Cyclonic Eddies Research Articles

Bacterial reworking of particulate organic matter in a dynamic marginal sea: Implications for carbon sequestration

Concentrations of particulate organic carbon (POC) and total hydrolyzable amino sugars (THAS) were measured along a transect of the dynamic South Yellow Sea (SYS) to investigate the bioreactivity and bacterial reworking of particulate organic matter (POM). Results showed that POM bioavailability was linked with primary production, as revealed by the significant correlation between chlorophyll-a concentrations and the diagenetic indicator glucosamine/galactosamine (GlcN/GalN). Production of bioavailable POM could rapidly stimulate microbial activity, generating hot spots of heterotrophic alteration. Lower GlcN/GalN ratios (<3) observed in the entire SYS indicate that POM underwent extensive microbial alteration. In particular, extremely low GlcN/GalN ratios (∼0.7) were found in the Yellow Sea Cold Water Mass, reflecting high bacterial alteration of POM. Estimates based on the bacterial biomarker muramic acid showed that on average ∼13% of POM in the SYS was of bacterial origin. Elevated bacterial contributions were found in both nearshore and offshore areas. Strong mixing in the nearshore and the presence of cyclonic eddies in offshore waters may increase the residence time of POM in the water column and thus promote bacterial transformation of POM. Overall, our findings indicate that bacterial reworking of POM varies with productivity and that the extensive bacterial transformation of the remaining POM observed in the water column probably enhances long-term carbon sequestration.

Subsurface imbalance stimulated in a mesoscale eddy. Part I: Observations

This study presents observational evidence of a stimulated imbalance below the mixed layer from a fast-tow cruise survey with approximately 4 km resolution that traverses a cyclonic mesoscale eddy in the central South China Sea. The observed physical and biochemical properties reveal both geostrophic and ageostrophic physics in the mesoscale eddy. In addition to the well-known eddy-induced features, banded structures of temperature and chlorophyll anomaly across the section indicate unbalanced motions. These features are notably active near the eddy's edge and show larger amplitudes of variance than those induced by the mesoscale eddy. The results also show remarkable unbalanced variances of temperature and salinity, as well as lateral temperature-salinity compensation on isopycnals, likely due to stirring associated with internal waves. Spectral analysis suggests that there exist significant unbalanced variances of temperature and chlorophyll at ∼13 km. Our further analysis suggests that symmetric instability may have been stimulated due to strong lateral buoyancy gradients and vertical shear of velocity, which may account for the imbalance signature in the pycnocline. The observed imbalances point to the instability due to the interaction between mesoscale, submesoscale, and internal wave dynamics, which may play an important role in catalyzing the forward energy cascade in the ocean.

A Large Winter Chlorophyll‐a Bloom in the Southeastern Bay of Bengal Associated With the Extreme Indian Ocean Dipole Event in 2019

AbstractA large Chlorophyll‐a (Chla) bloom developed in the southeastern Bay of Bengal (BoB) (4°N–10°N, 88°E−95°E) during winter 2019 and was observed from Moderate Resolution Imaging Spectroradiometer‐Aqua Chla data. By analyzing various observational and reanalysis datasets, we found that remote Equatorial Indian Ocean (EIO) upwelling waves and local Ekman pumping associated with the extreme Indian Ocean Dipole (IOD) event in 2019 were responsible for the Chla bloom. The extreme IOD 2019 favored strong upwelling Kelvin waves propagating from the eastern EIO into the BoB along the eastern boundary. These Kelvin waves were reflected into the central BoB as upwelling Rossby waves manifested by eddylike sea surface height propagating westward approximately along 5°N. In addition, local wind stress curl generated strong Ekman pumping in the southeastern BoB. As a result, upwelling processes associated with westward‐propagating Rossby waves and local Ekman pumping together uplifted the thermocline, breaking the stratification so that subsurface nutrient‐rich water was transported to the surface to cause a large Chla bloom in the southeastern BoB during winter 2019. The Chla bloom had a ring structure and its spatial shape was modulated by westward upwelling Rossby waves and cyclonic eddies. This study highlights the important role of IOD events in the formation of Chla bloom in the southeastern BoB during winter.

Frontal eddies provide an oceanographic triad for favorable larval fish habitat

AbstractFrontal eddies form on the shoreward edge of boundary currents, on average at weekly intervals and can last up to several weeks. This duration allows zooplankton production and completion of the larval stage of fish but may be too ephemeral for longer food chains with planktonic predators to establish. Therefore, frontal eddies may provide a suitable offshore nursery habitat, by entraining and retaining inner‐shelf water, preconditioned with coastal plankton and ichthyoplankton, into an upwelling favorable cyclonic eddy. Here, we briefly describe the behavior of frontal eddies formed by western boundary currents in the context of the fundamental ocean triad, which incorporates three processes for the successful reproduction of fish: nutrient enrichment, food concentration, and retention of larvae. We adapt this hypothesis for frontal eddies adjacent to the substantial fisheries production of continental shelves, creating conditions for enhanced recruitment potential of larval cohorts at a finer scale than previously considered. We review the evidence and investigations of frontal eddies in their capacity to entrain coastal water, sustain the plankton community through eddy uplift and retain distinctive coastal communities of larval fish offshore until larval development is complete. The process of frontal eddy formation is complex and such habitats are irregular yet ubiquitous, which present challenges and opportunities for their study. With the advance of ocean observing systems and integration of physical and biological sampling, frontal eddies provide a novel focus for understanding fisheries production and connectivity of coastal ecosystems.

Particulate organic carbon in the deep-water region of the Gulf of Mexico

Ocean eddies play a major role in lateral and vertical mixing processes of particulate organic carbon (POC), as well as in the transport of heat, salinity, and biogeochemical tracers. In the open waters of the Gulf of Mexico (GoM), however, there are limited observations to characterize how these mesoscale structures affect the spatial distribution of POC in the upper water column, which is important for organic matter cycling and export. We present the distribution patterns of POC relative to mesoscale features throughout the water column in the deep-water region of the GoM during three oceanographic cruises held during the summer months of 2015, 2016, and 2017. Samples were collected under well-stratified upper ocean conditions, which allowed us to assess the spatial and temporal distribution of POC as a function of non-steric sea surface height, density, apparent oxygen utilization, and chlorophyll fluorescence. We further explored the variability of integrated surface layer POC concentrations at stations located within the cores and the edges of cyclonic and anticyclonic eddies, and those collected outside these structures. Although our results indicate mesoscale eddies modulate several important physical and biogeochemical variables and POC concentrations in the upper ocean, these features do not fully explain the spatial distribution of POC concentrations throughout the deep-water region of the GoM. Relatively lower POC concentrations were observed in the border of the cyclonic and the center of the anticyclonic eddies, in contrast to the relatively higher POC concentrations at the center of the cyclonic and the border of anticyclonic eddies. We observed high variability in POC concentration variability outside mesoscale structures, which may be attributed to other processes such as upwelling over the shelves, and the contribution by rivers during the summer especially in the northern and southern GoM.

Satellite-Based Ocean Color and Thermal Signatures Defining Habitat Hotspots and the Movement Pattern for Commercial Skipjack Tuna in Indonesia Fisheries Management Area 713, Western Tropical Pacific

Understanding the mechanisms that determine the critical habitat of commercial species is one of the significant challenges in marine science, particularly for species that inhabit the vast ocean worldwide. Previous investigations primarily focused on determining skipjack habitats without considering the feasible size for sustainable fisheries. To define habitat hotspots and movement patterns for decently sized skipjack tuna (≥50 cm) in Indonesia Fisheries Management Area (IFMA) 713, Indonesia, we examined the remote sensing of synoptic sea surface temperature (SST) and chlorophyll-a concentration (Chl-a) measurements with catch data from 2007 to 2016. A new skipjack tuna habitat model was developed based on the link between the key satellite-based environmental data and the best tuna fishery performance using a combination of generalized additive models (GAMs) and kernel density estimates. The findings reveal that feasible skipjack catch sizes were found in approximately 27% of total fishing grounds and were significantly captured in areas with a Chl-a of 0.15–0.28 mg m−3 and an SST of 29.5–31.0 °C, corresponding with an elevated skipjack habitat index (SHI). The habitat hotspots for the commercial skipjack were particularly produced by favorable Chl-a and SST, in association with Chl-a front and anticyclonic and cyclonic eddies, especially in October, which coincided with the highest skipjack catch per unit effort (CPUE). Skipjack distributions were mostly found within 10 km of favorable feeding habitats. They used the hotspot area as an indicator of their dynamics and movement pattern in the environment. The observed CPUEs cross-validated the predicted SHI values, suggesting that the model provided a reliable proxy for defining the potential habitats and the spatial movement of mature skipjack schools. Our findings have global significance for locating ecological hotspots, monitoring sustainable skipjack fisheries, and tracking skipjack migration, especially within the western tropical Pacific.

Elastic Interaction Between a Mesoscale Eddy‐Pair and an Axisymmetrical Eddy

AbstractWe investigate numerically the elastic interaction between an eddy‐pair and an axisymmetrical eddy in inviscid isochoric two‐dimensional (2D) flows, as well as in three‐dimensional (3D) flows under the quasi‐geostrophic (QG) approximation. The eddy‐pair is a straight moving Lamb‐Chaplygin dipole where the absolute value of either its positive or negative amount of vorticity equals the vorticity of the axisymmetrical eddy. The results for the 2D and 3D cases show that interactions with almost no vorticity exchange, or vorticity loss to the background field between ocean eddies, but causing changes in their displacement velocity, are possible. When the eddy‐pair approaches the axisymmetrical eddy, their respective potential flows interact, the eddy‐pair's trajectory acquires curvature and their vorticity poles separate. In the QG dynamics, the eddies suffer little vertical deformation, being the barotropic effects dominant. At the moment of highest interaction, the anticyclonic eddy of the pair elongates, simultaneously the cyclonic eddy of the pair evolves toward spherical geometry, and the axisymmetrical eddy acquires prolate ellipsoidal geometry in the vertically stretched QG space. Once the eddy‐pair moves away from the axisymmetrical eddy, its poles close, returning to their original geometry, and the anticyclonic and cyclonic eddies continue as an eddy‐pair with a straight trajectory but along a new direction. The interaction is sensitive to the initial conditions and, depending on the initial position of the eddy‐pair, as well as on small changes in the vorticity distribution of the axisymmetrical eddy, inelastic interactions may instead occur.

Persistence of Cold Wedges in the Somali Current System

AbstractThe Somali Current system in the western Arabian Sea reverses seasonally with the South Asian Monsoon and is associated with localized upwelling cells or cold wedges during the summer southwest monsoon. Drifter trajectories in boreal summer and fall 2014 provide rare observational evidence that the northward Somali Current and associated cold wedges can persist into the boreal fall intermonsoon period. The near‐surface circulation and sea surface temperatures further suggest that the wedge‐like surface signatures may intermittently be capped and then reappear at a later time. Our observations show that the northward Somali Current system rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy at the coast in early November 2014. This eddy may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem through the transport of water properties and locally induced upwelling.

Eddy Characteristics and Vertical Structure in the Bay of Bengal during Different Monsoon Regimes

The evolution of mesoscale eddies in the Bay of Bengal (BoB) and their characteristics (number of eddies, radius, amplitude, and eddy kinetic energy) are addressed during all strong, normal, and weak monsoon regimes from 1993 to 2019. Their impacts on the 3–7-day synoptic oscillations of atmospheric precipitation and upper ocean heat content are also assessed. In the western Bay, eddies are located in the meandering East India Coastal Current (EICC). The propagation of coastally trapped Kelvin waves into the Andaman Sea varies with monsoon intensity. Eddies with smaller radii, weaker amplitudes, increased vertical mixing, and deeper vertical extents were found during weak monsoons. Eddy kinetic energy (EKE) of EICC anticyclonic eddies is high (1200–2000 cm2 s−2) in May and November-December during weak and normal monsoon regimes, and EKE attains a maximum off the Sri Lanka coast during the strong monsoon regime. Throughout the Bay, density anomalies at ~100 m depth are influenced by subsurface temperature anomalies, while those at the surface more closely follow salinity anomalies. Wavelet coherence analysis for all three monsoon regimes reveals stronger coherence between eddy amplitude, atmospheric precipitation, and ocean heat content than the number of eddies for both anticyclonic and cyclonic eddies.

Three-dimensional structure of mesoscale eddies and their interaction with Kuroshio based on observations from a CPIES array

Mesoscale eddies are widespread in the oceans; however, tracking and observing them remains a challenge. Additionally, there are few studies of eddy–Kuroshio interactions based on large-scale in situ data. Data from a large array of 39 CPIESs deployed around the Luzon Strait in 2018–2019 (12 deployed east of Taiwan within a 3 × grid) were used to reveal the three-dimensional structure of mesoscale eddies and their interaction with Kuroshio. The results show a strengthening (weakening) of the tilt of the pycnocline, which has also caused a deeper (shallower) pycnocline in eastern Taiwan following an anticyclonic (cyclonic) interaction with Kuroshio, which resulted in a strengthening (weakening) of the Kuroshio. During the anticyclonic eddy impinging Kuroshio, the downward (upward) movement at the center of the anticyclonic eddy (Kuroshio) results in a positive (negative) anomaly structure for temperature, with corresponding positive (negative) anomalies for salinity above 600 m in the intermediate layer and negative (positive) anomalies below 600 m. In contrast, when the cyclonic eddies interact with the Kuroshio, the temperature and salinity anomalies change simultaneously, with the temperature showing an overall negative anomaly and the salinity showing a negative and positive anomaly above and below 600 m, respectively. The research in this study provides the basis for further research on energy exchange during eddy–Kuroshio interactions.

Case where a mesoscale cyclonic eddy suppresses primary production: A Stratification-Lock hypothesis

Cyclonic eddies in the ocean often increase primary production and phytoplankton biomass. The main mechanism is the rise of the nutricline in the cyclone's core, which determines the intensification of the ascending flow of deep nutrients into the photic layer. However, some cyclones suppress primary productivity, and the natural processes underlying them are little studied and are of great fundamental interest. In August 2021, a vast cyclonic zone with two mesoscale cyclones occupied the Black Sea deep-water basin. A powerful quasi-tropical atmospheric vortex amplified one of the cyclones, which increased the Ekman pumping and led to strong sea surface cooling by 4 °C. However, the inflow of thermocline water into the euphotic layer and higher nutricline gradients did not lead to elevated depth-integratedconcentration of chlorophyll a (Chl)and phytoplankton biomass. Moreover, the surface Chl and the average primary production was less than in the outside waters. The proposed Stratification-Lock hypothesis explains this phenomenon. A combination of several factors led to its appearance. The drastic water cooling, followed by calm weather and vigorous summer heating, led to strong thermal stratification, dampening turbulent mixing in the near-surface layer. At the same time, the velocity and shear of current in and below the thermocline were low in the entire cyclonic zone, which prevented substantial diapycnal mixing generated at depth. As a result, a 'stratification lock' appeared in the entire water column, suppressing nutrients' upward flux. The proposed hypothesis is consistent with the effect of thermal stratification on primary productivity revealed in the entire cyclonic zone. The higher was the thermal stratification the lower was the depth-integrated total phytoplankton biomass and primary production. Also, diatoms developed in the upper layer in waters with weaker thermal stratification. The described stage lasted two weeks and was interrupted by strong wind mixing, which destroyed the 'stratification lock' and caused an increase in Chl at the sea surface. This stage of the cyclone life cycle occurs under a strictly defined combination of hydro-meteorological conditions, suggesting a relatively rare occurrence. Nevertheless, some evidence of the similar effect of a cyclone on primary productivity can be found in other studies, indicating that this natural mechanism, consisting of a complex weather-current-eddy combination, can operate in various marine areas.

Mesoscale eddies with anomalous sea surface temperature and its relation with atmospheric convection over the North Indian Ocean

AbstractMesoscale eddies are the important phenomenon of world oceans that play a prominent role in influencing the near‐surface atmosphere and transport of heat. Ocean and atmosphere responses due to the presence of oceanic mesoscale eddies are not well studied in the Northern Indian Ocean (NIO) region. The present study analysed the ocean and atmospheric response due to the presence of mesoscale eddies using a composite analysis. Generally, it is assumed that the anti‐cyclonic eddies are associated with the positive anomalies of sea surface temperature (SST), and cyclonic eddies are accompanied by negative anomalies. However, for the two marginal seas in the NIO, that is, the Bay of Bengal (BoB) and Arabian Sea (AS), the composite picture of SST anomalies associated with mesoscale eddies showed an abnormal response of SST. This unusual behaviour of cold SST near anti‐cyclonic eddies and warm SST near cyclonic eddies in BoB and AS is analysed with the mean composites of precipitation, outgoing longwave radiation (OLR), and turbulent fluxes. The mean composites over all anti‐cyclonic eddies reveal the presence of enhanced precipitation and convection (low OLR), whereas over cyclonic eddies, suppressed convection and negative precipitation anomalies are evident. The composites of latent heat flux (LHF), precipitation and OLR near surface cold anti‐cyclonic eddies indicate the loss of heat from the ocean to the atmosphere, intense precipitation along with cloudy conditions are responsible for the cooling of the ocean. Along with this, stronger winds are also evident during cloudy conditions could play a role in anomalous cooling of SST observed near anti‐cyclonic through intense surface mixing. Similarly, vice versa is observed near the surface warm cyclonic eddies. The overlying atmosphere and abnormal response of SST near oceanic eddies are essential for understanding the significant role of oceanic mesoscale eddies in influencing the large‐scale circulation of the atmosphere and climate.

Spatial and seasonal variations in the particulate sinking flux in the Bay of Bengal

Sediment traps have been widely used to study particles sinking through the oceanic water column. As part of the SIBER-INDIA program, sediment traps were deployed at three depths − 430 m (shallow), 1020 m (middle), and 1555 m (deep) on a mooring line in the northern BoB (Bay of Bengal Sediment Trap, BoBST). We present here data on fluxes, concentrations, and isotopic composition of carbon and nitrogen, for the period 2011–2012 for all three traps, and the shallow (505 m) and middle (1035 m) traps for 2019–2020. The mooring site was largely influenced by riverine water in 2011–2012 and the increase in flux during the southwest monsoon was attributed to both autochthonous production and a high influx of river-supplied material. On the other hand, the contribution of the lithogenic matter was less in 2019–2020. The weak sea level anomaly (SLA) signal showed that the influence of cyclonic eddies at the mooring location was minimal during the deployment period in 2011, whereas a cyclonic eddy seems to have contributed to the higher flux seen in May-June 2019. To delineate the long-term changes in the component fluxes, data collected during 2011–2012 and 2019–2020 were compared with earlier available data from the Northern Bay of Bengal Traps (NBBT-N). The annual carbonate fluxes in the present study (9.3 and 7.1 g m−2 y-1 in 2011 and 2019, respectively in the ∼ 1000 m trap, and 7.7 g m−2 y-1 in the deep trap in 2011) were low compared to earlier (1987–1997) data of NBBT-N (10.9–14.7 g m−2 y-1; avg. 13.1 ± 1.4 g m−2 y-1 for ∼ 1000 m traps and 12.0–16.1 g m−2 y-1; avg. 13.3 ± 1.3 g m−2 y-1 for deep traps). On the other hand, the opal fluxes were higher in 2011 as compared to earlier (1987–1997) and 2019 data. This is also reflected in the abundance of silicifiers, which was much higher in 2011 mainly supported by significant dissolved silica in the river runoff and from the lithogenic material dissolved in the seawater. The present study highlights the seasonal and inter-annual variations in riverine suspended particulate material in the BoB and warrants the continuation of time-series studies to infer long-term changes in the component fluxes.

Energetics during eddy shedding in the Gulf of Mexico

Using the Estimating Circulation and Climate of the Ocean (ECCO) Phase II product, this study investigates the energetic characteristics during eddy shedding in the Gulf of Mexico. Based on the sea level anomaly data between 1992 and 2016, a total of 34 eddy shedding events are identified. Drawing on multiscale energy and vorticity analysis method, the eddy kinetic energy (EKE) budgets are diagnosed based on the ensemble of 34 eddy shedding events. During the stage of eddy shedding, barotropic instability (BT) dominates the energy budget. Meanwhile, energy transfers from upper layer to the deep layer by vertical pressure work (PW), which is the main source of abyssal EKE. Before eddy detachment, cyclonic eddy appears at the southeastern side of the Loop Current. Even though buoyancy forcing (BF) dominates the energy budget, BT makes considerable contribution to the generation of cyclonic eddy. Baroclinic instability (BC) shares the similar horizontal distribution with BF which accounts for 32% of the value of BC.

Ocean responses to Shaheen, the first cyclone to hit the north coast of Oman in 2021

On October 3rd, 2021, cyclone Shaheen made landfall on the northeast coast of Oman as a first-category cyclone after crossing the Sea of Oman. It is the first time in more than 130 years that a storm has made such extraordinary landfall on the northeast coast of Oman. This study intends to examine the effects of cyclone Shaheen on ocean cooling and biological response in the Sea of Oman (SO) and the the Arabian Sea (AS) using satellite remote sensing data, an Argo float, and the HYCOM model. The validation analysis of the HYCOM model and the Argo float showed strong correlations between vertical temperature profiles (R = 0.99) and vertical salinity profiles (R = 0.89). Furthermore, winds during the cyclone caused mixing and upwelling leading to Sea Surface Temperatures (SST) dropping 2.5 °C along southern Iranian coastlines, decreasing 5 °C along northern Oman's coastlines, and rising 1 °C across the Arabian Sea. As a result of the exitance of the cyclonic eddies in the Sea of Oman, a significant drop of 5 °C in SST was observed along the coastline of Oman caused by cold upwelled water from southern Iranian coastlines. Cyclone Shaheen's heavy rainfall caused the Sea Surface Salinity (SSS) in the Sea of Oman to drop from 36.7 to 36.8 psu. According to the results, Ekman pumping and upwelling processes predominated along the southern Iranian coast resulting in nutrient uplift and phytoplankton blooms (3 mg/m3). In contrast, strong stratification near the coast of Oman prevented nutrient uplift and led to low Chl-a concentrations. Due to the unique nature of cyclone Shaheen, it is essential to examine the complex dynamic and interaction between the ocean and atmosphere in the Sea of Oman and the Arabian Sea.

Comparative analysis of four types of mesoscale eddies in the North Pacific Subtropical Countercurrent region - part II seasonal variation

The North Pacific Subtropical Countercurrent area (STCC) is high in mesoscale eddy activities. According to the rotation direction of the eddy flow field and the sign of temperature anomaly within the eddy, they can be divided into four categories: cyclonic cold-core eddy (CCE), anticyclonic warm-core eddy (AWE), cyclonic warm-core eddy (CWE) and anticyclonic cold-core eddy (ACE). CCE and AWE are called normal eddies, and CWE and ACE are named abnormal eddies. Based on the OFES data and vector geometry automatic detection method, we find that at the sea surface, the maximum monthly number of the CCE, AWE, CWE, and ACE occurs in December (765.70 ± 52.05), January (688.20 ± 82.53), August (373.40 ± 43.09) and August (533.00 ± 56.92), respectively. The number of normal eddies is more in winter and spring, and less in summer and autumn, while abnormal eddies have the opposite distribution. The maximum rotation velocity of the four types of eddies appears in June (11.71 ± 0.75 cm/s), June (12.24 ± 0.86 cm/s), May (10.63 ± 0.99 cm/s) and June (9.97 ± 0.91 cm/s), which is fast in winter and spring. The moving speed of the four types of eddies is almost similar (about 10 ~ 11 cm/s). The amplitude of normal and abnormal eddies is both high in summer and autumn, and low in winter and spring, with larger amplitudes in normal than abnormal eddies. The eccentricity (defined as the eccentricity of the ellipse obtained by fitting the eddy boundary) of the four types of eddies is also close to each other, and their variation ranges from 0.7 to 0.8, with no apparent seasonal variation. The vertical penetration depth, which has no significant seasonal difference, is 675.13 ± 67.50 m in cyclonic eddies (CCE and CWE), which is deeper than that 622.32 ± 81.85 m in anticyclonic eddies (ACE and AWE). In addition, increasing the defined temperature threshold for abnormal eddies can significantly reduce their numbers but does not change their seasonal variation trend.

Mechanism of oceanic eddies in modulating the sea surface temperature response to a strong typhoon in the western North Pacific

Eddies are ubiquitous mesoscale dynamics in oceans worldwide that prominently influence the upper ocean. Specifically, the presence of preexisting eddies can modulate oceanic responses to passing typhoons, for which the underlying mechanism requires further investigation. In 2013, Typhoon Soulik traveled over 2,000 km in the western North Pacific, encountering multiple eddies, offering a suitable opportunity for investigating the impact of eddies on typhoon-induced upper ocean responses. A coupled ocean-atmosphere model was developed for this event, and the simulations were validated with multiple observations. The surface cold wake after Typhoon Soulik was successfully reproduced by the coupled model, showing a multicore structure, which was due to the preexisting mesoscale cyclonic eddies along the typhoon path. An experimental case excluding eddies was also tested in the numerical model. The comparison between the control and experimental cases revealed that sea surface cooling was enhanced by eddies via the vertical mixing and horizontal advection of eddy-related currents. In particular, vertical mixing enhanced sea surface cooling beneath the typhoon center due to the stronger vertical stratification within the preexisting cyclonic oceanic eddies. The eddy-related current with a horizontal temperature gradient contributed to advection, enhancing sea surface cooling at the cooling center and leading to a rightward-shifted pattern of enhanced sea surface cooling. This study enriches the understanding of the complex upper ocean dynamics under the combined effects of typhoons and eddies.

Spatial-temporal variations of short-lived mesoscale eddies and their environmental effects

Mesoscale eddies (MEs) affect the transport and redistribution of oceanic matter and energy. The long-lived and long-distance propagation of individual eddies has garnered extensive attention; however, short-lived MEs (&amp;lt; 7 days) have been widely overlooked. In this study, the basic features of short-lived MEs and their spatial-temporal variations in a tropical eddy-rich region were extracted and analyzed for the first time. Short-lived cyclonic and anticyclonic eddies (CEs/AEs) were found to be widespread in two eddy belts in the tropical region of the western Pacific warm pool (WPWP). The CEs and AEs were formed by the shear instability between large-scale circulations and were distributed on both sides of the North Equatorial Countercurrent, with significant differences in spatial distribution. The variations in sea surface temperature, mixed layer depth, and surface chlorophyll-a concentration in the core of the WPWP were spatially and temporally related to the development of the two eddy belts. This new insight into short-lived MEs in the tropical region contributes to our current understanding of ocean eddies. The potential impacts of short-lived MEs on climate change, global air–sea interactions, and tropical cyclone formation should receive adequate attention and further assessment in future research.

Influence of mesoscale eddies on the cross-shelf phosphate transport of the Kuroshio Current northeast of Taiwan: A modeling study

The Kuroshio Current flows northeastward along the East China Sea (ECS) shelf break, carrying a large amount of nutrients, and is thus an important source of nutrients for the ECS. The mainstream and transport of the Kuroshio Current are significantly affected by mesoscale eddies. However, the influence of mesoscale eddies on the Kuroshio nutrient input into the ECS is unknown. We add constructed cyclonic and anticyclonic eddies to a hydrodynamic model to explore the influence of mesoscale eddies on cross-shelf Kuroshio phosphate input into the ECS. This model suitably reproduces the fate of mesoscale eddies and the variation in the Kuroshio Current during eddy-current interactions. The simulation results reveal that during the strong interaction between the Kuroshio Current and mesoscale eddy east of Taiwan, the cyclonic eddy reduces the on-shelf phosphate flux, while the anticyclonic eddy increases the Kuroshio phosphate input to the ECS. When the anticyclonic eddy moves to the Okinawa Trough, it reduces the Kuroshio phosphate input into the ECS.These basic features are not sensitive to the initial latitude of the eddy center east of Taiwan. The change in cross-shelf phosphate flux is caused by the changes in cross-shelf velocity and phosphate concentration along the shelf. Momentum balance analyses suggest that the change in cross-shelf velocity is mainly caused by the change in the pressure gradient term due to eddy-induced changes in sea surface height in the horizontal direction and isotherm tilting in the vertical direction. The advection-diffusion equation analysis shows that the change in phosphate concentration along the shelf is attributed to changes in the upper horizontal advection and lower vertical advection of phosphate, which are induced by the upper phosphate change and vertical velocity change along the shelf, respectively. This study has important implications for the possible response of the ECS ecosystem to mesoscale eddies that are partly triggered by enhanced typhoons east of Taiwan under global warming.

Imprints of tropical cyclone on three-dimensional structural characteristics of mesoscale oceanic eddies

The impact of tropical cyclones (TCs) on the three-dimensional characteristics of mesoscale oceanic eddies is investigated in this study on the basis of statistical analysis by satellite-based eddy information and Argo data. By comparing the three-dimensional structure of the temperature, salinity, and geostrophic velocity in the upper ocean above 1,000 m depth, it was found that there is a heat pump effect in the changes of eddy structure similar to that in tropical cyclones. Under the forcing of TC, the abnormal signals in the strong cold core (warm core) structure originally existing in the upper layer of the cyclonic eddy (anticyclonic eddy) are transmitted to the middle and lower layers of the eddy and form retention, making the eddy structure not recover to the original state in a short period of time. To a certain extent, this shows that the influence of TC on the eddy structure is not limited to the ocean surface. At the same time, the change of barrier layer in the eddy is explored, and it was found that the barrier layer thickness in both cyclonic eddy and anticyclonic eddy has increased, which also confirms the previous research.