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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]

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]

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]

Recent satellite sea surface height (SSH) and sea surface temperature (SST) observations have shown that abnormal eddies, that is, warm cyclonic eddies and cold anticyclonic eddies occur sporadically in some regions, which triggers an essential question on the spatiotemporal distribution of abnormal eddies in the global ocean. In this study, a deep learning framework was developed to systematically mine information from the synergy of satellite‐sensed global SSH and SST data over the 1996–2015, 20‐year period. Abnormal eddies account for a surprising one‐third of total eddies and are active along the Equatorial Current and high unstable currents. Normal (abnormal) eddies are stronger in winter (summer) in the North Hemisphere and vice versa in the Southern Hemisphere. The annual mean amplitudes of normal eddies are larger than that of abnormal eddies. Crucially, the daily number of normal (abnormal) eddies increased (decreased) 9.68 (11.80) every year. Plain Language Summary: Mesoscale eddies are broadly distributed in the global ocean and play a significant role in transporting energy, heat, and mass. This study is the first to demonstrate characteristics of both normal eddies (cold cyclonic eddies and warm anticyclonic eddies) and abnormal eddies (warm cyclonic eddies and cold anticyclonic eddies) in the global ocean. The study developed a deep learning‐based model to detect normal and abnormal eddies from the synergy of 20‐year satellite‐derived global sea surface height and sea surface temperature observations. Abnormal eddies account for a surprising one‐third of total eddies in the global ocean. Besides, the increase of normal eddies' number and the decrease of abnormal eddies' number are proposed to be related with the long‐term variation trend of ocean currents in the global ocean. Key Points: Abnormal eddies (warm cyclonic eddies and cold anticyclonic eddies) account for a surprising one‐third of total eddies in the global oceanThe spatiotemporal variabilities of abnormal eddies are significantly different from those of normal eddies in the global oceanThe daily number of normal (abnormal) eddies has an increasing (decreasing) trend during 1996–2015 [ABSTRACT FROM AUTHOR]

Field observations were conducted along three west‐east transects in the East China Sea Kuroshio Front in July 2017. The microstructure observations results show that the surface turbulence kinetic energy (TKE) dissipation rates in the northern and southern transects are about 2 orders larger than those in the middle transect. The sea surface temperature, mixed layer depth, and the submesoscale motions (with horizontal scales smaller than 30 km) also suggest the surface turbulences are more active in the northern and southern transects. The current velocity data and sea level anomaly show that the middle transect was located at the northern edge of a cyclonic eddy. The eddy‐generated cross‐front geostrophic current was observed to flow toward the opposite direction of the cross‐front Ekman transport and believed to counteract the Ekman buoyancy flux (EBF)‐induced turbulence enhancement. The TKE dissipation rates can be well scaled by a scaling considering the EBF and the wind in the northern and southern transects but wind scaling only in the middle transect, suggesting the EBF does not contribute to the surface turbulence in the middle transect. An improved empirical scaling considering the cross‐front geostrophic current is defined and can scale the TKE dissipation rates better, indicating an eddy plays a key role in surface turbulent mixing in the East China Sea Kuroshio front. Plain Language Summary: Ocean surface boundary layer is an important mediator for air‐sea interaction. Recent field observations have shown that the turbulence mixing within the ocean surface boundary layer can be significantly elevated at the front when the cross‐front Ekman transport carries dense cold water over less dense warm water. Our observations show that the mesoscale eddy also plays an important role in the surface turbulence at the front by generating cross‐front geostrophic current. This result may have important implications on the ocean model and climate forecast. Key Points: Expected Ekman buoyancy flux‐induced surface turbulence enhancement was not observed in the Kuroshio FrontEddy‐generated cross‐front geostrophic current counteracted the Ekman buoyancy flux‐induced surface turbulence enhancementNewly defined empirical scaling considering geostrophic current can better scale observed turbulent dissipation rates [ABSTRACT FROM AUTHOR]

This study investigates the mechanism of mesoscale eddies observed in the South China Sea (SCS) deep basin, which are characterized by high-frequency occurrence, long life, and regular distribution patterns. Geophysical fluid dynamics predicts that for an enclosed ocean basin, there exist the Rossby normal modes if considering the Earth rotation effect. We use a rectangular ocean basin with a uniform depth to simulate the SCS deep basin and obtain 2D (x - y) sea level distribution patterns of the Rossby normal modes, consisting of alternating cyclonic and anticyclonic eddies, as well as their dynamic parameters. The periods of the modes are evidenced by the Fourier period power density spectra of satellite altimeter sea level anomaly (SLA) data from 1993 to 2015. The 2D normal mode patterns are evidenced by SLA images. The phase speeds are evidenced by that derived from the westward movement of the wave/eddy patterns on SLA time-series images. [ABSTRACT FROM AUTHOR]

An Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO) mesoscale eddy trajectory atlas product is used to analyze the path type and temporal variability of the eddies that entered the continental shelf area of the northern South China Sea (SCS) from 1993 to 2016. A total of 184 mesoscale eddies entered the continental shelf area of the northern SCS during a 24-year period. We classify the mesoscale eddies into four types according to the motion trajectories: along-the-isobath type, intrusion-of-continental-shelf type, local wandering type, and shelf-internal-generation type. The occurrence numbers of these four types were 87, 38, 23, and 36, respectively. The mean amplitude and radius of the along-the-isobath type are the largest, about 18 cm and 153 km, respectively; furthermore, their average lifetime is also the longest, about 93 days. The mean amplitude, radius, and lifetime are the smallest for the shelf-internal-generation type, about 16 cm, 146 km, and 74 days, respectively. The direction and velocity of the background flow field affects the intrusion path of the mesoscale eddies onto the continental shelf of the northern SCS. The seasonal distribution of the mesoscale eddies quantity is also related to the direction and velocity of the corresponding background flow field. [ABSTRACT FROM AUTHOR]

The spatial and temporal variability of the Kuroshio surface axis northeast of Taiwan Island is investigated using 24 years of surface geostrophic currents derived from satellite altimeter data from 1993 to 2016. The Kuroshio surface axis is derived by an extraction method with three selected parameters, including the length of the subsidiary line, the intervals between two adjacent points, and the distance between the two adjacent subsidiary lines. The empirical mode decomposition analysis on the 24-year Kuroshio axes reveals that the mean periods of intra-seasonal and inter-annual variability, which are the two dominant components, are about 3.2 months and 1.3 years, respectively. The self-organizing map analysis reveals that the variation of Kuroshio axis northeast of Taiwan Island has four best matching unit (BMU) patterns: straight-path (BMUS), meandering-path (BMUM) and two transition stages (BMUT1 and BMUT2). The straight-path pattern shows strong seasonality: more likely occurring in summer. The meandering-path pattern is less frequent than straight-path pattern. During a typical period from November 26, 2012 to January 27, 2013, which is chosen as an independent example, the analysis on the satellite altimeter and sea surface temperature data shows that the patterns of the Kuroshio axis change successively in order of BMUT1→BMUM→BMUT2→BMUS, i.e., the Kuroshio axis migrates from the meandering-path to the straight-path pattern. During the typical period the warm water intrusion and a mesoscale eddy occur at the second stage corresponding to BMUM and migrate northwestward gradually at the last two stages corresponding to BMUT2 and BMUS. The transient order appears only during this typical period but it is not common for the whole study period. The monthly mean relatively vorticity is calculated and analyzed to evaluate the impact of the eddies on the Kuroshio surface axis variability, the results show that the anticyclonic (cyclonic) eddies can promote the Kuroshio surface axis to present the meandering-path (straight-path) pattern because of the potential vorticity conservation. The impacts of the anticyclonic eddies and the cyclonic eddies on the variability of the Kuroshio surface axis are opposite. The long-term day-to-day detection contributes to improving understanding the variability of Kuroshio surface axis northeast of Taiwan Island. [ABSTRACT FROM AUTHOR]