 150 km of the slope by two autonomous underwater gliders with maximum profile depths of 500 and 1000 m show the steepest slope near 43°S reflects almost all of the incident energy flux to form a standing wave. Starting from the slope and moving offshore by one wavelength (~150 km), potential energy density displays an antinode–node–antinode–node structure, while kinetic energy density shows the opposite.
Mission-mean mode-1 incident and reflected flux magnitudes are distinguished by treating each glider’s survey as an internal wave antenna for measuring amplitude, wavelength, and direction. Incident fluxes are 1.4 and 2.3 kW m
−1
from the two missions, while reflected fluxes are 1.2 and 1.8 kW m
−1
. From one glider surveying the region of highest energy at the steepest slope, the reflectivity estimates are 0.8 and 1, if one considers the kinetic and potential energy densities separately. These results are in agreement with mode-1 reflectivity of 0.7–1 from a model in one horizontal dimension with realistic topography and stratification. The direction of the incident internal tides is consistent with altimetry and modeling, while the reflected tide is consistent with specular reflection from a straight coastline.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
3.
Observations of the Transition Layer SHAUN JOHNSTON, T. M; RUDNICK, Daniel L
Journal of physical oceanography,
03/2009, Letnik:
39, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Abstract
The transition layer is the poorly understood interface between the stratified, weakly turbulent interior and the strongly turbulent surface mixed layer. The transition layer displays ...elevated thermohaline variance compared to the interior and maxima in current shear, vertical stratification, and potential vorticity. A database of 91 916 km or 25 426 vertical profiles of temperature and salinity from SeaSoar, a towed vehicle, is used to define the transition layer thickness. Acoustic Doppler current measurements are also used, when available. Statistics of the transition layer thickness are compared for 232 straight SeaSoar sections, which range in length from 65 to 1129 km with typical horizontal resolution of ∼4 km and vertical resolution of 8 m. Transition layer thicknesses are calculated in three groups from 1) vertical displacements of the mixed layer base and of interior isopycnals into the mixed layer; 2) the depths below the mixed layer depth of peaks in shear, stratification, and potential vorticity and their widths; and 3) the depths below or above the mixed layer depth of extrema in thermohaline variance, density ratio, and isopycnal slope. From each SeaSoar section, the authors compile either a single value or a median value for each of the above measures. Each definition yields a median transition layer thickness from 8 to 24 m below the mixed layer depth. The only exception is the median depth of the maximum isopycnal slope, which is 37 m above the mixed layer base, but its mode is 15–25 m above the mixed layer base. Although the depths of the stratification, shear, and potential vorticity peaks below the mixed layer are not correlated with the mixed layer depth, the widths of the shear and potential vorticity peaks are. Transition layer thicknesses from displacements and the full width at half maximum of the shear and potential vorticity peak give transition layer thicknesses from 0.11× to 0.22× the mean depth of the mixed layer. From individual profiles, the depth of the shear peak below the stratification peak has a median value of 6 m, which shows that momentum fluxes penetrate farther than buoyancy fluxes. A typical horizontal scale of 5–10 km for the transition layer comes from the product of the isopycnal slope and a transition layer thickness suggesting the importance of submesoscale processes in forming the transition layer. Two possible parameterizations for transition layer thickness are 1) a constant of 11–24 m below the mixed layer depth as found for the shear, stratification, potential vorticity, and thermohaline variance maxima and the density ratio extrema; and 2) a linear function of mixed layer depth as found for isopycnal displacements and the widths of the shear and potential vorticity peaks.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Sea surface temperature, determined remotely by satellite (SSST), measures only the thin “skin” of the ocean but is widely used to quantify the thermal regimes on coral reefs across the globe. In ...situ measurements of temperature complements global satellite sea surface temperature with more accurate measurements at specific locations/depths on reefs and more detailed data. In 1999, an in situ temperature-monitoring network was started in the Republic of Palau after the 1998 coral bleaching event. Over two decades the network has grown to 70+ stations and 150+ instruments covering a 700 km wide geographic swath of the western Pacific dominated by multiple oceanic currents. The specific instruments used, depths, sampling intervals, precision, and accuracy are considered with two goals: to provide comprehensive general coverage to inform global considerations of temperature patterns/changes and to document the thermal dynamics of many specific habitats found within a highly diverse tropical marine location. Short-term in situ temperature monitoring may not capture broad patterns, particularly with regard to El Niño/La Niña cycles that produce extreme differences. Sampling over two decades has documented large T signals often invisible to SSST from (1) internal waves on time scales of minutes to hours, (2) El Niño on time scales of weeks to years, and (3) decadal-scale trends of +0.2 °C per decade. Network data have been used to create a regression model with SSST and sea surface height (SSH) capable of predicting depth-varying thermal stress. The large temporal, horizontal, and vertical variability noted by the network has further implications for thermal stress on the reef. There is a dearth of definitive thermal information for most coral reef habitats, which undermines the ability to interpret biological events from the most basic physical perspective.
Winds generate inertial and near-inertial currents in the upper ocean. These currents dominate the kinetic energy and contain most of the vertical shear in horizontal currents. Subsequent shear ...instabilities lead to mixing. In the Bay of Bengal, the annual mean wind energy input and near-inertial mixed layer energy is almost as large as in the mid-latitude storm tracks. Also, mixing associated with these waves is known to affect mixed layer heat content, sea surface temperature, and, thus, precipitation in coupled global models. Therefore, the mechanisms leading to the decay of these currents in the mixed layer and below are of considerable importance. Two such decay mechanisms are examined here. One mechanism is the downward propagation of near-inertial internal waves, which is aided by the mesoscale circulation and is observed with a rapidly profiling float. In a few days (faster than at mid-latitudes), the near-inertial wave group propagated from the base of the mixed layer to 250 m depth in the stratified interior. Another decay mechanism is enhanced shear generation at the mixed layer base from periodic alignment of rotating, near-inertial current shear and winds, which is observed with a mooring and analyzed with a simple two-layer model.
Tidal flow through the Luzon Strait produces large internal waves that propagate westward into the South China Sea and eastward into the Pacific. Underwater gliders gathered sustained observations of ...internal waves during seven overlapping missions from April 2007 through July 2008. A particular focus is the high‐frequency internal waves, where the operational definition of high involves periods shorter than a glider profile taking 3–6 h. Internal wave vertical velocity is estimated from measurements of pressure and glider orientation through two methods: (1) use of a model of glider flight balancing buoyancy and drag along the glider path and (2) high‐pass filtering of the observed glider vertical velocity. By combining high‐frequency vertical velocities from glider flight with low‐frequency estimates from isopycnal depth variations between dives, a spectrum covering five decades of frequency is constructed. A map of the standard deviation of vertical velocity over the survey area shows a decay from the Luzon Strait into the Pacific. The growth of high‐frequency vertical velocity with propagation into the South China Sea is observed through two 2‐week time series stations. The largest observed vertical velocities are greater than 0.2 m s−1 and are associated with displacements approaching 200 m. The high‐frequency waves are observed at regular intervals of 1 day as they ride on diurnal tidal internal waves generated in the Strait.
Key Points
Temporal and spatial structure of internal waves observed by underwater gliders
Internal wave vertical velocity estimated using two methods
Waves decay from strait into Pacific and grow into the South China Sea
Temperature or salinity variability on isopycnals is referred to as spice, which acts as a tracer in the absence of mixing. The westward North Equatorial Current and eastward North Equatorial ...Countercurrent are incident upon Palau, where the meridional extent of the topography exceeds 100 km and the thermohaline structure on the up‐ and downstream sides of the islands can be distinct. Westward flow of high salinity water past two islands at the south point of Palau injects a spice anomaly into the wake in the form of a 15 km wide jet with changes of salinity of 0.1 psu across the headland and the jet. With three repeated surveys, several terms in the spice balance are estimated for the jet. Horizontal diffusivity has a typical value of about 11 m2 s−1 for 15‐km length scales, while the vertical diffusivity is about 10−5 m2 s−1 from the balance, with similar maximum estimates for the latter from a shear‐based parameterization. Typical vertical diffusivities at this latitude are about 10−6 m2 s−1, which is the area mean from the shear‐based parameterization. Amidst this typical background diffusion, the spice variance increases linearly in the wake over three successive surveys. The surveys occurred over similar but not identical areas, following the shifting flow. The spice anomaly is detected 100–200 km downstream and is consistent with the mesoscale flow obtained from altimetric absolute geostrophic currents. Spice anomalies are a useful way of tracing flows encountering abrupt topography.
Plain Language Summary
Dye tracers are used in experiments to follow flows over time. Here, a natural tracer is used to follow flow in an island wake. This tracer is salinity along an isopycnal, which is referred to as spice. Such salinity structure is set at the surface by processes like evaporation and precipitation. It is then subducted and stirred by currents. Density affects flow patterns and is dynamically active. Spice is not dynamically active and is only erased by mixing. At the south point of Palau, high salinity water on the upstream side was brought by mesoscale currents from about 250 km farther south. On the downstream side, there is local water with lower salinity. Over several days, the saltier water is injected into the wake and was followed by successive spatial surveys. The spice variance on the downstream side increases over time, which indicates stirring dominates. Horizontal and vertical diffusion values are typical. This stirring of salty water is noted 200 km downstream at Palau's north point, consistent with the mesoscale flow. Spice is a useful way of tracing flows encountering abrupt topography.
Key Points
On either side of a headland, spice contrasts are noted as salinity anomalies on an isopycnal and can trace the flow into the wake
Using the advection‐diffusion equation for spice, horizontal diffusivity at 15 km scale in the wake is 11 m2 s−1, a typical value
The spice anomaly is a useful tracer, is followed 200 km downstream, and is consistent with the mesoscale flow
Subtropical oceans contribute significantly to global primary production, but the fate of the picophytoplankton that dominate in these low-nutrient regions is poorly understood. Working in the ...subtropical Mediterranean, we demonstrate that subduction of water at ocean fronts generates 3D intrusions with uncharacteristically high carbon, chlorophyll, and oxygen that extend below the sunlit photic zone into the dark ocean. These contain fresh picophytoplankton assemblages that resemble the photic-zone regions where the water originated. Intrusions propagate depth-dependent seasonal variations in microbial assemblages into the ocean interior. Strikingly, the intrusions included dominant biomass contributions from nonphotosynthetic bacteria and enrichment of enigmatic heterotrophic bacterial lineages. Thus, the intrusions not only deliver material that differs in composition and nutritional character from sinking detrital particles, but also drive shifts in bacterial community composition, organic matter processing, and interactions between surface and deep communities. Modeling efforts paired with global observations demonstrate that subduction can flux similar magnitudes of particulate organic carbon as sinking export, but is not accounted for in current export estimates and carbon cycle models. Intrusions formed by subduction are a particularly important mechanism for enhancing connectivity between surface and upper mesopelagic ecosystems in stratified subtropical ocean environments that are expanding due to the warming climate.
Elevated mixing at a front Johnston, T. M. Shaun; Rudnick, Daniel L.; Pallàs-Sanz, E.
Journal of Geophysical Research: Oceans,
November 2011, Letnik:
116, Številka:
C11
Journal Article
Recenzirano
Odprti dostop
The mesoscale, submesoscale, and microscale structure of a front in the California Current was observed using a towed vehicle outfitted with microconductivity sensors. Thirteen >60 km cross‐front ...sections from 0 to 350 m in depth were covered in 3.5 days. Objectively mapped data are fit via the Omega (ω) equation to obtain vertical velocity. A composite cross‐front section shows elevated mixing on the dense side within 10–20 km of the front. Water downwells and gradients are elevated there: i.e., Rossby number (Ro), horizontal strain (α), spice gradients, and microscale thermal dissipation (χ). Thermal eddy diffusivity (KT) reaches 10−3 m2 s−1 and increases 3–10× from the anticyclonic to the cyclonic side with a depth mean of ∼10−4 m2 s−1. The spatial structure of KT, Ro, and α are similar on the dense side, suggesting an energy cascade from the mesoscale via the submesoscale to the microscale. However, it is unclear whether frontogenesis, internal wave blocking by elevated vorticity, or internal wave trapping by large α produces the elevated mixing. The mean turbulent heat flux opposes the mean restratifying, mesoscale heat flux of 10 W m−2 and may allow the front to persist. Turbulent nitrate fluxes are 0.1–0.3 mmol m−2 s−1. Chlorophyll fluorescence and beam transmission reveal a <6 km wide, ∼100 km long alongfront streamer which is a deep biomass maximum. Time scales for mixing and nutrient fluxes are 0.3–3 days, which are similar to phytoplankton growth rates and the time scale for frontal evolution.
Key Points
Elevated mixing on dense side related to Rossby number and strain
Mean turbulent heat flux opposes mean restratifying, mesoscale heat flux
Similar time scales for mixing, frontal evolution, and nutrient fluxes
Measuring vertical motions represent a challenge as they are typically 3–4 orders of magnitude smaller than the horizontal velocities. Here, we show that surface vertical velocities are intensified ...at submesoscales and are dominated by high frequency variability. We use drifter observations to calculate divergence and vertical velocities in the upper 15 m of the water column at two different horizontal scales. The drifters, deployed at the edge of a mesoscale eddy in the Alboran Sea, show an area of strong convergence (O $\mathcal{O}$(f)) associated with vertical velocities of −100 m day−1. This study shows that a multilayered‐drifter array can be an effective tool for estimating vertical velocity near the ocean surface.
Plain Language Summary
The study of vertical motions in the ocean is a key challenge as they are difficult to measure and predict although their impact is crucial on the exchange of water properties in the water column. Here, we use high resolution drifter observations to calculate vertical velocities in the upper layers of a surface density front. This study shows high values of downward speed combined with a high temporal variability at the surface layer.
Key Points
Horizontal divergence and vertical vorticity computed from drifter observations
Maximum downward speeds of 100 m day−1 in the upper 15 m on a subducting region
The vertical velocity time series shows a high temporal variability, varying from 50 to −100 m day−1 over 4 hr
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