Warm Eddy Research Articles

Physical-biological coupling in the waters surrounding the Prince Edward Islands (Southern Ocean)

The results of a macro-scale oceanographic survey conducted in the upstream and downstream regions of the Prince Edward Islands in austral autumn (April/May) 1989 are presented. During the investigation, the Subantarctic Front, upstream of the islands, was shown to lie initially south at 46°38′S, while downstream, the front remained in a northern position of approximately 46°S. Surface expressions of the front show that the Subantarctic Front forms a zonal band, while the subsurface expressions (200 m) show a distinct meander in both regions. In the upstream region of the islands, the northern branch of the Antarctic Circumpolar Current, the Subantarctic Front, influenced by the shallow bathymetry, was deflected around the northern edge of the islands. Water masses in this region were shown to modify gradually from Subantarctic Surface Water (7°C, 33.75) to Antarctic Surface Water (5°C, 33.70) as the Polar Frontal Zone was crossed. Downstream of the islands a wake was formed resulting in the generation of broad, cross-frontal meanders. As a consequence, warm Subantarctic Surface Water from north of the Subantarctic Front was advected southwards across the Polar Frontal Zone, while cooler waters, which had been modified in the transitional band of the Polar Frontal Zone, were advected northwards. In the downstream region a warm eddy consisting of Subantarctic Surface Water was observed. Its generation is possibly due to baroclinic instabilities in the meandering wake. Zooplankton species composition and distribution patterns during the investigation were consistent with the prevailing oceanographic regime. Four distinct groupings of stations were identified by numerical analysis. These corresponded to stations found north of the Subantarctic Front, within the warm eddy, located in the Polar Frontal Zone, and those stations associated with the meander. The groupings were separated by the Subantarctic Front, which appears to represent an important biogeographic boundary to the distribution of warm-water zooplankton species. Warm eddies in the downstream region of the islands may represent an effective mechanism for transporting warm water species across the Subantarctic Front.

The Influence of Mesoscale Features of the Sea Surface Temperature Distribution on Marine Boundary Layer Winds off the Scotian Shelf during the Superstorm of March 1993

Abstract This paper studies the mesoscale wind field during the blizzard of March 1993 off the east coast of North America and examines the influence of the sea surface temperature distribution on surface winds. Can the Gulf Stream and its meanders, by its strong influence on the marine boundary layer, generate mesoscale features in the wind field? Numerical simulations of the storm are carried out using the MC2, a fully elastic nonhydrostatic model. Simulations are conducted at different resolutions (50, 25, 10, 5, and 2 km) with both detailed and smoothed SST fields, so as to examine the influence of these parameters on the marine boundary layer winds. Results from these numerical simulations are compared with surface observations from buoys. The study reveals some mesoscale features in the wind field caused by the Gulf Stream’s meanders and the warm eddies of the SST field. In a stable boundary layer, the meanders shaped a 50–55-kt (26–28 m s−1) band of winds in a general 40–45-kt (21–23 m s−1) wind fi...

Ocean acoustic tomography using explosive charges to observe the submerged mesoscale oceanic variability in the East Sea of Korea

A tomography system consisting of aircraft-deployed shots as sources and a vertical line array moored by a receiver ship was used to survey a large area (120×120 km) where a mesoscale warm eddy appears frequently. The position of the shots was measured by the GPS on the aircraft and the detonation time (or depth) was monitored by sonobuoys which had been deployed close to the shot position. Results show that individual multipaths between a shot and the receiver contain substantial information for a vertical slice of the ocean with a warm eddy. With high accuracy, GPS positioning of shot locations, and receiver position, this rapidly deployable system can be used for monitoring the submerged structure of the warm eddy during the period of diffusive SST in summer in the East Sea of Korea. [Work supported by KOSEF.]

津軽海峡南西海域における対馬暖流の係留観測

Warm water of the Tsushima Current converges southwest of the Tsugaru Strait and flows relatively stable northward with most narrow cross section in the Japan Sea. Therefore, many Japanese oceanographers have evaluated the volume transport in this region to monitor the interannual or seasonal variation of the Tsushima Current system. However, the description on the water movements by the past studies have been only inferred from the dynamic caluclation on the basis of hydrographic data. From April 1992 to March 1993, we carried out the direct current measurement southwest of the Tsugaru Strait by using of a moored instrument at the depth of 130m (Apr.∼Oct.1992) and 180m (Oct.1992∼Mar.1993) to investigate the dynamical variability of the Tsushima Current. The large baroclinic events appear to be dominated mainly by passing through the seaward meander of the coastal current in May 1992 and by merging a warm eddy with the spatial scale of several hundred kilometers within the coastal curent in January 1993. Moreover, about one month-periodical fluctuations observed in the temparature and the on-offshore current corresponded with a Tsushima Current frontal eddy event that could be identified in the NOAA/AVHRR images. From a comparison between the current velocities observed by the current meter and the geostrophic ones, it is suggested that the Tsushima Current includes the evidence as the barotoropic northward flow with about 10 cm s-1 tobe unrelated with the seasonal changes of stratification. This implies that the actual Tsushima Current transports are likely to be larger than those estimated by the past.

Evolution of baroclinic planetary eddies over localized bottom topography in terms of JEBAR

Numerical experiments have been conducted to investigate the evolution of pure baroclinic eddies using two-layer planetary geostrophic equations in the presence of localized bottom topography. In particular, we interpret the eddy motion in terms of JEBAR (joint effect of baroclinicity and relief). It is shown that the emission of barotropic flows (or waves), westward acceleration and energy conversion occur when an eddy interacts with bottom topography. A meridional ridge acts as a repellor for a cold eddy and an attractor for warm eddy due to JEBAR-induced barotropic flows (or waves), which is consistent with the vorticity argument. This mechanism may provide a possible explanation not only for the global distribution of the phase speed of baroclinic Rossby waves observed in the Pacific by the TOPEX/POSEIDON satellite altimeter but also for the abrupt change in the trajectories of the westward propagating baroclinic eddies observed in the North Atlantic.

Interaction of a warm eddy with the coastal current at the eastern boundary area in the Tsushima Current region

The Tsushima Current region in the Japan Sea is one of the eddy-rich areas. According to Isoda (1994) ( Journal of Oceanography, 50, 1–15), when an eastward moving warm eddy reached its eastern boundary area, two or three mesoscale eddies frequently evolved and gradually disappeared into the coastal current. Such process associated with the eddy-current interaction are examined numerically using a reduced gravity model on the f-plane. Once a warm (anticyclonic) eddy influences the coastal current, the following two kinds of meanders evolve. In the nonlinear regime, an original eddy moves onshore and becomes part of the seaward meander. Furthermore, this eddy excites another small seaward meander or a new eddy at its northern part due to the vorticity control of the distorted coastal current. After that, a newly formed northern eddy travels downstream with an advective speed of the coastal current, while the potential vorticity of an original eddy becomes to lose within a larger potential vorticity anomaly onshore side of the coastal current.

「海の渦が見えた」 航空機により渦を捉える

「海の渦が見えた」 航空機により渦を捉える

衛星画像により暖水渦の発生・発達を初めて捉える

衛星画像により暖水渦の発生・発達を初めて捉える

Sea level variations induced by heating and cooling: An evaluation of the Boussinesq approximation in ocean models

In this paper a sigma coordinate ocean model is modified to remove the commonly used Boussinesq approximation so that the effect of thermal expansion is exactly included in the basic equations in order to cope with the seasonal heating cycle and the detection of climate change through variation in sea level height. Tests are performed to evaluate the differences between Boussinesq and non‐Boussinesq calculations under different heating and cooling conditions and different model domains. For an idealized case of a flat bottom, shallow ocean basin without wind forcing, simulations of a warm eddy show that the non‐Boussinesq dynamics have only a minor effect on the baroclinic current field. However, vertically averaged velocities, though small compared with the baroclinic velocities, are cyclonic for the Boussinesq calculation and anticyclonic for the non‐Boussinesq calculation. The results indicate that global or closed basin Boussinesq models should be able to simulate most of the observed steric sea level changes on seasonal or climate timescales, when corrected by a spatially uniform, time‐dependent factor calculated from the volume‐averaged density change. The seasonal variation of the globally averaged sea level calculated from climatological data is small, about 1 cm. Variations in steric sea level in regional models, both Boussinesq and non‐Boussinesq, may differ from those of global models owing to the unknown transport across their boundaries associated with the local heating and cooling. A spatially uniform, time‐dependent correction, similar to that associated with thermal expansion, is proposed to account for transport across open boundaries of regional models. Variations of sea level obtained from a Boussinesq model of the Atlantic Ocean approximate the seasonal signal due to the heating/cooling cycle of each hemisphere as observed by satellite altimeter data.

The anatomy of the Arctic Frontal Zone in the Greenland Sea

From a series of two‐dimensional expendable bathythermograph (XBT) surveys, conductivity‐temperature‐depth (CTD) sections, surface drifters, and acoustic Doppler current profiler (ADCP) observations, the properties and structure of the Arctic Frontal Zone in the Greenland Sea have been determined. The Arctic Frontal Zone appears to be a large‐scale, climatic “multifrontal” frontal zone. The structure of the frontal zone can be discerned from subsurface as well as from surface hydrographic parameters, even in summer, when a seasonal thermocline covers the subsurface hydrographie structure. The Arctic Frontal Zone consists of two semipermanent frontal interfaces with warm, saline Norwegian Atlantic Water to the east and Arctic Water from the Greenland Sea gyre to the west. The two frontal interfaces are bounding a band of shallow cyclonic cold eddies and anticyclonic warm eddies with horizontal scales of the order of 40–50 km. The typical diameter of the eddies can be scaled with the local internal Rossby radius of deformation. The eddy kinetic energy of the surface flow in the frontal zone is of the order of 60 to 85 cm2 s−2. The zonal density gradient in the Arctic Frontal Zone maintains a mean northward geostrophic transport of 3.8 Sv, averaged over a number of cruises. This transport is mainly connected with the frontal interface on the western side of the warm and saline Norwegian Atlantic Water. The estimated cross‐frontal eddy transports of heat and salt appear to be of considerable importance for the conditioning of the Greenland Sea gyre.

Gyres off the Somali coast and western boundary currents in the Bay of Bengal during the south-west monsoon

Abstract Satellite remotely-sensed sea surface temperature (SST) data were obtained during the initial phase of the onset of the south-west monsoon in the north-western Indian Ocean for the years 1987and 1988. Large wedge-like areas of upwelled water during 1988 were observed at 5°Nand 10°N after the Somali current spinup, indicative of a two-gyre circulation in the Somali current system. Satellite infrared observations of the Bay of Bengal during the early February 1990revealed the existenceof two bands of warm water that resembled a western boundary current (WBC) along the east coast of India. A warm core eddy, with a major axis of nearly 120km, appeared at 89°E and 19°N at the end of the axis of the current. The existence of the two gyre system ofT the Somali coast and the presence or absence of the WBC in the Bay of Bengal in response to the onset of the summer monsoon has been discussed in detail.

Long-Term Evolution of Elongated Warm Eddies

Abstract The purpose of this research is to investigate the evolution of elongated warm eddies. A shallow-water, reduced-gravity, primitive equation model is used to perform a multicase numerical experiment, which includes vortices of very different eccentricities and strengths. The above is partly motivated by the increased number of oceanic observations, performed in recent years, that show the wide variety of warm eddies. Under the assumption that an eddy can be approximately described as a solid-body rotating elliptical vortex, the evolution is followed using a least-squares fitting procedure. Results indicate that the majority of these eddies evolve toward less eccentric states, a process called axisymmetrization. A few of the most elongated eddies break up, and some of the less eccentric ones remain unchanged. Axisymmetrization seems to be triggered by instability and, in the space of the parameters that describe the elliptical vortex, the trajectories followed in each of the cases fairly approximat...

The Value of NDBC Observations during March 1993's “Storm of the Century”

National Data Buoy Center (NDBC) observations from the western Gulf of Mexico Provided several early indications of more rapid cyclogenesis on 12 March 1993 than was forecast by numerical weather prediction (NWP) models. Observations demostrated a closed circulation with gale-force winds shortly after the storm entered the Gulf of Mexico. Pressure measurements at two buoys off the Texas coast were 4-6 hPa lower than the 12-h NWP forecasts, a significant forecast error. Observations from NDBC's moored buoys and Coastal-Marine Automated Network stations revealed that the developing storm was significantly deeper than was indicated on the National Meteorological Center's automated surface analyses. Ocean wave observations reveal some of the steepest waves NDBC has ever measured, indicating phenomenonal wave growth and a high potential for damage to vessels and structures. A warm eddy caused sea surface temperatures (SST) to be several degrees above normal under the track of the storm, creating a strong SST gradient to the north. This provided ample energy and strengthened the baroclinity. NDBC observations showed the eddy to be somewhat larger and warmer than indicated by the most recent National Hurricane Center analysis. This event demonstrates the tremendous value of NDBC marine observations for accurately detecting the occurrence and strength of coastal cyclogenesis events.

Radar and infrared measurements of a cold eddy in the Tyrrhenian Sea

Abstract Simultaneous airborne measurements of sea surface temperature (SST) by an infrared (IR) radiometer and radar backscatter from a scattcromcter operating at the same C band as on ERS-1, were made over the Tyrrhenian Sea. The aircraft SST field showed part of the cold and warm eddy system characteristic of the region and compared well with ship-based measurements. The radar backscatter data were examined to investigate whether any dependence on SST could be discerned. It was found that the radar backscatter data gave a more intermittent view of the eddy and that view appeared to change from day to day. A change in the radar view of the eddy could occur due to changing wind fields or atmospheric stability patterns over the thermal front and this hypothesis was discussed with regard to two periods with different atmospheric conditions. For two days of very light winds and calm sea conditions the radar signature followed the main thermal gradient features and a positive, significant correlation was obt...

The Kuroshio Structure and Transport Estimated by the Inverse Method

Abstract Structure and transport of the Kuroshio are examined by the inverse method, based on a dataset around the Tokara Strait. Subsurface velocities measured by a shipborne acoustic Doppler current profiler are used together with CTD sections across the Kuroshio to estimate the absolute flow fields. The dataset was obtained along a closed line south off the Kyushu by the R/V Chofu Maru in September 1987. The Kuroshio transport through the Tokara Strait was 28 × 109 kg s−1 at the time of observations. The transport of warm water eddy south of the Kuroshio is estimated at larger than 39 × 109 kg s−1, but its precise value is difficult to determine because of strong transport dependency on the solution rank. The result also suggests a southward flow below the Kuroshio off Cape Toimisaki and in the Tokara Strait. In the section east of Amamioshima Island, a broad northward flow is suggested on the continental slope.

Warm eddy movements in the eastern Japan Sea

Warm eddy movements and their areal extent in the eastern Japan Sea were described by presenting space-time diagrams for the warm eddy locations and magnitudes. The analyzed data were compiled from Japan Maritime Safety Agency thermal maps at 200 m depth from 1985 to 1992. Two to four warm eddies always existed in the eastern Japan Sea and exhibited both internnual and annual signals. We found that warm eddies were generated in spring around Oki Spur at least three times during the analyzed period of eight years, moved eastward, and interacted with neighboring warm eddies, which were involved in coalescences or separations. The warm eddy distributions off Noto Peninsula have clear seasonal preference. Warm eddies moved eastward from Noto Peninsula in winter-spring to North Japan in the next winter, with mean translation speeds of 0.5–2 cm s−1. Warm eddies reaching North Japan typically decayed during a few month after splitting into two or three mesoscale warm eddies.

The northward intruding eddy along the East coast of Korea

The current structures and their seasonal variations in the East Korean Warm Current (EKWC) region, which plays a significant role in the northward transport of warm and saline waters, were described by combining the sea surface temperature (SST) data of consecutive satellite inferred (IR) images and hydrographic data. The SST patterns in winter-spring clearly showed that the small meander of thermal front originating from the Tsushima/Korea Strait formed close to the Korean coast and grew an isolated warm eddy with horizontal dimension of order 100 km. Such warm eddy began to intrude slowly northward from spring to summer. At that time, interactions with neighboring synoptic warm eddy [Ks] around the Ulleung Basin were found to have strongly influence the movement of the intruding eddy and its structural change. In autumn, after the northward movement stopped at the north of eddy [Ks], the relative stable northward current along the Korean coast were formed. The evidence from observational results does not support a persistent branching of the EKWC from the Tsushima/Korea Strait, but a seasonal episodic supply of warm and saline waters due to the northward intruding eddy process described above.

Upper ocean response to Hurricane Gilbert

The evolving upper ocean response excited by the passage of hurricane Gilbert (September 14–19, 1988) was investigated using current and temperature observations acquired from the deployment of 79 airborne expendable current profilers (AXCPs) and 51 airborne expendable bathythermographs from the National Oceanic and Atmospheric Administration WP‐3D aircraft in the western Gulf of Mexico. The sea surface temperatures (SSTs), mixed layer depths, and bulk Richardson numbers were objectively analyzed to examine the spatial variability of the upper ocean response to Gilbert. Net decreases of the SSTs of 3°–4°C were observed by the profilers as well as by the airborne infrared thermometer (AIRT) along the flight tracks and advanced very high resolution radiometer (AVHRR) imagery. The AXCPs indicated a marked cooling from 29°C to about 25.5°C on September 17, 1988, which was about 1.2 inertial periods (IP) following storm passage. This pool of cooler water (3.5°) was located further downstream in the hurricane wake by September 19 (2.7 IP following the storm) as a result of the near‐inertial currents in the mixed layer. While there was a bias of about 0.6°C and 1.7°C between the in situ and AVHRR‐derived SSTs, respectively, both the AVHRR images and the objectively analyzed fields indicated a rightward bias in the upper ocean cooling that extended from the storm track to about 4Rmax (where Rmax, the radius of maximum winds, is equal to 50 km). The larger SST offset of 1.7°C was due to the difference between the time of the AVHRR image and the time of the aircraft experiment on September 19. The SSTs derived from the AVHRR images and the AIRT also indicated large gradients between the cold wake and the warm eddy in the central Gulf of Mexico. The mixed layer deepened by about 30–35 m on the right side of the track during the storm and 1.2 IP later, with little evidence of continued deepening afterward. The mixed layer current vectors demonstrate that a strong, near‐inertially rotating current was excited by the passage of Gilbert, with maxima of about 1–1.4 m s−1. The currents, observed during and subsequent to (1.2 IP) the storm, diverged from the storm track, whereas the mixed layer current vectors 2.7 IP after storm passage converged toward the track, with relative maxima of 0.8–1 m s−1. This alternating pattern of convergence and divergence of the mixed layer current was associated with the upwelling and downwelling cycles of the baroclinic response. Considerable current shear existed between the mixed layer and the thermocline currents in the cool wake between the storm track and the 4Rmax. Estimates of the bulk Richardson numbers ranged between 0.2 and 1.0 during Gilbert and at 1.2 IP, which suggests that enhanced current shears were responsible for some of the mixed layer deepening.

Rotational modes for warm and cold eddies

Analytical solutions are found to describe a water parcel motion in both warm and cold eddies. An ƒ plane, reduced gravity model, with upper layer thickness and velocity fields respectively Gaussian and linear in space, was reduced to the eight‐dimensional system of nonlinear coupled ordinary differential equations which describe time evolution of differential kinematic parameters and eddy shape parameters. Analytical solutions for steady rotating modes for warm eddies and for cold eddies are discussed. For warm eddies the rigidly rotating ellipse mode solution corresponds to the “rodon” solution for lenslike warm eddies. For cold eddies the corresponding rigidly rotating mode solution exhibits triangular path with abrupt direction change. Elliptical paths for warm eddies and triangular paths for cold eddies were observed in Western Pacific Ocean Circulation Study III drifter tracking data, which may be interpreted to be a possible manifestation of the rotating mode solutions predicted theoretically.

Observations of an Arc-shaped Cloud Band over an Oceanographic Warm Eddy

Observations on January 7 and 8, 1991, of an arc-shaped cloud band outlining an area in the eastern Mediterranean within a warm ocean eddy were made using the NOAA-11 AVHRR. Clouds formed at the edge of the warm eddy's internal boundary layer as the wind traveled at right angles from the colder Aegean water toward the warm eddy. No clouds formed on the southern side of the warm eddy where sea surface temperatures were warmer and where the winds may not have flowed at right angles toward the warm eddy. An upper limit of 24 h was determined for the length of time the arc-shaped cloud stayed over the eddy.