Anticyclonic Eddies Research Articles

Study of upwelling and mixing process in the Somali coastal region using satellite and numerical model observations: A Lagrangian approach

During the summer monsoon, the Somali region undergoes a significant upwelling phenomenon that enhances plankton productivity, thereby benefiting fisheries. Wind and coastal dynamics initially drive this upwelling, but eventually, eddy flows influence it. Our study explores the interplay between ocean currents, eddies, and chlorophyll-a concentrations using the backward Finite-Size Lyapunov Exponents (bFSLEs) technique. We also delve into the specific role of Ekman transport in distributing chlorophyll-a across the region. The Great Whirl (GW), an anticyclonic eddy, predominantly causes strong downwelling, interrupting the summer monsoon upwelling along the Somali coast longitudinally. Despite the GW's significant impact on moving upwelled water offshore, the influence of downwelling diminishes northward. As a result, the northern Somali coast, especially around 9°N and 10°N, showcases the most extensive offshore upwelling, reaching as far as 55°E. Our findings highlight a robust connection between chlorophyll-a levels and oceanic dynamics, influenced by both currents and eddies, as evidenced by bFSLEs, and by cross-shore Ekman transport, particularly within chlorophyll-a concentrations ranging from 0.2 to 0.6 mg m−3. The data suggests that Ekman transport-induced upwelling primarily drives coastal phytoplankton biomass. Furthermore, bFSLEs analysis underlines the supportive role of ocean currents and eddies in the offshore distribution of chlorophyll-a, especially near the coast. Further examination of lagged correlations reveals a temporal lag between peak concentrations of chlorophyll-a and Ekman transport; the lag increases offshore and is at least 9 days near the coast.

Semidiurnal Internal Tide Interference in the Northern South China Sea

Multiwave interference plays a crucial role in shaping the spatial variations of internal tides. Based on a combination of in situ mooring and altimeter data, interference of semidiurnal internal tides was investigated in the northern South China Sea. Mooring observations indicate the observed kinetic-to-potential energy ratio and group speed are both relatively lower than the theoretical values of mode-1 semidiurnal internal tides, indicating the presence of partly-standing waves. This is consistent with the altimeter result that the mooring was located at the antinode within the interference pattern formed by the superposition of the westward and southward semidiurnal internal tides from the Luzon Strait and the continental slope of the southern Taiwan Strait. However, the kinetic-to-potential energy ratio and group velocity were notably changed when an anticyclonic eddy passed by the mooring. By employing the ray-tracing method, we identified that mesoscale processes may induce a phase difference in the semidiurnal internal tides between the Luzon Strait and the continental slope of the southern Taiwan Strait. This alteration further leads to changes in the positions of nodes and antinodes within the interference pattern of the semidiurnal internal tides.

A Gaussian Function Model of Mesoscale Eddy Temperature Anomalies and Research of Spatial Distribution Characteristics

Mesoscale eddies are ubiquitous oceanic phenomena and play an important role in ocean circulation, ocean dynamics, and the transport of material energy. Temperature anomalies are a crucial parameter that reflects the state of mesoscale eddies. This study proposes a Gaussian function model to fit the vertical temperature anomaly (TA) profile to facilitate the analysis of variations, and the principle of the model is based on the fact that each TA profile tends to fluctuate around one or more peaks. The model is extracted and validated using Argo profiles within cyclonic and anticyclonic eddies in the Northwest Pacific Ocean spanning over the period from 2002 to 2021. The validation demonstrates that the model can accurately recover the vertical TA profiles with a limited number of parameters. This makes it suitable for analysing the spatial distribution patterns that require a large sample count. The analysis indicates that eddies with different TA profiles have a spatial aggregation effect in geographic distribution. Eddies with lower extreme temperature anomalies, at depths of 200–300 m, are mainly distributed along two bands on the north side of the Kuroshio Extension (KE) and the North Equatorial Current. Eddies with extreme TAs at the deepest depth (500–600 m) are distributed along the KE.

Phytoplankton community responses to the combined effects of internal waves and anticyclonic eddies shed from the Kuroshio intrusion

Phytoplankton community responses to the combined effects of internal waves and anticyclonic eddies shed from the Kuroshio intrusion

The influence of the Black Sea rim current and the coastal quasi-stationary anticyclonic Eddie on the distribution of ichthyoplankton and jellyfish

This article describes the Black Sea quasi-stationary coastal anticyclonic eddy (CAE), with large accumulations of zooplankton and ichthyoplankton, which were registered in August 2021 off the northeastern Black Sea during the period of a tropical-type atmospheric mesocyclone. Intense rains (60–80 millimeters of precipitation) caused flooding and large volumes of terrestrial water entered the sea. Water flows came from rivers into the coastal area and flowed into the CAEs. As a result, there were favorable conditions for the growth of plankton, fish eggs and, larvae. Thus, in the Sochi CAE, against the background of a high concentration of chlorophyll a, and the abundance of small-sized fractions of the fodder zooplankton, there was an almost 30-fold increase in the number of fish larvae (947 ind.·m−2 in the CAE and 33.5 ind.·m−2 in the surrounding waters) and the accumulation of jellyfish Aurelia aurita (Linnaeus, 1758), which is their food competitor, with an abundance of up to 44 ind.·m−2. The data obtained indicates that the Black Sea coastal quasi-stationary anticyclones arise between the cyclonic rim current (Main Black Sea current) and the coast, which can be of great importance in the formation of fish abundance.

Mesoscale eddies in the Gulf of Mexico: A three-dimensional characterization based on global HYCOM

The Gulf of Mexico (GoM) is characterized by strong mesoscale eddy activities that have been studied extensively, yet the comprehensive three-dimensional (3-D) kinematic properties of GoM eddies are still not well documented. In this study, the 3-D mesoscale eddy activities in the upper layer (0–800 m) of the GoM are characterized using 14-year (1997–2010) global Hybrid Coordinate Ocean Model (HYCOM) outputs. Most eddies in the upper layer (both cyclonic and anticyclonic) have radii of ∼30–60 km and lifespans shorter than 30 days. The spatial distributions of GoM eddies do not vary much with depth, while their intensity decreases with depth. The size of cyclonic eddies does not vary much with depth, while the size of anticyclonic eddies decreases slightly with depth. Cyclonic eddies are often found to be generated in the eastern GoM (especially in the Loop Current region), the Bay of Campeche, and on the continental slope of the Campeche Bank, while anticyclonic eddies are often generated on the northeastern and northwestern GoM continental slopes, and in the central GoM (near 24°N) and the Bay of Campeche (92–94°W). In addition, long-lived GoM eddies (e.g., lifespan >150 days) tend to have intermediate eddy intensity (e.g., 0.13–0.32 for cyclonic eddies at the 10 m level). Both cyclonic and anticyclonic eddies are found to play an important role in the horizontal and vertical transport of heat and salt, and eddy-induced anomalies of water temperature and salinity at both surface and subsurface are generally more pronounced in the eastern GoM than in the western GoM.

Characteristics and generation mechanisms of anticyclonic eddies, cyclonic eddies and dipole eddies in the Mozambique Channel

The discovery of cyclonic and dipole eddies in the Mozambique Channel (MC) indicates that the understanding of the mesoscale eddy characteristics in the MC is incomplete. The distributions of anticyclonic, cyclonic, and dipole eddies along the MC were elucidated in this study using satellite observations. It was observed that these eddies exhibit a preference for emergence and movement in the western MC. The occurrence frequencies of anticyclonic and cyclonic eddies are four and three times per year, respectively, in the narrowest section of the MC. In contrast, the frequency of mesoscale eddies reaches its peak at nine times per year in the central region of the MC. The occurrence of dipole eddies also reaches its peak twice per year in the middle MC. Dipole eddies are more prevalent in the MC and exhibit larger dimensions and shorter lifespans compared to anticyclonic and cyclonic eddies. Mesoscale eddies, which traverse the narrowest section of the MC and propagate southward, are predominantly generated within the western Comoros Basin due to barotropic instability. The southward branch of the Northeast Madagascar Current (NEMC) plays a crucial role in transporting these eddies to the middle MC. The eastern middle MC is also a generation site for mesoscale eddies in addition to the Comoros Basin, where cyclonic eddies are generated twice per year. These cyclonic eddies are also generated due to barotropic instability.

Evaluating westward and eastward propagating mesoscale eddies using a 1/10° global ocean simulation of CAS-LICOM3

This research evaluates the performance of CAS-LICOM3 (Chinese Academy of Science, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics (LASG/IAP) Climate system Ocean Model, version 3) in simulating global coherent mesoscale eddies by comparison to satellite altimeter observations. The simulations of westward and eastward propagating eddies (WPEs and EPEs) and cyclonic and anticyclonic eddies (CEs and AEs) are separately analyzed. The results demonstrate that the simulated spatial-temporal variabilities in global mesoscale eddies agree roughly with the satellite observations. CAS-LICOM3 also reproduces the distinctive features between WPEs and EPEs or between CEs and AEs. However, some systematic biases are found. Globally, CAS-LICOM3 simulates a less frequent and weaker mesoscale eddy field than is observed. WPEs contribute more to these global biases than do EPEs. EPEs are relatively better reproduced than WPEs, exhibiting smaller underestimations and even overestimations in the energetic western boundary current and Antarctic circumpolar current regions. The simulation results for CEs resemble those of AEs, but AEs are comparatively less biased than CEs. These findings provide a basis for improving low-resolution and eddy-resolving ocean general circulation models (OGCMs) and developing submesoscale-resolving OGCMs.

Winter Convective Mixing Mediating Coupling of N‐Gain and ‐Loss in the Arabian Sea

AbstractMarine dinitrogen (N2) fixation fuels primary production and thereby influences the Earth's climate. Yet, its geographical distribution and controlling environmental parameters remain debatable. We measured N2 fixation rates from the two spatially and physicochemically contrasting regions of the Arabian Sea during the winter monsoon: (a) the colder and nutrient‐rich waters in the northern region owing to winter convection and (b) the warmer and nutrient‐poor waters in the southern region unaffected by winter convection. We found higher N2 fixation rates at the surface of northern region due to convective mixing driven supply of phosphate (intuitively iron also) from the underlying suboxic waters. N2 fixation was favored by high nutrient concentrations in the euphotic waters, whereas remained unaffected by nutrient availability in the aphotic waters. We conclude that diazotrophs dwelling in the euphotic zone chose phosphate over fixed nitrogen‐poor waters. However, we found that among oligotrophic waters, anticyclonic eddy extremes the barrier of fixed nitrogen supply, and thereby, elevates N2 fixation. While the Arabian Sea loses about 20%–40% of the global ocean fixed nitrogen, we estimate that N2 fixation in the Arabian Sea offsets only up to 42% of its fixed nitrogen‐loss by denitrification, but this offset could be higher if diazotrophic activity is further examined up to the deeper depths of the Arabian Sea.

The different dynamic influences of Typhoon Kalmaegi on two pre-existing anticyclonic ocean eddies

Abstract. Using multi-source observational data and GLORYS12V1 reanalysis data, we conduct a comparative analysis of different responses of two warm eddies, AE1 and AE2 in the northern South China Sea, to Typhoon Kalmaegi during September 2014. The findings of our research are as follows: (1) for horizontal distribution, the area and the sea surface temperature (SST) of AE1 and AE2 decrease by about 31 % (36 %) and 0.4 °C (0.6 °C). The amplitude, Rossby number (Ro = relative vorticity / Coriolis parameter) and eddy kinetic energy (EKE) of AE1 increase by 1.3 cm (5.7 %), 1.4×10-2 (20.6 %) and 107.2 cm2 s−2 (49.2 %) after the typhoon, respectively, while AE2 weakens and the amplitude, Rossby number and EKE decrease by 3.1 cm (14.6 %), 1.6×10-2 (26.2 %) and 38.5 cm2 s−2 (20.2 %), respectively. (2) In the vertical direction, AE1 demonstrates enhanced convergence, leading to an increase in temperature and a decrease in salinity above 150 m. The response below the mixed-layer depth (MLD) is particularly prominent (1.3 °C). In contrast, AE2 experiences cooling and a decrease in salinity above the MLD. Below the MLD, it exhibits a subsurface temperature drop and salinity increase due to the upwelling of cold water induced by the suction effect of the typhoon. (3) The disparity in the responses of the two warm eddies can be attributed to their different positions relative to Typhoon Kalmaegi. Under the influence of negative wind stress curl outside the maximum wind radius (Rmax) of the typhoon, triggering negative Ekman pumping velocity (EPV) and quasi-geostrophic adjustment of the eddy, the warm eddy AE1, with its center to the left of the typhoon's path, further enhances the converging sinking of the upper warm water, resulting in its intensification. On the other hand, the warm eddy AE2, situated closer to the center of the typhoon, weakens due to the cold suction caused by the strong positive wind stress curl within the typhoon's Rmax. The same polarity eddies may have different response to typhoons. The distance between eddies and typhoons, eddy intensity, and the background field need to be considered.

Characteristics of mesoscale eddies in the Mozambique Channel.

The mesoscale eddy characteristics of the Mozambique Warm Current were investigated by detecting and tracking satellite altimetry data from 2010 to 2019. A total of 1,086 eddies were identified in the Mozambique Channel, comprising 509 cyclonic eddies and 577 anticyclonic eddies. The results revealed that the bay area on the northwest coast of Madagascar was the main hotspot of eddy generation, and the mean amplitude and radius of the anticyclonic eddies in the Mozambique Channel were 24.23 cm and 82.7 km, respectively, which are larger than those of the cyclonic eddies. Local wind forcing had a significant impact on the formation of mesoscale eddies in the Mozambique Channel. In winter, the wind stress in the northern and southern areas of the Mozambique Channel exhibited a strong correlation with the distribution of eddy kinetic energy (EKE), where both monsoonal winds in the north and trade winds in the south could facilitate mesoscale anticyclonic eddy formation. In addition, the variability in the number of anticyclonic and cyclonic eddies in the Mozambique Channel may have exerted a significant influence on the seasonal anomalous fluctuations in local sea surface temperatures (SSTs). This study presented a novel analysis of the mesoscale eddy characteristics in the Mozambique Channel.

Temperature Structure Inversion of Mesoscale Eddies in the South China Sea Based on Deep Learning

Mesoscale eddies are common in global oceans, playing crucial roles in ocean dynamics, ocean circulation, and heat transport, and their vertical structures can affect the water layers from tens to thousands of meters. In this study, we integrated sea surface height and sea surface temperature data into deep learning methods to study the mesoscale eddy subsurface temperature structure and to explore the relationship between sea surface data and eddy subsurface layers. In this study, we introduce Dual_EddyNet, a deep learning algorithm designed to invert the subsurface temperature structure of mesoscale eddies. Using this algorithm, we explore the impact of the sea surface height and sea surface temperature on the subsurface temperature structure inversion of mesoscale eddies. Furthermore, we compare different data fusion strategies, namely single-stream neural networks and dual-stream neural networks, to validate the effectiveness of the dual-stream model. To capture the interrelations among surface data and integrate feature information across various dimensions, we introduce the Triplet Attention Mechanism. The experimental results demonstrate that the proposed Dual_EddyNet performs well in reconstructing the three-dimensional structure of mesoscale eddies in the South China Sea (within a depth of 1000 m), with an inversion accuracy of 91.44% for cyclonic eddies and 95.25% for anticyclonic eddies. This algorithm provides a new method for inverting the subsurface temperatures of mesoscale eddies, and can not only be directly deployed in systems, embedded in ship moving platforms, etc., but can also provide a data reference for assimilations and numerical simulations, demonstrating its rich application potential.

Experimental construction of eddy real-time structure based on gradient-dependent OI in the Kuroshio-Oyashio confluence region

The internal structures of mesoscale eddies are variable due to different generation mechanism, spatiotemporal scale, and movement characteristics. Based on the principle of data assimilation, this study provided a new approach for constructing the eddy real-time internal structure using the gradient-dependent optimal interpolation (OI) method. Three eddy cases with different types and locations were selected to validate the construction results. The reliability of construction was evaluated by satellite observation, in-situ comparisons, and contrast with the numerical output. The results showed that the real-time geostrophic current characters were basically consistent with the surface and deep current (observed by satellite and acoustic doppler current profiler, ADCP). The density features exhibited by constructed results (eddy centers, mean radii, and variation tendency) were similar to the expendable conductivity-temperature-depth (XCTD) survey data and numerical output. Compared to the composite structure, the real-time structures represented more small or mesoscale fluctuations. During eddy cases evolution, the mean radii at the same depth first increased and then decreased. The horizontal scale, influence depth, and internal stability of an anticyclonic eddy with low-latitude were more powerful than the other two eddy cases owing to the domination of warm water in the Kuroshio extension region. The results indicated that the gradient-dependent OI was a hopeful technique for representing the real-time internal features during eddy evolution.

Changing Humboldt Squid Abundance and Distribution at Different Stages of Oceanic Mesoscale Eddies

Humboldt squid, Dosidicus gigas, is one of the main economic cephalopod species off Peruvian waters, and their abundance and distribution are regulated by localized oceanic mesoscale dynamical processes. To this end, the present study employs normalization and frequency distribution methods, combining mesoscale eddy, oceanic environment, and D. gigas fishery data. This is the first exploration into the different stages of mesoscale eddies during their evolution off Peruvian waters and their influence on the regional marine environment, as well as the abundance and distribution of D. gigas resources. The results indicate that across the four stages of eddies, namely formation, intensification, mature, and aged, their kinetic energy and structure follow a pattern of “growth–equilibration–decay”. The abundance of D. gigas within the eddy’s covered zone undergoes an initial increase, followed by a decrease during the evolution of the four stages, peaking during the eddy’s mature stage. The abundance of D. gigas was higher in the anticyclonic eddies than that in the cyclonic eddies under different stages. The environmental factors conducive to D. gigas in eddies exhibited similar changes to D. gigas abundance throughout the eddy’s different stages. Our research emphasizes that anticyclonic eddies, during their evolution, exerted a more significant impact on the abundance and distribution of D. gigas in the Peruvian waters compared with cyclonic eddies. The eddy-induced changes in water temperature and productivity caused by the eddies may be the primary drivers of this impact.

Intraseasonal variability of the deep scattering layer induced by mesoscale eddy

The deep scattering layer (DSL), a stratum of the marine diel vertical migration (DVM) organisms inhabiting the mesopelagic ocean, plays a crucial role in transporting carbon and nutrients from the surface to depth through the migration of its organisms. Using 18 months of in-situ observations and altimeter sea level data, we reveal for the first time the intraseasonal variations and underlying mechanisms of the DSL and the DVM to the east of the Taiwan Island. Substantial vertical speeds acquired from the Acoustic Doppler Current Profiler were used to examine the distribution and variation of the DVM. Innovatively, the results for the power spectrum analysis of the scattering intensity demonstrated a significant intraseasonal variability (ISV) with an 80-day period in the DSL. Furthermore, the variation in the DVM was closely linked to the DSL and showed an 80-day ISV during the observation. A dynamic relationship between the ISV of the DSL east of Taiwan Island and the westward-propagating mesoscale eddies was established. Anticyclonic (cyclonic) eddy movement toward Taiwan Island triggers downward (upward) bending of the local isotherms, resulting in a layer of DSL warming (cooling) and subsequent upper boundary layer deepening (rising). These findings underscore the substantial influence of mesoscale eddies on biological activity in the mesopelagic ocean, establishing a novel understanding of ISV dynamics in the DSL and their links to eddy-induced processes.

Observational Analysis of Vertical Heat Flux Caused by Typhoon‐Induced Near‐Inertial Waves Under the Modulation of Mesoscale Eddies

AbstractTropical cyclones (TCs) induce heat pump and cold suction in the upper ocean layer. However, limited research investigated whether this heat can be effectively transported into the deep ocean. The present study shows that seawater at Station S2 is anomalously warmer in the deep ocean layer than at S1 and S3 during Typhoon Kalmaegi (2014) in the South China Sea. The turbulence‐induced vertical heat flux is estimated based on fine‐scale mixing parameterization but it cannot explain the warming rate below the thermocline. Therefore, a new method is proposed to estimate the vertical velocity, allowing us to calculate the heat flux more accurately. To elucidate the underlying causes of the observed differences, we analyze horizontal velocity to examine the role of mesoscale eddies in modulating near‐inertial waves (NIWs)‐induced vertical heat flux. Station S1 is located inside a cyclonic eddy while S2 and S3 are within two distinct anticyclonic eddies. According to the “Chimney Effect” theory, cyclonic (anticyclonic) eddies tend to limit (enhance) the vertical propagation of the NIWs. While this theory explains the confined heat flux at S1, it fails to explain the differences observed at S2 and S3. Further examination of the vertical structures and intensities of the two anticyclonic eddies reveals that the eddy at S2 extends much deeper and is stronger than that at S3, allowing the NIWs to propagate and transport heat deeper at S2 than at S3. The study demonstrates the role of TC‐induced NIWs in deep ocean heat transport under the influence of mesoscale eddies.

Role of Air‐Sea Interaction in the Energy Balance of Anticyclonic and Cyclonic Eddies in the Kuroshio Extension

AbstractUsing eddy‐resolving Community Earth System Model (CESM) simulations, this study investigates mesoscale eddy energetics and the role of air‐sea interaction for both anticyclonic and cyclonic eddies (AEs and CEs) in the Kuroshio Extension (KE) region. Based on energy budget analysis and eddy tracking algorithm, it is found that eddy energy balance depicts similar characteristics for AEs and CEs. The eddy kinetic energy (EKE) is generated through barotropic instability, vertical buoyancy flux, as well as transported from the upper stream Kuroshio. In addition, the temperature variance, which is directly related to eddy potential energy, is maintained by baroclinic instability. Air‐sea heat flux and wind stress act as eddy killers to move energy from oceanic eddies. For both AEs and CEs, heat exchange between atmosphere and oceanic eddies dominates the dissipation of temperature variance and accounts for more than 60% and 72% of the total dissipation, respectively. In comparison, the role of wind power in damping the EKE is relatively small. Only 14% (5%) of EKE dissipation in AEs (CEs) is attributed to eddy wind power.

Evolution of Internal Gravity Waves in a Mesoscale Eddy Simulated Using a Novel Model

Abstract We investigate the effect of wave–eddy interaction and dissipation of internal gravity waves propagating in a coherent mesoscale eddy simulated using a novel numerical model called the Internal Wave Energy Model based on the six-dimensional radiative transfer equation. We use an idealized mean flow structure and stratification, motivated by observations of a coherent eddy in the Canary Current System. In a spindown simulation using the Garret–Munk model spectrum as initial conditions, we find that wave energy decreases at the eddy rim. Lateral shear leads to wave energy gain due to a developing horizontal anisotropy outside the eddy and at the rim, while vertical shear leads to wave energy loss which is enhanced at the eddy rim. Wave energy loss by wave dissipation due to vertical shear dominates over horizontal shear. Our results show similar behavior of the internal gravity wave in a cyclonic as well as an anticyclonic eddy. Wave dissipation by vertical wave refraction occurs predominantly at the eddy rim near the surface, where related vertical diffusivities range from to . Significance Statement Using a novel model and observations from the Canary Current System of a coherent eddy of 100-km diameter, we explore the interaction between a realistic internal gravity wave field and this eddy. We study wave refraction and energy transfers between the waves and the eddy induced by the eddy’s lateral and vertical shear. Waves lose energy at the eddy rim by vertical shear and gain outside of the eddy rim by horizontal shear. We find large vertical wave refraction by vertical shear at the eddy rim, where waves break and mix density, which can have wide ranging effects on the ocean’s circulation. These results are important for understanding the ocean’s mixing and energy cycle and to develop eddy and wave parameterizations.

Role of Anomalous Ocean Warming on the Intensification of Pre‐Monsoon Tropical Cyclones Over the Northern Bay of Bengal

AbstractThe northern Indian Ocean, typically the Bay of Bengal (BoB), has experienced several stronger tropical cyclones during the pre‐monsoon season in the last few years. Similarly, in 2021, cyclone Yaas reached the Very Severe Cyclonic Storm (VSCS) stage despite forming above 14°N. In comparison to the other three cyclones that formed in 2007 (Akash), 2009 (Aila), and 2017 (Mora) with a similar cyclogenesis location and track, the pre‐cyclonic condition in 2021 is more favorable. It was found that from 2007 to 2021, the area associated with Sea Surface Temperature greater than 31°C increased from 0.1 percent to 29 percent. The Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction buoy at 15°N also recorded an increasing trend of 0.1°C per year in both average and maximum temperatures during May. The warming in 2021 is primarily linked with Marine Heat Wave events, which extensively cover the north BoB for a longer duration starting from the first week of May. Such warming extends even to the subsurface, providing the heat required to convert the depression into VSCS within 2 days. The weaker western boundary current during 2021 made the northern BoB fresher and favorable for cyclone, intensification. The associated anticyclonic eddy along the track extended the abnormal distribution of low saline water even up to the subsurface (∼100m), which is vital in increasing the ocean heat content. These simultaneous pre‐cyclonic oceanic conditions can be incorporated into the weather model to understand and predict the rapid intensification within a shorter duration.

Controls on terrigenous sediment supply to northwestern South China Sea based on a sediment trap record at Xisha Trough

The South China Sea (SCS) is the largest marginal sea in the western Pacific and comprises an ideal natural laboratory for studying deep-sea processes. Previous investigations in the region have primarily focused on surface currents and to a lesser extent on intermediate currents, while observations of bottom currents are scarce, hindering our understanding of deep ocean sediment transport and deposition mechanisms. Here, we carried out grain size measurement and end-member modelling analysis (EMMA) on samples collected by a sediment trap at a water depth of 1500 m in the Xisha Trough in the northwestern South China Sea. Grain size data shows that terrigenous material has particle sizes ranging from 0.3 μm to 745.9 μm and a peak at 4–11 μm. The EMMA result suggests that finer sediment accounting for an average of 48.2% of the sediment represents material transported by deep water currents (End-member 1; EM1), and this is primary sediment source in the region. Coarser sediment contributing to 32.9% of the sediment is the second most significant source (EM3), representing the deposits of surface currents influenced by monsoons and deposited close to the source. A third sediment type (EM2) constituting approximately 18.9% of the sediment exhibits a wide range of grain sizes and rapid changes, indicating transport in the bottom nepheloid layer triggered by anticyclonic eddies. Our study identifies three primary sediment transport mechanisms to the northwestern SCS, bridging present-day observations with paleoceanography and providing a crucial basis for future research on paleoceanographic reconstructions.