Mesoscale eddies are ubiquitous in the world ocean and well researched both globally and regionally, while their properties and distributions across the whole Indonesian Seas are not yet fully understood. This study investigates for the first time the spatiotemporal variations and generation mechanisms of mesoscale eddies across the whole Indonesian Seas. Eddies are detected from altimetry sea level anomalies by an automatic identification algorithm. The Sulu Sea, Sulawesi Sea, Maluku Sea and Banda Sea are the main eddy generation regions. More than 80% of eddies are short-lived with a lifetime below 30 days. The properties of eddies exhibit high spatial inhomogeneity, with the typical amplitudes and radiuses of 2–6 cm and 50–160 km, respectively. The most energetic eddies are observed in the Sulawesi Sea and Seram Sea. Eddies feature different seasonal cycles between anticyclonic and cyclonic eddies in each basin, especially given that the average latitude of the eddy centroid has inverse seasonal variations. About 48% of eddies in the Sulawesi Sea are highly nonlinear, which is the case for less than 30% in the Sulu Sea and Banda Sea. Instability analysis is performed using high-resolution model outputs from Bluelink Reanalysis to assess mechanisms of eddy generation. Barotropic instability of the mean flow dominates eddy generation in the Sulu Sea and Sulawesi Sea, while baroclinic instability is slightly more in the Maluku Sea and Banda Sea.

Breaking internal tides and induced mixing are critical to shelf dynamics, including heat and mass exchanges. Spatiotemporal variability of internal tides and modulation factors for the southern East China Sea shelf were examined based on a combination of a three-month mooring velocity and satellite altimeter data. Semidiurnal and diurnal internal tides exhibited distinct temporal trends, with the semidiurnal internal tides enhanced by an order of magnitude during the latter half of the record, while the diurnal internal tides followed quasi spring-neap cycles with a generally stable intensity except for two specific periods of strengthening. These internal tides probably originated remotely over the shelf-slope area northeast of Taiwan. Time-varying stratification was the most important factor for the internal tidal magnitude. In addition, varying background currents influenced the diurnal critical latitude band, which explains the slightly enhanced diurnal internal tides during the two periods. Although both semidiurnal and diurnal internal tides were mode-1 dominated, the semidiurnal internal tides were surface intensified while the diurnal tides were bottom intensified. The proportion of higher mode internal tides increased during robust eddy activities. Stronger background vertical shear corresponded to high-frequency events and energy transfers from tidal frequencies to high frequencies associated with turbulent mixing.

As the Arctic Ocean continues to warm, both the extent and thickness of sea ice have dramatically decreased over the past few decades. These changes in ice have an impact on sea ice motion, including sea ice inertial oscillations (SIIO). However, the spatial pattern and temporal variations of Arctic SIIO remain poorly understood. In this study, the spatiotemporal characteristics of Arctic SIIO between 1979 and 2019 are revealed based on the sea ice drifting buoy dataset from the International Arctic Buoy Program (IABP). The results indicate the significant enhancement of SIIO during 1979–2019, with the trend of 7.84 × 10⁻³ (±3.34 × 10⁻³) a⁻¹ (a⁻¹ means per year) in summer and 1.92 × 10⁻³ (±0.80 × 10⁻³) a⁻¹ in winter. Compared with the first 30 years, the magnitude of SIIO in 2009–2019 increases by 66% in summer and 21% in winter. Spatially, the remarkable enhancement of SIIO during 2009–2019 is found in most of the Arctic Ocean. Especially in summer, SIIO are significantly intensified in marginal seas, including the Beaufort Sea, East Siberian Sea and Laptev Sea, which is mainly correlated with the decrease of sea ice concentration in recent years. This study is anticipated to provide insights for spatiotemporal variation of Arctic sea ice inertial motion in recent decades.