Thermocline Position Research Articles

Experimental investigation on thermal stratification characteristic of steam condensation through multi-hole sparger

Thermal stratification induced by steam direct contact condensation is experimentally investigated through a multi-hole sparger of horizontal injection with different steam mass flux, initial water temperature, and submergence depth. The exitance of chugging regime and oscillatory bubble regime causes the differences in flow patterns of pool water through multi-hole sparger. The four stages of temperature evolution, complete mixing, gradual stratification, stable stratification, and downward extension of the heated region stage, are found. Thermal stratification vanishes at water subcooling lower than 3℃. The influence of key factors on the thermal stratification and thermocline dynamic characteristic is discussed and analyzed. With steam mass flux increasing, the maximum temperature difference of pool water increases first and then decreases, and the inflection point of temperature growth increases. Besides, thermocline downward velocity increases with steam mass flux increasing resulting from the enhanced turbulence effect. Larger initial water temperature and smaller distance from sparger to pool bottom help to reduce the maximum temperature difference. The thermocline velocity increases with the initial water temperature rising due to the reduced density difference. Lower sparger position can effectively move down the thermocline position, and the thermocline velocity decreases resulting from the weakened turbulence motion. The initial water level has a limited effect on thermal stratification and thermocline velocity. In addition, the Richardson number, used to judge whether thermal stratification occurs, reaches the maximum at water temperature 65℃. The results are helpful to understand the mechanism of thermal stratification and provide an effective reference for the design of the suppression system.

Internal solitary waves on the NW African shelf: A heuristic approach to localize diapycnal mixing hotspots

Turbulent mixing on continental shelves plays roles in the structure and dynamics of marine ecosystems, nutrient cycling, primary production and dispersion of pollutants. Describing and understanding internal wave dynamics enables improved mapping of mixing over continental shelves, especially in complex environments where many processes may interact, such as in upwelling systems. This paper describes internal wave propagation and dissipation in the Moroccan upwelling system using a comprehensive set of hydrographic observations made during two in situ surveys. The acoustic backscatter of the echosounder is shown to be a rapid and convenient survey tool for detecting internal solitary waves (ISWs) on large spatial scales, through the high-frequency oscillation of the zooplankton layer during nighttime conditions. Along ISW trains, enhanced diapycnal mixing episodes are observed with increased dissipation coefficients in the thermocline (O(10-7 W kg-1)), associated with overturning up to 6 m. Mixing due to internal wave soliton packets shows large spatial and temporal variability, but contributes to the overall mixing processes and is responsible for large intermittent variations in the thermocline position. The joint use of a multifrequency echosounder and a current profiler ADCP allows mixing to be quantified (via the Richardson number) on large spatial scales. Validation and use of this method in other coastal regions could be useful to determine regional mixing parameterization in numerical models.

Particulate organic matter as causative factor to eutrophication of subtropical deep freshwater: Role of typhoon (tropical cyclone) in the nutrient cycling

Intense storms pose a serious threat to ecosystem functioning and services. However, the effects of typhoons (tropical cyclones) on the biogeochemical processes mediating risk of eutrophication in deep freshwater ecosystems remain unclear. Here, we conducted a three-year study to elucidate linkages between environmental change, stable isotopes and the stoichiometry of particulate organic matter (POM), and nutrient cycling (i.e., carbon, nitrogen and phosphorus) in a subtropical deep reservoir subjected to typhoon events. The typhoons significantly changed the nutrient levels in the deep waters as well as the thermocline position. Increased typhoon-driven organic matter input, algae sinking and heterotrophic decomposition interacted with each other to cause steep and prolonged increases of total nitrogen, ammonium nitrogen and total phosphorus in the bottom waters of the reservoir. Small-sized or pico-sized POM (i.e., 0.2–3 μm) showed a substantial increase in bottom waters, and it exhibited stronger response than large-sized POM (i.e., 3–20, 20–64, 64–200 μm) to the typhoons. Our results also indicated that typhoons boost the nutrient cycling in deep waters mainly through pico-sized POM.

Phytoplankton response to experimental thermocline deepening: a mesocosm experiment

A number of modelling results suggested thermocline shifts as a consequence of global climate change in stratifying lakes. Abundance and composition of the phytoplankton assemblage is strongly affected by the stratification patterns, and therefore, change in the thermocline position might have a substantial effect on this community or even on the whole lake ecosystem. In this study, thermocline depths in large mesocosms installed in Lake Stechlin (Germany) were deepened by 2 meters and phytoplankton changes were analysed by comparing changes to untreated mesocosms. Higher amounts of SRP were registered in the hypolimnion of treatment mesocosms than in the controls, and there were no differences in the epilimnion. Small but significant changes were observed on the phytoplankton community composition related to the effect of deepening the thermocline; however, it was weaker than the yearly successional changes. The most remarkable differences were caused by Planktothrix rubescens and by chlorophytes. P. rubescens became strongly dominant at the end of the experiment in the mesocosms, and in the open lake as well. The results of the experiment cannot clearly support the proliferation of cyanobacteria in general; however, the deepened thermocline can modify the behaviour of some species, as was observed in case of P. rubescens.

Acoustic monitoring of the thermocline height by acoustic intensity interference-pattern

Relationship between water depth and acoustic intensity interference-pattern in a shallow water environment with a homogeneous SSP was proposed by D.E.Weston many years ago. In this paper, we extend the idea to monitor temporal variation of thermocline position by combining the concept of up turning depth for lower order normal modes. Broadband signal is used and acquired with only single hydrophone. To validate the idea, both simulation and AEYFI-05(Acoustics Experiment of Yellow sea Oceanic Front and Internal Waves 2005)experiment data processing are performed. In the AEYFI-05 experiment, broadband signals were transmitted from a fixed source and received at fixed receivers located at 37 km along a cross-shelf propagation track. The acoustic intensity pattern indicates regular frequency shift that is used to predict the variation of thermocline position monitored by a thermistor chain at the same time. Taking into account of adiabatic approximation, an excellent agreement between inverted thermocline position and that monitored by the thermistor chain suggests a potential of long-term acoustic monitoring of the thermocline position in summer conditions.

Low-latitude northern hemisphere oceanographic andclimatic responses to early shoaling of the Central American Seaway

The opening and closing of ocean gateways has played a major role in shaping global climate by altering oceanic and atmospheric circulation. This study uses planktonic foraminifer assemblage-based sea surface temperature (SST) estimates (Modern Analog Technique) to examine the eastern tropical Pacific (ETP) and subtropical Northwest Atlantic (NWA) during the early stages of shoaling of the Central American Seaway (CAS). In the subtropical NWA (DSDP 103 and ODP 1006), the 5.2 to 5.1 Ma interval is characterized by an increase in SST and sea surface salinity, indicating a strengthening of the Florida Current (FC) and Gulf Stream (GS). Sea surface temperature in the ETP Warm Pool (DSDP Site 84) remained relatively stable between 6.9 and 5.1 Ma, during which El Niño-like conditions persisted. A slight cooling is observed after this interval (with synchronous warming in the NWA), followed by the onset of major cooling at ∼3.2 Ma, both of which are preceded by a shallowing of the thermocline. Stepwise cooling is attributed to enhanced Atlantic meridional overturn cicrulation (AMOC), which caused a shoaling of the main tropical thermocline, thereby strengthening the Walker Circulation and weakening the Pacific North Equatorial Counter Current. Antithetical changes in surface current strength in the NWA and ETP suggest a southward migration of the Intertropical Convergence Zone ∼4.3 Ma. During the mid-Pliocene, SST in the Panama Basin was ∼0.8°C cooler than today, while the subtropical NWA was only ∼1.1°C warmer. This corroborates evidence for reduced meridional SST gradients during the mid-Pliocene as well as the hypothesis that more vigorous ocean circulation-particularly in the NWA-was critical during this period. The timing of SST changes in the ETP and NWA (∼5.1 Ma) suggest that the termination of permanent El Niño and enhanced AMOC did not contribute significantly to the onset of major Northern Hemisphere glaciation (NHG), as both of these events occur well before the beginning of the glacial cycles. However, these processes may have contributed to the development of the small ice sheets of the late Miocene and early Pliocene, but were most likely only preconditioning factors for the onset of major NHG. However, changes in SST and relative thermocline position suggest that high-latitude processes and global cooling may have influenced thermal structure in the ETP. The SST estimates provided indicate that even in its early stages, the shoaling of the CAS had significant implications for low-latitude ocean circulation and thermal structure, as well as for some of the most significant global climate events of the late Neogene.

Relationships between fish larvae and siphonophores in the water column: effect of wind-induced turbulence and thermocline depth

Abstract Sanvicente-Añorve, L., Alatorre, M. A., Flores-Coto, C., and Alba, C. 2007. Relationships between fish larvae and siphonophores in the water column: effect of wind-induced turbulence and thermocline depth. – ICES Journal of Marine Science, 64: 878–888. The relationship between the abundance of fish larvae and siphonophores in relation to wind-induced turbulence and thickness of the mixed layer in the southern Gulf of Mexico were studied during two periods of different wind conditions: April (5.25 m s−1) and October (6.5 m s−1). The Spearman correlation between fish larvae and siphonophores revealed a random relationship in the 0–10 m layer during April and in the 0–20 m layer in October. This structure presumably persists while turbulent forces remain at sufficient strength. Positive patterns were observed deeper in the water column. Whereas thermocline position did not correspond with the depth separating random and positive relationships, low turbulence values did. Observations indicate that turbulent kinetic energy values above 4 × 10−4 J kg−1 might promote a random distribution between potential prey and predator zooplankton taxa. In surface waters, contact rates between siphonophores and fish larvae showed that turbulence enhances encounters 2.5 (1.2) times in April and 3.3 (1.3) times in October for prey velocities of 0.003 (0.011) m s−1. The positive relationship between fish larvae and siphonophores could be caused by a high degree of spatial overlap, enough food for both groups, and limited predation on larvae in the presence of alternative prey. Seasonal variability in the vertical structure of distribution patterns was attributed mainly to aggregative feeding behaviour of organisms and disruption of patches as a consequence of small-scale water turbulence.

Vertical distribution of fish larvae in the Canaries-African coastal transition zone in summer

This study reports the vertical distribution of fish larvae during the 1999 summer upwelling season in the Canaries-African Coastal Transition Zone (the Canaries-ACTZ). The transition between the African coastal upwelling and the typical subtropical offshore conditions is a region of intense mesoscale activity that supports a larval fish population dominated by African neritic species. During the study, the thermal stratification extended almost to the surface everywhere, and the surface mixed layer was typically shallow or non-existent. Upwelling occurred on the African shelf in a limited coastal sub-area of our sampling. The vertical distributions of the entire larval fish population, as well as of individual species, were independent of the seasonal thermocline. Fish larvae and mesozooplankton were concentrated at intermediate depths regardless of the thermocline position, probably because of its weak signature and spatial and temporal variability. Day/night vertical distributions suggest that some species did not perform diel vertical migration (DVM), whereas others showed either type I DVM or type II DVM. The opposing DVM patterns of different species compensate for each other resulting in no net DVM for the larval fish population as a whole.

Vertical heterogeneity in zooplankton community structure: a variance partitioning approach

The vertical structure of a freshwater zooplankton community was analysed quantitatively, in order to test the hypotheses that 1) vertical multivariate variability in community composition coincided with vertical variations in water temperature and that 2) the fundamental nature of the spatial structure in community composition would differ in mixed and stratified conditions. Multivariate methods were used to partition the variability in a spatio-temporal species dataset (12 sampling depths, 19 dates, 6 species) into pure spatial, temporal, environmental and shared components. Using this approach, 51 % of the variation of the zooplankton community could be explained. Broad-scale spatial organization in the zooplankton community (i.e. water column scale variation that could be explained as a simple first-order function of depth) accounted for 23 % of the explained variation. Approximately half of this variation was shared with depth-wise variations in water temperature variables. Due to the discontinuity of the thermally stratified water column, there existed a component of the shared variation between water temperature variables and zooplankton community structure that could not be explained by a simple first-order function of depth. By statistically modelling the discontinuous nature of the thermally stratified water column for a series of discrete time periods, thermocline position was found to explain between 9 and 22 % of the total variation in community structure. This indicated that spatial variations in the community composition included a discontinuous component, that coincided with the thermocline.

Simulation of double cold cores of the 35°N section in the Yellow Sea with a wave-tide-circulation coupled model

Based on the MASNUM wave-tide-circulation coupled numerical model, the temperature structure along 35°N in the Yellow Sea was simulated and compared with the observations. One of the notable features of the temperature structure along 35°N section is the double cold cores phenomena during spring and summer. The double cold cores refer to the two cold water centers located near 122°E and 125°E from the depth of 30m to bottom. The formation, maintenance and disappearance of the double cold cores are discussed. At least two reasons make the temperature in the center (near 123°E) of the section higher than that near the west and east shores in winter. One reason is that the water there is deeper than the west and east sides so its heat content is higher. The other is invasion of the warm water brought by the Yellow Sea Warm Current (YSWC) during winter. This temperature pattern of the lower layer (from 30m to bottom) is maintained through spring and summer when the upper layer (0 to 30m) is heated and strong thermocline is formed. Large zonal span of the 35°N section (about 600 km) makes the cold cores have more opportunity to survive. The double cold cores phenomena disappears in early autumn when the west cold core vanishes first with the dropping of the thermocline position.

Variability in thermocline depth and strength, and relationships with vertical distributions of fish larvae and mesozooplankton in dynamic coastal waters

In this paper the dynamic nature of thermoclines is documented, and their influences on the vertical distributions of larval fishes and mesozooplankton in coastal waters off Sydney, southeastern Australia are tested. Significant small-scale spatial and temporal variability in thermocline depth and strength was observed, and even though there were strong depth-related differences in abundances of fish larvae and mesozooplankton, there were no predictable effects of thermoclines on their vertical distributions. Peak concentrations of some fish larvae were observed in thermoclines, but these patterns were not consistent among sampling locations and time periods. Fish larvae and mesozooplankton were most concentrated in the upper 30 m of the water column, regardless of thermocline position, and therefore trophic interactions among fish larvae and zooplankton would be strongest in this depth strata. Consequently, thermoclines were not considered the most important interface for trophic interactions in this dynamic coastal zone. We argue that thermoclines are probably not critical to the survival of fish larvae in relatively shallow coastal waters characterised by dynamic oceanographies where perturbations in the position and intensity of thermoclines are frequent.

A modelling assessment of potential for eutrophication of Lake Riñihue, Chile

Lake Rinihue is a large (A = 90 km 2 ), deep (z max = 323 m) lake in the high rainfall region of the Andean pre-cordillera of southern Chile. A combination of daily meteorological readings, monthly measurements of inflows and water column composition, and a 7-month period of thermistor chain readings over a 2.5-year study period were used to assess physical and biogeochemical characteristics of Lake Rinihue and potential for change in trophic status. A coupled hydrodynamic-water quality model, DYRESM-WQ, was used to reproduce the observed changes in water temperature in the lake over this period. The model output yielded insights into the timing of stratification and mix is. Lake Rinihue did not mix completely over the 2.5-year period of the study (1995-1997). From late spring to autumn, a well developed thermocline was observed at depths of 20-40 m. Thermistor chain readings indicated that the thermocline position was quite variable on a day-to-day basis. In late winter the thermocline deepened rapidly in response to a sustained net negative heat flux, due largely to reduced solar radiation. Biological and chemical variables in DYRESM-WQ were calibrated over a period of 7 months and a further period of 2 years was used for validation. The simulation output reinforced the oligotrophic nature of Lake Rinihue ([chl-a] <6 mg/m -3 ), the general timing of changes in chlorophyll-a and the depletion of dissolved inorganic nutrients from the surface layer during summer. The DYRESM-WQ model and a bulk loading model were used to assess the potential for eutrophication of Lake Rinihue in response to increases in nutrient loads from inflows, The response to doubling of phosphorus (PO 4 -P and TP) concentrations in the inflows was to increase mean levels of depth-integrated chlorophyll-a near the water surface (0-58 m), from 1.36 mg/m 3 to 1.95 mg/m 3 . The simulated 1.4-fold increase in chl-a concentrations in response to the increased phosphorus load was considerably less than that predicted from a combination of a bulk loading model for TP and a regression against chl-a (2.8-fold increase). The DYRESM-WQ model indicated that the trophic response to increased nutrient loading would be complicated by growth limitation by light and nitrogen.

Structure of the glacial thermocline at Little Bahama Bank

Within the thermocline of the western North Atlantic, bathymetric gradients of hydrographic properties are controlled by the lateral movement of water along isopycnal surfaces1–4. This movement is driven by air–sea interaction processes :Ekman pumping and sur-face heat (buoyancy) flux. By affecting the intensity of these processes, glacial7ndash;interglacial climatic change should also affect the bathymetric gradients of properties within the thermocline. Benthic foraminifera live on carbonate bank margins that intersect the thermocline. These foraminifera record the hydrographic gradients in their isotopic shell chemistry, providing the means to reconstruct changes in thermocline structure and circulation. We have measured the δ18O compositions of a near-surface planktonic foraminifer, Globigerinoides ruber, and a shallow-water benthic foraminifer, Cibicidoides floridanus, in a core recovered from Little Bahama Bank. Cibicidoides floridanus exhibits a glacial–inter-glacial range in δ18O that is 0.5% less than G. ruber's, because individuals of this species lived and calcified ~110m shallower on the thermocline ~ 18,000 yr ago. After glacio-eustatic changes in thermocline position are accommodated, we estimate that temperature at 540 m palaeodepth cooled by ~ 2 ° C. Globigerionoides ruber δ18O values suggest that surface water temperature at this location also cooled by ~2 °C. This uniform cooling of the upper 600 m of the water column is consistent with the observation that surface water where isopycnals outcrop was ~2 °C cooler during the glacial maximum5–8.

Seasonal Thermal Cycle of Lake Erie

A summary of the seasonal water temperature characteristics of Lake Erie and the 1979 and 1980 thermal structure in the central basin is described. Ice cover extends over 90% of Lake Erie most winters. Minimum surface temperature usually occurs in February (0.1° C) but fully mixed conditions at 1°C or less occur in January with isothermal conditions at (1°C) occurring from mid-February to mid-March. The thermal bar advance lasts about 5 to 6 weeks from April to mid-May and permanent stratification usually begins in mid-June with maximum heat storage in mid-August and overturn in mid-September. The central basin thermocline position varies significantly from year to year, the variability of the upper and lower mesolimnion boundaries being as large as 10 m. Thermocline position shows some dependence on prevailing meteorological conditions and has implications to the development of central basin anoxia. Temperature increases and decreases depicted on isotherm plots for stations in the central basin show correspondence with peak wind stress events. During fragile stability conditions, even moderate wind stresses of less than 0.5 dynes/cm 2 are capable of producing upper layer deepening. Episodes of complete vertical mixing in response to high wind stresses of 3 dynes/cm 2 during storm periods are observed. Double thermoclines are evident at several locations within the basin and temperature changes resulting from an influx of hypolimnetic water from the Pennsylvania Ridge is documented. Periods of hypolimnetic entrainment are clearly observed along with thermocline tilting of 1 to 2 meters toward the south.

Distribution and movement of Japanese sardine in relation to oceanographic conditions. II. Horizontal distribution of the Japanese sardine in relation to oceanic front at the purse seine fishing grounds southeast of Hokkaido.

Between July and September in 1980, four survey cruises were carried out in the fishing grounds southeast of Hokkaido by embarking on a purse seiner with DBT, XBT and echo sounders to observe sardine Sardinops melanosticus (TEMMINCK et SCHLEGEL) school distributions in relation to oceanic front. Fishing information (such as reports of catch, location, time and SST), called QRY by fishermen, were analysed to find the fishing grounds fromation and their fluctuation between July and October from 1977 to 1980. Sarding schools were usually observed and caught in and around a front. Moreover, their distribution changes responded to seasonal variations of the front location. It is hence considered that the front and thermocline position have a considerable impact on sardine distribution.

A FICKIAN ANALYSIS OF LAKE SEDIMENT UPSURGE1

ABSTRACT. The fundamental hypothesis of this paper is that the stratified‐mixed states which occur annually in lakes play a dominant role in the movement of nutrient‐rich material from the lake bottom upward into the water column. The stratified‐mixed history has been used previously to develop a Fourier type, semi‐infinite slab energy model to represent the lake surface temperature, thermal profile, and thermocline position with Julian day (Thibodeaux, 1975). The present paper extends the semi‐infinite slab idea to include a Fickian type model for the upsurge of nutrients from the sediment‐water interface upward. Relations will be developed for the rate of nutrient upwelling into the epilimnion and the photoactive zone of a lake. A lake time constant emerges from the model equations. This constant is thought to be important in predicting the likelihood of a fall algal bloom. Results of model verification attempts on three lakes are presented.