Mesoscale eddies are common in the ocean and their surface characteristics have been well revealed based on altimetric observations. Comparatively, the knowledge of the three-dimensional (3D) structure of mesoscale eddies is scarce, especially in the open ocean. In the present study, high-resolution field observations of a cyclonic eddy in the Kuroshio Extension have been carried out and the anatomy of the observed eddy is conducted. The temperature anomaly exhibits a vertical monopole cone structure with a maximum of −7.3 °C located in the main thermocline. The salinity anomaly shows a vertical dipole structure with a fresh anomaly in the main thermocline and a saline anomaly in the North Pacific Intermediate Water (NPIW). The cyclonic flow displays an equivalent barotropic structure. The mixed layer is deep in the center of the eddy and thin in the periphery. The seasonal thermocline is intensified and the permanent thermocline is upward domed by 350 m. The subtropical mode water (STMW) straddled between the seasonal and permanent thermoclines weakens and dissipates in the eddy center. The salinity of NPIW distributed along the isopycnals shows no significant difference inside and outside the eddy. The geostrophic relation is approximately set up in the eddy. The nonlinearity—defined as the ratio between the rotational speed to the translational speed—is 12.5 and decreases with depth. The eddy-wind interaction is examined by high resolution satellite observations. The results show that the cold eddy induces wind stress aloft with positive divergence and negative curl. The wind induced upwelling process is responsible for the formation of the horizontal monopole pattern of salinity, while the horizontal transport results in the horizontal dipole structure of temperature in the mixed layer.

Marine heatwaves (MHWs) are persistent, discrete, extreme high-temperature events in the ocean, which can destructively affect marine ecosystems. Using satellite remote sensing data and reanalysis data from 1982 to 2021, we find that six indices characterizing the MHWs are in a remarkable increasing trend in the Japan/East Sea (JES), which shows that the most severe MHW events take place in the East Korean Bay (EKB) in the summer of 2021. Based on this finding, the present study focuses on the characteristics and mechanisms of the MHWs in the EKB and its adjacent areas from June to August 2021. The analysis reveals that the total days and mean intensity of MHWs that occur in the EKB are 1.84 and 1.47 times more than those averaged in the JES, respectively. It is shown that mechanisms for the occurrences of the MHWs in the summer of 2021 are caused by the atmospheric high-pressure system moving to the EKB area. Other reasons also decrease the water cooling: the net positive lateral heat fluxes across open boundaries, and the weak sea surface wind over the EKB area. Other possible reasons which cause the summer MHW events in 2021 need the oceanic numerical models to further investigate the issue.

Introduction: The Indonesian Throughflow (ITF) connects the Pacific Ocean and the Indian Ocean. It plays an important role in the global ocean circulation system. The interannual variability of ITF transport is largely modulated by climate modes, such as Central-Pacific (CP) and Eastern-Pacific (EP) El Niño and Indian Ocean Dipole (IOD). However, the relative importance of these climate modes importing on the ITF is not well clarified.Methods: Dominant roles of the climate modes on ITF in specific periods are quantified by combining a machine learning algorithm of the random forest (RF) model with a variety of reanalysis datasets.Results: The results reveal that during the period from 1993 to 2019, the average ITF transport derived from high-resolution reanalysis datasets is -14.97 Sv with an intensification trend of -0.06 Sv year^-1, which mainly occurred in the upper layer. Four periods, which are 1993–2000, 2002–2008, 2009–2012 and 2013–2019, are identified as Niño 3.4, Dipole Mode Index (DMI), no significant dominant index, and DMI dominated, respectively.Discussion: The corresponding sea surface height differences between the Northwest Tropical Pacific Ocean (NWP) and Southeast Indian Ocean (SEI) in these three periods when exist dominant index are -0.50 cm, 0.99 cm and -3.22 cm, respectively, which are responsible for the dominance of the climate modes. The study provides a new insight to quantify the response of ITF transport to climate drivers.

Internal solitary waves (ISWs) with features such as large amplitude, short period, and fast speed have great influence on underwater thermohaline structure, nutrient transport, and acoustic signal propagation. The characteristics of ISWs in hotspot areas have been revealed by satellite images combined with mooring observation. However, the ISWs in the Timor Sea, which is located in the outflow of the ITF, have not been studied yet and the characteristics are unrevealed. In this study, by employing the Synthetic Aperture Radar (SAR) images taken by the Sentinel-1 satellite from 2017 to 2022, the temporal and spatial distribution characteristics of ISWs in the Timor Sea are analyzed. The results show that most of ISWs appear in Bonaparte basin and its vicinity. The average wavelength of the ISWs is 248 m, and most of the wave lengths are less than 400 m. The peak line of ISWs is longer in deeper water. The underwater structures of two typical ISWs are reconstructed based on the Korteweg–de Vries (KdV) equation combined with mooring observation. This shows that, compared with the two-layer model, the continuous layered model is more suitable for reconstructing the underwater structures of ISWs. Further analysis shows that both the rough topography and the spring-neap tides contribute to the generation of ISWs in the Timor Sea. This study fills a gap in knowledge of ISWs in regional seas, such as the Timor Sea.

Acoustic rays are modified while propagating through oceanic eddies. However, due to the lack of field synchronous observation, the impact of mesoscale eddy on the acoustic propagation is less clarified. To address the issue, an eddy-acoustic synchronous observation (EASO) field experiment for a mesoscale warm eddy was carried out in the slope of the South China Sea (SCS) in October, 2021. During the field experiment, a total of 105 conductivity-temperature-depth (CTD) stations, as well as a zonal acoustic survey line through the center of the warm eddy, were obtained. The vertical structures of temperature and salinity indicate that the warm eddy is surface-intensified with temperature and salinity cores confined within depths from 70 m to 200 m and 10 m to 70 m, respectively. The acoustic observation shows two obvious convergency zones (CZs) at about 39 km and 92 km in the eastern half acoustic line, and one convergency zones (CZ) at about 25 km in the western half acoustic line. By comparing with the none eddy circumstance, the respective impacts of the topography and warm eddy are quantitatively analyzed with a ray-tracing model. The results indicate that the topography shortens the horizontal span of the CZ by 11.4 km, while the warm eddy lengthens it by 1.7 km. Additionally, the warm eddy shallows the depth and broadens the width of the CZ by 32 m and 1.4 km, respectively. The anisotropy of 3D sound fields jointly influenced by the warm eddy and the local topography show that the distance differences of the first CZs in different horizontal directions can be as long as 31 km.