The current system in the Cosmonaut Sea off East Antarctica is subjected to large-scale climate change, which in turn affects the nutrient availability and productivity of the region. The Copernicus merged absolute dynamic topography (ADT) data were employed to analyze the variability of major currents and potential driving mechanisms behind. The main currents in the Cosmonaut Sea are revealed to be the Antarctic Circumpolar Current (ACC), the Weddell Gyre Eastern Branch (WGeb) and the Antarctic Slope Current (ASC); and strong seasonal and interannual variations are also found associated with these currents. ACC and ASC both are stronger in autumn and weaker in spring and summer, while the WGeb expends eastward in winter and retreats in summer. At the interannual timescale, the Southern Annular Mode (SAM) could reinforce westerlies during its positive phase, pushing ACC southward and suppressing westward ASC, and vice versa. Studies on related mechanisms suggest the crucial role of winds in the barotropic modulation of ocean dynamics. The westerlies affect the meridional gradient of the sea level through Ekman transport, inducing the change of barotropic geostrophic currents. The changing atmospheric and oceanic circulations have possible influence on the southward Sverdrup transport maintaining the Antarctic Slope Front (ASF) crucial to the ASC thereafter. The WGeb might also perform an intermediate role in the southward cascading of variability from ACC to ASC. This indicates that the response of regional ocean dynamics to circumpolar atmospheric variations could be understood from the view of cascading processes.

The seasonal Predictability Barrier (PRB) and Persistence Barrier (PEB) in the El Niño-Southern Oscillation (ENSO) phenomenon are of recent interest, yet the distinction between the PRB in observations and PEB in ensemble forecast models remains unexplored. Using observational and North American Multimodel Ensemble data since the 1980s, we examined the seasonal PRB and PEB, focusing on intensity, timing, decadal variations, and spatial patterns. Although the intensity of the ENSO spring PRB in dynamic models is notably lower than the spring PEB intensity, the temporal variations, spatial patterns and barrier timing of the PRB and PEB are similar. The chaotic nature of ENSO systems exhibits comparable decadal variations and spatial patterns to the seasonal PEB and PRB, suggesting potential control by chaotic behavior for both seasonal PRB and PEB. Hence, the seasonal PEB of ENSO can still provide useful benchmarks for the predictability study of ENSO in dynamic models.

Eddy activities are particularly prominent in the Southern Ocean due to the instabilities of the Antarctic Circumpolar Current (ACC), which plays a critical role in energy transport of the global ocean. The Indian sector of the Southern Ocean is not only a typical eddy-rich region with strong Eddy Kinetic Energy (EKE) and associated energy conversions among different energy reservoirs (kinetic energy and potential energy of the eddy and mean flow), but also events of extreme EKE. In this study, a systematic energetics analysis framework is employed to examine the notable anomalies of an intensified EKE event observed in the southwest region of the Kerguelen Plateau in 2017 based on a reanalysis product. The EKE anomaly existing at all depths emerges in April, reaches its peak during the austral winter, and persists into the following summer. Energetics analysis indicates that the strong anomalous EKE is primarily determined by baroclinic instability, with distinct governing mechanisms at the surface and in the internal ocean. The anomalous intrusion of warm Circumpolar Deep Water intensifies the baroclinic energy conversion in the subsurface, which contributes to the observed EKE anomalies. Moreover, the anomalous strong wind-induced Ekman pumping serves to amplify the lifting of isopycnals, which enhances the baroclinic instability and subsequently intensifies the EKE anomalies. This study sheds new light on underlying mechanisms governing local polar dynamics and provides insights into the intricate interaction between ocean dynamics and energy distribution in the Antarctic.

Using long-term satellite altimeter data, a new streamline-based algorithm is developed to identify the Kuroshio intrusion types and describe the seasonal variations of related dynamical properties. Results from this new classification show that a mixing of leaping, looping and leaking streamlines is the dominant form of Kuroshio intrusion into the South China Sea (SCS). The leaping path is very stable and crosses the Luzon Strait mainly through the Balintang Channel regardless of seasons, while the streamlines leaking into the SCS is more likely to intrude via the channel between the Babuyan Island and the Camiguin Island. Large seasonal variations are found with the percentage of each kind of streamline and the Luzon Strait Transport (LST), but not with the intensity, width and current axis position of the Kuroshio. The along-streamline analysis reveals that the seasonal intrusion of the Kuroshio is essentially the seasonal variation of the cyclonic shear part of the flow. A possible physical mechanism is proposed to accommodate these seasonal characteristics based on globally the vorticity (torque work) balance between the basin-wide wind stress and the lateral friction, as well as locally the loss of balance between the torques of interior stresses and normal stresses both provided by the wall boundary, together with a plausible conjecture that the seasonally-reversing monsoon can significantly modify the torque of the interior stresses within the cyclonic shear part of the flow and thus responsible for the seasonal variation of the Kuroshio intrusion.

The seasonal Predictability Barrier (PB) of Sea Surface Temperature Anomaly (SSTA) is characterized by a rapid loss of prediction skills at a specific season in dynamic models. To investigate whether this PB phenomenon is caused by the inherent nonlinearity of the air-sea coupled system that leads to chaos under certain conditions, a statistical method - Sample Entropy, was introduced to investigate the spatial-temporal distribution of the chaotic degree of SSTA time series in the tropic Pacific. The results showed that high chaotic values existed in Niño 3 and Niño 3.4 regions in April and May, and in Niño 4 region in May and June, which matched the PB timing previously reported in these regions. Furthermore, the chaotic signal moves westward from March to June longitudinally in the tropical Pacific, leading to a similar linear variation of PB timing along the longitude.