Decadal variation of thermocline-sea surface temperature feedback in the tropical Indian Ocean and the underlying mechanisms

The thermocline-sea surface temperature(SST)feedback is the most important component of the Bjerknes feedback,which plays an important role in the development of the air-sea coupling modes of the Indian Ocean.The thermocline-SST feedback in the Indian Ocean has experienced significant decadal variations over the last40 a.The feedback intensified in the late twentieth century and then weakened during the hiatus in global warming at the early twenty-first century.The thermocline-SST feedback is most prominent in the southeastern and southwestern Indian Ocean.Although the decadal variations of feedback are similar in these two regions,there are still differences in the underlying mechanisms.The decadal variations of feedback in the southeastern Indian Ocean are dominated by variations in the depth of the thermocline,which are modulated by equatorial zonal wind anomalies.Whereas the decadal variation of feedback in the southwestern Indian Ocean is mainly controlled by the intensity of upwelling and thermocline depth in winter and spring,respectively.The upwelling and thermocline depth are both affected by wind stress curl anomalies over the southeastern Indian Ocean,which excite anomalous Ekman pumping and influence the southwestern Indian Ocean through westward propagating Rossby waves.

DYNAMIC CHARACTERISTICS OF SEASONAL THERMOCLINE IN THE DEEP SEA REGION OF THE SOUTH CHINA SEA

The dynamic characteristics of the seasonal thermocline in the deep sea region of the South China Sea were analyzed by using seasonal mean temperature climatology. The thermocline undergoes remarkably seasonal variation throughout a year, is thinnest and weakest in winter, and thickest in spring, strongest in summer and fall. Due to the upper Ekman transport caused by monsoon over the SCS, the thermocline slopes upward(downward) in winter(summer) from northwest to southeast, but there is no pileup of upper warm water along the monsoon direction. In addition, the intrusion of the Kuroshio loop through the Luzon Strait, and some local eddies in the SCS, can notably affect the depth, thickness and strength of the thermocline in the deep sea region of the SCS.

Analysis of interdecadal variation of tropical Pacific thermocline based on assimilated data

The interdecadal variation of Pacific thermocline represented by depth anomalies of 25σθ isopycnal surface calculated from SODA data set is analyzed. The climatological depth of 25σθ isopycnal surface is quite close to the depth of 20 ℃ isotherm in the tropical Pacific. The EOF1 mode of the 25σθ isopycnal surface accounts for 26. 4% of the total variance and its associated pattern is of east-west direction. The centers of positive and negative extremes are located near 10oS over the southern Pacific and the correlation coefficient with zero-lag between the corresponding EOF1 time coefficient and PDO index is -0.67. This shows that there is very close relation between the southern tropical Pacific and PDO. The wavelet analysis of detrended EOF1 time coefficient reveals that there are two dominant time scales of about 3～7 and 30 a respectively. An apparent abruptness of mean value occurred in the late 1970s. EOF2 mode accounts for 12.4% of the total variance and its pattern is an ENSO-related one. The correlation coefficient between the EOF2 time coefficient and NINO3 index is -0.68. The wavelet analysis of EOF2 time coefficient reveals that there are two leading time scales of about 2～7 and 10～15 a respectively. On an interdecadal scale, the zonal change is consistent along the equator and is seesaw along 10oS; there is consistent polarity in the tropics along 165oE, but reverse polarity between around equator and other tropical region along 120oW. In all the four profiles mentioned above, the regime shift occurred in the late 1970s. The evolving characteristics of anomalies can be explained mostly by the anomalies of ocean currents during a complete cycle on an interdecadal scale.

A numerical study on seasonal variations of the thermocline in the South China Sea based on the ROMS

On the basis of the regional ocean modeling system （ROMS）, the seasonal variations of the thermocline in the South China Sea （SCS） were numerically investigated. The simulated hydrodynamics are in accordance with previous studies： the circulation pattern in the SCS is cyclonic in winter and anticyclonic in summer, and such a change is mostly driven by the monsoon winds. The errors between the modeled temperature profiles and the observations obtained by cruises are quite small in the upper layers of the ocean, indicating that the ocean status is reasonably simulated. On the basis of the shapes of the vertical temperature profiles, five thermocline types （shallow thermocline, deep thermocline, hybrid thermocline, double thermocline, and multiple thermocline） are defined herein. In winter, when the northeasterly monsoon prevails, most shallow shelf seas in the northwest of the SCS are well mixed, and there is no obvious thermocline. The deep region generally has a deep thermocline, and the hybrid or double thermocline often occurs in the areas near the cold eddy in the south of the SCS. In summer, when the southwesterly monsoon prevails, the shelf sea area with a shallow thermocline greatly expands. The distribution of different thermocline types shows a relationship with ocean bathymetry： from shallow to deep waters, the thermocline types generally change from shallow or hybrid to deep thermocline, and the double or multiple thermocline usually occurs in the steep regions. The seasonal variations of the three major thermocline characteristics （the upper bound depth, thickness, and intensity） are also discussed. Since the SCS is also an area where tropical cyclones frequently occur, the response of thermocline to a typhoon process in a short time scale is also analyzed.

Analysis of monthly variability of thermocline in the South China Sea

This study analyzes monthly variability of thermocline and its mechanism in the South China Sea(SCS). The study is based on 51-year(1960–2010) monthly seawater temperature and surface wind stress data from Simple Ocean Data Assimilation(SODA), together with heat flux, precipitation and evaporation data from the National Centers for Environmental Prediction(NCEP), the National Oceanic and Atmospheric Administration(NOAA) and the Woods Hole Oceanographic Institution, respectively. The results reveal that the upper boundary depth(Z_(up)), lower boundary depth(Z _(low)), thickness(?Z) and intensity( T _z) of thermocline in the SCS show remarkable monthly variability. Being averaged for the deep basin of SCS, Z_(up) deepens gradually from May to the following January and then shoals from February to May, while Z low varies little throughout the whole year. Further diagnostics indicates that the monthly variability of Z_(up) is mainly caused by the buoyancy flux and wind stress curl. Using a linear method, the impacts of the buoyancy flux and wind stress curl on Z_(up) can be quantitatively distinguished. The results suggest that Z_(up) tends to deepen about 4.6 m when the buoyancy flux increases by 1×10^(-5) kg/(m?s ~3), while it shoals about 2.5 m when the wind stress curl strengthens by 1×10-^(7) N/m3.

Interannual Thermocline Signals and El Nio-La Nia Turnabout in the Tropical Pacific Ocean

One of the fundamental questions concerning the nature and prediction of the oceanic states in the equatorial eastern Pacific is how the turnabout from a cold water state （La Nino） to a warm water state （El Nino） takes place, and vice versa. Recent studies show that this turnabout is directly linked to the interannual thermocline variations in the tropical Pacific Ocean basin. An index, as an indicator and precursor to describe interannual thermocline variations and the turnabout of oceanic states in our previous paper （Qian and Hu, 2005）, is also used in this study. The index, which shows the maximum subsurface temperature anomaly （MSTA）, is derived from the monthly 21-year （1980-2000） expendable XBT dataset in the present study. Results show that the MSTA can be used as a precursor for the occurrences of E1 Nino （or La Nino） events. The subsequent analyses of the MSTA propagations in the tropical Pacific suggest a one-year potential predictability for E1 Nino and La Nino events by identifying ocean temperature anomalies in the thermocline of the western Pacific Ocean. It also suggests that a closed route cycle with the strongest signal propagation is identified only in the tropical North Pacific Ocean. A positive （or negative） MSTA signal may travel from the western equatorial Pacific to the eastern equatorial Pacific with the strongest signal along the equator. This signal turns northward along the tropical eastern boundary of the basin and then moves westward along the north side of off-equator around 16°N. Finally, the signal returns toward the equator along the western boundary of the basin. The turnabout time from an E1 Nino event to a La Nino event in the eastern equatorial Pacific depends critically on the speed of the signal traveling along the closed route, and it usually needs about 4 years. This finding may help to predict the occurrence of the E1 Nino or La Nino event at least one year in advance.

A dipole mode at thermocline layer in the tropical Indian Ocean

Temperature data at different layers of the past 45 years were studied and we found adiploe mode in the thermocline layer (DMT): anomalously cold sea temperature off the coast of Sumatra and warm sea temperature in the western Indian Ocean. First, we analyzed the temperature and the temperature anomaly (TA) along the equatorial Indian Ocean in different layers. This shows that stronger cold and warm TA signals appeared at subsurface than at the surface in the tropical Indian O-cean. This result shows that there may be a strong dipole mode pattern in the subsurface tropical Indian Ocean. Secondly we used Empirical Orthogonal Functions (EOF) to analyze the TA at thermocline layer. The first EOF pattern was a dipole mode pattern. Finally we analyzed the correlations between DMT and surface tropical dipole mode (SDM), DMT and Nino 3 SSTA, etc. and these correlations are strong.

The Southwest Indian Ocean Thermocline Dome in CMIP5 Models:Historical Simulation and Future Projection

Using 20 models of the Coupled Model Intercomparison Project Phase 5 （CMIP5）, the simulation of the Southwest Indian Ocean （SWIO） thermocline dome is evaluated and its role in shaping the Indian Ocean Basin （IOB） mode following E1 Nifio investigated. In most of the CMIP5 models, due to an easterly wind bias along the equator, the simulated SWIO thermocline is too deep, which could further influence the amplitude of the interannual IOB mode. A model with a shallow （deep） thermocline dome tends to simulate a strong （weak） IOB mode, including key attributes such as the SWIO SST warming, antisymmetric pattern during boreal spring, and second North Indian Ocean warming during boreal summer. Under global warming, the thermocline dome deepens with the easterly wind trend along the equator in most of the models. However, the IOB amplitude does not follow such a change of the SWIO thermocline among the models; rather, it follows future changes in both ENSO forcing and local convection feedback, suggesting a decreasing effect of the deepening SWIO thermocline dome on the change in the IOB mode in the future.

Review on Thermocline Storage Effectiveness for Concentrating Solar Power Plant

In this paper, a literature review on thermocline storage performance for Concentrating Solar Power (CSP) plant storage systems has been conducted. The efficiency of materials to store heat depends on the storage process like sensible heat storage, latent heat storage and thermochemical one and also on their properties. This study has been focused on sensible heat storage materials especially thermocline storage system (DMT) using eco-materials which has a high potentiality (35%) to reduce CSP cost. There is a possibility to use natural rocks, industry waste and to develop also materials for a thermocline storage within a bed called packed bed using one tank. The thermal storage materials should have some optimum parameters (particle diameter less than 2 cm and good thermo-physical properties) to achieve better thermal storage performance (thermal cycle efficiency, extraction factor). However, the size and the shape of natural rocks are uncontrollable (big diameter) and can drive to thermocline degradation, catastrophic thermal ratcheting and poor thermal stratification due to the variability of the storage system porosity and the stress on the storage tank wall. Also a better thermal storage efficiency is achievable at low velocity and with good thermo-physical properties of the HTF. The ratio H/D, the height, the porosity, the shape and the position of the tank should be optimized to increase the storage efficiency.

Thermocline Model for Estimating Argo Sea Surface Temperature Thermocline Model for Estimating Argo Sea Surface Temperature

Argo has become an important constituent of the global ocean observation system.However,due to the lack of sea surface measurements from most Argo profiles,the application of Argo data is still limited.In this study,a thermocline model was constructed based on three key thermocline parameters,i.e,thermocline upper depth,the thermocline bottom depth,and thermocline temperature gradient.Following the model,we estimated the sea surface temperature of Argo profiles by providing the relationship between sea surface and subsurface temperature.We tested the effectiveness of our proposed model using statistical analysis and by comparing the sea surface temperature with the results obtained from traditional methods and in situ observations in the Pacific Ocean.The root mean square errors of results obtained from thermocline model were found to be significantly reduced compared to the extrapolation results and satellite retrieved temperature results.The correlation coefficient between the estimation result and in situ observation was 0.967.Argo surface temperature,estimated by the thermocline model,has been theoretically proved to be reliable.Thus,our model generates theoretically feasible data present the mesoscale phenomenon in more detail.Overall,this study compensates for the lack surface observation of Argo,and provides a new tool to establish complete Argo data sets.

Exploring thermocline and water masses variability in southern South China Sea from the World Ocean Database(WOD)

Study about water characteristics(temperature and salinity) from the World Ocean Database(WOD) was conducted in the area of southern South China Sea(SSCS), covering the area of 0°–10°N, 100°–117°E. From interannual analysis, upper layer(10 m) and deep water temperature(50 m) increased from 1951 until 2014. Monthly averaged show that May recorded the highest upper layer temperature while January recorded the lowest. It was different for the deep water which recorded the highest value in September and lowest in February. Contour plot for upper layer temperature in the study area shows presence of thermal front of cold water at southern part of Vietnam tip especially during peak northeast season(December–January). The appearances of warm water were obviously seen during generating southwest monsoon(May–June). Thermocline study revealed the deepest isothermal layer depth(ILD) during peak northeast and southwest monsoon. Temperature threshold at shallow area reach more than 0.8°C during the transitional period. Water mass study described T-S profile based on particular region. Water mass during the southwest monsoon is typically well mixed compared to other seasons while strong separation according to location is very clear. During transitional period between northeast monsoon to southwest monsoon, the increasing of water temperature can be seen at Continental Shelf Water(CSW) which tend to be higher than 29°C and vice versa condition during transitional period between southwest monsoon to northeast monsoon. Dispersion of T-S profile can be seen during southwest monsoon inside Tropical Surface Water(TSW) where the salinity and temperature become higher than during northeast monsoon.

EFFECTS OF A SLOPING THERMOCLINE AND RAYLEIGH FRICTION ON EQUATORIALLY TRAPPED KELVIN WAVES

Based on the observed equatorial ocean dynamic characteristics, the effects of a sloping thermocline and Rayleigh friction on the equatorially trapped free Kelvin waves were theoretically studied with a linear one and one half layer reduced gravity model, the multiple scale method and a small parameter expansion technique. Assuming that main thermocline depth (MTD) variations are slow, i.e. the changes of MTD over one wavelength are smaller than that of the wave amplitude and that wave reflections are negligible, the authors showed by their analytical results that the wavelengths and amplitudes of Kelvin waves are significantly modified by the MTD variations and Rayleigh friction. The results also showed that for an eastward shallowing thermocline, the zonal velocity of the Kelvin waves varies with thermocline depth to the power -7/8. The eastward shallowing of the thermocline depth strengthens Kelvin wave entrapment at the equator. Rayleigh friction reduces the Kelvin wave’s eastward velocity while the thermocline acts in the opposite way. The friction causes dispersion of the Kelvin wave, whose dissipation factor does not depend on its wavelength. The friction increases the lateral decay length and causes phase lines of Kelvin waves to slant westward in parabolic arcs.

A THREE-DIMENSIONAL THERMOCLINE MODEL FOR THE YELLOW SEA AND NORTHERN EAST CHINA SEA

A time-dependent, three-dimensional finite difference model is presented for simulating the stratifiedYellow Sea and northem East China Sea. The mode is forced by time-dependent observed wind, surfaceflux of heat, and tidal turbulence. With this model, momentum and temperature distribution can be computed,and an approximation for the sub-grid scale effects is introduced by the use of mass and momentumexchange coefficients. The vertical exchanges are quite dependent on these assumed coefficents, whichare complicated functions of the turbulence energy of tide and wind, of the stratified strength and otherfactors. This model was applied to describe the mechanics of the variations in strength and thickness ofthe thermocline covering almost the whole Yellow Sea and northern East Chna Sea in summer. Comparisonsof the computed output with obtained survey data led to some important conclusions.

Thermohaline intrusions in the thermocline of the western tropical Pacific Ocean

The existing high-resolution hydrographic data in the western tropical Pacilit; Ocean are used to explore the spatial distribution and primary characteristics of thermohaline intrusions in the thermocline. Statistics show that the vertical scales of intrusions are 20-40 m in the upper thermocline （22.0-26.0δ0） and 40-80 m in the lower thermocline （26.0-27.2δ0）. In the upper thermocline, the most intensive intrusions exist at the equatorial front （EF） where north/sonth Pacilic water masses converge, anti Ihe westward spreading of the north Pacilic tropical waler （NPTW） in the Philippines Sea also produces patches of intrusions surrounding its high-salinity tongue. In the lower thermocline, intrusions are also strong at the tropical front （TF） which is the boundary between the north Pacilic subtropical/tropical waters. At the bottom of the thermocline （at about 27.0δ0）, intrusions mainly exist near the western boundary, which are produced by intermediate water convergence through the advection of subthermocline western boundary Ilows. Most strikingly a ＂C＂-shape distribution of intrusions at around 26.4δ0 is revealed, covering the vicinity of the EF the TE and the Mindanao Current （MC）, i.e., tile western boundary pathway ol the norlh Pacilic subtrnpical cell （STC）. Synoptic section analysis reveals that intrusions are more prominent on the warm/sally flank ot the fronts, implying more cross-front tongues of cold/fresh water. Among the intrusions, those at the EF are of best lateral coherence which implies a unique driving mechanism involving near-inertial velocity perturbations near the equator.

Distribution Characteristics of Mean Seawater Temperature Related to the Thermocline in the Deep-Water Area of Nansha

On the bisis of determining the there elements of thermocline ( depth [upper bound depth ], thickness and intensity ) and the maximum vertical temperature gradient of the surveying station, the paper calculates the mean temperature of the Nansha deep-water surveying station within the upper-bound depth layer of thermocline and the mean temperature below the lower-bound depth of thermocline between the 300m and 800m layers,respectively. Analysis indicates that the horizontal distribution of mean seawater temperature shows a distinct trend of the low-temperature seawater slowly moving from the northeast to the southeast of Nansha,which seems to have been driven by the Northeast Monsoon. The larger the vertical temperature radient is, the greater is its capability of preventing the heat of the upper seawater from diffusing into the deeper layers on the vertical direction.

Features of Internal Waves in a Shoaling Thermocline

Observations and numeric modeling of internal wave generation and transformation in the shelf zone of sea show that the main part of tidal energy is transported to shores in form of internal gravitational waves. Long-term measurements of temperature and current velocity fluctuations at many levels in the near-bottom thermocline were carried out during the periods when stable seasonal thermocline was present. Analysis of the measurements permits us to understand mechanisms of internal wave destruction with turbulent motion generation and corresponding rebuilding of velocity and density mean fields in the stratified near-bottom layer. Spectral analysis of temperature fluctuations shows that in shoaling internal waves the low-frequency maxima disappear, maxima at higher frequencies appear, and the spectra slope in the high frequency range changes with depth. Taking into account the concurrent analysis of near-bottom pressure fluctuations and current velocity fluctuations from surface till bottom we come to the conclusion that breaking internal waves in a near-bottom thermocline generate not only small-scale three-dimensional turbulence, but also quasi-horizontal turbulence of larger scales, which considerably contributes into mixing and sediments, alluvium, and nutrients transport in the shelf zone of sea.

Diagnostic study of the summer thermocline depth in the Huanghai Sea and the East China Sea

In this paper, the depth of the summer thermocline of the South Huanghai Sea and the East China Sea is calculated with two kinds of one-dimentional models, and the formation reasons are explained for the summer thermocline depth distribution characteristics in the study area. It is proved that in the shelf area of the East China Sea, tidal mixing has an important impact on the thermocline depth. And a new explanation for certain phenomena of the so-called coastal upwelling in the East China Sea is proposed.

Numerical modelling of the depth variation and the fluctuation of the thermocline under the effects of wind in the Bohai Sea

-In this paper, numerical modelling of the fluctuation of the thermocline in the Bohai Sea has been made using a two-dimensional nonlinear model in stratified ocean and the model for the depth of the thermocline under the effects of wind stirring. The computed results depict the variations of the fluctuation of the thermocline driven by different kinds of wind fields. The fluctuation of the thermocline in the Bohai Sea varies somewhat with different directions, paths and locations of typhoon (cyclone). Under the effects of strong wind, the thermoclines both sink due to mixing and fluctuate. Furthermore, the fluctuation of the thermocline speeds up mixing. At last, the thermoclines disappear after 12-15 h when the strong wind increases from Force 6 to Force 9.

Evolution of planktonic foraminifera and thermocline in the southern South China Sea since 12 Ma (ODP-184,Site 1143)

Down-core variation in planktonic foraminifera (PF) at Site 1143 (ODP 184) has dis closed the evolution of upper water-column structure over the last 12 Ma in the southern South China Sea. In the early Late Miocene (～10.6-7.8 Ma), there existed a lower percentage of total deep-dwelling species, reflecting a water thermocline deeper than that in the Middle Miocene,which resulted from the closure of Indonesian seaway and relevant intensification of the equatorial warm current. After the increase in deep-dwelling PF and the rising of thermocline during the late Late Miocene (7.6-6.4 Ma), the total deep-dwelling species decreased gradually from late Late Miocene (6.4 Ma) to the Pliocene, implying the deepening of water thermocline. The evolution of thermocline depth in the southern South China Sea reflected by the PF at Site 1143 might be a good indicator of the change of west Pacific 'warm pool'.

Evidence for thermocline ventilation in the South China Sea in winter

Based on analysis of Levitus data in the South China Sea (SCS), this note addresses the seasonal feature of the mixed layer and thermocline in the northern SCS in winter, and demonstrates thermocline ventilation during the winter period. The ventilated thermocline is isopycnic layers between sigma 22.0 and 23.5, the other layers beneath is not ventilated yet. The process of thermocline ventilation is accompanied by detrainment water from the mixed layer into the thermocline at the outcropping line and moves to the south. Forced by subduction, the seasonal southward propagating signal also appears in the unventilated thermocline. The horizontal component of the signal propagatesalong the isoline of potential vorticity (PV), in other words, moves around a seasonal PV pool.