ALBERT

Description: Intensified climate change is already affecting water quality around the world. The fundamental goal of water quality engineering is to ensure water safety to human and the environment. Traditional water quality engineering includes monitoring, evaluation, and control of key water quality parameters, most of which however do not consider potential health impact of mixture pollutants - a reality faced by terminal water users. To focus on one of the original goals of water quality engineering—human health and environmental protection—here we advocate toxicity-oriented water quality monitoring and control. This presentation showcases some of our efforts to achieve this goal. Specifically, as a complement to traditional water quality parameters, we assessed water toxicity using high-sensitivity toxicological endpoints, and subsequently investigated the performance of some water quality intervention strategies in modulating water toxicity. Furthermore, we applied the toxicity concept to existing water treatment design theories to facilitate toxicity-oriented water quality control designs. Suggestions for the next steps are also discussed. We hope that our work will stimulate the interest of water quality scientists and engineers in improving and adopting the toxicity-oriented approach to water quality monitoring and control.

Description: Ice sheet melting into the Southern Ocean increases the upper ocean stratification and sea ice production, and thus can change the formation and properties of the Antarctic Bottom Water (AABW). Indeed, both the AABW freshening, and sea ice expansion have been observed in the Southern Ocean from the late 1980s until 2014, raising the question as to what extent increasing ice sheet melting could explain the observed patterns. However, it is challenging to address this question using observations due to the scarcity of direct observations along the Antarctic shelf and slope. Until recently, most ocean models could not form AABW efficiently along the Antarctic shelf either, precluding further investigation. In this study, we use a global ocean & sea ice model to explore how increasing ice sheet melting can alter the AABW properties and sea ice production Here, we explore three observation-based ice sheet melting scenarios based on historical observations (i.e., 5, 12, and 18%) for the historical period of 1958-2017. Our model results indicate a 12% increase in ice sheet melting may lead to a freshening of AABW at a rate similar to that observed since the early 1980s. This suggests that the observed AABW freshening could be driven by the increasing Antarctic meltwater discharge. Sea ice did not show any robust response to any of the three ice sheet melting scenarios.

Description: Since the inception of the international South Atlantic Meridional Overturning Circulation initiative in the 21st century, substantial advances have been made in observing and understanding the Southern Hemisphere component of the Atlantic Meridional Overturning Circulation (AMOC). Here we synthesize insights gained into overturning flows, interocean exchanges, and water mass distributions and pathways in the South Atlantic. The overturning circulation in the South Atlantic uniquely carries heat equatorward and exports freshwater poleward and consists of two strong overturning cells. Density and pressure gradients, winds, eddies, boundary currents, and interocean exchanges create an energetic circulation in the subtropical and tropical South Atlantic Ocean. The relative importance of these drivers varies with the observed latitude and time scale. AMOC, interocean exchanges, and climate changes drive ocean warming at all depths, upper ocean salinification, and freshening in the deep and abyssal ocean in the South Atlantic. Long-term sustained observations are critical to detect and understand these changes and their impacts.

Description: The freshwater transport(M〈sub〉ov〈/sub〉) by the Atlantic Meridional Overturning Circulation (AMOC) across 34.5ºS is computed using observations from 49 eXpendable BathyThermograph (XBT) AX18-lines between 2002-2019. The M〈sub〉ov〈/sub〉is used as an indicator of the AMOC stability at 34.5ºS. XBT data present a negative M〈sub〉ov〈/sub〉 mean of -0.15 ± 0.09 Sv, indicating a bi-stable AMOC regime. Moreover, we have investigated the time variability of the M〈sub〉ov〈/sub〉, as well as its correlation with the MOC and meridional heat transport (MHT), estimating a strong linear relationship between them. Results are complemented with data from Argo floats, numerical ocean models, and coupled models. The M〈sub〉ov〈/sub〉 estimation is very sensitive to the dataset used, with some of the coupled models estimations providing a positive M〈sub〉ov〈/sub〉. To clarify the causes of the opposite sign of the M〈sub〉ov〈/sub〉, we have investigated the differences in the vertical profiles obtaining fresher upper and saltier deep waters, and higher MOC/MHT values in models with positive M〈sub〉ov〈/sub〉.

Description: XBT has been proven to be an invaluable tool in the measurement of temperature in the upper layers of the ocean. The effortlessness and low cost of XBT-based surveys has made possible the build-up of an impressive dataset used for inferring dynamic properties such as dynamic height, geostrophic velocity, and sound speed. However, for the proper estimation of such properties, the corresponding salinity fields are also required. Due to the inherent difficulty of in-situ measurements, the salinity can be estimated from XBT data based on the close relationship that exists between temperature and salinity in most of the ocean’s waters. The salinity field can be also estimated by objective analysis based in coarse in-situ observations or by data assimilation techniques, combining numerical observations and models. In recent years, with the increasing power of computers and the availability of immense amounts of data, the use of Neural Networks has been increasingly used to solve complex problems. An effort is underway to develop a Machine Learning (ML) model for the estimation of salinity along temperature profiles measured by XBTs. The preliminary results of a model using a stack of fully connected neural networks are promising. Using data from the AX18 and AX08 lines in the South Atlantic, it is found that when the input variables include the XBT location (longitude and latitude) and the depths, the accuracy is considerably increased as compared with when only temperature is used. In the sequence, the model will also incorporate remotely sensed surface properties.

Description: The South Atlantic Meridional Overturning Circulation (SAMOC) observing system has evolved tremendously since 2007, and has substantially improved our understanding of the dynamics and variability of the upper, deep, and abyssal South Atlantic circulation from daily to interannual time-scales. However, the SAMOC daily time series derived from moored arrays are still relatively short and are only available at 11°S and 34.5°S. To expand the SAMOC time series in space and time, we derived monthly zonal trans-basin temperature (T) and salinity (S) sections since 1993 at four latitudes (20°S, 25°S, 30°S, and 34.5°S) based on historical relationships between T, S, and satellite sea level. The resulting meridional overturning circulation (MOC) and meridional heat transport (MHT) estimates at 20°S, 25°S, and 30°S are significantly correlated with each other at near zero lag, however correlations with the estimates at 34.5°S are somewhat lower. Although the overturning contribution dominates changes in the MHT at all four latitudes, the gyre contribution increases southward, reaching 30% of the explained MHT variability at 34.5°S. These 30-year monthly records indicate that the dominant mechanism controlling the MOC/MHT variability alternates between wind forcing and internal ocean dynamics. Therefore, both mechanisms must be monitored to fully capture changes in the MOC/MHT. These estimates demonstrate a linkage between the tropical Pacific forcing and heat content changes in the subtropical South Atlantic, as well as the impact of the MOC/MHT on extreme weather events, and provide context for measurements obtained from the SAMOC moored arrays.

Description: Marine heatwaves and cold spells are extreme surface temperature events that have been associated with adverse societal and ecosystem impacts in several regions around the globe. Predicting these events presents a challenge because of their generally short-lived nature and dependence on air-sea interactions, both locally and remotely. Here we analyze oceanic propagating features that promote the occurrence of marine heatwaves and cold spells in the western subtropical South Atlantic. The main interannual feature detected from satellite sea level data since 1993 shows a westward propagating zonal pattern with a periodicity of 3-5 years. The pattern has a significant in-phase correlation with sea surface temperature (SST) anomalies in the western South Atlantic, explaining 77% of the daily extreme warm (90%) and cold (10%) SST anomalies and consequently modulating interannual variations in the intensity and duration of marine heatwave and cold spell events. It is found that meridional oceanic advection plays an important role in the regional heat budget associated with the westward-propagating mode, modulating the meridional exchange of tropical (warm) and extratropical (cold) waters in the western subtropical South Atlantic region and thereby setting a baseline for temperature extremes on interannual timescales. This propagating mode is well correlated (r 〉 0.6) with the strength of the meridional overturning circulation at 25°S and 30°S with a lag of approximately 5 to 9 months. The lagged response provides a potential for predictability of extreme events in the western South Atlantic.

Description: The Ordos Block in the western part of the North China Craton is enigmatic in having contrasting topographic structure in the northern and southern parts, while previous geophysical studies show little difference in crustal and upper mantle structure across the region. Here we present a new model of upper mantle structure in the Ordos Block region in order to test the importance of mantle heterogeneity for topographic differences. Our model is based on P- and S-wave seismic receiver functions calculated for data from 171 stations. It documents the presence of an uppermost mantle low-velocity zone between the Mid Lithospheric Discontinuity (MLD) and the Lehmann discontinuity. Clear converters at the 410 and 660 km discontinuities show constant Mantle Transition Zone (MTZ) thickness within the Ordos Block region, which indicates that no deep mantle thermal anomaly affects its present dynamics. However, the amplitude of the MTZ-converters is higher in the southern than the northern Ordos Block. In contrast, the conversions from MLD and the Lehmann discontinuity are strongest in Northern Ordos, which we interpret as a block with essentially preserved cratonic lithospheric mantle. We speculate that smaller amplitudes of the MLD and Lehmann converters in Southern than Northern Ordos may be related to either rheological weakening of cratonic lithosphere during the Mesozoic convergence between the North and South (Yangtze) China Cratons, or northeast extrusion of Tibetan lower crust and upper mantle in the Cenozoic caused by the India-Asia collision.