Bohai Sea, with the highest suspended sediment concentration in the world, has experienced a sharp decrease in sea surface suspended sediment concentration (SSSC) in the latest two decades. However, limited by the shortage of long‐term field observations, the spatiotemporal characteristics and mechanisms of the long‐term variation in the SSSC are still unclear. Based on remote sensing data, this study reconstructed the spatiotemporal SSSC distribution from 2003 to 2016, and found significant spatial difference in decadal SSSC variation in the Bohai Sea. The largest decadal SSSC decrease was observed in the southern Bohai Sea with an average annual decrease of 1.3 mg/L (decrease of 4.4% per year), while significant SSSC increase was observed in the central Bohai Sea with largest annual increase of 0.4 mg/L (increase of 5.0% per year). Significant seasonal differences in the SSSC decreasing were also revealed, and the SSSC decrease in winter and spring (0.46 mg/L/a) was 27.8% higher than that in summer and autumn (0.36 mg/L/a). Decadal wind speed data measured at 14 meteorological stations around the Bohai Sea indicated that wind speed reduction played the most important role in the decadal decrease in SSSCs, and significant SSSC decrease can only be found in sea areas with a significantly reduced wind speed. In addition, although the runoff and sediment discharge of the Yellow River also decreased during 2003–2016, they were not related to the spatiotemporal variation of SSSCs and their direct impact was limited to the range of 15 km from the estuary.

The recent dramatic decline of sea ice in the western Arctic Ocean changes the transfer of momentum across the ice‐ocean boundary layer. The surface stress energy input through the surface geostrophic current in the Beaufort Gyre (BG) based on a numerical model is 0.03 mW/m2 in 1992–2004 versus 0.23 mW/m2 in 2005–2017. This energy input is primarily concentrated over the southern Canada Basin and the Chukchi Sea. It is 1.38 × 1016 J in observations versus 4.90 × 1016 J in the model in the BG during 2003–2014. We find that some well‐known freshwater changes in the BG over 1992–2017 resulted from episodic extrema of energy input in 2007, 2012, and 2016. In particular, most of the energy input in 2007 was transformed into potential energy (57%) which resulted in a new state of freshwater budget. Our study suggests that as of 2016, the BG had not yet reached a saturated freshwater state.

As a main source of nitrate for the pycnocline in the global ocean, the ocean south of 30°S plays a pivotal role in the global marine biogeochemical cycle. Nitrate in the pycnocline often shows a more stable relationship with density than with depth. Hence, it is important to understand the nitrate structure and its relationship with density in the ocean south of 30°S. In the present study, the nitrate‐density relationship and the mechanisms for its variability are studied based on the data from more than 60,000 profiles. Through analysis, we find that nitrate is approximately linearly correlated with potential density in the nitracline, except for the western boundary current zones. The upper bounds of the nitracline depend on the mixed layer and euphotic layer depth. The lower bounds are related to the depth of maximum nitrate, which mainly vary meridionally with sharp changes near the Subantarctic Front. The diapycnal gradients of nitrate also show meridional variability, with large magnitudes corresponding to thick Subantarctic Mode Water, low diapycnal mixing in the nitracline, and high biological uptake in the euphotic layer. The nitrate‐density relationship can be applied to predict nitrate concentrations based on the observed temperature and salinity profiles and support some detailed studies on the physical‐biogeochemical interactions. Two examples show that the predicted nitrate data could help better resolve the effects of ocean circulations and mesoscale eddies on nitrate than the existing observed nitrate profiles.

The Arctic is currently experiencing unprecedented changes across all components of the climate system, primarily driven by global warming. As an important indicator of climate change in the Arctic, sea level reflects variations in both the atmosphere and ocean. This paper analyzes the sea level variation of the Arctic Ocean over the past four decades using ORAS5 data, which is the product of the latest reanalysis-analysis system produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). ORAS5 accurately reproduces the main spatial features of the climatology and temporal evolution of sea surface height (SSH) in the Arctic Ocean, as observed by satellite altimeters, and reveals that seasonal variability is the most significant property of the sea level variation in this region. The seasonal cycle of SSH is closely linked to atmospheric circulation and sea ice formation. The first two dominant modes of the annual-mean SSH in the Arctic Ocean exhibit significant decadal variability. The first mode can be explained by the Ekman transport of wind related to the Arctic Oscillation (AO), which leads to antiphase changes in SSH on the continental shelves and in the deep basins. The second mode shows an antiphase oscillation of SSH between the Eurasian and Canadian Arctic Archipelago (CAA) sides and is driven by the wind anomaly associated with the Arctic dipole anomaly (DA). Due to the decadal variations associated with climate modes, particularly the AO, sea level in the Arctic Ocean has been continuously rising since the mid-1990s or early 2000s, with the most rapid sea level rise occurring in the Beaufort Sea.