This study presents submeso-scale eddies (SMEs) over the Izu-Ogasawara ridge off Japan mainland using a double-nested Regional Oceanic Modeling System (ROMS) with a horizontal resolution of 1 km. Both of the Kuroshio and tides largely influence physical processes in the study site, e.g., the Kuroshio generate strong turbulent mixing resulting in upward nutrient supplies, barotropic tides cause nonlinear internal tides leading to enhanced turbulent mixing. However, SMEs, and interaction between the Kuroshio and tides have not been investigated well. In this study, mean and eddy kinetic energy components (MKE and EKE) are extracted by a horizontal Gaussian filter with a filter length scale (standard deviation) of 9 km. The kinetic energy spectra show that the Gaussian filter extracts SMEs less than approximately 50 km. Although spatially averaged EKE over the whole model domain is one order lower than MKE, EKE is comparable to MKE over the ridge and island wakes. EKE on the Kuroshio downstream is approximately 2 times larger than that on the upstream. Tidal forcing significantly influences magnitude of MKE and EKE in the study area. EKE is enhanced by 10% due to tidal forcing. On the other hand, MKE and total kinetic energy (MKE + EKE) are suppressed by tidal forcing. This implies that tidal forcing enhances energy transfer from MKE to EKE and contributes to dissipate total kinetic energy. EKE generation is dominated by the Reynolds stress due to the lateral velocity shear (barotropic conversion) for the summer season. For the winter season, both of the barotropic conversion and baroclinic conversion (eddy potential energy into EKE) contribute to EKE generation.

Mesoscale and submesoscale processes play important roles in the transport and redistribution of oceanic heat. We propose a hypothesis that submesoscale-induced vertical heat transport can be done via the modulation of mesoscale eddies. The Estimating Circulation and Climate of the Ocean, Phase II (ECCO2) product with a nominal horizontal grid spacing of 1/48°, is applied to analyze a pair of a cyclonic and an anticyclonic mesoscale eddies in the Kuroshio Extension region, which can penetrate into 2000 meters in depth. Delicate submesoscale structures are found along the edges of the mesoscale eddies. Both dynamic analysis and spectral analysis of the high-resolution numerical product reveal the important role of submesoscale processes in the vertical heat transport, which has significant implications for the climate modeling and understanding of the global warming trend.

Realistic simulation of kinetic energy transport from the wind (mostly near inertial kinetic energy, NIKE) to the ocean’s interior is one of the most important issues in numerical studies of climate change. The present study finds that the horizontal resolution of a model can significantly impact the depth towards which NIKE can be transported. The data recorded by two buoys in the South China Sea, which survived Typhoon Kalmaegi (2014), show that the NIKE can be transported into the deeper ocean in an anticyclonic than a cyclonic eddy. Numerical experiments with three horizontal resolutions reveal that the increase in the model resolution enhances the enstrophy, deepening NIKE transport and reproducing the NIKE vertical distribution as observed.

Among Western Boundary Currents (WBCs), the East Australian Current (EAC) has a more energetic eddy field relative to its mean flow. The mechanisms responsible for the inter-annual modulation of upstream transport and downstream eddy kinetic energy (EKE) and the relationship between transport and EKE are still unclear. We investigate the inter-annual modulation of downstream EKE in the EAC's typical separation region (Tasman EKE Box) (33.1°S-36.5°S) based on a long-term (22-year), high-resolution (2.5-6 km) model simulation and satellite altimeter observations from 1994 to 2016. Our results show that the EAC poleward transport at 28°S is regulated by negative sea level anomalies and leads the EKE in the Tasman EKE Box by 90-110 days. Barotropic instabilities are the primary sources of EKE in the EAC system. Anticyclonic eddies shed from the EAC dominate the Tasman EKE Box in the high-EKE periods, but in low-EKE periods anticyclonic eddies penetrate further south by ~2°.

Mesoscale eddies are important for regulating oceanic energy. Variation in eddy shapes leads to uncertainty in calculations of heat or energy content. In this study, we investigated the deformation of a warm eddy in the northern South China Sea from April to June 2018 and elucidated the mechanism governing the deformation. Satellite altimetry images showed that the warm eddy originated from Luzon Strait (LS eddy), migrated westward, and then moved along 500‐m isobaths until it approached the east of Hainan Island. Thereafter, the LS eddy deformed, moved southward, merged with other eddies, and finally dissipated. Using ship and virtual‐mooring Chinese underwater glider observations, we examined the three‐dimensional structure of the LS eddy. The warm eddy had a low‐density core that reached a depth of 250 m. The LS eddy gave rise to a front along its eastern edge, and two associate submesoscale eddies with horizontal radii of approximately 10 km were found at the front. The warm eddy was circular before deformation but morphed into an egg shape after deformation. This shape change allowed the eddy to entrain additional water mass (approximately 10¹⁴ kg). The deformation event was able to be forecasted by the vorticity and deformation index, as the eddy deformed by leaking into the zone with a high vorticity and deformation index. We used modeling and energy transformation calculations to analyze the mechanism of the warm eddy deformation. Our results revealed that baroclinic instability played a primary role in the deformation event.  

The seasonal velocity variations of the Kuroshio from the sea surface to a depth of 1000 m from the region southeast of Luzon Island to south coast of Japan were investigated based on data analysis and numerical models. It was found that the seasonal velocity variations exhibit coherent features spanning most of the regions along the Kuroshio path: the seasonal velocity variations above ∼500 m depth (upper layer) reach a maximum in July while those below ∼500 m depth (lower layer) reach a maximum in winter. To clarify the essential mechanisms underlying the observed seasonal velocity variations of the Kuroshio, numerical experiments were performed using a realistic general circulation model. It was shown that seasonal Kuroshio velocity variations in the upper layer are mainly caused by the response to local wind stress upon the Kuroshio. Focusing on this result, we considered a hypothesis that the seasonal Kuroshio velocity variations in the upper layer are forced by thermocline variations due to nonlinear Ekman pumping and examined this hypothesis using a rigid-lid reduced-gravity analytical model with Ekman layer dynamics. The analytical model showed that the asymmetry of the jet profile has a marked effect on the seasonal Kuroshio velocity variations in the upper layer: with a higher wavenumber on the west side of a northward Kuroshio, the velocity is shown to increase under summer wind condition while decrease under winter wind condition. On the other hand, seasonal Kuroshio velocity variations in the lower layer can be explained by the Sverdrup theory, in which barotropic responses to the wind stress curl over the area west of the Izu-Ogasawara Ridge are responsible.

The presence of deep barrier layer due to warm and freshwater in upper ocean provides favourable conditions to cyclogenesis in post-monsoon months over the Bay of Bengal (BoB). Intense cyclone winds play a significant role in surface transport and uplifting of subsurface cold nutrient rich water to the surface. According to classification by India Meteorological Department, a very severe cyclonic storm Titli over the BoB occurred during 8 – 12 October 2018 and made its landfall in toward morning on 11^th October 2018 near Palasha, Andhra Pradesh. In this study, high resolution (5 km) nitrogen-based nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecological model embedded with Regional Ocean Modeling System (ROMS) is utilized to analyze upper oceanic biophysical responses to Titli cyclone. Advantages of numerical models are continuous monitoring of oceanic processes during cyclones and quantifications of changes. Model is forced with Scatsat-1 satellite winds, which has proven to have to capture cyclone structures well compared to other reanalysis products. Spatial maps of satellite-derived and model simulated sea surface temperature, salinity, chlorophyll-a concentration and height anomaly are well matched. Along track-depth variation of physical (temperature and salinity) and biogeochemical (chlorophyll-a, dissolved oxygen, and NPZD) parameters at pre-, peri- and post-cyclone weeks clearly depict that cyclone induced upwelling occurs at 220 km away from its genesis location and exists for two weeks after passing it. At this location surface temperature difference between pre- and peri-cyclone period is 2^oC and during first week of post-cyclone subsurface chlorophyll maxima uplifts to a very shallow depth of 5 m with a large concentration ~3 micromol/kg. As a consequence of upwelling, during post-cyclone weeks upward shifts in 23^oC isotherm, oxycline and nutricline are observed. This upliftment of nutricline helps near surface layer to reach its maximum phytoplankton concentration with value of 1 micromol/kg.

The wave conditions in the West Africa coast are dominated by swells. The extreme wave determination therein is different from that along the China coast where the extreme waves are induced by typhoons. In order to determine the near-shore extreme waves along such swell-dominated coasts, this research applies two kinds of winds as the driving force in the numerical wave simulation, i.e., the winds with the same return period as the waves, and the winds accompanying with the extreme waves derived through wind-wave joint distribution. Eventually the average of the extreme waves calculated from these two winds is recommended for coastal engineering designs.