Common occurrences of subsurface heatwaves and cold spells in ocean eddies.

Extreme ocean temperature events are becoming increasingly common due to global warming, causing catastrophic ecological and socioeconomic impacts1-5. Despite extensive research on surface marine heatwaves (MHWs) and marine cold spells (MCSs) based on satellite observations6,7, our knowledge of these extreme events and their drivers in the subsurface ocean-home to the majority of marine organisms-is very limited8,9. Here we present global observational evidence for the important role of mesoscale eddies in the occurrence and intensification of subsurface MHWs and MCSs. We found that 80% of measured MHWs and MCSs below a depth of 100 m do not concur with surface events. In contrast to the weak link between surface MHWs (MCSs) and ocean eddies, nearly one-third of subsurface MHWs (MCSs) in the global ocean, and more than half of such events in subtropical gyres and mid-latitude main current systems, occur within anticyclonic (cyclonic) eddies. These eddy-associated temperature extremes have intensified at rates greater than background level in past decades, suggesting a growing impact of ocean eddies on subsurface MHWs and MCSs with ongoing global warming.

Enhancing impacts of mesoscale eddies on Southern Ocean temperature variability and extremes.

As a major sink of anthropogenic heat and carbon, the Southern Ocean experienced pronounced warming with increasing extreme temperature events over the past decades. Mesoscale eddies that strongly influence the uptake, redistribution, and storage of heat in the ocean are expected to play important roles in these changes, yet observational evidence remains limited. Here, we employ a comprehensive analysis of over 500,000 historical hydrographic profile measurements combined with satellite-based eddy observations to show enhanced thermal eddy imprints in the Southern Ocean. Our observations reveal that anticyclonic (cyclonic) eddies are responsible for nearly half of the subsurface high (low)-temperature extremes detected, although only 10% of the profiles are located in eddy interiors. Over the past decade (2006 to 2019), both mean and extreme temperature anomalies within eddies in the Antarctic Circumpolar Current increased significantly, promoting the rise in subsurface ocean temperature variability. This enhanced role of eddies is likely a result of enhanced eddy pumping due to the increase in eddy intensity and ocean stratification caused by ocean warming. Our analysis underscores the crucial role of eddies in amplifying ocean temperature variability and extremes, with their effects expected to be even more pronounced as global warming persists.

Impact of anthropogenic activities on morphological and deposition flux changes in the Pearl River Estuary, China.

The evolution of the Pearl River Estuary (PRE), China in recent decades has been dominated by human activities. Historical admiralty charts and remote sensing images indicated that from 1936 to 2017, the tidal flat area and water area decreased by 23.6 × 107 m2 and 60.7 × 107 m2, respectively. The average advancing rate of the coastline of the PRE to the sea from 1972 to 2017 reached approximately 64.8 m/year, which is several times or even dozens of times that since the mid-Holocene. Land reclamation was the main reason for the dramatic changes in the water area and coastline. Although the water volume of the PRE showed a decreasing trend from 1936 to 2017, the water volume reduction rates for 1996-2005 and 2005-2017 were only 29% (1.27 × 107 m3/year) and 12% (0.53 × 107 m3/year), respectively, of that for 1936-1972. The combined influences of channel dredging, sand mining, and sediment load reduction caused by dam construction have contributed to this change. From the perspective of the filling up of the estuary, channel dredging, sand mining, and dam construction in the river basin are beneficial for prolonging the life of the estuary.

Impacts of climate change and human activities on the water discharge and sediment load of the Pearl River, southern China.

Global climate change and human activities have important effects on the water discharge and sediment load of the Pearl River. In this study, the water discharge and sediment load were investigated by using hydro-meteorological data from 1954 to 2018. The linear regression, Mann-Kendall abrupt test and double mass curve were employed to detect trends and abrupt change-points in water discharge and sediment load and to quantify the effects of climate change and human activities on water discharge and sediment load. The results revealed that the annual sediment load exhibited a significant decreasing trend at a rate of - 2.24 × 104 t/year, regardless of water discharge, and an abrupt change occurred in 1998. Human activities, especially dam construction contributed 96% to this change, while 4% was due to climate change. El Niño/Southern Oscillation (ENSO) events are often associated with low precipitation, resulting in low water discharge and sediment load, indicating that changes in ENSO periodicity could affect the inter-annual periodic variations of water discharge and sediment load. As population and economy boom, more dams are being built in the Pearl River basin, and special attention should be paid to the management and mitigation of the effects of dams on sediment load.