Subsurface Temperature Variability Research Articles

Relative contributions of heat flux and wind stress on the spatiotemporal upper-ocean variability in the tropical Indian Ocean

High-resolution ocean general circulation model (OGCM) simulations are employed to investigate interannual variability of the upper-ocean temperature in the tropical Indian Ocean (20°S–20°N). The seasonal cycle and interannual variability in the upper-ocean temperature in the tropical Indian Ocean in the forced ocean simulation are in good agreement with available observation and reanalysis products. Two further sensitivity OGCM simulations are used to separate the relative contributions of heat flux and wind stress. The comparison of the model simulations reveals the depth-dependent influences of heat flux and wind stress on the ocean temperature variability in the tropical Indian Ocean. Generally, heat flux dominates the temperature variability in the top 30 m, while wind stress contributes most strongly to the subsurface temperature variability below 30 m. This implies that a transition depth should exist at each location, where the dominant control of the ocean temperature variability switched from heat flux to wind stress. We define the depth of this transition point as the ‘crossing depth’ and make use of this concept to better understand the depth-dependent impacts of the heat flux and wind stress on the upper-ocean temperature variability in the tropical Indian Ocean. The crossing depth tends to be shallower in the southern tropical Indian Ocean (20°S-EQ), including the Seychelles-Chagos Thermocline Ridge (SCTR) and the eastern part of the Indian Ocean Dipole (IOD), suggesting the dominance of forcing due to wind stress and the resulting ocean dynamical processes in the temperature variability in those regions. The crossing depth also shows prominent seasonal variability in the southern tropical Indian Ocean. In the SCTR, the variability of the subsurface temperature forced by the wind stress dominates largely in boreal winter and spring, resulting in the shallow crossing depth in these seasons. In contrast, the intensified subsurface temperature variability with shallow crossing depth in the eastern part of the IOD is seen during boreal autumn. Overall, our results suggest that the two regions within the tropical Indian Ocean, the SCTR and the eastern part of the IOD, are the primary locations where the ocean dynamics due to wind-stress forcing control the upper-ocean temperature variability.

Drivers of Subsurface Temperature Variability in the Northern California Current

AbstractJISAO's Seasonal Coastal Ocean Prediction of the Ecosystem (J‐SCOPE) was developed to carry out experimental seasonal forecasts of ocean conditions in the Northern California Coastal System (N‐CCS). This system relies on NOAA's Coupled Forecast System (CFS) global climate model to provide initial and boundary conditions for dynamical downscaling with a high‐resolution numerical ocean model. Experiments with J‐SCOPE show that the bottom temperature on the shelf is more predictable than the sea surface temperature (SST) on seasonal time scales. While seasonal forecasts have been shown to have positive skill in terms of SST for the CCS due to El Niño‐Southern Oscillation (ENSO), the mechanism(s) responsible for subsurface predictability in the N‐CCS have yet to be determined. This study quantifies the relative importance of North Pacific and Tropical Pacific properties in determining the variations in temperatures at depth (TDs) on the N‐CCS shelf. A multivariable linear regression (MLR) model with selected predictors explains almost 83% of the N‐CCS TD variability with oceanic teleconnections from the North Pacific dominating the variance. The N‐CCS TD is related to (i) coastally trapped waves (CTWs) related to ENSO, originating in Tropical Pacific, (ii) west‐central Pacific spice anomalies, (iii) seasonal Pacific Decadal Oscillation (PDO) variation, and a more local influence from (iv) anomalies south of N‐CCS, originating in North Pacific. ENSO and CTW can each impact the depth of the California Undercurrent (CUC) core with the latter most prominently following ENSO events. The results on potential sources of predictability are relevant to the continued development of current seasonal forecasting efforts off the U.S. West coast.

Intraseasonal variability of the equatorial Pacific Ocean and its relationship with ENSO based on Self-Organizing Maps analysis

We investigated the intraseasonal variability of equatorial Pacific subsurface temperature and its relationship with El Nino-Southern Oscillation (ENSO) using Self-Organizing Maps (SOM) analysis. Variation in intraseasonal subsurface temperature is mainly found along the thermocline. The SOM patterns concentrate in basin-wide seesaw or sandwich structures along an east-west axis. Both the seesaw and sandwich SOM patterns oscillate with periods of 55 to 90 days, with the sequence of them showing features of equatorial intraseasonal Kelvin wave, and have marked interannual variations in their occurrence frequencies. Further examination shows that the interannual variability of the SOM patterns is closely related to ENSO; and maxima in composite interannual variability of the SOM patterns are located in the central Pacific during CP El Nino and in the eastern Pacific during EP El Nino. These results imply that some of the ENSO forcing is manifested through changes in the occurrence frequency of intraseasonal patterns, in which the change of the intraseasonal Kelvin wave plays an important role.

Indian Ocean Warming Trends and Forcing Mechanism with Emphasis on Northeastern Tropical Indian Ocean

Tejas, D.M.; Gnanaseelan, C.; Rashmi A.K., and Deepa, J.S., 2020. Indian Ocean warming trends and forcing mechanism with emphasis on northeastern tropical Indian Ocean. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 15-19. Coconut Creek (Florida), ISSN 0749-0208.Subsurface temperature variability in the northeastern Tropical Indian Ocean (TIO) is addressed in this study. The recent decade witnessed strong positive trends in the warming of subsurface waters of Indian Ocean. On the other hand the net heat flux in to the ocean is showed a negative trend in the last decade citing the oceanic process as the primary cause of temperature variability and trends. The present study is focused on understanding the temporal evolution of the subsurface temperature structure of the northeastern TIO from the latter part of the twentieth century. Long term trends and time series are examined to see whether they result from low-frequency variability. It is seen that, strong warming in the northeastern TIO is closely related with phase change in the decadal oscillation of temperature as well as equatorial Indian Ocean dynamics. Associated circulation over this region supports the findings. The equatorial Kelvin wave after mid 2000s also seems to play a crucial role in the evolution of subsurface temperature patterns.

Asymmetry in the tropical Indian Ocean subsurface temperature variability

The empirical orthogonal function (EOF) analysis of subsurface temperature shows a dominant north-south mode of interannual variability in the Tropical Indian Ocean (TIO) at around 100 m depth (thermocline). This subsurface mode (SSM) of variability evolves in September-November (SON) as a response to Indian Ocean Dipole and intensifies during December-February (DJF) reinforced by El Niño and Southern Oscillation (ENSO) forcing. The asymmetry in the evolution of positive and negative phases of SSM and its impacts on the modulation of surface features are studied. The asymmetry in the representation of anomalous surface winds along the equator and off-equatorial wind stress curl anomalies are primarily responsible for maintaining the asymmetry in the subsurface temperature through positive and negative phases of the SSM. During the positive phase of SSM, downwelling Rossby waves generated by anticyclonic wind stress curl propagate towards the southwestern TIO (SWTIO), the thermocline ridge region of mean upwelling. The warmer subsurface water associated with the downwelling Rossby waves upwells in the region of mean upwelling and warms the surface resulting in strong subsurface-surface coupling. Such interaction processes are however weak during the negative phase of SSM. The asymmetry in the subsurface-surface interaction during the two phases of SSM and its impact on the modulation of surface features of TIO are also reported. In addition to the ENSO forcing, self-maintenance of SSM during DJF season is evident in the positive SSM (PSSM) years through modulation of subsurface surface coupling and air-sea coupling. This positive feedback during PSSM years is maintained by the deepening thermocline, warm SSTs and convection. The asymmetry in the thermocline evolution is more evident in the SWTIO and southern TIO.

Multidecadal to decadal variability in the equatorial Indian Ocean subsurface temperature and the forcing mechanisms

The Tropical Indian Ocean (TIO) is seen to exhibit robust warming after the 1950s. Most of the previous studies on the Indian Ocean (IO) surface and subsurface temperature focussed on long-term trends and interannual variability. The multidecadal to decadal variability in the Eastern Equatorial Indian Ocean (EEIO, 80°E–110°E, 8°S–5°N, 50-110 m) subsurface temperature is studied using reanalysis products and ocean general circulation model (OGCM) simulations. Prior to 1990s, the EEIO subsurface temperature is found to be dominated mostly by the multidecadal variability. However, since the mid-1990s, significant decadal variability is observed, something that has not been reported before. The mechanisms responsible for the observed decadal and multidecadal variability in the EEIO subsurface temperature are studied in detail. Wind forcing along the equator, and the associated ocean dynamics are found to be the major factors responsible for the EEIO subsurface temperature variability in the decadal and multidecadal time scales, emphasizing the importance of local atmospheric forcing on this variability. Alongshore winds along the Sumatra coast also contribute considerably to the decadal changes in the EEIO subsurface temperature. It is also found that the observed decadal and multidecadal changes are independent of the phase reversal of Interdecadal pacific oscillation (IPO). OGCM sensitivity experiments also suggest that the decadal variability of EEIO subsurface temperature is largely unaffected by the oceanic pathways such as Indonesian Through Flow (ITF). Hence, remote forcing through these pathways is insignificant in the TIO subsurface temperature north of 10°S as compared to the TIO wind forcing. Consistent with EEIO subsurface temperature, equatorial IO (EIO) winds also exhibit decadal variability post mid-1990s. The major contributors for the decadal variability in EIO winds are found to be the decadal variability in the Mascarene high, the low level jet, and the large scale off-equatorial circulations.

Revisit of seasonal variability of subsurface temperature in the tropical Pacific with Argo data

Abstract Seasonal variability of temperature in the tropical Pacific is studied with Argo data from 2001 to 2017. It is found that the strongest seasonal variation is in the subsurface layer corresponding to the thermocline, which is located in three zonal bands centering at 12°N, 5°N and 5°S across the entire tropical Pacific. Further analysis indicates that seasonal variation of subsurface temperature is caused by the vertical movement of the thermocline, and the signal is generated in the eastern Pacific and propagates westward to the western Pacific. A linear wind-driven first-mode baroclinic Rossby wave model is then employed to investigate the seasonal fluctuation of thermocline. The seasonal shoaling and deepening of thermocline and associated temperature variations are well captured by this Rossby wave model. Basin-scale seasonal anomalies of wind stress curl in the tropical Pacific produce significant thermocline fluctuations, which propagate westward in the form of annual Rossby wave, and cause the seasonal variation of subsurface temperature. Thermocline fluctuations are revealed to be tightly associated with the meridional shift of equatorial currents such as North Equatorial Current, North Equatorial Countercurrent and South Equatorial Current.

Role of Kuroshio Current in fish resource variability off southwest Japan

Western boundary currents in the subtropics play a pivotal role in transporting warm water from the tropics that contribute to development of highly diverse marine ecosystem in the coastal regions. As one of the western boundary currents in the North Pacific, the Kuroshio Current (hereafter the Kuroshio) exerts great influences on biological resource variability off southwest Japan, but few studies have examined physical processes that attribute the coastal fish resource variability to the basin-scale Kuroshio variability. Using the high-quality fish catch data and high-resolution ocean reanalysis results, this study identifies statistical links of interannual fish resource variability off Sukumo Bay, Shikoku island of Japan, to subsurface ocean temperature variability in the Kuroshio. The subsurface ocean temperature variability off the south of Sukumo Bay exhibits vertically coherent structure with sea-surface height variability, which originates from the westward-propagating oceanic Rossby waves generated through surface wind anomalies in the Northwest Pacific. Although potential sources of the atmospheric variability remain unclarified, the remotely-induced oceanic Rossby waves contribute to fish resource variability off Sukumo Bay. These findings have potential applications to other coastal regions along the western boundary currents in the subtropics where the westward-propagating oceanic Rossby waves may contribute to coastal ocean temperature variability.

Spatial and temporal variations in female size at maturity of a Southern Rock Lobster (Jasus edwardsii) population: A likely response to climate change.

The size at which sexual maturity is reached is a key population parameter used to guide the setting of minimum legal size limits in fisheries. Understanding spatial and temporal variations in size at maturity is fundamental to management because the relationship between size at maturity and minimum legal size limits affects the fraction of the mature population biomass that is harvested, and resulting egg production, larval settlement and recruitment. This study measured the size at maturity of female Southern Rock Lobster (Jasus edwardsii) across South Australia between 1991 and 2015 in relation to known oceanographic characteristics, surface and subsurface temperature data, and relative changes in lobster abundance. There was pronounced north to south spatial variation in estimates of size at maturity. Larger average size at maturity was recorded in warmer north-western areas of the fishery relative to the cooler waters of the south-east. Estimates of size at maturity also differed over 25 years across the fishery. However, the nature of temporal responses varied spatially, and were more consistent with variations in surface and subsurface water temperature at local-scales than changes in lobster density. In the well-mixed waters of the north-western, western and south-eastern parts of the fishery, relatively high rates of increase in sea-surface temperature and size at maturity were recorded since 1991, indicating that size at maturity may be responding to ocean warming associated with global climate change. In more central parts of the fishery, contrasting temporal signals in sea-surface temperature (positive) and bottom temperature (negative) indicated increases in upwelling strength over the study period, and formation of a bottom cold pool below a warm surface layer, with corresponding decreases in size at maturity recorded. The spatio-temporal changes in size at maturity measured in this study highlight the need for oceanographic information to be integrated into future stock assessment models to enhance harvest strategy development, allow timely adaptive management decisions and increase the resilience of fisheries to the impacts of climate change.

Experimental modeling of subsurface gas traps on Mars

Methane seasonal variation observable by MSL mission and possible variations of atmospheric mass on timescale (105-106 years) are among the most intriguing problems in Mars exploration. These variations are connected with hypothetical biosphere activity in the subsurface Martian soil and existence of liquid water on the Martian surface within modern era. Stability of liquid water on surface request higher atmospheric pressure in comparing to modern value. CO2 cannot loss with known mechanisms of atmospheric escape. Therefore, the main part of necessary CO2 must be buried in upper layers of the Martian soil. Local and seasonal time variable sources and fast methane destruction are needed to explain high seasonal variations of methane concentration in air at the Martian surface. Gas reservoirs, containing biogenic or abiogenic methane could be possible seasonal sources of methane as well. In this work we experimental study stability of the gas reservoirs covered of mixture of regolith and water ice with perchlorates. Thickness of covered regolith layer was about 10mm. In experimental runs we increased a temperature of gas traps and monitored a possible diffusion of gases through the isolated layer with mass spectrometer. The gas traps stay stable at gas pressure up to 1 bar. We did not discover any diffusion process before mechanical destruction of reservoirs at gas pressure over 1 bar. In this work we show that the big subsurface gas reservoirs can exist for a long time before cracking due to slow process of the water ice sublimation by climate and seasonal variation of subsurface temperature.

Improvement on an analytical finite line source model considering complex initial and boundary conditions: Part 1, model development and validation

For a ground coupled heat pump system, the variations of ground surface temperature and undisturbed subsurface temperature have pronounced effects on the heat transfer between the ground heat exchanger and surrounding soil. However, how to quantify these effects analytically has long been a tricky issue. In this paper, finite line source model under more complex initial and boundary conditions (FLSCC) is presented and solved based on an existing model, and transient solutions of sub-problems are given, to investigate the effects of ground surface temperature and subsurface temperature variations. To examine the performance of this model, mean borehole surface temperature simulated by FLSCC was compared to that from five conventional models in a case study. Results show that long term behavior of FLSCC is the most close to the reference duct ground heat storage model (DST). The relative mean deviation between the two models at each simulation time step is less than 4.02% from 10 h to 100 years, and the mean value throughout the simulated 100 years is only 1.05%. Also, its short term behavior is close to a validated numerical mode. In addition, the new model is also validated with experimental data from two thermal response tests. Errors between tested and simulated mean fluid temperature calculated by FLSCC are smaller than that of conventional models. Conclusively, the improved model is suitable for dealing with simulation projects at any locations and from any starting point in a year, and also deal with thermal response test data interpretation. It is an effective supplement to finite line source model, and is helpful for fast and reliable practical engineering design.

Three-Dimensional Temperature Field Change in the South China Sea during Typhoon Kai-Tak (1213) Based on a Fully Coupled Atmosphere–Wave–Ocean Model

Studying the sea–air interaction between the upper ocean and typhoons is crucial to improve our understanding of heat and momentum exchange between the atmosphere and the ocean. There is a strong heat flux exchange between the atmosphere and the ocean during the impact of a typhoon, and the physical fields, such as the wind field, wave field, flow field, and SST field, also interact with each other. A fully coupled Atmosphere–Wave–Ocean model in the South China Sea was established by the mesoscale atmospheric model WRF, wave model SWAN, and the regional ocean model ROMS based on the COAWST model system. Typhoon Kai-tak was simulated using this fully coupled model and some other coupled schemes. In this paper, the variation of sea surface temperature (SST) and ocean subsurface temperature caused by Typhoon Kai-tak is analyzed by the fully coupled model, and the basic characteristics of the response of the upper ocean to the typhoon are given. The simulation results demonstrate that the fully coupled WRF-SWAN-ROMS model shows that the typhoon passes through the sea with obvious cooling. In the cold eddy region, the sea surface temperature cools 4 to 5 °C, and the cooling zone is concentrated on the right side of the track. The change of sea surface temperature lags more than 12 h behind the change of sea surface height. The decrease of SST on the left side of the track was relatively small: ranging from 1.5 to 2.5 °C. The disturbance of typhoon causes the subsurface water to surge to the surface, changes the temperature distribution of the surface, and causes the mixing layer to deepen about 40 m to 60 m. The simulation results reveal the temporal and spatial distribution of sea temperature and mixed layer depth. The sea surface temperature field has an asymmetrical distribution in space and has a lag in time. The heat exchange at the air–sea interface is very strong under the influence of the typhoon. The heat exchange between the air and sea is divided into latent heat and sensible heat, and the latent heat generated by water vapor evaporation plays a dominant role in the heat exchange at the air–sea interface, which shows that the heat carried by the vaporization of the sea surface is one of the important factors for the decrease of sea temperature under the influence of the typhoon.

Impact of Tropical Cyclones on the Global Ocean: Results from Multidecadal Global Ocean Simulations Isolating Tropical Cyclone Forcing

Tropical cyclone (TC)-induced ocean vertical mixing can alter the upper-ocean temperature structure, influencing ocean heat content variability and meridional ocean heat transport. TC–ocean interactions can influence tropical variability on seasonal to interannual time scales. Here the impacts of TCs on the global ocean and the associated feedbacks are investigated using a hierarchy of high-resolution global ocean model simulations featuring the Community Earth System Model (CESM). The aim is to understand the potential impact of the model’s self-generated transient TC events on the modeled global ocean. Two ocean-only simulations are performed using the atmosphere boundary conditions from a fully coupled preindustrial CESM simulation configured with 0.25° atmosphere resolution and the nominal 1° ocean resolution (with ~0.25° meridional resolution in the tropics). The high-resolution coupled model is capable of directly simulating TC events with wind structure and climatology generally consistent with observations. TC effects at the ocean–atmosphere boundary are filtered out in one of the ocean simulations (OCN_FILT) while fully retained in the other (OCN_TC) in order to isolate the effect of the TCs on regional and global ocean variability across multiple time scales (from intraseasonal to interdecadal). Results show that the model-simulated TCs can 1) alter surface and subsurface ocean temperature patterns and variability; 2) affect ocean energetics, including increasing ocean mixed layer depth and strengthening subtropical gyre and meridional overturning circulations; and 3) influence ocean meridional heat transport and ocean heat content from seasonal to interannual time scales. Results help provide insights into the model behavior and the physical nature of the effect of TCs within the Earth system.

Biases in the Tropical Indian Ocean subsurface temperature variability in a coupled model

In this study, the subsurface temperature variability in the Tropical Indian Ocean (TIO) is examined in the Climate Forecast System version 2 (CFSv2) coupled model. The observations and reanalysis show a north–south dominant mode of variability in the TIO subsurface temperature during September–November, the season when the dominant east–west surface mode (Indian Ocean Dipole; IOD) peaks. The nature of the north–south dipole in TIO subsurface temperature is successfully captured by CFSv2. The observations however indicate that this subsurface mode, in general, persists for the next two seasons with stronger signals during December–February, whereas such tenacity is not seen in the model, instead rapid decay of the mode is seen in the model. It is found that the misrepresentation of both equatorial surface wind anomalies and associated Ekman transport as well as the Ekman pumping in the model have close association with the early weakening of the mode in CFSv2. The surface easterlies are generally modulated by the presence of twin anticyclones on both sides of the equator. The model captured these anticyclones with weaker than observed intensity and the northern anticyclone is confined over much smaller region than observed. Association of the subsurface mode with El Nino Southern Oscillation (ENSO) and IOD is further examined in this study. The anomalously prolonged decay phase of El Nino in CFSv2 is found only during the El Nino, IOD co-occurrence years, which was not reported before. This paves way for addressing an important modeling issue which is common in many coupled climate models including CFSv2. The analysis suggests the possible role of coupled air–sea interaction over the TIO on the El Nino cycle in the Pacific. It is also found that the misrepresentation of subsurface variability in CFSv2 during December–February is closely associated with the rapid decay of El Nino forced TIO warming.

Observed diurnal variations in Mars Science Laboratory Dynamic Albedo of Neutrons passive mode data

The Mars Science Laboratory Dynamic Albedo of Neutrons (DAN) experiment measures the martian neutron leakage flux in order to estimate the amount of water equivalent hydrogen present in the shallow regolith. When DAN is operating in passive mode, it is sensitive to neutrons produced through the interactions of galactic cosmic rays (GCR) with the regolith and atmosphere and neutrons produced by the rover’s Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). During the mission, DAN passive mode data were collected over the full diurnal cycle at the locations known as Rocknest (sols 60–100) and John Klein (sols 166–272). A weak, but unexpected, diurnal variation was observed in the neutron count rates reported at these locations. We investigate different hypotheses that could be causing these observed variations. These hypotheses are variations in subsurface temperature, atmospheric pressure, the exchange of water vapor between the atmosphere and regolith, and instrumental effects on the neutron count rates. Our investigation suggests the most likely factors contributing to the observed diurnal variations in DAN passive data are instrumental effects and time-variable preferential shielding of alpha particles, with other environmental effects only having small contributions.

Deglacial Tropical Atlantic subsurface warming links ocean circulation variability to the West African Monsoon

Multiple lines of evidence show that cold stadials in the North Atlantic were accompanied by both reductions in Atlantic Meridional Overturning Circulation (AMOC) and collapses of the West African Monsoon (WAM). Although records of terrestrial change identify abrupt WAM variability across the deglaciation, few studies show how ocean temperatures evolved across the deglaciation. To identify the mechanism linking AMOC to the WAM, we generated a new record of subsurface temperature variability over the last 21 kyr based on Mg/Ca ratios in a sub-thermocline dwelling planktonic foraminifera in an Eastern Equatorial Atlantic (EEA) sediment core from the Niger Delta. Our subsurface temperature record shows abrupt subsurface warming during both the Younger Dryas (YD) and Heinrich Event 1. We also conducted a new transient coupled ocean-atmosphere model simulation across the YD that better resolves the western boundary current dynamics and find a strong negative correlation between AMOC strength and EEA subsurface temperatures caused by changes in ocean circulation and rainfall responses that are consistent with the observed WAM change. Our combined proxy and modeling results provide the first evidence that an oceanic teleconnection between AMOC strength and subsurface temperature in the EEA impacted the intensity of the WAM on millennial time scales.

On the dynamics of the Sri Lanka Dome in the Bay of Bengal

AbstractEast of Sri Lanka, in the northern Indian Ocean, a cold dome, known as the Sri Lanka Dome (SLD), develops during southwest monsoon season (June–September). The SLD first forms around May, matures in July, and decays around September, in association with the strong cyclonic wind stress curl. In this study, the structure and dynamics of SLD in response to the climatic events such as the Indian Ocean Dipole (IOD) are examined. Our results reveal that these climatic events modulated the subsurface temperature variability in the thermocline at ∼100 m depth, whose signature was also evident in the satellite‐derived sea surface height (SSH) anomaly in the SLD region. We find that the mechanisms for the formation of SLD are consistent with previous research, and there is year‐to‐year variability in the SLD dynamics. This study also reveals that the atmospheric parameters including vertical wind shear and midtropospheric relative humidity are high over the SLD region and adjacent southern Bay of Bengal and show an upward (increasing) trend over the decades. This has impacted the atmospheric parameters over the northern Bay of Bengal over the decades and as a consequence the total number of monsoon depressions (June–September) decreased over the decades from 1980 to 2015, as reported by the India Meteorological Department, New Delhi. Thus, the new insight emerged from this study is the variability in the cyclogenesis and the occurrence of total number of monsoon depressions over northern Bay of Bengal over the decades is much related to the SLD dynamics.

Variation in subsurface water temperature and its link to the Kuroshio Current in the Okinawa Trough during the last 38.5 kyr

An 820.4-cm long piston core (YK12-15 PC-01) was obtained from site OT-1B (28°07.15′N, 127°11.97′E, 1050 m water depth), located in the middle of the Okinawa Trough in the southern East China Sea (ECS). For core YK12-15 PC-01, oxygen and carbon isotopes (δ18O and δ13C) of planktic foraminifera Globigerinoides ruber sensu stricto (250–355 μm size fraction) and Globorotalia inflata (>250 μm size fraction) together with total organic carbon (TOC), nitrogen content (TN), and δ13C of organic matter were analyzed for the last 38.5 thousand years (kyr). The δ18O records of G. ruber (δ18Oruber) and G. inflata (δ18Oinflata) were used to estimate the temperature/salinity of the surface and subsurface waters, respectively. In addition, we estimated the vertical temperature gradient between the surface and the subsurface based on the difference in δ18O between G. ruber and G. inflata (Δδ18Oruber-inflata). The δ18Oinflata and Δδ18Oruber-inflata indicate millennial-scale variations in association with the oscillations in Antarctic temperatures during the Marine Isotope Stage (MIS) 3. These results suggest that variations in subsurface temperature and vertical temperature gradient in the upper water column in the ECS were closely related to the Atlantic bipolar seesaw during MIS 3. The subsurface temperature in the ECS was higher at the beginning of Greenland stadials when the Antarctic temperatures showed the lowest values. The warming of the subsurface is likely attributed to the stronger Kuroshio transport. High peaks in the TOC and TN records coincide with the low peaks in δ18Oinflata (high temperature), suggesting higher primary productivity, which is in good agreement with the interpretation of the intensified Kuroshio. However, the subsurface warming was not recognized during Herinch events, indicating different temperature responses in the upper water column from other stadials.

Multiyear Variability in the Tasman Sea and Impacts on Southern Hemisphere Climate in CMIP5 Models

Naturally occurring multiyear to decadal variability is evident in rainfall, temperature, severe weather, and flood frequency around the globe. It is therefore important to understand the cause of this variability and the extent to which it can be predicted. Here internally generated decadal climate variability and its predictability potential in an ensemble of CMIP5 models are assessed. Global hot spots of subsurface ocean decadal variability are identified, revealing variability in the southern Tasman Sea that is coherent with variability in much of the Pacific Ocean and Southern Hemisphere. It is found that subsurface temperature variability in the southern Tasman Sea primarily arises in response to preceding changes in Southern Hemisphere winds. This variability is multiyear to decadal in character and is coherent with surface temperature in parts of the Southern Hemisphere up to several years later. This provides some degree of potential predictability to surface temperature in the southern Tasman Sea and surrounding regions. A few models exhibit significant correlation between subsurface variability in the southern Tasman Sea and zonally averaged precipitation south of 50°S; however, the multimodel mean does not exhibit any significant correlation between subsurface variability and precipitation. Models that exhibit stronger subsurface variability in the southern Tasman Sea also have a stronger interdecadal Pacific oscillation signal in the Pacific.

Experimental investigation into lubrication properties and mechanism of vegetable-based CuO nanofluid in MQL grinding

Application of vegetable-based nanolubricants in machining operations has been promoted due to the environmental concerns and higher demand for better quality of machined parts. In this study, lubrication properties of copper oxide nanofluids in surface grinding of AISI 1045 hardened steel are investigated. These nanofluids were synthesized by submerged electro discharge process, exposed to ultrasonic agitation in the presence of Tween 20 as dispersant and sprayed in grinding position using minimum quantity lubrication system. The base fluid is an emulsion of canola oil and distilled water with vegetable oil acting as triglyceride agent. Convective heat transfer coefficients of different lubrication systems in grinding are measured using an innovative approach to better explore the cooling mechanisms involved in grinding process with or without the application of nanolubricants. The variation of grinding forces and sub-surface temperature of workpiece are recorded at different lubrication conditions. These parameters along with surface roughness, micro-hardness, and microscopic observations of ground surfaces are employed to evaluate the performance of synthesized nanofluids as lubricants in grinding. The results show that the synthesized nanofluids are effective in reducing the grinding forces and temperatures especially in extreme machining conditions. Better surface integrity of ground parts is observed in all grinding conditions through the application of CuO nanofluids as lubricant in minimum quantity lubrication system.