High-resolution Temperature Observations Research Articles

High-Resolution Temperature Observations of a Shallow Lagoon in the South Pacific (Bora Bora)

van Haren, H., 2020. High-resolution temperature observations of a shallow lagoon in the South Pacific (Bora Bora). Journal of Coastal Research, 36(3), 536–544. Coconut Creek (Florida), ISSN 0749-0208.The daily cycle of heating and cooling of the near-surface ocean may be quite different in the open ocean compared with a shallow lagoon with a seafloor that is a few meters deep and can be heated directly by the sun. This solar radiation can affect the local benthic communities. To study the physical processes associated with the daily cycle of the South Pacific lagoon surrounding Bora Bora, a vertical string of five high-resolution temperature sensors is moored at a 2-m-deep site for 3 weeks. Besides standard ocean warming (mostly during the day) and cooling (mostly at night), the sensors show relatively highest temperature near the lagoon floor during the warming phase and weakly stable stratification toward the end of the cooling phase. During the warming phase, highly variable stratification is observed extending into the water column, but only under conditions of calm weather and turbid waters. Under trade winds and clear waters, the lowest sensor or sensors show consistently higher temperature variability than higher sensors with spectral slopes indicative of shear and/or convective turbulence. During the cooling phase, the lower sensor shows consistently very low variance (nonturbulent), while other sensors show a spectral slope around the buoyancy frequency, evidencing weakly stratified waters supporting internal waves. These observations contrast with open-ocean near-surface observations of stable stratification during the warming phase and turbulent free convection during the cooling phase. Thus, lagoons seem to resemble the atmosphere more than the ocean in daytime thermodynamics and possibly act as a natural solar pond with bottom conductive heating (when salinity compensates for unstable temperature variations).

Instability observations associated with wave breaking in the stable-stratified deep-ocean

High-resolution temperature observations above underwater topography in the deep, stably stratified ocean have revealed two distinctive turbulence processes. These processes are associated with different phases of a large-scale (here tidal) internal gravity wave: (i) highly nonlinear turbulent bores during the upslope propagating phase, and (ii) Kelvin–Helmholtz billows, at some distance above the slope, during the downslope phase. Whilst the former may be associated in part with convective turbulent overturning following Rayleigh–Taylor instabilities ‘RTi’, the latter is mainly related to shear-induced Kelvin–Helmholtz instabilities. In this paper, details are particularly presented of rare (convective) RTi penetrating stable density stratification under high-frequency internal waves. Such ‘apparent RTi’ can be explained using both stability parameterization of entrainment across a density interface, and, more relevant here, internal wave acceleration overcoming the reduced gravity.

High-resolution temperature observations of a trapped nonlinear diurnal tide influencing cold-water corals on the Logachev mounds

A high-resolution thermistor string mooring of 120m length was used to measure turbulence processes in the water layer above the foot of a densely populated cold-water coral mound at 919m depth at the southeast slope of Rockall Bank in the Logachev area, North-East Atlantic Ocean. As expected from previous current observations, the temperature data reveal a dominant diurnal (tidal) periodicity associated with topography-trapped, weakly bottom-intensified waves. These baroclinic diurnal waves are driven to (near-) resonance around the mound and have vertical amplitudes exceeding the mooring line. Their horizontal excursion length matches the size of the mound, which causes a residual current around and flux up the mound. As their particle velocities also match the phase speed of the wave traveling around the mound, these waves can become highly nonlinear and show largest turbulence due to wave-breaking at the transition from warming downslope to cooling upslope tidal phase. Averaged over the entire 9-day time series and the 120m vertical range, mean turbulent kinetic energy dissipation amounts to 2.2±1.1×10−7Wkg−1 (and mean vertical eddy diffusivity to 9±5×10−3m2s−1) with short-term variations over four orders of magnitude. Such large turbulence, more than 100 times larger than open-ocean values and comparable with that observed in tidally energetic shelf break-shallow sea areas, will affect the nutrient replenishment of the cold-water corals.

Convective and shear-induced turbulence in the deep Kane Gap

[1] The boundary layer above a 4569 m deep slope in the near-equatorial N-Atlantic Ocean Kane Gap, a throughflow for Antarctic Bottom Water (AABW), is characterized by two distinct turbulent regimes that differ by an order of magnitude in intensity depending on the direction of throughflow. During southward and downward flow, vertical mixing is vigorous. This is inferred from high-resolution temperature observations between 6 and 132 m above the bottom. For a representative case study of 2 days, average values are found for dissipation rate of e?=?2.1?±?1 × 10-9 W kg-1 and eddy diffusivity of Kz?=?7?±?4 × 10-4 m2 s-1. The mixing is across relatively large vertical overturns. During northward and upward flow, smaller overturns are more horizontal as in stratified shear flow (with representative 2 day mean e?=?6?±?3 × 10-11 W kg-1, Kz?=?4?±?2 × 10-5 m2 s-1). Stratification is approximately the same during both flow directions. Although the different turbulence regimes are partially associated with frictional boundary layers of large-scale flows above sloping topography, but not with those over flat bottoms, and partially with flow across a hill-promontory, internal waves are a dominant process in promoting turbulence. In addition, internal waves are observed to push stratification toward the bottom thereby importantly contributing to the mixing of AABW.

Stratified turbulence and small-scale internal waves above deep-ocean topography

When ocean's internal tidal waves “beach” at underwater topography, they transform from more or less linear into highly nonlinear waves that can break with generation of vigorous turbulent mixing. Although most mixing occurs in the half hour around a steep (bottom-)front leading the upslope moving internal tide phase, relatively large mixing also occurs some distance of several tens of meters off the bottom, just prior to the downslope moving internal tide phase and initiated by high-frequency “small-scale” internal waves. Details of this off-bottom small-scale mixing in a stratified natural environment and some of its variability per tidal period are presented here in two case studies using high-resolution temperature observations (61 sensors at 1 m intervals; <10−3 °C precision) at a 969 m deep site south of New Zealand. The observations shed some light on stratified turbulence that is generated in a relatively thick (∼30 m) weakly stratified layer and in the strongly stratified interfaces above and below. The interfacial internal waves generate turbulence with largest dissipation rate and temperature variance at the edge of the upper interface and the weakly stratified layer. When these waves steep nonlinearly, immediate moderate turbulence generation is observed below, throughout the weakly stratified layer. Largest turbulence is generated by 25 m high asymmetric Holmboe overturns.

Quantifying spatial and temporal discharge dynamics of an event in a first order stream, using distributed temperature sensing

Abstract. Understanding the spatial distribution of discharge can be important for water quality and quantity modeling. Non-steady flood waves can, particularly as a result of short high intensity summer rainstorms, influence small headwater streams significantly. The aim of this paper is to quantify the spatial and temporal dynamics of stream flow in a headwater stream during a summer rainstorm. These dynamics include gains and losses of stream water, the effect of bypasses that become active and hyporheic exchange fluxes that may vary over time as a function of discharge. We use an advection-dispersion model coupled with an energy balance model to simulate in-stream water temperature, which we compare with high resolution temperature observations obtained with Distributed Temperature Sensing. This model was used as a learning tool to stepwise unravel the complex puzzle of in-stream processes subject to varying discharge. Hypotheses were tested and rejected, which led to more insight in the spatial and temporal dynamics in discharge and hyporheic exchange processes. We showed that, for the studied stream infiltration losses increase during a small rain event, while gains of water remained constant over time. We conclude that, eventually, part of the stream water bypassed the main channel during peak discharge. It also seems that hyporheic exchange varies with varying discharge in the first 250 m of the stream; while further downstream it remains constant. Because we relied on solar radiation as the main energy input, we were only able to apply this method during a small summer storm and low flow conditions. However, when additional (artificial) energy is available, the presented method is also applicable in larger streams, during higher flow conditions or longer storms.

A distributed stream temperature model using high resolution temperature observations

Abstract. Distributed temperature data are used as input and as calibration data for an energy based temperature model of a first order stream in Luxembourg. A DTS (Distributed Temperature Sensing) system with a fiber optic cable of 1500 m was used to measure stream water temperature with 1 m resolution each 2 min. Four groundwater inflows were identified and quantified (both temperature and relative discharge). The temperature model calculates the total energy balance including solar radiation (with shading effects), longwave radiation, latent heat, sensible heat and river bed conduction. The simulated temperature is compared with the observed temperature at all points along the stream. Knowledge of the lateral inflow appears to be crucial to simulate the temperature distribution and conversely, that stream temperature can be used successfully to identify sources of lateral inflow. The DTS fiber optic is an excellent tool to provide this knowledge.

High-resolution vertical imaging of the troposphere and lower stratosphere using the new MU radar system

Abstract. In the present paper, a new application of the range imaging technique called Frequency Interferometry Imaging (FII) or Range Imaging (RIM), performed in April 2005, is shown using the new 46.5-MHz Middle and Upper (MU) atmosphere radar system (Shigaraki, Japan). Height-time images of brightness distribution have been computed at the highest resolution ever obtained for imaging with VHF radars in the troposphere and, for the very first time, in the lower stratosphere, up to about 22 km. The images were produced by processing signals obtained with an initial range-resolution of Δr=150 m and five equally-spaced frequencies within Δf=1.0 MHz, with the adaptive Capon method. These values represent an improvement of a factor 2 over all the previous published experiments at VHF, which were performed with Δr=300 m and Δf=0.5 MHz. The Capon images present realistic and self-consistent features, and reveal many more organized structures than the height-time SNR plots at the initial range-resolution. For example, the Capon images show persistent enhanced brightness layers significantly thinner than 150 m in the stratosphere, which are impossible to track with the standard single-frequency mode owing to a lack of range resolution. These observations thus support the idea of strong stratification even at vertical scales much smaller than 100 m, as suggested by recent high-resolution temperature observations by balloons (Dalaudier et al., 1994). We also present comparisons of Capon images with patterns obtained from the dual-FDI technique and two parametric methods (the MUSIC algorithm and the newly-introduced Maximum Entropy Method based on an auto-regressive (AR) model). The comparisons confirm the insufficiencies of the dual-FDI technique and indicate that parametric methods such as MEM and the MUSIC algorithm can help to validate the Capon images when the parametric methods provide similar patterns.

Internal waves near the buoyancy frequency in a narrow wave-guide

High-resolution temperature observations from the northern North Sea were used to study increased isotherm displacement variance above the internal wave spectral continuum near the background buoyancy frequency (N). Existing linear WKB-theories attributed such increase to internal reflection at a turning point within a moderately wide wave-guide or to resonant wave generation from outside the wave-guide. In contrast to linear theory and partially following previous suggestions, the present observations showed that in a narrow wave-guide this increase is due to just a few passages of near-monochromatic, mode-1 interfacial waves. Furthermore, it was observed for the first time that these interfacial waves can occasionally grow and increase their frequency (σ) until σ = N B ≪ N 00, where N 00 denotes the frequency of the thin interface of thickness Δz 00 moving up and down and N B is the 5-day mean Eulerian background stratification. Maximum-displacement variance reached levels 2 decades above the long-term average value. The maximum-amplitude H of the σ ≈ N B-waves was half the vertical scale of the mean pycnocline (and inertial shear layer) Δz B = 2H delineated by depths z for which N(z) ≥ N B, the ‘wave-guide’. As a result, the high-frequency waves partially determined the background stratification because further growth was not possible, leading to wave breaking at the edges of the wave-guide and thereby decreasing N. This mechanism for reduction of the pycnocline is different from accepted mechanisms such as boundary mixing and internal mixing following inertial shear. However, as it occurs within a finite thick pycnocline it may be responsible for much of the relatively slow turbulent exchange of nutrients across pycnoclines in shelf seas.