Temperature Fine Structure Research Articles

Deepwater dynamics and mixing processes during a major inflow event in the central Baltic Sea

AbstractIntrusions of large amounts of dense and oxygen‐rich waters during so‐called Major Baltic Inflows (MBIs) form an essential component of the Baltic Sea overturning circulation and deepwater ventilation. Despite their importance, however, detailed observations of the processes occurring in the central basins during an MBI are virtually lacking. Here data from a long‐term deployment of an autonomous profiling platform located in the center of one of the main basins are presented, providing the first direct and detailed view of the deepwater modifications and dynamics induced by one of the largest MBIs ever recorded (MBI 2014/2015). Approximately, 21 Gmol of oxygen were imported during three distinct inflow phases with an unexpectedly large contribution of oxic intrusions at intermediate depth. Oxygen consumption rates during the stagnation period immediately following the inflow phase was found to be 87 g m−2 yr−1 with a dominant contribution of sedimentary oxygen demand. The most energetic deepwater processes (topographic and near‐inertial waves) were only marginally affected by the inflow; however, subinertial energy levels associated with intrusions and eddies were strongly enhanced. Turbulence microstructure data revealed that the deep interior regions remain essentially nonturbulent even during the energetic conditions of an MBI, emphasizing the importance of boundary mixing. Warm intrusions frequently showed a temperature fine structure with vertical scales of the order of 0.1 m, without any signs of active turbulence. At the upper flanks of these intrusions, double‐diffusive staircases were often found to develop, suggesting an important alternative mixing process during inflow conditions.

Temperature finestructure in the Indonesian Seas

Twenty‐one years of eXpendable BathyThermograph temperature profile data are used to examine the spatial and temporal variability in finestructure throughout the Indonesian Seas region. Finestructure is the variation of vertical properties on scales from several meter to tens of meter thickness, and is associated, for example, with internal wave activity, an evolving system that can be a precursor to turbulent vertical mixing. As might be expected, on average, the highest temperature finestructure values are observed within 5 km of the shelf‐slope boundary, especially within straits, and the lowest values at distances greater than 35 km. However, the long‐period time variability in the temperature finestructure reveals significant, unexpected, depth‐dependent monsoonal and interannual modulations. In the upper 100 m, there is a 63% enhancement in the temperature finestructure during the windy Southeast Monsoon months of June, July, and August (in particular during La Niña years), relative to the low‐wind intermonsoon months of September, October, and November (in particular during El Niño years). Below 100 m, the monsoon influence is much reduced, but surprisingly significant interannual temperature finestructure variability is observed down to 600‐m depth with a signal opposite to that in the upper layer of the ocean: a 6‐month lagged 13% enhancement associated with El Niño relative to La Niña time periods between 100‐ and 600‐m depths. These results provide a background framework in which to interpret modeling and observational vertical mixing studies in the Indonesian Seas region.

Simultaneous seismic reflection and physical oceanographic observations of oceanic fine structure in the Kuroshio extension front

New simultaneous seismic reflection and physical oceanographic observations east of Japan demonstrate the utility of the seismic reflection method in mapping oceanic fine structure. Synthetic seismograms calculated from temperature and salinity data confirm that seismic reflections correlate with physical oceanographic structures. Seismic reflections at the boundary between the warm Kuroshio and the cold Oyashio water masses correspond to well developed, ∼10 m scale, temperature fine structure. Vertical current profiles suggest that this fine structure is caused by interleaving of these two water masses. We compare our seismic images with acoustic Doppler current profiler (ADCP) intensity maps and discuss similarities and the differences between seismic images and the ADCP maps. Our study demonstrates that even relatively low‐energy seismic sources, in this case a 3.4 l (210 in3) generator‐injector (GI) airgun, can be used to image upper oceanic fine structure.

Two‐dimensional mapping of fine structures in the Kuroshio Current using seismic reflection data

Multi‐channel seismic reflection data acquired in the Pacific Ocean off the Muroto peninsula of Shikoku Island, Japan reveal the two‐dimensional distribution of fine structures in the Kuroshio Current. Eighty‐one seismic sections, each extending 80 km perpendicular to the current and separated by 100 m, were acquired from 20 June to 15 August 1999 (57 days). The seismic data clearly show that fine structures extend over 40 km perpendicular to the current in almost all of the profiles. A simulation study using acoustic model from CTD data demonstrates that fine structure of temperature and salinity identified in CTD data acquired from the Kuroshio Current off the Ashizuri peninsula yield a synthetic seismic profile with characteristics similar to the Muroto transect profiles.

Detection and estimation of the istropy degree of temperature scales in a turbulent thermal plume by ultrasound scattering

A non-intrusive acoustical approach was used to characterize fine structure of temperature and the isotropy degree in a turbulent thermal plume. For this a suitable coefficient y was defined. Results give indication about anisotropy over the whole scale range considered in this study, which is attributed to shear stress for larger scales while buoyancy acts on smaller temperature scales

Exposure to ultraviolet radiation in aquatic ecosystems: estimates of mixing rate in Lake Ontario and the St. Lawrence River

Vertical eddy diffusion coefficients (Kz) were determined for the surface waters at several sites in Lake Ontario and along the Upper St. Lawrence River using the water column distribution patterns of hydrogen peroxide. Values of Kzranged from 0.45 × 10-3to 23 × 10-3m2·s-1in Lake Ontario and from 0.75 × 10-3to 2.1 × 10-3m2·s-1along the St. Lawrence River. The residence time for bacterioplankton and phytoplankton in the surface waters was then determined from the Kzvalues and incorporated into a spectral model to determine the continuous biologically effective exposure to ultraviolet radiation (E*UVR). The values of E*UVRfor stations where the temperature fine structure profiles indicated near-surface warming (diurnal thermocline formation) were higher (149.1 J·m-2) than at stations with isothermal surface waters (3.0 J·m-2). Model calculations for two contrasting bays of a lake underscored the dominant role of diurnal thermocline formation in increasing the duration of exposure to continuous damaging ultraviolet radiation exposure. The E*UVRvalue for the near-surface bacterioplankton in a humic stained bay was higher (219.2 J·m-2) than in a larger bay with lower concentrations of chromophoric dissolved organic matter (47.83 J·m-2).

Wavenumber and depth properties of temperature finestructure in the region around the Ryukyu Islands

Forty-two CTD profiles gathered in summertime of 1997 were used for finestructure studies around the Ryukyu Islands indicating that finestructure properties varied with depth; and that the wavenumber spectra density is a negative (−3 ≈ −4) power function of the wavenumber in the high wavenumber range. The identical finestructure spectral features in different stations indicate considerable water exchange through the Kerama Trench. The wavelet spectrum's spatial structure and intermittency is related to the vertical mixing of water.

Medium-scale heat fluxes across the Antarctic Circumpolar Current in the Drake Passage and western Scotia Sea

Closely spaced continuous temperature profiles from expendable bathythermographs launched along two sections across the Drake Passage and western Scotia Sea in the summer 1981–1982 are used to examine the vertical medium-scale (∼10–100 m) temperature fine structure. The large-scale temperature structure across the frontal regimes characteristic of the Antarctic Circumpolar Current and the cross-frontal structure of the upper ocean are discussed. In the Drake Passage the heat content drops about 0.5 × 102 Kcal cm−2 (2 × 109 J m−2) across the Subantarctic Zone and 0.9 × 102 Kcal cm−2 (3.6 − 109 J m−2) across the Polar Front. In the Scotia Sea the heat content changes across the front are not as prominent. The statistical model of Joyce (1977) is used to quantify the heat fluxes across the fronts produced by the medium-scale temperature interleaving. In the Drake Passage the estimated heat flux is 0.32 × 10−3 °C m s−1 (1.3 × 103 W m−2) across the Subantarctic Front and 0.46 × 10−3 °C m s−1 (1.9 × 103 W m−2) across the Polar Front. In the Scotia Sea the estimated heat flux is larger in the Polar Front reaching 0.71 × 10−3 °C m s−1 (2.9 × 103 W m−2). The medium-scale fine structure heat fluxes are about 10% of the existing estimates of the mesoscale eddy heat fluxes and comparable to heat fluxes associated with the meridional flow of deep and bottom waters across the Antarctic Circumpolar Current.

Ther mohaline finestructure and its relation with the water masses and currents system in the Northern East China Sea

Eighty-one CTD profiles gathered in springtime were used for northern East China Sea thermohaline finestructure studies indicating that the finestructure properties varied with region and depth, as shown in finestructure spectra, distribution of Cox numbers, etc., Some results closely related to distribution of water masses and currents. Analysis of two typical profiles revealed differences in autospectra of temperature, salinity and potential density gradients, probability distribution of temperature finestructure gradient, Cox numbers, etc. The probability density function of vertical temperature gradients, which varied with sample interval, is given. The variances of temperature finestructure gradient are used to estimate the lateral diffusivity {ie128-1} and lateral temperature flux, which were 10.3 (m2/s) and 5.5×10−4 (°C m/s). respectively.

Internal waves, fossil turbulence, and composite ocean microstructure spectra

Composite vertical shear spectra of Gargett et al. (1981) and composite vertical temperature-gradient spectra of Gregg (1977) are compared with the fossil-turbulence model of Gibson (1980–6). Both the shear and temperature-gradient spectra show high-wavenumber microstructure bumps which are identified by Gargett et al. (1981) and Gregg (1980) as due to turbulence in the fluid at the time of measurement. However, using γ [ges ] 5N as the criterion for turbulence to exist in a stratified fluid, where γ is the rate of strain and N is the Brunt-Vaisala frequency, the largest-scale fluctuations of the microstructure bumps may actually be remnants of previous turbulence persisting in fluid that is no longer turbulent at these scales: such fluctuations are termed fossil vorticity turbulence (a class of internal waves) and fossil temperature turbulence respectively. Both composite spectra exhibit k−1 subranges which are identified by their low amplitudes as subsaturated (two-three)-dimensional internal waves and resulting temperature fine structure by comparison with saturated three-dimensional internal-wave subranges proposed by Gibson (1980):7N2k−1 for the saturated vertical shear spectrum and , also indicating extreme undersampling of the turbulence and mixing.

A Statistical Description of Temperature Finestructure in the Presence of Internal Waves

We have developed a statistical model describing a random field of internal waves passively oscillating a random, locally horizontally uniform temperature finestructure. Here we define finestructure to be non-internal wavelike temperature fluctuations of any vertical scale caused by phenomena such as horizontal intrusions or geostrophic eddies. The model allows one to examine the effects of finestructure upon all relevant measurable statistical quantities of the temperature field as a function of the vertical scale of the finestructure. Also, the effects of the time variation of the finestructure itself are considered. The model was fit to data obtained during the 3-week Mid-ocean Acoustic Transmission Experiment (MATE) in summer 1977 new Cobb seamount in the northeastern Pacific. Various spectra and coherences estimated from temperature time series and vertical profiles were used to make an assessment of the finestructure as well as establish the consistency of the model. Temperature variance measured at frequencies above the local Väisäiä frequency was used to estimate the magnitude of the high vertical wavenumber finestructure. This high wavenumber finestructure (0.05–1 m−1) with a (wavenumber)−−2.5 dependence can consistently explain all the observed variance in the measured high vertical wavenumber spectra. At lower wavenumbers (0.002–0.020 m−1) a (wavenumber)−2 dependence was observed, and the spectral level found to be approximately equally divided between finestructure and internal waves advecting a constant temperature gradient. The contribution of the finestructure effects to the internal wave frequency spectrum was found to be about a factor of 10 less than that of internal waves advecting a constant temperature gradient.

Small‐scale shear measurements during the fine and microstructure experiment (Fame)

The turbulent kinetic energy dissipation rate ε is estimated from measurements of small‐scale shear taken with a vertical profiler during the Fine and Microstructure Experiment (Fame). Typical profiles of ε are presented for the different oceanographic regions sampled, the Gulf Stream, a mid‐Sargasso site, and locations within and without the 1000 fathom (∼2000 m) contour about the island of Bermuda. Heavily averaged values of ε are presented as a function of mean Väisälä frequency , a fundamental scaling parameter for the oceanic internal wave field. A dependence of is found for an ensemble of stations near Bermuda; functional dependence for an ensemble of stations at the mid‐Sargasso site is less clear, with results exhibiting an undesirable sensitivity to infrequent large events. Dissipation is found to increase as the island of Bermuda is approached from any direction: the density of measurements is insufficient to determine any azimuthal variation resulting from the anisotropic mean flow field about the island at the time. A set of three profiles across the Gulf Stream suggests that this is not a region of abnormally high dissipation, a conclusion supported by previous and concurrent measurements of temperature finestructure and microstructure.

Acoustic fluctuations due to the temperature fine structure of the ocean

The effect of oceanic temperature fine structure on sound transmission is investigated. The model used assumes that the layered fine structure is advected horizontally and vertically by the internal waves. Taylor’s frozen turbulence hypothesis is then used to determine the space–time variations in sound speed. We compare our results to those of Ewart’s Cobb Seamount experiment [J. Acoust. Soc. Am. 60, 46 (1976)]. The agreement between the calculated spectrum of the log-intensity fluctuations and the experiment is excellent except at low frequencies (ω≲0.3 cph) and extends, at high frequencies, even beyond the internal wave frequency range. Previous calculations based on the internal wave model of turbulence have consistently underestimated these fluctuations in this frequency range. The agreement between the observed and the previously calculated phase fluctuations is not affected; that is, the fine structure adds little to the phase fluctuations.

Detecting Temperature Fine Structure and Mesoscale Frontal Regimes along Line P in the Northeastern Pacific Ocean

Temperature observations obtained at 13 line P stations by Canadian weatherships for the period 1969–1977 show evidence for two quasi-permanent mesoscale frontal regimes west of Vancouver Island, British Columbia. Findings are based on vertically smoothed digitized STD profiles and normalized variances for temperature fine structure having vertical scales of a few tens of metres. A major frontal boundary crosses line P in the vicinity of 128°40′W longitude to depths of 700 m and in the vicinity of 130°40′W longitude between depths of 700 and 1300 m. Over certain depth ranges, location of the front varies seasonally, with a tendency for the front to be further offshore in winter than in summer. There is also some suggestion of a secondary, poorly defined frontal boundary in the region of 136–139°W longitude.Key words: ocean fronts, line P, northeast Pacific, temperature fine structure

A moored instrument for vertical temperature gradients

Vector averaging current meters have been modified to measure the temperature gradient in addition to current and absolute temperature. The modified instrument measures the temperature difference between sensors spaced vertically 1.74 m apart (or closer) using thermistors in a Wheatstone bridge circuit. Calibrations provide corrections for drift and static offset resulting in temperature gradient measurements accurate to about 0.5 m °C/m. Originally developed to infer vertical velocities, the instrument has also been successfully used to study mixed layer motions and thickness variations and to investigate temperature fine structure.

Temperature fine structure near the Sea‐ice margin of the Chukchi Sea

Temperature fine structure with peak‐to‐peak fluctuations often exceeding 2°C is prevalent throughout the marginal sea‐ice zone of the shallow Chukchi Sea in midsummer. It is normally found within 20–35 km outside of the ice and seldom farther than 5–10 km inside the ice margin. It has also been infrequently found more than 100 km south of the ice. Temperature and salinity fronts of all degrees of sharpness occur in the same general area, but the two types need not be coincident. Considerable variability in time and length scales of fine structure elements has been noted with persistences ranging from 2 hours or less to more than 20 hours and length scales ranging from 600 m to more than 20 km. While much of this variability is intrinsic, part can be accounted for by the inability to sample in the direction of flow and the relative motion between the sampling platform and fine structure element. Fine structure lenses are considerably longer parallel to the ice edge with similarity in temperature structure noted over distances of 80 and 100 km.

On the internal wave variability during the internal wave experiment (IWEX)

The relation between internal wave variability and larger and smaller scales of motion is investigated, using the IWEX data set. To investigate the role of internal waves in the vertical diffusion of large scale momentum, the time variability of the vertical flux of horizontal internal wave momentum (estimated from temperature and current data) is compared to that of the mean vertical shear. It is found that internal waves cannot cause a vertical viscosity as large as proposed by Müller (1976), but that the data are too noisy to detect a possible wave‐induced viscosity in absolute value of the order of 10−2 m2 s−1 or less. Similarities in the time behavior of the total internal wave energy and that of the square mean vertical shear suggest that some kind of dynamical coupling exists between internal waves and larger scale flows. There is some evidence that the level of temperature finestructure activity also varies in a related way. An analysis of CTD station data taken during Mode demonstrates the mappability of the finestructure activity, and again suggests a relation with the geostrophic eddy flow.

Finestructure of Precipitation and Temperature in a Stratocumulus Cloud

Abstract A stratocumulus cloud located above a cold front in a midlatitude depression has been observed simultaneously by a K-band radar, instrumented aircraft and radiosondes. The radar recording suggests that convective generating cells near the cloud top present a quasi-periodic organization. This is confirmed by a spectral analysis of reflectivity fluctuations within the generating layer. Aircraft data collected in the precipitation streamers under this layer exhibit systematic fluctuations both in temperature and hydrometeor number density fields. A positive correlation is shown to exist between these fluctuations. A simple numerical model is used to estimate the temperature increase resulting from the release of latent heat by deposition of water vapor on the precipitating crystals. Satisfactory agreement is obtained with the observed temperature and radar reflectivity data.

Temperature finestructure and microstructure observations in a coastal upwelling region during a period of variable winds (Oregon, summer 1974)

Profiles of temperature and temperature gradient made during two 4-day cruises with a freely-falling probe, in conjunction with conductivity-temperature-depth (CTD) and current meter records, are used to examine a 100-m water column on the Oregon shelf for evidence of finestructure-microstructure correlation, shear-induced mixing, and double-diffusive convecting layers. The rate of dissipation of temperature fluctuations, ε ν , is founnd to be as large as 2 × 10 −5 °C 2 s −1 in a well-stirred surface layer, and as low as 7 × 10 −11 °C 2 s −1 in a region undisturbed by intrusions or boundary stirring. For the surface mixed layer, a decade-long, + 1 slope was observed in the temperature gradient spectrum for vertical wavelengths greater than about 5 mm. If this is assumed to be a viscous-convective subrange, the rate of dissipation of mechanical energy, ε, is calculated to be 0.06 cm 2 s −3 . Transient patches of intense microstructure, 1 to 2 m in extent, can occur in the thermocline. Variations in temperature variance of 10 3 can occur over small vertical separations in several intrusions, but when normalized by the local finestructure gradient (yielding the Cox number) the range is considerably less. A 4-m thick ‘hole’ in the microstructure appears in a shallow, cold intrusion. A warm intrusion has a complicated, time-dependent vertical structure and a low Cox number (less than 10). An interleaving at about 60 m produces a 0.10°C, 10-m thick inversion with many small-scale features indicative of overturning; the Cox number is about 600 in this case. Although all the temperature inversions have T-S gradients favorable for double-diffusive convection, no systems of layers are found. In general, the results are similar to those from other observations of oceanic microstructure associated with the different types of finestructure.

Eddies, islands, and mixing

As part of a field study of the relation between fine scale and large‐scale variations of water properties in the western North Atlantic, the waters in the vicinity of Bermuda were investigated in detail. Previous work in the area had revealed regions of intense temperature fine structure confined to the sides of the island. Generally quieter levels of activity elsewhere in the midocean have suggested that significant mixing might only occur at the solid and fluid boundaries of the ocean. During the course of our investigation, two Gulf Stream rings were found in the vicinity of the island. The exchange of water between them caused three regions of strong alongshore flow. In these three areas we find elevated levels of temperature fine structure in the upper 800 m as measured by the variance in the temperature gradient normalized by the square of the mean temperature gradient over the interval. The normalized temperature variances on small scales (0.2–1 m) are most energetic in patches tightly bound to the island sides, whereas the fine structure on larger scales (5–25 m) is also energetic away from the island in a region of outflow. Velocity profiles show that vertical scales shorten as one approaches the island, and the energy increases in the counterclockwise component. There is no correlation evident between the shear measurements of the internal wave field and the intensity of the fine structure. Possible mechanisms for the production of fine structure are explored within the context of these observations.