Cyclonic Eddies Research Articles

The effect of normal and abnormal eddies on the mixed layer depth in the global ocean

Mesoscale eddies are broadly distributed over the global ocean and play a significant role in modulating the spatiotemporal evolution of mixed layer depth (MLD). The presence of abnormal eddies in the ocean has been shown; however, the precise quantification of the effect of eddies on MLD, given the case of abnormal eddies, has not been carried out thus far. Differently from the previous approach to identify abnormal eddies through sea surface temperature, we therefore, proposed a method to identify abnormal eddies, using potential density based on Argo data (Array for Real-time Geostrophic Oceanography) for 15 years from 2003 to 2017. Results showed that abnormal anticyclonic eddies (AAE) and abnormal cyclonic eddies (ACE) accounted for 21.67% and 20.17% of total matching anticyclonic eddies (TAE) and the total matching cyclonic eddies (TCE), respectively, in the global ocean. The proportions of abnormal eddies were relatively higher in tropical regions but lower in regions with the boundary current and strong eddy kinetic energy. The MLD changes caused by normal and abnormal eddies were estimated combining satellite altimetry data. The normal eddies were the total matching eddies with the removal of the abnormal eddies, separately called normal anticyclonic eddies (NAE) and normal cyclonic eddies (NCE). The overall influence of NAE (NCE) was more significant on MLD deepening (uplifting) than that of TAE (TCE). Globally, NAE (NCE) changed MLD deepening (uplifting) from ~66 m (~54 m) to ~67 m (~53 m) and exhibited a more pronounced change in the Indian Ocean sector of the Southern Ocean region, from ~111 m (~94 m) to ~115 m (~92 m) in the winter. AAE (ACE), exerted a relatively weak but opposite effect on MLD deepening (uplifting). In other words, the global average MLD caused by them shoaled (deepened) from ~66 m (~54 m) to ~59 m (~59 m), and the North Pacific Ocean shoaled (deepened) from ~61 m (~47 m) to ~49 m (~57 m) in winter. Given the above results, abnormal eddies should be accounted for when the impact of ocean eddies is evaluated on the global climate system.

Eddies and fronts influence pelagic communities across the eastern Pacific ocean

The behaviors of micronekton and zooplankton across the eastern Pacific Ocean are poorly known, as pelagic fauna in this region are rarely sampled. One particularly understudied aspect of these community dynamics is their distribution and responses to mesoscale oceanographic features. To explore this, we conducted active acoustics and oceanographic surveys at two locations within the eastern Pacific Ocean with Saildrone uncrewed surface vehicles. Using 38 and 200 kHz data collected by these platforms, we evaluated the vertical structure and migration patterns of micronekton and large zooplankton in the upper 1000 m of the water column within survey areas at the western and eastern ends of this region. We found stronger and shallower acoustic backscatter and stronger DVM intensity within the eastern area, likely driven by its significantly lower midwater oxygen and higher surface productivity compared to the western area. The passage of cyclonic and anticyclonic eddies through the sampling areas was found to correlate with changes in the vertical structure of micronekton in both survey areas by shoaling and deepening acoustic scattering layers, respectively. Further, the strength of mesopelagic scattering layers was greatly increased along the northern frontal boundary of a strong, westward near-surface current in our eastern study area. Our results highlight the considerable natural variability of pelagic community dynamics on daily to weekly scales in the eastern Pacific Ocean in addition to broader regional patterns. Such variation is inherent to open ocean conditions and plays a large role in aggregation and predator–prey dynamics. Describing the interactions between mesoscale features and pelagic fauna may help us understand how these communities will respond to a changing ocean environment— induced by anthropogenic activities and broader climate-related shifts. This is the first study to use Saildrones to examine micronekton in the open ocean and provides some of the only remote sampling of pelagic prey communities in the eastern Pacific Ocean.

Cold ridge formation mechanisms on the Agulhas Bank (South Africa) as revealed by satellite-tracked drifters

The formation of the cold ridge, a seasonal mid-shelf upwelling feature on the Agulhas Bank, is investigated from a combination of satellite-tracked surface drifters, in situ current and temperature data, as well as satellite sea surface temperature (SST) and chlorophyll-a (Chl-a). The observational data showed coastal upwelling along the Tsitsikamma coast to be induced by the easterly wind in austral summer, with a coexistent westward flowing coastal current. Under this wind and current regime, surface drifters moved west and onto the mid-shelf with the cold water, thereafter following the southward-curved 100 m isobath. Satellite observations in March–April 2008 showed increased south-westward advection of wind-driven upwelled water during an Agulhas Current intrusion that resulted in the formation of a classically shaped cold ridge. Also of interest was the passing of a Natal Pulse along the south-eastern shelf edge of the Agulhas Bank in July 2007. This generated a large cyclonic eddy in the Agulhas Bight with an attendant warm water plume that swept over the adjacent shelf. Two idle drifters positioned on the mid-shelf at the time, immediately west of the Natal Pulse, were rapidly advected offshore along the curved 100 m isobath, confirming the long-thought association of the cold ridge with an Agulhas Bight cyclone. A moored current meter adjacent to the Tsitsikamma coast (1.5 km offshore) showed the effect of the Natal Pulse on the nearshore. Drifter trajectories show accelerated south-westward flow on the offshore side of the cold ridge and recirculation towards the east on its shoreward side. These findings strongly support the Westward Transport Hypothesis where squid paralarvae are advected westward from the inshore breeding grounds, to the food maxima on the interior of the central Agulhas Bank, to improve their chances of survival and can therefore have important implications for the management of the South African chokka squid fishery.

Phytoplankton Blooms Triggered by Anticyclonic Eddy and Cyclonic Eddy during Tropical Cyclone Nada

Tropical cyclone (TC) Nada occurred in the southwestern Bay of Bengal (BOB) in the winter of 2016. With high photosynthetically available radiation (PAR) and minimal precipitation, phytoplankton blooms occurred from offshore to nearshore respectively. In this study, there was an pre-existed ocean eddy pair on the left and right sides of Nada’s path before its passage, we studied the different mechanisms of phytoplankton blooms caused by TC Nada and the ocean eddy pair with satellite observation, multisource reanalysis products, and Argo float data. An anticyclonic eddy near Sri Lanka was weakened during Nada and horizontal transport of chlorophyll-a (Chl-a) balanced the inhibition effect of anticyclone on Chl-a. Near the anticyclonic eddy’s periphery, an upwelling above the thermocline was enhanced by Nada which promoted the uplift of nutrient-rich deep-water. Meanwhile, the large cyclonic eddy in the northern Bay of Bengal was strengthened by Nada, with the vorticity increasing from 0.19 s^–1 to 0.28 s^–1. Significant inertial oscillation (~2 days period) happened in the subsurface layer, leading to strong vertical mixing and upwelling and subsequently causing the offshore surface Chl-a bloom. This study provides new insights to evaluate typhoons and ocean eddies that induce biological responses in the future.

Wind- and eddy-driven upwelling over submarine canyons inshore of the northern Agulhas Current

The iSimangaliso Wetland Park, located along the coastline of northern KwaZulu-Natal, hosts South Africa’s only subtropical coral reef system. Episodic upwelling events are observed inshore and at the heads of Diepgat, Leadsman and Leven canyons within the iSimangaliso Wetland Park and may assist in larval dispersal and act as a buffer against coral bleaching. Over a 12-month period (1 June 2018-31 May 2019), two cooling events were identified from surface and subsurface temperature measurements extracted from the Sea Surface Temperature (SST) product of the Operational Sea Surface Temperature and Ice Analysis (OSTIA) satellite and data collected from underwater temperature recorders, respectively. Lagged correlations between subsurface temperatures confirmed that the anomalous cooling events were upwelling events (of 2-7 hours). The approximate upwelling rate was greatest at Diepgat Canyon, which suggests that upwelling events are enhanced by the physical structure of the canyon head and its shallow termination point which may influence the affected ecosystems. Wind measurements from the Mbazwana weather station and satellite sea-level anomalies were used to investigate the respective roles of wind stress and mesoscale ocean variability as driving forces of the upwelling events. The results indicate that the less intense and shorter cooling event was driven by a cyclonic eddy event, but that the stronger cooling event was driven by a combination of both forcing mechanisms.

Eddy Deformation and Kinetic Energy Conversion During the Eddy‐Kuroshio Interaction East of Taiwan

AbstractEddy‐mean flow interactions have always been of interest to researchers because they are among the causes of energy cascades. Different from most previous studies, this research concentrates on the Kuroshio Current's effect on an eddy during multiple independent elliptical eddy‐Kuroshio interactions. Eddy deformation during the eddy‐Kuroshio interaction is applied as the statistical criterion to select the typical eddies that interact with the Kuroshio east of Taiwan. There are 9 anticyclonic eddies (AEs) and 15 cyclonic eddies (CEs) that satisfy the criteria from the 1993 to 2018 satellite altimeter data. The blocking effect caused by the anti‐phase velocity at the contact interface of the CE and Kuroshio results in the eddy kinetic energy (EKE) at the CE edge being obviously inhomogeneous, while the AE does not result in the same effect. Moreover, the temporal variation in EKE during the eddy‐Kuroshio interaction precedes the AE/CE deformation by approximately 0.55/0.27 interaction cycles. The Kuroshio has different modulation mechanisms for different eddy polarities. Barotropic and baroclinic instabilities contribute to the generation of EKE, with barotropic instabilities providing more than 10 times as much as baroclinic instabilities for the AE and 4 times as much for the CE. The advection effect of the AE on the Kuroshio is in anti‐phase with the effect of the Kuroshio on the AE, which eventually makes the AE shrink. They are in phase for the CE and lead the CE to eventually become meandering.

Phytoplankton transport out of the euphotic zone by frontal subduction and gravitational sinking in the Santa Barbara Channel, CA, USA

Production of particulate organic carbon (POC) in nutrient-rich coastal waters over continental shelves, its export to depth, and its transport to deeper ocean waters is a poorly quantified component of the global carbon cycle. A critical step in quantifying this vertical transport is identifying shelf processes that export phytoplankton out of the euphotic zone. During cruises of the Santa Barbara Coastal Long Term Ecological Research project, we discovered substantial chlorophyll a (chl a) below the euphotic zone in the Santa Barbara Channel, a part of the southern California Current System. Observations from towed, undulating vehicles revealed deep chlorophyll layers near fronts where upwelled waters from central California converged with lower-density waters from the Southern California Bight. The mean fraction ± 1 standard deviation (SD) of chlorophyll biomass below the euphotic zone spanning the entire Santa Barbara Channel was ~7 ± 9% during 13 cruises averaged across all seasons. In one spring cruise, the fraction was ~30%, and in other cruises the layers were absent. Phytoplankton export out of the euphotic zone by subduction was indicated by spatial coherence between chl a and sloping density surfaces. Vertical plumes of chl a crossing density surfaces indicated enhanced gravitational export within cyclonic eddies. Chl a in water samples below the euphotic zone, away from fronts and cyclonic flows, suggested additional phytoplankton export. Our results emphasize the importance of subduction in the export of phytoplankton and POC out of the euphotic zone in coastal upwelling systems.

Uncertainties in Altimetry Observations of Eddy Changes Induced by Tropical Cyclones

Abstract The effects of tropical cyclones (TCs) on preexisting eddies are generally quantified by comparing post-TC and pre-TC altimetry-based eddy amplitudes and radii. The dynamical and technical uncertainties in this quantification have been revealed by the altimetry-based and simulated eddy characteristics of five cyclonic ocean eddies (COEs) and two anticyclonic ocean eddies (AOEs). Although demonstrated by eddy cases, both the uncertainties should be universal in principle. The dynamical uncertainty primarily arises from the highly variable eddy characteristics associated with the post-TC quasigeostrophic evolutions driven by the inevitable pattern discrepancy between TC-injection and preexisting eddy’s potential vorticity (PV). The technical uncertainty is due to the artificial smoothness in the altimetry-based eddy characteristics produced by the mismatch between sparse data interpolation and sudden injection of TC-induced effects. Beyond the uncertainties, the amplitudes and radii of both the COEs and AOEs were damped directly by a rectilinear-track TC. After the TC passage, the COEs may strengthen again or remain in the damped state, depending on whether the COEs can absorb the TC-injected PV. By contrast, the AOEs remained in the damped state because the TC-injected positive PV cannot excite them to enhance and enlarge. More importantly, the above damped state of the perturbed COEs and AOEs may be the result of the developing geostrophic turbulence, not meaning the decay of the TC-induced effects. This fact, together with the dynamical and technical uncertainties, implies that the previously used quantification may significantly underestimate the TC-induced effects. Significance Statement Typhoons/hurricanes inject their effects on ocean eddies and further modulate the ocean circulation and climate by the accumulated effects. These effects are generally quantified by altimetry observations. Two uncertainties in this quantification are illustrated by using several eddy cases. The first uncertainty is caused by the eddy evolution, while the second is by the artificial smoothness in the altimetry-based eddy characteristics. These findings suggest that the effects of typhoons/hurricanes may be underestimated due to the two uncertainties and underscore that a new method based on physical understanding is necessary to quantify these effects.

Identification of eddy-induced chlorophyll in the Northeastern Natuna Sea

We detected a robust cyclonic eddy in the northeastern Natuna Island in the Northwest Monsoon during an extremely negative El Niño-Southern Oscillation (ENSO) period (i.e., extreme La Niña event) using the Copernicus Marine Environment Monitoring Service (CMEMS) reanalysis results. In the intense La Niña event from 1999 to 2001, we also found an increase in the abundance of chlorophyll coincided with the eddy strength pattern using the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data. Furthermore, this chlorophyll intensity may be related to the upwelling of water mass to the surface at the eddy’s center. The intensification of chlorophyll compared to the average value at the corresponding year is 83,4% (1999), 37.8% (2000), 37.3% (2008), and 72.5% (2011). In this paper, we revisit the identification of solid eddies in extreme La Niña events from 1999 to 2012 and further discuss the mechanism of the relationship between chlorophyll intensity and eddy strength, especially during the months of the extreme La Niña event.

Oceanographic conditions of the continental slope and deep waters in Santos Basin: the SANSED cruise (winter 2019)

This work describes the circulation over the continental slope and the São Paulo Plateau in the Santos Basin during the SANSED winter 2019 survey. The cruise consisted of four legs in the period between June, 11 and August, 03 2019. The observed circulation is dominated by the Atlantic southwestern boundary current regime and remotely-generated anticyclones and cyclones. The former is composed by the Brazil Current, the Intermediate Western Boundary Current and their mesoscale meanders; the latter are 300km vortical rings with origin in the eastern side of the South Atlantic Basin. A Lagrangian scheme applied over satellite altimeter maps indicate that the origin of these rings is primarily the Cape Basin of South Africa. The interaction between the boundary currents, their cyclonic meanders, and the anticyclonic rings is complex, and varies widely. During the SANSED winter 2019 survey period, three anticyclones interacted with the Brazil Current, instabilizing it, forming dipoles with the current cyclonic meanders, leading to their downstream propagation. Ancienter cyclonic eddies within Santos Basin may interfere with the propagation of the large anticyclones further south. In addition, the continuous arrival of remotely-originated anticyclones, the larger portion over the São Paulo Plateau presented a tendency of counter-clockwise circulation during the whole cruise period.

Effects of Mesoscale Eddies in the Northern South China Sea on Phytoplankton Size and Physiological Status

Mesoscale eddies have essential effects on the distribution of the different sizes of phytoplankton and the status of phytoplankton physiology. The impact of mesoscale eddies on phytoplankton size and physiological level in the northern South China Sea is analyzed based on satellite data and HYCOM-simulated results from 2003 to 2018. The results show that there are higher nanophytoplankton levels for high and low nonlinearity in the center of cyclonic eddies. At the same time, the growth rate of phytoplankton increased, and the assimilation of phytoplankton decreased. Moreover, in the center of anticyclonic eddies, a lower nanophytoplankton level is observed in both high and low nonlinearity. At the same time, the growth rate of phytoplankton decreased, and the assimilation of phytoplankton increased. In addition, there is a higher nanophytoplankton level in the mode-water eddy, while the growth rate of phytoplankton is increased, and the assimilation of phytoplankton is decreased.

Corrections of Mesoscale Eddies and Kuroshio Extension Surface Velocities Derived from Satellite Altimeters

Oceanic datasets derived from satellite altimeters are of great significance to physical oceanography and ocean dynamics research and the protection of marine environmental resources. Ageostrophic velocity induced by centrifugal force is not considered in altimeter products. This study introduces an iterative method to perform cyclogeostrophic corrections of mesoscale eddies’ surface velocities derived from satellite altimeters. The corrected eddy velocity field and geostrophic velocity field were compared by combining eddy detection and mathematical statistics methods. The results show that eddies with small curvature radii, high roundness, or Rossby number larger than 0.1 illustrate that cyclogeostrophic correction is required. The cyclogeostrophic velocity is greater (less) than the geostrophic velocity in anticyclonic (cyclonic) eddies. Additionally, the iterative method is applied to cyclogeostrophic-corrected multi-year (1998–2012) Kuroshio surface velocities. The effect of cyclogeostrophic correction is significant for the Kuroshio Extension region, where the maximum relative difference of velocities with and without correction is about 10% and the eddy kinetic energy is 20%.

Major Migration Channels of Mesoscale Eddies in the Southern Ocean Derived From Satellite Altimetry

AbstractThe propagation pathways of mesoscale eddies have an essential impact on the meridional transport of freshwater, carbon, nutrients, and other tracers. However, existing methods have limitations in exactly expressing their major propagation pathways. To tackle this problem, a direction‐based spatial clustering algorithm for the trajectory is proposed according to the partition‐and‐group framework. The idea is to extract the major eddy channels quantitatively, thus effectively representing the eddy trajectories with similar propagation pathways in a given spatial distance and direction, from the eddy identification and tracking datasets obtained by satellite altimetry spanning nearly 20 years in the Southern Ocean. The major eddy channels of the Southern Ocean are more geographically correlated rather than determined by polarity (anticyclonic eddies (AEs) and cyclonic eddies (CEs)). Importantly, the main axes of zonal propagation for eddy channels are extracted in this study. In the area north of 45°S (30°S–45°S), the zonal propagation (both for AEs and CEs) is intrinsically westward along ∼42°S ± 2° (main axes). While in those south of 45°S (45°S–80°S), it is reversed to eastward, propagating zonally along ∼56°S ± 2° (main axes) and obviously showing a gyre pattern. The bottom topography significantly effects on the propagation pathways of eddy channels in the Southern Ocean. Mainly, their meridional propagation pathways are pronouncedly steered by the shape of oceanic ridges around isobath −3900m (AEs for −3,877 m, CEs for −3,922 m).

Distribution and transport of Fish larvae at the entrance of the Gulf of California (September, 2016)

Fish larvae distribution and transport of dominant species were analyzed at the entrance to the Gulf of California in September 2016. Using the Sea Surface Temperature (SST) gradient, a frontal system was defined at the entrance to the Gulf. The most relevant gradient (∼0.06 °C km−1) in the region was transverse to the gulf axis (W-E), from south of Cabo San Lucas to the mainland, south of the Pescadero Basin. This SST gradient was associated with an anticyclonic flow (∼0.4 m s−1) that was connected with a cyclonic eddy outside the Gulf. Fish larvae of mesopelagic species such as Vincigueria lucetia and Benthosema panamense were widely distributed and increasing in abundance from north to south, while the mesopelagic Bregmaceros Bathymaster and the pelagic Auxis spp. showed an inverse abundance gradient. Most of these larvae were absent from the zone of strong SST gradient. A Lagrangian particle tracking model applied at the sampling stations where the fish larvae were collected, showed that during the first 5 days of trajectory, the transport was weak. But for the next 5 days, most of the particles were trapped by mesoscale flows that dominated the region. An unexpected result was that no particles crossed the transverse front, coinciding with the absence or low abundance of larvae there. The particles near to this front were transported west out the Gulf. We concluded that when the mesoscale flows are extended along the transverse axis of the Gulf entrance, these might trap inert particles and zooplankton organisms with little mobility, such as fish larvae in early phases of development, preventing their transport along the Gulf parallel axis (NW-SE), and therefore, affecting the species distribution.

What happened around an inverted V-shaped track turning of the tropical cyclone Madi?

The tropical cyclone (TC) Madi with an inverted V-shaped track happened over the Bay of Bengal (BoB) from Dec. 5 to Dec. 12, 2013. In this paper, the strong oceanic and atmospheric responses during the turning of the inverted V-shaped track of Madi were studied using the multisource satellite remote sensing and Argo float data. The results indicate that the response during the pre-turn phase (00:00 Dec.5 to 06:00 Dec.10) is more acute than that during the post-turn phase (06:00 Dec.10 to 18:00 Dec.12) of Madi. During the pre-turn phase, the cumulative precipitation reached 206 mm, the sea surface temperature (SST) decreased from 27.7 °C to 24.5 °C, and the maximum wind speed (MWS) increased from 7.71 m·s−1 to 43.72 m·s−1. At the turning corner on Dec.10, a symmetrical distribution of temperature drop, with greater precipitation on the left than that on the right. Meanwhile, the deep vertical wind shear was 17.13 m·s−1 with a northwestward moving cyclonic eddy. The barrier layer thickness was reduced from 25.82 m (24.88 m) to 23.8 m (12.38 m) during the pre-turn (post-turn) phase. Moreover, the potential vorticity (PV) caused by the ocean eddy disturbance is closely related to the intensity of TC. This study helps to estimate the sea-air interactions in the western BoB.

Characteristics of Eulerian mesoscale eddies in the Gulf of Mexico

Although numerous studies on Eulerian mesoscale eddies with closed contours of sea surface height (SSH) or streamline have been conducted in the Gulf of Mexico (GoM), a comprehensive study on their temporal and spatial characteristics is still lacking. In this study, we combine three eddy detection algorithms to detect Eulerian eddies from the 26-year SSH record in the GoM and examine their characteristics. We find distinct characteristics between Loop Current Eddies (LCEs), Loop Current Frontal Eddies (LCFEs), and mesoscale eddies that are not directly related to the Loop Current (LC). Many characteristics of LCEs and LCFEs in the eastern GoM are closely related to the LC. More LCFEs are formed in January to July than in August to December, likely related to the seasonal variation of the northward penetration of the LC. However, the formation of non-LCFE cyclonic eddies shows a biannual variability, which could be linked to the position and strength of the background current in the western GoM. Nevertheless, the seasonal variability of the Eulerian eddies shows large uncertainties (not significant at the 95% confidence level). Low-frequency (interannual to multidecadal) variability is also detected. In the eastern GoM, the extent of northward penetration of the LC can affect the generation of LCFEs and result in low-frequency variations. In the western GoM, the low-frequency variability of eddy occurrence and amplitude could be related to the surface circulation strength.

Contrasting phytoplankton composition and primary productivity in multiple mesoscale eddies along the East Australian coast

Mesoscale eddies drive variability in phytoplankton functional trait composition and primary productivity (PP) relative to adjacent waters. Offshore waters in southeast Australia are subject to substantial mesoscale eddies that form when the East Australian Current (EAC) travels poleward along the coast, forming distinctive habitats in the upper ocean. Eddies provide an important enrichment mechanism in the nitrogen-limited waters of the Tasman Sea, yet there is limited knowledge of PP within cold- and warm-core eddies in the region and how physico-chemical and biological factors affect phytoplankton communities in this variable environment. We addressed the scarcity of observations by quantifying net PP using 13C isotopic enrichment incubations of surface waters over 10 degrees of latitude, comparing phytoplankton species composition in five different environments: a coastal shelf station, an oceanic cold and warm-core eddy, and a coastal dipole. Cold-core (cyclonic) eddies were significantly more productive than their warm-core (anticyclonic) counterparts (∼35 versus 0 mg C m−3 d−1), with centric diatoms the most prominent phytoplankton group, with a relatively high centric:pennate ratio. The diffuse attenuation coefficient, Kd (PAR), and silicate were the best overall predictors of phytoplankton composition, explaining 88% of variation based on pigment analysis and size fractionation. Variance in net PP did not correlate significantly with physico-chemical parameters frequently used in PP models (temperature, Kd [PAR], Chl a), yet inclusion of size-fractionated Chl a generated substantial improvement in our statistical model (from 36 to 77%). We show that cold-core eddies play a key role in regulating PP in eastern Australian waters and highlight a need for eddy-resolving models to incorporate descriptors of phytoplankton size structure to improve the accuracy of PP forecasts in eddy intensive regions.

Characteristics of Mesoscale Eddies in the Vicinity of the Kuroshio: Statistics from Satellite Altimeter Observations and OFES Model Data

Mesoscale eddies propagate westward in the northwestern Pacific Ocean and interact with the Kuroshio in the vicinity of the western boundary of the ocean. However, the processes affecting the eddy properties and the detailed structure of the eddies when they encounter the Kuroshio remain unclear. In this study, we analyze the statistics of the eddy properties around the Kuroshio using 25 years of satellite altimeter data and the eddy-resolving OFES model product. The spatial compositions of the eddies in the northwestern Pacific show that, as the eddies propagate westward, their radius and amplitude decrease sharply when they approach the Kuroshio region. The radius, amplitude, and kinetic energy of the eddies reaching the Kuroshio region decay much faster during their lifespan compared with the eddies in the interior of the Pacific Ocean. Furthermore, the three-dimensional structure of the eddies obtained from the OFES model data shows that the maximum temperature anomalies in the cyclonic and anticyclonic eddies occur at ~300 m, and the maximum depth reduces as a result of the interaction between the eddies and the main Kuroshio current.

Refraction of the M2 internal tides by mesoscale eddies in the South China Sea

Mesoscale eddies (MEs) are active in the northern South China Sea, yet it is hard to separate their impacts on the internal tides (ITs) from those of other subtidal circulation. This study examines the role of an anticyclonic eddy and a cyclonic eddy in modulating the M2 ITs based on numerical simulations. In idealized experiments, MEs that differ only in polarity are located on the main beam of the M2 ITs in the northern South China Sea. Results show that the anticyclonic eddy and cyclonic eddy cause northward and southward refractions of the M2 ITs, respectively, which echoes the realistic simulation. The most dramatic changes in the M2 IT energy fluxes occur on the continental slope to the west of the MEs. Theoretical analysis based on the ray-tracing model and the empirical model for estimating wave front locations reveals that the MEs cause refraction of the M2 ITs by changing the phase speeds. Further investigation shows that the currents related to the MEs make a greater contribution to the IT refraction than the ME-associated stratification. These results have important implications for investigation of spatial and temporal variations in the magnitude and direction of the M2 IT energy fluxes.

Effects of numerical model's horizontal resolution on the vertical transport of near-inertial energy

Reproducing kinetic energy transport from the wind (mostly near inertial kinetic energy, NIKE) to the interior ocean is one of the most important issues in climate change numerical studies. Data recorded by two buoys in the South China Sea, which survived Typhoon Kalmaegi (2014), show that the NIKE can be transported into the deeper ocean in an anticyclonic rather than in a cyclonic eddy. Numerical experiments with three horizontal resolutions reveal that the increase in the model resolution enhances the enstrophy and advection, deepening NIKE transport and better reproducing the NIKE vertical distribution as observed. The present study finds that the horizontal resolution of a model can significantly impact the depth towards which the NIKE can be transported, further implicating that the increase of the model resolution is an essential step to improve the accuracy of a climate model.