Oceanic mesoscale eddies with horizontal scales of 50–300 km are the most energetic form of flows in the ocean. They are the oceanic analogues of atmospheric storms and are effective transporters of heat, nutrients, dissolved carbon, and other biochemical materials in the ocean. Although oceanic eddies have been ubiquitously observed in the world oceans since 1960s, our understanding of their three-dimensional (3D) structure, generation, and dissipation remains fragmentary due to lack of systematic full water-depth measurements. To bridge this knowledge gap, we designed and conducted a multi-months field campaign, called the South China Sea Mesoscale Eddy Experiment (S-MEE), in the northern South China Sea in 2013/2014. The S-MEE for the first time captured full-depth 3D structures of an anticyclonic and cyclonic eddy pair, which are characterized by a distinct vertical tilt of their axes. By observing the eddy evolution at an upstream versus downstream location and conducting an eddy energy budget analysis, the authors further proposed that generation of submesoscale motions most likely constitutes the dominant dissipation mechanism for the observed eddies.

A pair of mesoscale eddies, namely, an anticyclonic eddy (AE) and a cyclonic eddy (CE), generated southwest of Taiwan was investigated using long‐term moored observations augmented with satellite and reanalysis data. AE (CE) increased the upper‐ocean current to 100 cm/s (50 cm/s) and generated temperature anomaly (T′) up to 7.5°C (−3.0°C). The vertical structure of AE presented a phase difference between T′ and velocity, with the core of T′ located south of the zero‐velocity line. This phase difference induced a −28.5 ± 12.0 TW (1 TW = 10¹² W) westward eddy heat transport southwest of Taiwan. AE also presented strong nonlinearity in the upper ocean and yielded an annual mean volume transport of 0.86 ± 0.15 Sv. Energy analysis showed that both baroclinic and barotropic instabilities were important for the generation and growth of the eddy pair, and contribution from wind work was relative small. In the deep sea, AE (CE) intensified the deep currents and generated large positive (negative) T′. The deep and surface currents flowed in opposite directions during the eddy events, which is consistent with the interpretation that the eddy pair is dominated by the first baroclinic mode. The observed deep‐penetrating eddy pair could greatly influence the deep circulation of the northeastern South China Sea.

Sheddings of Kuroshio Loop Current (KLC) eddies in the northeastern South China Sea (SCS) are investigated using mooring arrays, multiple satellite data, and data-assimilative HYCOM products. Based on altimeter sea surface heights between 1992 and 2014, a total of 19 prominent KLC eddy shedding (KLCES) events were identified, among which four events were confirmed by the concurrent moored and satellite observations. Compared to the leaping behavior of Kuroshio, KLCES is a relatively short-duration phenomenon that primarily occurs in boreal autumn and winter. The KLC and its shedding anticyclonic eddy (AE) trap a large amount of Pacific water with high temperature–salinity and low chlorophyll concentration in the upper layer. The corresponding annual-mean transport caused by KLCES reaches 0.24–0.38 Sv (1 Sv ≡ 10⁶ m³ s⁻¹), accounting for 6.8%–10.8% of the upper-layer Luzon Strait transport. Altimeter-based statistics show that among ~90% of the historical KLCES events, a cyclonic eddy (CE) is immediately generated behind the AE southwest of Taiwan. Both energetics and stability analyses reveal that because of its large horizontal velocity shear southwest of Taiwan, the northern branch of KLC is strongly unstable and the barotropic instability of KLC constitutes the primary generation mechanism for the CE. After CE is generated, it quickly grows and gradually migrates southward, which in turn facilitates the detachment of AE from KLC. The intrinsic relationship between KLC and CE explains well why eddy pairs are commonly observed in the region southwest of Taiwan.

Oceanic eddy-induced meridional heat transport (EHT) is an important process in the Southern Ocean heat budget, the variability of which significantly modulates global meridional overturning circulation (MOC) and Antarctic sea-ice extent. Although it is recognized that mesoscale eddies with scales of ~40–300 km greatly contribute to the EHT, the role of submesoscale eddies with scales of ~1–40 km remains unclear. Here, using two state-of-the-art high-resolution simulations (resolutions of 1/48° and 1/24°), we find that submesoscale eddies significantly enhance the total poleward EHT in the Southern Ocean with an enhancement percentage reaching 19–48% in the Antarctic Circumpolar Current band. By comparing the eddy energy budgets between the two simulations, we detect that the primary role of submesoscale eddies is to strengthen mesoscale eddies (and thus their heat transport capability) through inverse energy cascade rather than directly through submesoscale heat fluxes. Due to the submesoscale-mediated enhancement of mesoscale eddies in the 1/48° simulation, the clockwise upper cell and anti-clockwise lower cell of the residual-mean MOC in the Southern Ocean are weakened and strengthened, respectively. This finding identifies a potential route to improve the mesoscale parameterization in climate models for more accurate simulations of the MOC and sea ice variability in the Southern Ocean.

One year long, full‐depth velocity measurements were used to examine the spatial structure and temporal variability of the zonal flow in the Luzon Strait (LSZF). The observation revealed a renewed mean flow structure: in the upper (2000 m) layers, the LSZF was mostly westward; in the intermediate layer (500–2000 m), it was dominated by an eastward flow in the south but a westward flow in the north. The volume transport across the observed section between 19.8°N and 21.2°N exhibited strong seasonal and intraseasonal variability. On the seasonal time scale, the upper‐layer transport showed a clear annual cycle, strongest in January and weakest in June; the intermediate‐layer transport also showed a semiannual cycle, attaining its peaks (troughs) in January and June (April and October). On the intraseasonal time scale, both the upper‐layer and intermediate‐layer transports showed significant energy peaks at about 60 and 10–30 days. Further analysis indicated that the ∼60 day variability might be attributed to the impinging mesoscale eddies from the Pacific, while the 10–30 day variability appeared to obtain its energy from local intraseasonal wind forcing and baroclinic instability of the background current. The 10–100 day upper‐layer and intermediate‐layer transport variabilities were highly anticorrelated, suggesting a baroclinic nature of the intraseasonal variabilities of the LSZF.

The Surface Water and Ocean Topography (SWOT) mission provides a good opportunity to study fine‐scale processes in the global ocean but whether it can detect balanced submesoscale eddies is uncertain due to the “contamination” by unbalanced inertial gravity waves. Here, based on concurrent observations from SWOT and a mooring array in the northwestern Pacific, we successfully captured two submesoscale cyclonic eddies with negative sea level anomalies (SLAs) in spring 2023. We find that the SLA amplitude and equivalent radius of the first (second) eddy are 2.5 cm and 16.0 km (2.0 cm and 18.8 km), respectively. For both eddies, their vertical scales are around 150 m and their horizontal velocities and Rossby numbers exceed 15.0 cm/s and 0.4, respectively. Further analysis suggests that similar submesoscale eddies can commonly occur in the northwestern Pacific and that SWOT is capable to detect larger submesoscale eddies with scales greater than ∼10 km.