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  1. null (Ed.)
    Abstract The Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the Southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004-2018. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking at σ θ =26.7-26.8 kg m −3 in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004-2018. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy loss. An increase in the mixed-layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre. 
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  2. null (Ed.)
    Abstract This study uses sea surface salinity (SSS) as an additional precursor for improving the prediction of summer [December–February (DJF)] rainfall over northeastern Australia. From a singular value decomposition between SSS of prior seasons and DJF rainfall, we note that SSS of the Indo-Pacific warm pool region [SSSP (150°E–165°W and 10°S–10°N) and SSSI (50°–95°E and 10°S–10°N)] covaries with Australian rainfall, particularly in the northeast region. Composite analysis that is based on high or low SSS events in the SSSP and SSSI regions is performed to understand the physical links between the SSS and the atmospheric moisture originating from the regions of anomalously high or low, respectively, SSS and precipitation over Australia. The composites show the signature of co-occurring La Niña and negative Indian Ocean dipole with anomalously wet conditions over Australia and conversely show the signature of co-occurring El Niño and positive Indian Ocean dipole with anomalously dry conditions there. During the high SSS events of the SSSP and SSSI regions, the convergence of incoming moisture flux results in anomalously wet conditions over Australia with a positive soil moisture anomaly. Conversely, during the low SSS events of the SSSP and SSSI regions, the divergence of incoming moisture flux results in anomalously dry conditions over Australia with a negative soil moisture anomaly. We show from the random-forest regression analysis that the local soil moisture, El Niño–Southern Oscillation (ENSO), and SSSP are the most important precursors for the northeast Australian rainfall whereas for the Brisbane region ENSO, SSSP, and the Indian Ocean dipole are the most important. The prediction of Australian rainfall using random-forest regression shows an improvement by including SSS from the prior season. This evidence suggests that sustained observations of SSS can improve the monitoring of the Australian regional hydrological cycle. 
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  3. The long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on inter-seasonal to inter-annual time scales, and to locate the source of moisture. Seasonal composites during El Niño Southern Oscillation/Indian Ocean Dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies towards Australia. During co-occurring La Niña and negative-IOD events, salty anomalies around the maritime continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, there is the moisture transport divergence anomaly over Australia and results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean-atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g. 2010-11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall. 
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  4. Abstract. Over the past decade, our understanding of the IndianOcean has advanced through concerted efforts toward measuring the oceancirculation and air–sea exchanges, detecting changes in water masses, andlinking physical processes to ecologically important variables. Newcirculation pathways and mechanisms have been discovered that controlatmospheric and oceanic mean state and variability. This review bringstogether new understanding of the ocean–atmosphere system in the IndianOcean since the last comprehensive review, describing the Indian Oceancirculation patterns, air–sea interactions, and climate variability.Coordinated international focus on the Indian Ocean has motivated theapplication of new technologies to deliver higher-resolution observationsand models of Indian Ocean processes. As a result we are discovering theimportance of small-scale processes in setting the large-scale gradients andcirculation, interactions between physical and biogeochemical processes,interactions between boundary currents and the interior, and interactions between thesurface and the deep ocean. A newly discovered regional climate mode in thesoutheast Indian Ocean, the Ningaloo Niño, has instigated more regionalair–sea coupling and marine heatwave research in the global oceans. In thelast decade, we have seen rapid warming of the Indian Ocean overlaid withextremes in the form of marine heatwaves. These events have motivatedstudies that have delivered new insight into the variability in ocean heatcontent and exchanges in the Indian Ocean and have highlighted the criticalrole of the Indian Ocean as a clearing house for anthropogenic heat. Thissynthesis paper reviews the advances in these areas in the last decade. 
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  5. null (Ed.)