Cyclonic Eddies Research Articles

Response of a Mesoscale Dipole Eddy to the Passage of a Tropical Cyclone: A Case Study Using Satellite Observations and Numerical Modeling

Mesoscale eddies occurring in the world’s oceans typically exist in pairs known as mesoscale dipole eddies or simply dipole eddies. Tropical cyclones (hereafter TCs) that move over the world’s oceans often encounter and interact with these dipole eddies. Through this interaction, TCs induce significant perturbations in the mesoscale eddies. However, the specific influences that the passage of a TC on a dipole eddy have not been addressed. In this paper, a case study of the dipole eddy’s response to the passage of a TC is conducted by using satellite observations and numerical simulation. The passage of a TC induces a long-duration response in the dipole eddy. First, the cyclonic ocean eddy component (COE) of the dipole is amplified, and the anticyclonic ocean eddy component (AOE) is weakened or even destroyed during the interaction. The amplification of the COE and weakening of the AOE primarily manifests as a change in their amplitudes and radii and as the adjustment of their vertical structure. The dipole eddy’s response to the interaction with a TC manifests as an upwelling anomaly and the injection of positive relative vorticity. Following the passage of the TC, the COE gradually stabilizes, and AOE slowly recovers after the disturbance energy from the interaction dissipates, which facilitates the reestablishment of the dipole eddy. The dipole reaches an equilibrium state through a quasi-geostrophic adjustment process. As a consequence, the overall effect of the interaction of the dipole with the TC leads to an asymmetric signature on the dipole eddy. The eddy–eddy interaction in a dipole may allow it to stabilize in a shorter time relative to that of a solitary eddy.

Characterizing Mesoscale Eddies of Eastern Upwelling Origins in the Atlantic Ocean and Their Role in Offshore Transport

Motivated by the recurrent formation of eddies in the eastern upwelling areas, we examine cross-basin connectivity that is promoted by coherent, long-lived and long-propagating mesoscale eddies in the Atlantic Ocean. By applying the TOEddies detection and tracking algorithm to daily satellite observations (AVISO/DUACS) of Absolute Dynamic Topography (ADT), we characterize mesoscale eddy activity and variability in the North and South Atlantic. This method provides a robust eddy-network reconstruction, enabling the tracking of eddies formed in the Atlantic eastern upwelling systems together with any merging and splitting events they undergo during their lifetime as long as they remain detectable in the altimetry field. We show that during the years of observations, mesoscale eddies are long-lived coherent structures that can ensure oceanic connectivity between the eastern and the western boundaries, as a result of complex inter-eddy interactions. Moreover, alignment of South Atlantic eddies of eastern boundary origins with available Argo floats achieves a mean cross-basin connectivity signal from both anticyclonic and cyc5lonic eddies which is particularly evident at depth, along thermocline isopycnal layers of γn = 26 - 27 kg m–3. We explore two individual cyclonic eddy trajectories from in-situ measurements gathered by different Argo profiling floats trapped inside the eddy cores. Our results support the hypothesis that mesoscale eddies sustain and transport water masses while subducting during their westward propagation.

Anomalously low diatom fluxes during 2009–2010 at Alfonso Basin, Gulf of California

A long-term information baseline is necessary to identify the seasonal and interannual variability of diatom production driven by environmental dynamics. This motivated our compiling hydrographic data and concurrent diatom fluxes at the Alfonso Basin sub-zone. The diatom data time series spanned from February 2008 to September 2012. These data were analyzed along with primary productivity and chlorophyll-a concentration estimates derived from satellite imagery from 2002 to 2012. Planktonic diatom flux (range: 1.62 × 106–3.55 × 107 valves·m-2d-1) was significantly correlated (r2 = 0.37) with chlorophyll-a concentration. The bimodal production season (December–July) exhibited a winter mixing period influenced by water from the Gulf of California. The diatom flux was dominated by fast-growing surface species such as Chaetoceros spp. and Thalassiosira spp. The second peak in May occurred under the influence of southwest winds concurrent with the entry of tropical surface water and a cyclonic eddy that injected nutrients into the base of the euphotic zone, eliciting a bloom of Pseudo-nitzschia spp. The summer stratification resulted in the deepening of Subtropical Subsurface Water, and the diatom flux included large species of oceanic affinity such as Chaetoceros (Phaeoceros), Rhizosolenia acuminata, and Rh. imbricata. The environment at Alfonso Basin promotes diatom sinking; thus, material from sediment traps accurately reflects the primary production cycle associated with hydrographic processes. Interannual variability was characterized by anomalously low diatom fluxes between August 2009 and July 2010. A significant negative association between diatom flux anomalies and El Niño Modoki Index values suggests that oceanographic changes are related to this event. Additional reductions in diatom fluxes were coeval with SST positive anomalies recorded from July to February 2008–2009 and 2011–2012, which showed a significant relationship with the tropical Pacific Meridional Mode. The temporal trends like the additive signals of interannual (CP-El Niño) and decadal (Pacific Meridional Mode) time scales indicate that climatic disturbances affected the local ecosystem in the basin, reducing the vertical flux of diatoms initially dominated by Pseudo-nitzschia spp., and boosting the presence of oceanic warm-water species and small highly silicified species such as Thalassionema nitzschioides var. parva.

A Case Study on the Dynamics of Phytoplankton Blooms Caused by Tropical Cyclones in the Southeastern Arabian Sea

A phytoplankton bloom was observed in the southeastern Arabian Sea after the passage of the anti-S-type track Tropical cyclone ARB 06 in 1998, which lasted for 22 days. The dynamic mechanism of the phytoplankton bloom was investigated with high-resolution satellite remote sensing data, reanalysis data and buoy data. The results show that the double thermocline structure hindered the uplift of deep nutrients to the surface before the tropical cyclone arrival. During and after the passage of the cyclone, the shallower thermocline disappeared, the deeper thermocline changed from 45.7 m to 76.5 m, the mixed layer deepened, and the upper oceanic stratification weakened. Additionally, a weak ocean eddy pair was enhanced gradually after the passage of cyclone. The relative vorticity of the ocean eddy pair was calculated. In the oscillation with a period of two days, the vorticity of the cyclonic eddy was stronger than that of the anticyclonic eddy, which enhanced a strong upwelling at a depth of more than 55 m for more than 10 days. The vertical current shear generated by the oscillation not only enhanced the vertical mixing, but also plays an important role in the distribution of phytoplankton blooms. The phytoplankton bloom was triggered by the nutrient below the thermocline into the sea surface, where a strong upwelling and weakened oceanic stratification occurred after the passage of cyclone. This study offers new insights on the mechanism of phytoplankton bloom induced by tropical cyclones and will be helpful for evaluating tropical cyclone-induced biological responses in future.

Effects of mesoscale eddies on intraseasonal variability of intermediate water east of Taiwan

The intraseasonal variability of the intermediate water (IW) and its characteristics east of Taiwan are studied utilizing 17 months of long-term, continuous and synchronous measurements of temperature, salinity and current from mooring sites deployed at 122° E/23° N from January 2016 to May 2017. Direct measurements revealed water masses east of Taiwan alternately show complete South China Sea Intermediate Water or North Pacific Intermediate Water (NPIW) characteristics, mostly in a mixed state, with NPIW dominating 70% of the time. For the first time, it is demonstrated that the variation of IW with periods of ~ 80 days is mainly related to mesoscale eddies. Anticyclonic (cyclonic) eddies corresponding to an increase (decrease) in temperature and salinity in the intermediate layer. Further mechanism analysis indicates the vertical motion of the water mass inside the eddies is one of the reasons for the thermohaline change in the intermedaiter layer. In addition, the anticyclonic eddies may increase the salinity concentration gradient across the Luzon Strait, and the enhanced advection is favorable to the outflow of water masses in the South China Sea. When the cyclonic eddies acts on the eastern part of Taiwan, the influence of the northward advection is weakened and the southward flow on its left side is more favorable to the transport of NPIW.

Optimal Precursor Triggering Kuroshio Large Meander Decay Obtained in a Regional Ocean Model

AbstractThis study investigates the optimal precursor (OPR) triggering Kuroshio large meander (LM) decay using a conditional nonlinear optimal perturbation (CNOP) approach and a regional ocean modeling system (ROMS). The results show that the large amplitudes of the OPR are mainly concentrated at 1,000–2,500 m in the LM upstream region (west of 137°E), and their propagation downstream along the Kuroshio axis contributes to triggering the transition from the LM path to the nonlarge meander (NLM) path. Using the growth of the OPR, it is found that baroclinic instability contributes to the fast vertical development of the perturbations in the early stage of the LM decay, while barotropic processes play a leading role during the cyclonic ring shedding of the LM decay. In particular, the LM decay process is evaluated by the depth‐integrated vorticity equation, implying that the advection term dominates over the beta effect in the LM decay. Furthermore, the vorticity balance associated with the cyclonic eddy indicates that the advection and divergence terms represent a main balance in the spin‐down process of the cyclonic eddy. Under the effects of both, high potential vorticity water flows out from the LM meander tip and then the amplitude of the LM experiences a remarkable decrease. With the detachment of a cyclonic ring, a transition from the LM path to the NLM path occurs. This study further deepens our understanding of the LM decay and provides us some useful hints about designing the observation system of the LM.

Modeling Mesoscale Eddies Generated Over the Continental Slope, East Antarctica

Mesoscale eddies are abundant over the Antarctic continental slope, with the potential to regulate the water masses transport, mixing, and energy transfer. Over the relatively cold and fresh shelf regions around the Antarctic margins, in the absence of dense overflows, the baroclinic instability of the Antarctic Slope Current is still favorable in the generation of mesoscale eddies. However, mesoscale eddies are barely observed over the fresh shelf regions due to the sparsity of in situ hydrographic observations. Based on an idealized eddy-resolving coupled ocean-ice shelf model, this study investigates the characteristics of mesoscale eddies and corresponding influences on the local hydrographic properties over the continental slope, East Antarctica. With the aid of an automated eddy detection algorithm, bowl-shaped eddies are identified from the simulated velocity vector geometry. The Cyclonic Eddies (CE) has a barotropic vertical structure extending to more than 2,500 m depth, while the vertical shear of the Anticyclonic Eddies (AE) velocity is strong at the upper 200 m layer. Mesoscale eddies can trap the cold and fresh water in the southern flank of the Antarctic slope front and flow offshore to the relatively warm and saline region. Therefore, the influences of eddies on the hydrographic properties are not only governed by the eddy polarities but also the eddy-induced heat and salt transport.

Cross‐Slope Buoyancy Fluxes Cause Intense Asymmetric Generation of Submesoscale Eddies on the Periphery of the Black Sea Mesoscale Anticyclones

AbstractSubmesoscale dynamics significantly affect the turbulence regime and is characterized by pronounced time variability, the reasons of which are still poorly known. In our study, we use high‐resolution (1 km) numerical model NEMO and satellite data in the Black Sea to demonstrate that one of the main processes impacting on the intensity of submesoscale dynamics in the marginal seas is the cross‐slope advection by mesoscale anticyclones. Such large eddies entrain the brackish shelf waters into their orbital motion and cause the strong rise of vertical and horizontal gradients of buoyancy. The emergence of baroclinic instability causes the generation of periodic submesoscale structures, among which cyclonic eddies have the largest vorticity. Our analysis shows that the presented mechanism is one of the main reasons for the intraannual variability of submesoscale dynamics over the continental slope of the Black Sea. This process also causes a strong asymmetry in the vertical structure of the mesoscale anticyclones. The generation of the submesoscale eddies and related vertical and horizontal exchange are significantly more intense on that side of the anticyclone, where current is directed offshore. Intensification of submesoscale motions in the zone of the brackish water entrainment is one of the probable reasons for the rapid and effective mixing of the shelf and open sea waters, often observed in satellite data.

Using Argo Floats to Characterize Altimetry Products: A Study of Eddy-Induced Subsurface Oxygen Anomalies in the Black Sea

The identification of mesoscale eddies from remote sensing altimetry is often used as a first step for downstream analyses of surface or subsurface auxiliary data sets, in a so-called composite analysis framework. This framework aims at characterizing the mean perturbations induced by eddies on oceanic variables, by merging the local anomalies of multiple data instances according to their relative position to eddies. Here, we evaluate different altimetry data sets derived for the Black Sea and compare their adequacy to characterize subsurface oxygen and salinity signatures induced by cyclonic and anticyclonic eddies. In particular, we propose that the theoretical consistency and estimated error of the reconstructed mean anomaly may serve to qualify the accuracy of gridded altimetry products and that BGC-Argo data provide a strong asset in that regard. The most recent of these data sets, prepared with a coastal concern in the frame of the ESA EO4SIBS project, provides statistics of eddy properties that, in comparison with earlier products, are closer to model simulations, in particular for coastal anticyclones. More importantly, the subsurface signature of eddies reconstructed from BGC-Argo floats data is more consistent when the EO4SIBS data set is used to relocate the profiles into an eddy-centric coordinate system. Besides, we reveal intense subsurface oxygen anomalies which stress the importance of mesoscale contribution to Black Sea oxygen dynamics and support the hypothesis that this contribution extends beyond transport and involves net biogeochemical processes.

Distribution, abundance, and diversity of euphausiids and their relationships with hydrodynamic processes in Campeche Canyon, Gulf of Mexico

We analyzed the abundance, diversity, and distribution of euphausiids and their relationship with hydrodynamic processes from February 2011 to October 2012 in the Campeche Canyon in the southern Gulf of Mexico. Sampling was conducted at 55 oceanographic stations and zooplankton were captured using horizontal trawls. Environmentalvariables (temperature, salinity, and oxygen) and phytoplankton fluorescence were recorded at different depths. Twenty-four species of euphausiids representing the genera Euphausia, Nematobrachion, Nematoscelis, Stylocheiron, and Thysanopoda were identified. Stylocheiron carinatum was abundant in February and October, with the furcilia stage always the dominant one. Differences were observed among the 4 zones identified based on the bathymetric criteria; zone III (axis and head of the canyon) presented the highest diversity of euphausiids between 100 and 200 mwith 2.14 bits Ind-1 in February and 1.99 bits Ind-1 in October. The spatial-temporal distribution of euphausiids was heterogeneous in water bodies with temperatures between 15 and 22 °C and low dissolved oxygen concentrations, which were distributed outside the maximum concentrations of chlorophyll-a (2,127 mg m-3) and had an affinity for the frontal regions between cyclonic and anticyclonic eddies.

Counter-rotating eddy pair in the Luzon Strait

Abstract. Based on satellite remote-sensing observation data and Hybrid Coordinate Ocean Model (HYCOM) re-analysis data, we studied the counter-rotating eddy pair in the Luzon Strait (LS). Statistical analysis reveals that when an anti-cyclonic mesoscale eddy (AE) (cyclonic mesoscale eddy [CE]) in the Northwest Pacific (NWP) gradually approaches the east side of the LS, a CE (an AE) gradually forms on the west side of the LS, and it is defined as the AE (CE) mode of the counter-rotating eddy pair in the LS. The counter-rotating eddy pair exhibits obvious seasonal variation: the AE mode mainly occurs in the summer half of the year, while the CE mode mainly occurs in the winter half of the year. The mean durations of the AE and CE modes are both approximately 70 d. Based on the vorticity budget equation and energy analysis, the dynamic mechanism of counter-rotating eddy-pair occurrence is determined to be as follows: the AE (CE) on the east side of the LS causes a positive (negative) vorticity anomaly through horizontal velocity shear on the west side of the LS, and the positive (negative) vorticity anomaly is transported westward by the zonal advection of the vorticity, finally leading to the formation of the CE (AE) on the west side of the LS.

Influence of cyclonic and anticyclonic eddies on plankton in the southeastern Mediterranean Sea during late summertime

Abstract. Planktonic food webs were studied contemporaneously in a mesoscale cyclonic (upwelling, ∼ 13 months old) and an anticyclonic (downwelling, ∼ 2 months old) eddy as well as in an uninfluenced background situation in the oligotrophic southeastern Mediterranean Sea (SEMS) during late summer 2018. We show that integrated nutrient concentrations were higher in the cyclone compared to the anticyclone or the background stations by 2–13-fold. Concurrently, Synechococcus and Prochlorococcus were the dominant autotrophs abundance-wise in the oligotrophic anticyclone (∼ 300 × 1010 cells m−2). In the cyclone, functional groups such as dinoflagellates, Prymnesiophyceae and Ochrophyta contributed substantially to the total phytoplankton abundance (∼ 14 × 1010 cells m−2), which was ∼ 65 % lower at the anticyclone and background stations (∼ 5 × 1010 cells m−2). Primary production was highest in the cyclonic eddy (191 mgCm-2d-1) and 2–5-fold lower outside the eddy area. Heterotrophic prokaryotic cell-specific activity was highest in the cyclone (∼ 10 fgCcell-1d-1), while the least productive cells were found in the anticyclone (4 fgCcell-1d-1). Total zooplankton biomass in the upper 300 m was 10-fold higher in the cyclone compared with the anticyclone or background stations (1337 vs. 112–133 mg C m−2, respectively). Copepod diversity was much higher in the cyclone (44 species), compared to the anticyclone (6 small-size species). Our results highlight that cyclonic and anticyclonic eddies show significantly different community structure and food-web dynamics in oligotrophic environments, with cyclones representing productive oases in the marine desert of the SEMS.

Variation in biogenic calcite production by coccolithophores across mesoscale eddies in the Bay of Bengal

Coccolithophore calcite production (CP) was investigated for the first time in the Bay of Bengal. Against expectation, calcite production was not fueled by the nutrient-enriched cold eddy because of the reduced light penetration. CP rate was observed to be higher at the anticyclonic eddy possibly benefited from rare species production. The adjoining river-induced shallow mixed-layer depth and eddy activity co-influenced CP rate. On average, the integrated CP rates were 0.04, 0.15, and 0.07 mmol C m−2 h−1 for the cyclonic eddy, anticyclonic eddy, and outer area, respectively. In the upper photic waters, CP rates showed a strong correlation with primary productivity (PP). However, a decoupling of CP and PP was observed in the lower photic zone due to differential light and nitrogen preferences in calcification and carbon fixation. Our findings could help to understand the biogenic response of eddy and inorganic-organic carbon association in the pelagic biogeochemical cycles.

Impacts of mesoscale eddies on the nitrate distribution in the deep-water region of the Gulf of Mexico

Mesoscale eddies play an important role in modulating the vertical distributions of nutrients and primary productivity in the ocean. In this study, we evaluated the role that eddies play in determining the vertical distributions of Nitrate + Nitrite (N + N), dissolved oxygen (DO), conservative temperature (CT), and absolute salinity (S A ) in the deep-water region of the Gulf of Mexico (GoM). We compared water-column profiles up to 1000 m depth and calculated the N + N stock in the euphotic layers within cyclonic eddies (CEs), anticyclonic Loop Current eddies (LCEs), and waters free of eddy influence (NOE). Stations were sampled based on a fixed grid during two oceanographic campaigns carried out under winter mixing and thermal stratification conditions and were classified as CE, LCE, or NOE stations based on a cluster analysis using dynamic height (DH) and the depth of the 25.5 kg m −3 isopycnal. Average profiles of the stations closest to the eddy cores (those with the highest or lowest dynamic heights and isopycnal depths of 25.5 kg m −3 ) were compared with those of NOE stations. Differences between structures were observed in multiple variables up to 1000 m, although the most notable differences occurred in the euphotic and upper mesopelagic zones (< 250 m). The average concentration of N + N between 0 and 50 m was ~0.03 μmol kg −1 , which was close to the detection limit, regardless of the presence or absence of eddies. The onset of the nitracline, defined as the depth at which the N + N concentration was 0.5 μmol kg −1 , was observed at ~56 m, ~ 82 m, and between ~100–120 m in CE, NOE, and LCE waters, respectively, which resulted in contrasting N + N stocks in the intermediate euphotic layer (50–100 m) among CE (158–208 mmol m −2 ), LCE (6–9 mmol m −2 ), and NOE (~ 28 mmol m −2 ) stations. As a result, fluorescence in the deep chlorophyll maximum was markedly higher in CE waters compared to that in NOE and LCE waters. In the summer campaign, a subsurface DO maximum between 30 and 50 m, which is characteristic of oligotrophic regions, was observed as a product of photosynthesis most likely associated with a positive anomaly of preformed N + N and positioned around the 24.3 kg m −3 isopycnal. In subsurface CE, LCE, and NOE waters, negative anomalies of preformed N + N were observed at different depths that remained between the same isopycnals (24.8–26.2 kg m −3 ), reaching up to −3.4 μmol kg −1 in CEs. As has been suggested for other oligotrophic regions, this anomaly could be due to the uptake of N + N by migrating phytoplankton without DO production, the use of N + N by heterotrophic bacteria that are limited by dissolved organic nitrogen, the remineralization of transparent exopolymeric particles (TEPs) with high C:N ratios, or to a combination of these processes, which remain to be studied within the GoM. • N + N stocks in the Gulf of Mexico are strongly modulated by mesoscale eddies. • Shallow/deep nitraclines within cyclonic (56 m)/Loop Current (LC;100–120 m) eddies. • N + N stocks (mmol m −2 ) in cyclonic eddies (158–208) greater than in LC (6–9) eddies. • Deep chlorophyll maximum fluorescence markedly high in cyclonic eddies. • Subsurface preformed N + N negative anomaly at 24.8–26.2 kg m −3 isopycnals in summer.

Tracking the Space‐Time Evolution of Ocean Acidification Extremes in the California Current System and Northeast Pacific

AbstractOcean acidification is punctuated by episodic extremes of low pH and saturation state with regard to aragonite (ΩA). Here, we use a hindcast simulation from 1984 to 2019 with a high‐resolution regional ocean model (ROMS‐BEC) to identify and track ocean acidification extremes (OAX) in the northeast Pacific and the California current system (CCS). In the first step, we identify all grid‐cells whose pH and ΩA are simultaneously below their first percentile over the analysis period (1984–2019). In the second step, we aggregate all neighboring cells with extreme conditions into three‐dimensional time evolving events, permitting us to track them in a Lagrangian manner over their lifetime. We detect more than 22 thousand events that occur at least once in the upper 100 m during their lifetime, with broad distributions in terms of size, duration, volume, and intensity, and with 26% of them harboring corrosive conditions (ΩA &lt; 1). By clustering the OAXs, we find three types of extremes in the CCS. Near the coast, intense, shallow, and short‐lasting OAXs dominate, caused by strong upwelling. A second type consists of large and long‐lasting OAX events that are associated with westward propagating cyclonic eddies. They account for only 3% of all extremes, but are the most severe events. The third type is small extremes at depth arising from pycnocline heave. OAXs potentially have deleterious effects on marine life. Marine calcifiers, such as pteropods, might be especially impacted by the long‐lasting events with corrosive conditions.

Temporal variability of plutonium in surface waters of the Sea of Japan

Long-term temporal variations of plutonium in Sea of Japan (SOJ) surface waters have been examined with the aim to better understand its behavior during several decades. The first observation is that 239,240Pu activity concentrations in surface waters of the SOJ during 1977–2019 were 6.5 ± 4.7 mBq m−3 in average, and 5.1 mBq m−3 as the median, whereas 137Cs and 90Sr activity concentrations decreased with time, except of the perturbation due to the 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Another observation is that sporadic high 239,240Pu activity concentrations occurred in the east Japan Basin, ranging from 1 to 39 mBq m−3. The spatial distribution of 239,240Pu activity concentrations in surface waters revealed that high 239,240Pu levels (>20 mBq m−3) occurred in 1994 in the northern SOJ, which was considered to be due to winter convection. To elucidate factors controlling the temporal variability of surface 239,240Pu levels in the SOJ, a relationship between surface 239,240Pu activity concentrations and vertical diffusion coefficients was examined. The results revealed that this relationship could be classified into two groups: one group did not show a change with increasing diffusion coefficient, while the other group showed a positive correlation. The vertical 239,240Pu distribution in SOJ waters suggests that the high surface 239,240Pu levels occurred due to the upwelling of cyclonic eddy. The rapid recycling of deeper plutonium occurred in the SOJ due to deep winter convection and upwelling associated with cyclonic eddy. The plutonium levels in the SOJ have been found to be sensitive to climate changes. Warming of the SOJ may cause a reduction of winter convection and eddy activity as a result of increasing sea surface temperature. This leads to a decline of recirculation of plutonium and other bioavailable elements from Japan Sea Proper Water (JSPW) to surface water layers. Plutonium would be, therefore, an important indicator of biogeochemical processes in the marine environment, helping to assess climate change impacts on marine ecological systems.

Spatial Patterns of Larval and Juvenile Fish Assemblages in an Eddy Area in the Western South China Sea

The objective of this study was to investigate influence of meso-scale eddies in the western South China Sea on species composition and spatial patterns of fish larvae and juveniles based on the data collected during summer and autumn in 2012. Nintyeight species belonging to 38 families were collected, including 78 species in summer and 41 species in autumn. The dominant species in summer were Vinciguerria sp. (39.70%), Stephanolepis cirrhifer (11.21%), Caranx sp. (5.45%) and Benthosema pterotum (1.97%). The dominant species in autumn were Gobiidae sp. (15.32%), Ceratoscopelus townsendi (7.26%), Diaphus sp. (6.45%), Astronesthes sp. (5.65%) and Callionymidae sp. (4.03%). Abundance of fish larvae and juveniles in summer was higher than that found in autumn. Similarity cluster analysis results indicated that larval fish assemblages in summer can be divided into two communities: (1) larval fish in the cyclonic eddy; (2) larval fish in the anticyclonic eddies. While in autumn, the larval fish assemblages can be divided into three communities: (1) larval fish without influence of the cyclonic eddy; (2) larval fish influenced slightly by the cyclonic eddy; (3) larval fish gathered near the center of the cyclonic eddy. The variations of abundance and assemblage of larval fish might be attributed to sea surface temperature (SST), sea surface salinity (SSS) and sea level anomaly (SLA).

Surface Seawater pCO2 Variation after a Typhoon Passage in the Kuroshio off Eastern Taiwan

In this study, two cruises were conducted across the mainstream of the Kuroshio off eastern Taiwan before and after the passage of Typhoon Saola in summer 2012. The continuous underway pCO2 (i.e., partial pressure of CO2) measurements revealed that surface seawater pCO2 (SS pCO2) displayed spatial variations in response to typhoon passage. The simulated results showed that the mixed-layer deepening after typhoon passage had a minor effect on SS pCO2 variation because pCO2 decrease driven by temperature dropdown and enhanced biological production fueled by nutrients input was largely compensated by pCO2 increase caused by salinity increase and dissolved inorganic carbon input from the subsurface layer. By contrast, the advection pattern showed significant change before and after the typhoon, which could play a major role in controlling the variation of SS pCO2. In the exit area of the cyclonic eddy, SS pCO2 decreased, while in the area of its arrival, SS pCO2 increased. Besides, the discharge of freshwater and the intrusion of the South China Sea subsurface could result in SS pCO2 increase in the nearshore area. The present study highlights that more advection changes need to be considered to better understand the impact of the typhoon on SS pCO2, especially in the strong current area, such as the Kuroshio.

On the Evolution of the Gulf of Mexico Loop Current Through Its Penetrative, Ring Shedding and Retracted States

How the Gulf of Mexico Loop Current (LC) transitions from penetrative to ring shedding to retracted configurations remains an active research topic. Satellite altimetry suggests a set of states, some more persistent than others, identified as Campeche Bank Anchoring, West Florida Shelf Anchoring, Extended, and LC Ring, plus transitional phases. Histograms show that the most frequent states are Campeche Bank Anchoring and West Florida Shelf Anchoring. Two canonical evolutions are defined. The first is a transition from Campeche Bank Anchoring to West Florida Shelf Anchoring. The second is a transition from Campeche Bank Anchoring to Extended to LC Ring. Common to both are cyclonic eddies. The first canonical transition requires cyclones formed both east and west of the LC, acting as pincers far enough south to dislodge the LC from the Campeche Bank. The second has a single cyclone, formed on either side of the LC, that cuts through either from the east, thereby not dislodging the LC from the Campeche Bank, or from the west, but too far north to dislodge the LC. Notable is the importance of the Campeche Bank and Florida escarpments. Cyclones may generate either by instability or downslope perturbation, but whether these lead to protracted penetration, or just jostling from one anchoring state to another, depends on where they are generated and how they combine to shed a LC Ring. Thus, these findings suggest that furthering understanding and prediction requires observations and analyses over the two escarpment regions and the LC domain south of about 25°N.

Energetics Analysis of the Eddy–Kuroshio Interaction East of Taiwan

Abstract The three-dimensional energetics evolution during eddy–Kuroshio interactions east of Taiwan is systematically investigated in a time-dependent theoretical framework using outputs from an eddy-resolving ocean general circulation model. Composite analyses are conducted based on 17 anticyclonic eddies (AEs) and 19 cyclonic eddies (CEs). These westward propagating mesoscale eddies impinge on the Kuroshio at ∼22°N, ∼124.5°E and interact with the Kuroshio with a mean duration of ∼70 days. During the interaction, all the eddy energy reservoirs and eddy–mean flow energy conversions exhibit complex spatial–temporal variations. In particular, during the strong interaction period (days 18–54), both AEs and CEs are deformed into an elliptic shape, with the major axis in the northeast–southwest direction due to the squeeze of surrounding eddies, and obtain kinetic energy from the mean flow. Overall, the eddies are weakened gradually after encountering the Kuroshio, with the energy of CEs decreased more rapidly than that of AEs. The eddies decay through two pathways: transferring ∼8% of eddy available potential energy (EPE) to the mean flow, and converting ∼64% of EPE to eddy kinetic energy (EKE) via the baroclinic instability with the majority of the EKE finally dissipated. The results suggest that although the time-dependent energy conversion terms vanish upon time averaging, they play important but opposite roles in the evolution of AEs and CEs. The analysis in this work is on the synoptic and intraseasonal time scales; hence, it provides a basis for understanding the long-term variations of the eddy–Kuroshio interaction and associated climate change.