ALBERT

Description: The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea-ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate.This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Description: Monthly precipitation prediction is of great significance to bridge the gap between short-term weather forecast and seasonal forecast. However, due to the complexity of the climate system, there is still a great deal of uncertainty in the prediction of precipitation at the monthly scale. In order to reduce the uncertainty of the monthly precipitation predictions results，we analyzed the corrective effect of BJP (Bayesian Joint Probabilistic ) and EMOS (Ensemble Model Output Statics) model on precipitation bias prediction using S2S (sub-seasonal to seasonal scale) near-real-time overall forecast data and re-forecast data from five global data centers, including ECMWF, NCEP, UKMO, JMA and KMA. According to the advantages of different models the integrated multi-structure monthly precipitation prediction model was constructed based on EM (Expectation-Maximum) algorithm. The prediction application of monthly precipitation was carried out during the flood season of 1981~2010 in the middle and lower reaches of the Yangtze River, and the result showed as follows. (1)The monthly precipitation prediction results of the ensemble model showed that the Nash efficiency coefficient reached more than 0.6 and the correlation coefficient was 0.83. It indicates that the prediction sequence and the measured sequence of the ensemble model had good consistency. (2) The average relative deviation of the monthly precipitation prediction results is 25%, which effectively reduces the uncertainty of the monthly precipitation prediction results of a single model compared with the monthly precipitation forecast results of a single model. The results provide scientific support for improving the accuracy of drought or flood prediction.

Description: The M〈sub〉S〈/sub〉 6.0 earthquake in Changning, Sichuan, China, on 17 June 2019 was the largest recorded earthquake in the stable Sichuan Basin. It occurred in a complicated region with salt mine and shale gas production. Whether this earthquake is induced raises concerns among the public and the scientific community. Furthermore, the relation between this earthquake and nearby industrial activities has also been of great interest. To address these questions, we estimated the nonstationary background seismicity rate and inverted for spatiotemporal stress changes. The results show that the background rate dramatically increased after hydraulic fracturing and remained at a high level until the present. Starting in 2005, the study region experienced an accelerating stress increase, and the rates of cumulative modified Coulomb stress changes were approximately 0.11 MPa/year from January 2005 to January 2015 and 0.24 MPa/year from January 2015 to December 2018. The 2019 Changning earthquake produced a stress step of 0.32 MPa. A clear difference between seismicity induced by salt mine injection and by hydraulic fracturing is documented. Our results suggest that the Changning sequence might have been induced by long-term injection for salt production. Furthermore, the seismicity-stress inversion method provides a tool for using seismicity rate changes as a stress meter to monitor human-induced seismicity.

Description: Wetland offset credit trading (WOCT) has emerged as a key method for wetland restoration, yet its ability to achieve "No Net Loss" (NNL) remains disputed. The effectiveness of WOCT in achieving NNL relies heavily on the ability of the offset credit accounting method to be consistent with the wetland ecosystem changes, while the method should be convenient to quantify for offset credit transaction. Therefore, taking the Yellow River Delta (YRD) as the case study area, this study intends to analyze the suitability of different offset credit accounting schemes in NNL achieving and wetland offset marketization, and factors that affect wetland NNL are analyzed. Specifically, various offset credit accounting schemes are developed based on area, function, and value. Spatial-temporal land use data of YRD wetland from 2000 to 2020 is obtained by utilizing remote sensing images. By combining land use changes, wetland function evaluation, wetland ecosystem service value accounting, and wetland economic value accounting, the spatial-temporal evolution of offset credit under different accounting schemes are analyzed. This study further explores causes of YRD wetland changes, discusses the suitability of each accounting scheme for representing wetland variation and market transaction, and analyzes factors that affect wetland NNL achieving. The research findings can provide more insights in understanding wetland changes, and can contribute to the development of the wetland offset credit market and wetland restoration.

Description: The construction activities of hydropower projects in the Yangtze River basin in China have reached a peak in the last two decades, and the construction achievements have attracted international attention. However, it has also significantly altered the ecological output levels in the project impact areas. Therefore, an assessment of the spatial and temporal distribution and evolutionary patterns of ecological gains and losses induced by hydropower projects is essential. The study takes large and medium-sized hydropower development projects in the Yangtze River basin as the research object, and utilizes remote sensing images from 2000 to 2020 to form a dataset of land use changes in the project impact areas. Following the analysis of the changes of surface cover types in these areas, the evolution patterns of ecological gains and losses in the project and impact areas are accounted for over 20 years. On this basis, we further analyze the factors influencing the spatial and temporal evolution of ecological gains and losses of hydropower projects by socio-economic statistics. The results of the study have important implications for optimizing hydropower development methods and improving the eco-efficiency of hydropower development.

Description: Wetlands are important ecosystems on the planet, undertaking a large number of functions in water regulation, climate mediation and maintaining ecological balance. However, social development and changes in natural conditions have severely affected the Ecosystem Service Value (ESV) of wetlands. Identifying and assessing the drivers of ESV change is important for sustainable social development and human well-being. This study selects the Yellow River Delta as a case study, which is the most extensive, ideal and youngest wetland ecosystem in the warm temperate region of the world, and is an important area for studying wetland ecosystems. This paper first uses a deep learning approach (CNN) to classify the land use of the Yellow River Delta and then obtains the changes in various ESV in the Yellow River Delta through ecological economics methods. The LDMI method is then used to analyze the influence of selected factors such as population, industrial development, local rainfall, upstream water use and ecological recharge projects on the changes in ESV in the Yellow River Delta. Finally, this study discusses the current problems in water management and possible directions for future improvement. This study aims to provide a scientific reference for the study of the factors influencing similar wetlands and the future management of water resources.

Description: Tropical rainfall is important for regional climate around the globe. In a warming climate forced by rising CO〈sub〉2〈/sub〉, the tropical rainfall will increase over the equatorial Pacific where sea surface warming is locally enhanced. Here, we analyze an idealized CO〈sub〉2〈/sub〉 removal experiment from the Carbon Dioxide Removal Model Intercomparison Project and show that the tropical rainfall change features a stronger pattern during CO〈sub〉2〈/sub〉 ramp-down than ramp-up, even under the same global mean temperature increase, such as the 2°C goal of the Paris Agreement. The tropical rainfall during CO〈sub〉2〈/sub〉 ramp-down increases over the equatorial Pacific with a southward extension, and decreases over the Pacific intertropical convergence zone and South Pacific convergence zone. The asymmetric rainfall changes between CO〈sub〉2〈/sub〉 ramp-down and ramp-up result from time-varying contributions of the fast and slow oceanic responses to CO〈sub〉2〈/sub〉 forcing, defined as the responses to abrupt CO〈sub〉2〈/sub〉 forcing in the first 10 years and thereafter, respectively, in the abrupt-4xCO2 experiment. The fast response follows the CO〈sub〉2〈/sub〉 evolution, but the slow response does not peak until 60 years after the CO〈sub〉2〈/sub〉 peak. The slow response features a stronger El Niño-like pattern, as the ocean dynamical thermostat effect is suppressed under stronger subsurface warming. The delayed and stronger slow response leads to stronger tropical rainfall changes during CO〈sub〉2〈/sub〉 ramp-down. Our results indicate that returning the global mean temperature increase to below a certain goal, such as 2°C, by removing CO〈sub〉2〈/sub〉, may fail to restore tropical convection distribution, with potentially devastating effects on climate worldwide.

Description: The Weddell Gyre is a prominent feature of the Southern Ocean and an important component of the planetary climate system; it regulates air-sea exchanges, controls the formation of deep and bottom waters, and hosts upwelling of relatively warm subsurface waters. It is characterised by extremely low sea surface temperatures, ubiquitous sea ice formation, and widespread salt stratification. At present, the remote and often sea-ice-covered Weddell Gyre is one of the most poorly-sampled regions of the global ocean, highlighting the need to extract as much information as possible from existing observations. Here, we apply a profile classification model (PCM), which is an unsupervised classification technique, to a Weddell Gyre profile dataset to identify coherent regimes in temperature and salinity. We find that, despite not being given any positional information, PCM identifies four spatially coherent thermohaline domains that can be described as follows: (1) a circumpolar class, (2) a transition region between the circumpolar waters and the Weddell Gyre, (3) a gyre edge class, and (4) a gyre core class. We also find signatures of Circumpolar Deep Water inflow and the presence of Weddell-Scotia Confluence waters. Compared with the circumpolar class, the transition class is relatively more affected by brine rejection from sea ice formation and the associated vertical mixing. By using PCM to examine the structure of the Weddell Gyre, we show that PCM can act as a useful hypothesis generation tool and a complement to existing expertise-driven approaches for characterising oceanographic data.

Description: Antarctic Bottom Water (AABW) is a key water mass in the global overturning circulation, flooding most of the world ocean abyss and pivotal for deep ocean ventilation and oceanic heat and carbon exchanges on multi-decadal to millennial timescales. The Weddell Sea contributes nearly half of global AABW through Weddell Sea Deep Water (WSDW), which along with denser underlying Weddell Sea Bottom Water (WSBW), is formed on the continental shelves via complex processes that include sea ice production. Here we report a multi-decadal decline of WSBW volume in repeat hydrographic sections. A 30% reduction of WSBW volume since 1992 is found, with the most significant volume decrease seen in the densest WSBW classes. This is likely driven by a multi-decadal reduction in dense water production on the Weddell Sea continental shelf associated with a 〉40% decline in the sea ice formation rate there. The ice production decrease is driven by northerly wind trends, partly in response to a negative polarity of the Interdecadal Pacific Oscillation since the early 1990s, with variability from the Amundsen Sea Low superposed. These results reveal key influences on the export of waters to the Atlantic abyss and their sensitivity to large-scale, multi-decadal climate variability.

Description: Fault zone head waves (FZHWs) are an essential diagnostic signal which can provide high-resolution imaging of fault interface properties at seismogenic depth. In this study, we investigate the existence of bi-material interfaces along the rupture zone of the 2022 Mw6.8 Luding earthquake at the southern Xianshuihe Fault and determine the cross-fault velocity contrast. We employ a semi-automatic workflow to detect FZHWs and the direct-P waves. We improve the identification ability of potential FZHWs in the automatic picking process with a combination of STA/LTA functions and a kurtosis detector in a “forward-detecting and backward-picking” strategy. We confirm 206 FZHWs at two stations. Results indicate average velocity contrasts of ~5 % along the northern segment of the rupture zone while it decreases to ~3% in the south, with the northeast side having a faster P-wave velocity, in agreement with tomographic results. We also noticed that some direct-P waves do not polarize parallel to the back azimuth, indicating a complex site effect beneath the station. The distribution of FZHW-generated events indicates the existence of a single continuous interface in the seismogenic zone with length about 100km, which might be conducive to the preparation of large earthquakes and further influence the dynamic rupture processes. The velocity contrast across the rupture zone indicates a statistically preferred NW propagation direction for typical subshear ruptures. However, the rupture direction of the Mw6.8 Luding earthquake was primarily from NW to SE, suggesting that other factors, such as heterogeneous stress distribution along the fault, might impact the rupturing direction.