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In the eastern off-equatorial Indian Ocean, deep current intraseasonal variability within a typical period of 10–20 days was revealed by a mooring at 5°N, 90.5°E, accounting for over 50% of the total bottom subtidal velocity variability. The 10–20-day oscillations were more energetic in the cross-isobathic direction (STD = 3.02 cm s−1) than those in the along-isobathic direction (STD = 1.50 cm s−1). The oscillations were interpreted as topographic Rossby waves (TRWs) because they satisfied the TRWs dispersion relation that considered the smaller Coriolis parameter and stronger β effect at low latitude. Further analysis indicated significant vertical coupling between the deep cross-slope oscillations and cross-isobathic 10–20-day perturbations at the depth of 300–950 m. The 10–20-day TRWs were generated by cross-isobathic motions under the potential vorticity conservation adjustment. The Mercator Ocean output reproduced the generation of kinetic energy (KE) of deep current variability. The associated diagnostic analysis of multiscale energetics showed that the KE of TRWs was mainly supplied by vertical pressure work. In the seamount region (2°–10°N, 89°–92°E), vertical and horizontal pressure works were identified to be the dominant energy source (contributing to 94% of the total KE source) and sink (contributing to 98% of the total KE sink) of the deep current variability, transporting energy downward and redistributing energy horizontally, respectively. [ABSTRACT FROM AUTHOR]

Observations of currents and temperatures from four moorings deployed around the deep slope (∼2500 m) of Caiwei Guyot in the Pacific Prime Crust Zone were utilized to investigate topographically trapped waves at low-latitude seamounts. Contrasting with commonly reported persistent diurnal seamount-trapped wave cases at middle and high latitudes, the subinertial variability in deep currents and temperatures at the slope of Caiwei Guyot was primarily characterized by two distinct lower-frequency bands (i.e., 13–24 and 3.3–4.7 days). These subinertial variabilities are interpreted as intermittent seamount-trapped waves and topographic Rossby waves (TRWs). During certain time periods, the observations include key signatures of seamount-trapped waves, such as near-opposite phases of azimuthal velocity (and temperature) on opposite flanks of the seamount, and patterns of temporal current rotation consistent with counterrotating cells of horizontal current propagating counterclockwise around the seamount. After comparing these observations to idealized seamount-trapped wave solutions, we conclude that the 13–24-day (3.3–4.7-day) energy is mainly due to radial–vertical mode 5 (3) for azimuthal wavenumber 1 (3). Sometimes the subinertial energy remained pronounced at only one flank of the seamount, primarily explained as TRWs with 192–379-m vertical trapping scale and 14–28-km wavelength. Upper-layer mesoscale perturbations might provide energy for deep seamount-trapped waves and TRWs. This study highlights the role of topographically trapped waves in modulating the deep circulation at low-latitude seamounts. [ABSTRACT FROM AUTHOR]

The presence of seamounts impacts the ocean's hydrodynamic environment. A 5‐year numerical simulation is conducted over the Northwestern Pacific Ocean (NWPO) to investigate the influence of seamounts. The results are validated against satellite remote sensing data and in‐situ measurements. Sensitivity experiments show that the presence of seamounts increases the complexity of the flow field, leading to enhanced relative vorticity, divergence, and strain rate. Additionally, eddy activity is enhanced while eddy propagation is suppressed. Five main physical processes that affect the hydrodynamic environment are examined around the Caiwei (CS), Weijia (WS) and Niulang (NS) seamounts. (a) Seamount‐induced negative vorticities are found above the three seamounts most of the time and extend to about 600 m above the summit of the seamounts; (b) density fronts, a few kilometers in width, are present in the bottom boundary layer of the three seamounts, with accompanying jets occurring at the same locations; (c) seamount‐induced topographic Rossby waves (TRWs) signals are extracted when a 7‐ to 10‐day band‐pass filter is applied to the temperature and velocity anomaly fields around CS (WS and NS) at 5,350 m (5,350 m and 4,500 m); (d) seamount‐eddy interactions result in abundant eddies generated and trapped around the three seamounts; and (e) lee waves are found at the three seamounts. The systematic discussion of these physical processes near the deep seamounts provides a new understanding of how these processes enhance biological connectivity among seamounts. Plain Language Summary: A numerical simulation, and sensitivity experiments with and without seamounts, are conducted in the Northwestern Pacific Ocean to demonstrate the influence of seamounts on the hydrodynamic environment. The results show that the presence of seamounts results in a more complex flow field structure. Five physical processes that significantly impact the hydrodynamic environment are further discussed: seamount‐induced negative vorticities above seamounts (Taylor Caps), density fronts and associated jets around seamounts, topographic Rossby waves, eddies around seamounts and seamount‐induced lee waves. These processes have substantial implications for the biological connectivity among the seamounts in the area. Key Points: Five main physical processes influenced by seamounts are investigatedThe presence of seamounts increases the number of generated eddies and biological connectivity among different seamounts in deep waterTidal effects are different for different seamounts [ABSTRACT FROM AUTHOR]

Strong deep‐current variabilities with periods of 8–25, 16–40, and 30–80 days observed in the northern South China Sea (NSCS) are interpreted as topographic Rossby waves (TRWs) based on the moored observations from July 25, 2017 to August 13, 2018. The TRWs present significantly spatial distribution in the frequency and intensity, which are largely controlled by the mesoscale perturbations in the upper layer. Longer‐period (30–80‐day) TRWs observed to the west of the Dongsha Islands were excited by locally enhanced perturbations with the periods of 30–80 days in the upper ocean. The bottom currents in the periods of 8–25 days observed on the slope of the northeastern South China Sea (NESCS) were influenced by upper‐layer mesoscale perturbations on their northeastern side through TRW propagation in the deep ocean in addition to local processes. In the Luzon Strait, deep currents with the periods of 16–40 days were associated with locally strong perturbations in the upper layer. The distribution of the bottom current variability was closely related to the spatial variability of the upper‐layer fluctuations. The 8‐25‐day bandpass‐filtered surface eddy kinetic energy (EKE) was overall maximal in the Luzon Strait and weakened westward to the east side of the Dongsha Islands, which explained why TRWs with the periods of 8–25 days were more energetic in the Luzon Strait. The surface fluctuations with the periods of 30–80 days were dominant in the west of the Dongsha Islands, and triggered the strong TRWs with the frequency band. Plain Language Summary: Topographic Rossby waves (TRWs) play an essential role in deep ocean circulation variability and potentially contribute to the abyssal mixing. Strong variations in the bottom currents with periods of 8–25, 16–40, and 30–80 days are interpreted as TRWs, explaining over 50% of the velocity variance in the northern South China Sea (NSCS). The TRWs present significantly spatial distribution in the frequency and intensity on the slope of the NSCS. The dominant periods of bottom currents varied geographically, with longer‐period (30–80 days) TRWs observed to the west of the Dongsha Islands and shorter‐period (8–25 days) TRWs active near the Dongsha Islands and in the Luzon Strait. Local perturbations from upper layers were thought to be the main energy source for the generation of TRWs. However, strong TRWs in the periods of 8–25 days were sometimes observed on the slope of the northeastern South China Sea when upper‐ocean fluctuations were weak. Based on a ray‐tracing model, distant surface fluctuations were identified as energy sources, impacting bottom currents through TRW propagation. These spatial patterns of TRWs are explained by the differences in surface fluctuations. Key Points: Observations reveal strong abyssal current variabilities in the northern South China Sea, which are interpreted as topographic Rossby wavesHigh frequency topographic Rossby waves weaken westward from Luzon Strait; those with low frequency are strong to west of Dongsha IslandsThe spatial difference of upper‐layer fluctuation dominates the distribution of topographic Rossby waves [ABSTRACT FROM AUTHOR]

Topographic Rossby waves (TRWs) are reported to make a significant contribution to the deep-ocean current variability. On the northern South China Sea (NSCS) continental slope, TRWs with peak spectral energy at ~14.5 days are observed over about a year at deep moorings aligned east–west around the Dongsha Islands. The TRWs with a group velocity of O(10) cm s−1 contribute more than 40% of total bottom velocity fluctuations at the two mooring stations. The energy propagation and source are further identified using a ray-tracing model. The TRW energy mainly propagates westward along the NSCS continental slope with a slight downslope component. The possible energy source is upper-ocean 10–20-day fluctuations on the east side of the Dongsha Islands, which are transferred through the first baroclinic mode (i.e., the second EOF mode). These 10–20-day fluctuations in the upper ocean are associated with mesoscale eddies. However, to the west of the Dongsha Islands, the 10–20-day fluctuations in the upper ocean are too weak to effectively generate TRWs locally. This work provides an interesting insight toward understanding the NSCS deep current variability and the linkage between the upper- and deep-ocean currents. [ABSTRACT FROM AUTHOR]

The vertical propagation of middepth (200–1,500 m) zonal velocity in the equatorial Indian Ocean (EIO) and its relationship with surface wind forcing are investigated. We focus on semiannual time scales, using in situ mooring observations and an oceanic reanalysis product that can reasonably well reproduce the structure and magnitude of the observed middepth zonal velocity in the EIO. The pronounced semiannual cycle in observed middepth zonal currents indicates a clear upward phase propagation with particularly robust signatures at depths of 500–1,000 m between 3°S to 3°N, 70 to 85°E. The main characteristics of the middepth currents are consistent with vertically propagating first‐meridional‐mode Rossby wave at semiannual period. Theory suggests that equatorial Kelvin waves, forced by zonal wind variations in the western half of the basin, transfer energy downward and eastward, reflecting into Rossby waves at the eastern boundary. These reflected Rossby waves transfer energy downward and westward, contributing greatly to observed phase propagation at middepths in the EIO. Key Points: The observed middepth (200‐1,500 m) zonal currents in the equatorial Indian Ocean are characterized by a pronounced semiannual cycle exhibiting upward phase propagationThe vertical propagation of middepth zonal currents is controlled by first‐meridional‐mode semiannual period Rossby wave that propagates energy downwardThe middepth Rossby waves are generated by wind‐forced semiannual Kelvin waves that reflect at the eastern boundary [ABSTRACT FROM AUTHOR]