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The spontaneous generation of near-inertial waves (NIWs) from quasi-geostrophic mesoscale eddies in a stratified fluid is investigated and the process and mechanism clarified by numerical modeling and a theoretical analysis. Mesoscale eddies, as slow balance flows, are inevitably accompanied by unbalanced near-inertial oscillations (NIOs) in their long period evolution. A mesoscale eddy supplies kinetic energy to NIOs through its interaction with perturbations, which exist universally in the world oceans. Afterward, NIWs are generated spontaneously via the nonlinear coupling of the eddy and NIOs. The baroclinicity of mesoscale eddies is an essential condition of this spontaneous NIW generation mechanism. The resonance of the NIWs and NIOs enables the internal wave continuum to be dominated by NIWs, which share the same horizontal wavenumbers with the eddy. After generation, the NIWs radiate energy toward the eddy center and remain stationary as a whole, relative to the propagating eddy. Generally, an anticyclonic mesoscale eddy can emit much stronger NIWs than does a cyclonic eddy. The NIW intensity strengthens exponentially with the Rossby number. The essence of this spontaneous NIW generation mechanism is the nonlinear interaction between slow balance flows and the accompanying fast motions. It represents an effective pathway for mesoscale eddy energy sinks and makes a non-negligible contribution to the global NIW energy. [ABSTRACT FROM AUTHOR]

This study aims to analyze statistical behavior of the continental shelf wave motions, including continental shelf waves (CSWs) and arrested topographic waves (ATWs), in the northern South China Sea. The baseline consists of tidal gauge data from stations Kanmen, Xiamen, Shanwei, Hongkong, and Zhapo, as well as along-track sea level anomaly (SLA) data derived from multi-satellite altimeters from 1993 to 2020. The subtidal signals propagating along the coast with periods shorter than 40 d and phase speeds of about 10 m s-1 are interpreted as CSWs. The cross-shelf structure of along-track SLA indicates that mode 1 of CSWs is the predominant component trapped in the area shallower than about 200 m. The amplitudes of CSWs reach the maximum 0.6 m during July–September, and minimum 0.2 m during April–June. The inter-seasonal and seasonal signals represent ATWs. The amplitudes of ATWs reach 0.10 m during October–December, twice of that during July–September. These observations are well interpreted by the framework of linear wave theory. The cross-shelf structures of CSWs and ATWs derived from along-track SLA illustrate that the methods are suitable for observing dynamic behavior of the CSWs. [ABSTRACT FROM AUTHOR]

Using satellite observations from 2003 to 2020 and cruise observations from 2019 and 2021, this study reveals an unexpected minor role of upwelling in seasonal and interannual variations in chlorophyll a (Chl a) concentrations in the coastal upwelling region east of Hainan Island (UEH) in the northwestern South China Sea (NWSCS). The results show strong seasonal and interannual variability in the Chl a concentration in the core upwelling area of the UEH. Different from the strongest upwelling in summer, the Chl a concentration in the UEH area reaches a maximum of 1.18 mg m -3 in autumn and winter, with a minimum value of 0.74 mg m -3 in summer. The Chl a concentration in summer increases to as high as 1.0 mg m -3 with weak upwelling, whereas the maximum Chl a concentration in October increases to 2.5 mg m -3. The analysis of environmental factors shows that, compared to the limited effects of upwelling, the along-shelf coastal current from the northern shelf and the increased precipitation are crucially important to the Chl a concentration variation in the study area. These results provide new insights for predicting marine productivity in upwelling areas, i.e., multiple mechanisms, especially horizontal advection, should be considered in addition to the upwelling process. [ABSTRACT FROM AUTHOR]

The influences of grouped anticyclonic eddies (AEs) in the ocean on the intensity of tropical cyclone (TC) in the atmosphere are investigated in the present study. The research was carried out using numerical methods taking the development of TC Hato (2017) in the northern South China Sea (SCS) as an example. Three AEs were observed, one at the TC track (defined as the inner area) and the other two in the south of the TC center (defined as the outer area), at about three times the radius of maximum wind speed. The results show that the outer AEs suppress TC development, but the inner AEs greatly favor TC development. This opposite influence primarily results from the different responses of TC secondary circulation to the AE‐induced local sea surface warm anomaly in the inner and outer areas. The outer AE triggers a low sea level pressure anomaly and convergent wind toward the AE, which weakens TC inflow at the lower layers and further weakens TC secondary circulation. Consequently, the energy conversion from ocean heat energy to TC kinetic energy is decreased by the weakened TC secondary circulation. The AE in the outer area also extends the TC eyewall, weakens the TC warm core, and increases the outer precipitation. As a result, the TC intensity is weakened. The inner AE causes opposite changes in TC secondary circulation, eyewall, warm core, and outer precipitation, so that favors TC development. These results provide evidence for the suppression effect of the outer AEs on TC development, providing a new perspective toward improving TC intensity forecasts. Plain Language Summary: The intensity of tropic atmospheric cyclone, that is, hurricane or typhoon is influenced by oceanic warm eddies on its track. The presence of warm eddies on the tropical cyclone track suppresses tropical cyclone‐induced ocean surface cooling and provides extra heat flux, which intensifies the tropical cyclone. However, it remains uncertain how tropical cyclone intensity change when a warm eddy occurs outside of the tropical cyclone track. This study investigates the changes in tropical cyclone intensity when a warm eddy is located near the cyclone center and finds that the tropical cyclone is intensified if the warm eddy is close to the tropical cyclonic center but weakened if it is distant. These results can help understand how eddies affect tropical cyclonic intensity when the eddy is positioned in different areas of the cyclone and accurately forecast cyclonic track and intensity. Key Points: Oceanic anticyclonic eddy located in inner and outer areas of tropic cyclone has favor and inhibiting opposite effect on cyclone developmentWeather Research and Forecasting model and Price‐Weller‐Pinkel ocean model are combined to simulate typhoon Hato (2017) in South China SeaModel results reveal that cyclone intensity change is due to changes in secondary circulation and accompanying kinetic energy conversion [ABSTRACT FROM AUTHOR]

Surface waves and internal tides have a great contribution to vertical mixing processes in the upper ocean. In this study, three mixing schemes, including non-breaking surface-wave-generated turbulent mixing, mixing induced by the wave transport flux residue and internal-tide-generated turbulent mixing, are introduced to study the effects surface waves and internal tides on vertical mixing. The three schemes are jointly incorporated into the Marine Science and Numerical Modeling (MASNUM) ocean circulation model as a part of the vertical diffusive terms, which are calculated by the surface wave parameters simulated from the MASNUM wave model and the surface amplitudes of the mode-1 M 2 internal tides extracted from satellite altimetry data using a two-dimensional plane wave fit method. The effects of the mixing schemes on Indian Ocean modeling are tested by five climatological experiments. The surface waves and internal tides enhance the vertical mixing processes in the sea surface and ocean interior, respectively. The combination of the mixing schemes is able to strengthen the vertical water exchange and draw more water from the sea surface to the ocean interior. The simulated results show significant improvement in the thermal structure, mixed layer depth and surface currents if the three schemes are all adopted. [ABSTRACT FROM AUTHOR]

Previous in situ observations have suggested that bottom water temperature variations in shelf seas can drive significant ocean bottom heat flux (BHF) by heat conduction. The BHF-driven bottom water temperature variations, however, have been overlooked in ocean general circulation models. In this study, we established a sea-sediment fully coupled model through incorporating the BHF. The coupled model included a sediment temperature module/model, and the BHF was calculated based on the sediment heat content variations. Meanwhile, we applied temporally varying BHF in the calculation of the bottom water temperature, which further determined the sediment temperature. The two-way coupled BHF process presents a more complete and physically reasonable heat budget in the ocean model and a synchronously varying sediment temperature profile. The coupled model was validated using a one-dimensional test case, and then it was applied in a domain covering the Bohai and Yellow Seas. The results suggest that when a strong thermocline exists, the BHF can change the bottom water temperature by more than 1°C because the effects of the BHF are limited to within a shallow bottom layer. However, when the water column is well mixed, the BHF changes the temperature of the entire water column, and the heat transported across the bottom boundary is ventilated to the atmosphere. Thus, the BHF has less effect on water temperature and may directly affect air–sea heat flux. The sea-sediment interactions dampen the amplitude of the bottom water temperature variations, which we propose calling the seabed dampening ocean heat content variation mechanism (SDH). [ABSTRACT FROM AUTHOR]

Using satellite observations from 2003 to 2020 and cruise observations in 2019 and 2021, this study reveals an unexpected minor role of upwelling in seasonal and interannual variations in chlorophyll-a (Chl-a) concentrations in the coastal upwelling region east of Hainan Island (UEH) in the northwestern South China Sea (NWSCS). The results show strong seasonal and interannual variability in the Chl-a concentration in the core upwelling area of the UEH. Different from the strongest upwelling in summer, the Chl-a concentration in the UEH area reaches a maximum of 1.18 mg m−3 in autumn and winter, with a minimum value of 0.74 mg m−3 in summer. The summer Chl-a concentration increases to as high as 1.0 mg m−3 with weak upwelling during El Niño years, whereas the maximum Chl-a concentration in October increases to 2.5 mg m−3 during La Niña years. The analysis of environmental factors shows that compared to the limited effects of upwelling, the along-shelf coastal current from the northern shelf and the increased precipitation are crucially important to the Chl-a concentration variation in the study area. These results provide new insights for predicting marine productivity in upwelling areas, i.e., multiple mechanisms, especially horizontal advection, should be considered in addition to the upwelling process. [ABSTRACT FROM AUTHOR]

The evolution of mesoscale eddies is crucial for understanding the ocean energy cascade. In this study, using global reanalysis sea surface velocity data and a mesoscale eddy trajectory product tracked by satellite altimeters, we aimed to reveal the coherence of eddy kinetic energy (EKE) variation to low-frequency ageostrophic energy during the eddy life span. The variation in EKE throughout the eddy life span was highly coherent to that of the seven-day low-passed ageostrophic kinetic energy, with a correlation coefficient of −0.94. The low-frequency ageostrophic motions supplied 38% of the EKE variation in the growing stage of mesoscale eddies and absorbed 42% in the decaying stage. The evolution rate of the EKE during the eddy life span was consistent with the barotropic conversion rate of the low-frequency ageostrophic motions, further confirming the dominant role of low-frequency ageostrophic motions in eddy growth and decay. [ABSTRACT FROM AUTHOR]