Westward Drift Research Articles

Westward plasma drift in the midlatitude ionospheric F region in the midnight‐dawn sector

In this paper we report the observations of a sharp ionospheric plasma velocity reversal boundary at subauroral latitudes during magnetic storms in the midnight‐dawn sector. The zonal plasma drift was eastward poleward of the velocity boundary and became westward equatorward of the boundary. The latitudinal extent of the westward drifts was ∼20°. A local maximum of the drift occurred only 2°–5° below the boundary; the magnitude of the maximum velocity was 200–400 m s−1 between 2100 and 0300 local time (LT) and as high as 780 m s−1 near midnight. The large westward plasma drift immediately equatorward of the velocity boundary first occurred at 2100 LT and continuously extended across midnight to the dawn sector. The location of the maximum velocity coincided with the minimum density within the midlatitude ionospheric trough. The latitude of the velocity boundary decreased from geographic latitude ∼53° to ∼43° (corresponding to geomagnetic latitude ∼65° to ∼55°) between 2100 and 0800 LT. We propose that the large westward drift at subauroral latitudes is driven by the polarization jet electric fields in the ring current and that the lower‐latitude westward drift with moderate magnitude is caused by the disturbance wind dynamo.

A study of the dynamics of the equatorial lower stratosphere by use of ultra‐long‐duration balloons: 1. Planetary scales

In the late southern winter of 1998, Centre National d'Études Spatiales (CNES), the French Space Agency, released six 10‐m‐diameter, superpressure balloons from a location near Quito, Ecuador. Three balloons collapsed soon after launching, but the remaining three drifted westward for a few weeks at altitudes between 19 and 20 km. Two of those balloons crossed the Pacific Ocean before falling above the “maritime continent,” while the other completed a revolution around the Earth and crossed the Pacific for a second time before its final fall. Despite the small number and the relatively short duration of the flights, the balloons provided a unique in situ data set for the lower equatorial stratosphere. This part 1 of a two‐part paper describes this data set and analyzes outstanding features in the planetary scales. Part 2 focuses on gravity‐wave scale. It is argued that balloon trajectories over the Pacific are primarily determined by the westward drift during the easterly phase of the equatorial quasi‐biennial oscillation (QBO) and the meridional velocity field of a mixed Rossby‐gravity (Yanai) wave with an apparent period of 4 days and zonal wave number 4. This wave appears to have two episodes of amplification during the balloon flights. It is also argued that the balloons show evidence of oscillations with periods between 2 and 4 days and of a Kelvin wave with an apparent period close to 10 days and zonal wave number 1. In this way, the balloon behavior provided a pictorial view of air parcel trajectory in the equatorial lower stratosphere. It is stated that larger balloon campaigns can provide excellent in situ data sets for studies on the dynamics and composition of the middle atmosphere.

The zonal drift associated with time-dependent particle motion on the earth

The zonal drift associated with the time-dependent particle velocity on the rotating earth, such as the inertial motion–westward in midlatitudes and eastward in the tropics–is shown to result from the conservation of angular momentum. This finding follows from a transformation of Newton's second law of motion on the earth into a dynamical system where the angular momentum is a system variable, a procedure similar to that used in the description of a spinning top's dynamics. In near-geostrophic zonal motion (i.e. time-dependent motion in the presence of a fixed meridional pressure gradient), where the angular momentum is conserved, and for angular momentum that is below a threshold value, the dynamical system possesses two elliptic points located off the equator. The fixed points of the dynamical system correspond to the steady, geostrophic, zonal motion so the time-dependent velocity of the small-amplitude motion (when the latitudinal particle excursion is not too large) deviates only slightly from the geostrophic velocity. The explicit expressions derived for the drift near the elliptic points imply that in near-geostrophic, eastward (westward) motion in midlatitudes the geostrophic, steady, speed provides an overestimate (underestimate) of the temporally averaged drift. This difference between the steady motion and the average of the periodic motion characterizes nonlinear systems. At large values of the angular momentum a single elliptic point exists on the equator and the drift is directed eastwards. This drift, too, can be quantified near the fixed point associated with a fixed zonal velocity on the equator, where the Coriolis force vanishes. At large oscillation amplitude in midlatitudes, when the oscillation encompasses a large latitude band and the instantaneous velocity deviates significantly from the geostrophic velocity, the zonal drift is strongly affected by the presence of a hyperbolic point at the equator. Explicit expressions for the zonal drift are not available in this case, but numerical results show that the effect of this nonlinearity is to greatly enhance the westward drift in midlatitudes compared with the small-amplitude case.

The zonal drift associated with time‐dependent particle motion on the earth

AbstractThe zonal drift associated with the time‐dependent particle velocity on the rotating earth, such as the inertial motion–westward in midlatitudes and eastward in the tropics–is shown to result from the conservation of angular momentum. This finding follows from a transformation of Newton's second law of motion on the earth into a dynamical system where the angular momentum is a system variable, a procedure similar to that used in the description of a spinning top's dynamics. In near‐geostrophic zonal motion (i.e. time‐dependent motion in the presence of a fixed meridional pressure gradient), where the angular momentum is conserved, and for angular momentum that is below a threshold value, the dynamical system possesses two elliptic points located off the equator. The fixed points of the dynamical system correspond to the steady, geostrophic, zonal motion so the time‐dependent velocity of the small‐amplitude motion (when the latitudinal particle excursion is not too large) deviates only slightly from the geostrophic velocity. The explicit expressions derived for the drift near the elliptic points imply that in near‐geostrophic, eastward (westward) motion in midlatitudes the geostrophic, steady, speed provides an overestimate (underestimate) of the temporally averaged drift. This difference between the steady motion and the average of the periodic motion characterizes nonlinear systems. At large values of the angular momentum a single elliptic point exists on the equator and the drift is directed eastwards. This drift, too, can be quantified near the fixed point associated with a fixed zonal velocity on the equator, where the Coriolis force vanishes.At large oscillation amplitude in midlatitudes, when the oscillation encompasses a large latitude band and the instantaneous velocity deviates significantly from the geostrophic velocity, the zonal drift is strongly affected by the presence of a hyperbolic point at the equator. Explicit expressions for the zonal drift are not available in this case, but numerical results show that the effect of this nonlinearity is to greatly enhance the westward drift in midlatitudes compared with the small‐amplitude case.

Drifts and Intensity Variations of the Geomagnetic Field

AbstractThe Briggs' method of correlation analysis of moving random pattern is modified. The characteristics of the westward drift and intensity variations of the non‐dipole geomagnetic field and 6 planetary‐scale geomagnetic anomalies are studied. During 1900‐2000, the non‐dipole field drifted westward at an average speed of 0:15°/α, and intensified by 29%. Six planetary‐scale magnetic anomalies exhibited different characteristics in the drift. The African Anomaly (AF) drifts westward at an average speed of 0:26°/α. The Australia Anomaly (AUS) drifts at an average speed of 0:23°/α. The slowest drift occurs in the Eurasia Anomaly (EA) at an average speed of 0:09°/α. As drifting westward, most anomalies drift northward slowly. During 1945‐1955, the anomalies EA, NAM, NAT in the northern hemisphere and AF near the equator changed its drift directions, from the westward to northwestward; a little later, the anomalies AUS and SAT in the southern hemisphere slowed down. In the 20 th century, the intensities of AUS, SAT, AF and EA, the largest in area and greatest in intensity anomalies, were increased, while the intensities of other two anomalies were decreased.

Steady core flow in an azimuthally drifting reference frame

SUMMARY Flows at the top of the Earth’s core generating the observed geomagnetic secular variation (SV) can be deduced in the frozen-flux approximation with various nonuniqueness-reducing assumptions such as tangential geostrophy and steadiness. Steady flows are attractive because they require only a small number of parameters to explain the gross features of the SV. However, they are unable to reproduce the fine detail contained in the SV, and cannot be used to explain observed decadal changes in the length of day. Here we find flows steady in a local reference frame within the core, but the frame is allowed to rotate relative to the mantle. Previous investigations have studied steady drift with respect to the mantle, introducing just a single extra parameter into the calculation; here we also allow the drift to vary with time. We then minimize a linear combination of the fit to time-varying coefficients expressing the SV, a measure of the complexity of the flow and the drift acceleration. The resulting non-linear inverse problem is solved in a two-stage iterative process—for a given drift, we solve for the best-fitting steady flow, and then adjust the drift to improve the fit. We seek solutions for the intervals 1900–1980 and 1840–1990; over both epochs, allowing the reference frame of a steady flow to drift gives a strikingly improved fit. For flows with a relatively high misfit, the frame drift is westwards at a rate similar to the observed ‘westward drift’ rate of the geomagnetic field at the Earth’s surface (approximately 0.2u yr x1 ). Requiring a tighter fit, and hence a more complex flow, gives rise to two solutions, one with a westward frame drift with respect to the mantle, the other eastward, and the relative drift rates gradually increase to a maximum of 0.9u yr x1 as the misfit decreases. Flows in the mantle reference frame are similar to those deduced previously by any of the nonuniqueness-reducing assumptions. However, in the drifting frame, the flows are almost completely dominated by the drift between the two reference frames. Although the time dependence of the drift is weak and results in only a small additional misfit reduction over a uniform drift, by assuming that the variation in drift reflects variation in solid body rotation of the whole core, it can explain decadal length-of-day changes almost as well as, and prior to 1900 perhaps better than, fully time-dependent tangentially geostrophic flows with vastly more free parameters. We examine the significance of these models in terms of large-scale wave motion in the core.

The storm‐substorm relationship: Ion injections in geosynchronous measurements and composite energetic neutral atom images

We have analyzed isolated and storm time ion injections using geosynchronous particles, energetic neutral atom (ENA) data, and Dst. There are both surprising similarities between the two classes of events as well as important differences that bear directly on the relationship between storms and substorms. The average geosynchronous ion responses during the growth phase, at onset, and in the ≈15 min following onset are nearly identical in intensity, spectral hardness, and temporal profile. ENA observations confirm that similarity and additionally show that the two classes of injections span nearly the same extent in local time. The two classes of injections differ primarily in the subsequent behavior of the ion fluxes. For the isolated injections the fluxes return to preevent levels within about an hour, and exhibit the expected westward drift and dispersion. For the storm time injections the fluxes remain elevated for at least several hours following the initial injection. Additionally, the ENA observations show new evidence that the region of new particle injections expands eastward (opposite to the ion drift direction) to encompass most of the nightside. Within 3 hours, ENA emissions are observed coming from most of the inner magnetosphere but have still not formed a symmetric, trapped distribution. Within those same 3 hours Dst decreased an average of 40 nT with the initial decrease observed in the same hour as the initial injection. The isolated injections did not produce a measurable Dst signature. These results show that despite many remarkable similarities, storm time ion injection events are different from isolated injection events

Secular Variations of the Planetary‐Scale Geomagnetic Anomalies

AbstractThe characteristics of a planetary‐scale geomagnetic anomaly are described by using “unsigned magnetic flux” through the anomaly region. The eighth generation of the international geomagnetic reference filed (IGRF) is analyzed to study the secular variations of 5 planetary‐scale geomagnetic anomalies during the 1900‐2000 period. The results show that the magnetic fluxes through the southern Atlantic anomaly (SAT), Australian anomaly (AUS), and African anomaly (AF) have increased by more than 200 MWb, the increase of the flux through the Eurasian anomaly (EA) is smaller (157MWb), while the flux through the North America anomaly (NAM) has decreased by 50MWb. On the other hand, the variations of the anomaly areas are within 10%. The westward drift at the core‐mantle boundary (CMB) is less than 0.1°/a, being much slower than that at the Earth's surface (0.2°/a). This contrast may be attributed to the different westward drift velocities of the various harmonics in the geomagnetic field: the westward drift at the Earth's surface is controlled by the dominant low‐degree harmonics, while the westward drift at the CMB largely depends on high‐degree harmonics. Consequently, it should be careful to take the westward drift velocity at the Earth's surface as the characteristic velocity of the fluid flow in the outer core.

Effects of the relative lithosphere–asthenosphere motion on the global tectonic features

The relative eastward rotation of the asthenosphere with respect to the lithosphere is explained by the westward-directed off-center rotation of the spinning Earth around the gravitational center of the Earth–Moon system (principle of hypocycloid gearing), and by the lateral viscosity variations at the base of the lithosphere. Independent geologic data favor a global westward drift of the lithosphere relative to the asthenosphere. Therefore, any attempt to investigate the mode of origin of the global tectonic features and the relationships among their tectonic components must consider this relative lithosphere–asthenosphere motion. As the angular velocity of the relative motion increases from the poles toward the equator, its effect on the global tectonic features also increases toward the equator. Consideration of the diverse effects of this relative lithosphere–asthenosphere motion on the west and east-facing subductions, on ternary plots, allows classification of east-facing island arcs into simple vs complex, and proximal vs distal.

Eddy Shedding from a Boundary Current around a Cape over a Sloping Bottom

Abstract The authors discuss laboratory experiments that elucidate the mechanism of formation and westward drift of anticyclonic baroclinic vortices from a buoyant surface current flowing along a lateral boundary and around a cape. Experiments were carried out with a sloping bottom in order to simulate the topographic β effect. They showed how a vortex can be generated from the current where it separates and reattaches to the cape and that, under some conditions, the eddy is able to detach from the cape and drift westward following isobaths. Two important timescales regulate the flow: the time tf that the current takes to generate a vortex and the time td that the vortex takes to drift westward for a distance equal to its radius. When these two timescales are either of the same order of magnitude or tf td the vortex was able to form at the cape but it did not detach and drift westward. The influence of the depth of the lower layer, h0, on the flo...

A phylogenetic and biogeographic study of Euniphysa (Eunicidae, Polychaeta)

Fourteen species have either been described in, or referred to, the genus Euniphysa. Seven of these are here re-described based on type material and two new species, E. quadridentata and E. filibranchia, are described. Euniphysa oculata is found to be a subjective synonym of E. spinea, and E. unicusa is a subjective synonym of E. aculeata. Euniphysa taiwanensis and E. megalodus are correctly assigned to the genus, but cannot be described due to lack of material. Euniphysa misakiensis, E. tubicola and E. tubifex are transferred to Eunice. A key is given to the nine identifiable species retained in Euniphysa. Coding strategies for polymorphic and inapplicable characters, as well as problems associated with shared absences, are discussed. A phylogenetic analysis of Euniphysa based on 24 morphological characters yielded two most parsimonious trees (CI = 0.902, RI = 0.905). The tree topology separates Euniphysa into two distinct groups. Group I includes E. filibranchia n. sp., E. italica, E. jeffreysii, E. quadridentata n. sp. and E. spinea, it is supported by five equivocal similarities. Group II is supported by five unequivocal synapomorphies and two equivocal similarities, it includes E. aculeata, E. auriculata, E. falciseta and E. tridontesa. Based on the phylogenetic topology, Paraeuniphysa and Heterophysa are considered as junior synonyms of Euniphysa. The recognition of a separate family for Euniphysa is not warranted. All species of Euniphysa are fragile, shallow, warm water species. They have been collected mainly from sandy sediments of the Northern Hemisphere. The greatest diversity is from the South China Sea area; other species are found throughout the Indian Ocean, the Mediterranean Sea and the East Atlantic Ocean coasts suggesting the genus may have originated in the Tethys Sea. A few species have also been found in the Gulf of Mexico and the West Atlantic Ocean coast again suggesting a Tethyan origin associated with the westward drift of the North American continent.

The interaction of two baroclinic geostrophic vortices on the β–plane

The interaction of two anticyclonic baroclinic vortices on the beta-plane is studied, taking into account the effects of vortex-vortex advection and the radiation of barotropic Rossby waves. Quasi-geostrophic dynamics is assumed in each of the two layers and the vortices are represented by delta-function anomalies in the upper-layer potential vorticity. The vortices are strong (i.e, their swirl velocities are large compared with the long baroclinic Rossby wave speed), but are sufficiently widely separated so that the advection of one vortex due to another is of the same order as the meridional drift caused by Rossby wave radiation. Centre of mass arguments are used to demonstrate that the velocity of a vortex is due to the sum of (a) advection due to the other vortex and (b) drift due to wave radiation.The far-field structure of the Rossby wave wake of two zonally aligned vortices (i.e. they have zero meridional displacement relative to each other) is found as a function of their relative position enabling the total drag on the centre of mass to be found. The drag and subsequent meridional drift of each vortex due to the wave radiation alone is then deduced.Equilibria are found in which the vortices have the same southward and westward drift, thus preserving their zonal alignment. Depending on the vortex separation distance, the equilibrium configuration may have meridional drift significantly different from that of a single vortex by itself. For example, when the ratio of the upper-layer depth to bottom-layer depth is 0.4, the meridional drift may be as little as 35% or as much as 117% of the single-vortex drift speed. The stability of the equilibria as a function of the vortex separation distance is found.

Westward drift of the lithosphere: not a result of rotational drag

It is shown that any non-zero torque resulting from differences in angular velocity between individual shells in the Earth would be an extremely short transient phenomenon as a consequence of the viscosity of the asthenosphere. Consequently, it cannot be a factor in the origin of the toroidal velocity field of degree one (‘westward drift’) of the lithosphere.

Emergence of modons from collapsing vortex structures on the β-plane

The evolution of unstable barotropic vortices is studied numerically. Exact solutions to the equation of potential vorticity conservation under the rigid lid condition, as well as nonsteady-state configurations, are set as initial states in the evolutionary experiments. The examined shielded modon structures usually collapse within one to several synoptic periods and radiate vortex pairs propagating westward and eastward. The latter are shown to be modons of Larichev and Reznik. The westward dipoles are identified as nonlocal modons, that is, vortical cores of stationary nonlinear Rossby waves. In the case of standing Stern modons, some small initial perturbations induce slow westward drift and subsequent collapse of the vortex structure due to the Rossby wave radiation, others lead to their transformation into Larichev and Reznik's modons. This conclusion is supported by the results of a numerical integration of the linear stability problem.

Rotation of the geomagnetic field about an optimum pole

Since 1693, when Halley proposed that secular change was the result of the westward drift of the main field, his simple model has undergone many refinements. These include different drift rates for dipole and non-dipole parts; separation into drifting and standing parts; latitudinal dependence of drift rate; northward drift of the dipole; and non-longitudinal rotations of the individual harmonics of the geomagnetic field. Here we re-examine the model of Malin and Saunders, in which the main field is rotated about an optimum pole which does not necessarily coincide with the geographical pole. The optimum pole and rotation angle are those that bring the main field for epoch T1 closest to that for T2 , as indicated by the coefficients of correlation between the spherical harmonic coefficients for the two epochs, after rotation. Malin and Saunders examined the pole positions and rates of rotation using data from 1910 to 1965, and noticed a number of trends. We show that these trends are confirmed by recent IGRF models, spanning the interval 1900–2000 and to degree and order 10. We also show that the effect of the level of truncation is small.

Westward Drift of the Geomagnetic Anomaly in East Asia

AbstractThe westward drift of East Asia geomagnetic anomaly is studied by using the technique of Correlation Analysis of Moving Random Pattern (CAMRAP) based on the International Geomagnetic Reference Field (IGRF) from 1900 to 1995. The drift velocity of the anomaly is calculated for X, Y and Z components, respectively. The drift velocity of Z component anomaly, which illustrates best the geomagnetic drift, is 0.07°/a in average for the 100 years. This is distinctly slower than the global average westward drift of 0.2°/a. At the same time the Z component anomaly slowly drifts northward at a speed of 0.02°/a. Further analyses indicate three stages in the drift of East Asia geomagnetic anomaly. In the first stage from 1900 to 1930 the westward drift was fast, with an average speed of 0.10°/a. Then, the drift changed to northwestward from 1930 to 1980. The westward speed was 0.07°/a and northward speed was 0.04°/a during this period. Finally, the drift almost stopped around 1980, and seems to change to eastwards.

A critique of frozen-flux inverse modelling of a nearly steady geodynamo

SUMMARY Most inversions of geomagnetic secular variation for £uid £ow at the core’s surface utilize the so-called frozen-£ux approximation where diiusion of the magnetic ¢eld is neglected. Here we note that the frozen-£ux approximation can fail miserably for the geophysically relevant case of a nearly steady dynamo. We draw this conclusion from an inspection of the induction equation, after expanding the velocity and magnetic ¢elds in terms of temporal mean and £uctuating parts (as is standard in mean-¢eld electrodynamics).The resulting pair of equations, one describing steady dynamo action and the other describing secular variation, are coupled, and since steady dynamo action relies on diiusion, secular variation cannot be approximately described by the (diiusive-free) frozen-£ux approximation. In support of this conclusion we present two new kinematic dynamo models. The ¢rst dynamo exhibits no secular variation even though it has a non-zero surface £ow; for this dynamo no purely poloidal surface £ow model can be constructed by using the frozen-£ux approximation, but the actual surface £ow is purely poloidal.The second dynamo exhibits westward drift of the magnetic ¢eld but has surface £ow that is eastwards. Both of these models have magnetic Reynolds numbers much greater than unity, and so satisfy the criterion usually invoked for justifying the use of the frozen-£ux approximation, and yet both of these models are contradictory to the frozen-£ux approximation. Thus, inversions for core £ow should consider the eiects of diiusion. Core £ow models deduced assuming that the frozen-£ux approximation is valid, and any results dependent on such £ow models, should be treated with a great deal of caution.

The pattern and variability of Antarctic sea‐ice drift in the Indian Ocean and western Pacific sectors

Sea‐ice drift physically redistributes pack ice and changes ice extent, concentration, and, through deformation, the ice‐thickness distribution. In this paper, data are presented from 39 satellite‐tracked buoys, deployed during various seasons from 1985 to 1996 in the sea ice of the Southern Ocean off East Antarctica between 20° and 160°E longitude. The dominant features of the ice motion in the region are a westward drift parallel to the bathymetry near the Antarctic continent, a cyclonic circulation cell in Prydz Bay, and eastward drift of the ice to the north of the zonal shear zone. The oceanic circulation along the coast is generally barotropic and the ice drift is well correlated with bottom topography. Northward outflows, the locations of which are determined by both bottom topography and the seasonally varying position of the zonal shear zone, allow the discharge of sea ice from the westward drift in the south into the northerly belt of eastward flow, but with considerable variability in the net northward ice transport. The ice translation monitored by the buoys is used to derive the spatial pattern of the ice‐velocity field. The daily average ice‐drift speed in the westward flow is 0.23 m s−1 (19.8 km d−1), with considerable spatial and temporal variability, and in the eastward flow the average is 0.17 m s−1 (15.1 km d−1). Seasonal and interannual ice‐drift variabilities are analyzed. The results are compared with satellite data of sea‐ice extent and concentration over the same time, as well as with hydrographic observation of the position of the Antarctic Divergence.

Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration

Numerical calculations of fluid dynamos powered by thermal convection in a rotating, electrically conducting spherical shell are analyzed. We find two regimes of nonreversing, strong field dynamos at Ekman number 10−4 and Rayleigh numbers up to 11 times critical. In the strongly columnar regime, convection occurs only in the fluid exterior to the inner core tangent cylinder, in the form of narrow columnar vortices elongated parallel to the spin axis. Columnar convection contains large amounts of negative helicity in the northern hemisphere and positive helicity in the southern hemisphere and results in dynamo action above a certain Rayleigh number, through a macroscopic α2 mechanism. These dynamos equilibrate by generating concentrated magnetic flux bundles that limit the kinetic energy of the convection columns. The dipole‐dominated external field is formed by superposition of several flux bundles at middle and high latitudes. At low latitudes a pattern of reversed flux patches propagates in the retrograde direction, resulting in an apparent westward drift of the field in the equatorial region. At higher Rayleigh number we find a fully developed regime with convection inside the tangent cylinder consisting of polar upwelling and azimuthal thermal wind flows. These motions modify the dynamo by expelling poloidal flux from the poles and generating intense toroidal fields in the polar regions near the inner core. Convective dynamos in the fully developed regime exhibit characteristics that can be compared with the geomagnetic field, including concentrated flux bundles on the core‐mantle boundary, polar minima in field intensity, and episodes of westward drift.

Lagrangian Exploration of the California Undercurrent, 1992–95

Abstract During the period 1992–95, nineteen isobaric RAFOS floats, placed in the California Undercurrent at intermediate depths (150–600 m) off Monterey and San Francisco, California, reveal a region of varying width of subsurface, poleward flow adjacent to the continental margin. The float trajectories exhibit three patterns: poleward flow in the undercurrent; reversing, but predominately alongshore, flow adjacent to the continental margin;and, farther offshore, anticyclonic motion accompanied by slow westward drift. Flow continuity of the undercurrent exists between Pt. Reyes and at least Cape Mendocino with an average speed dependent on the float depth. Speeds were variable, but common features were acceleration occurring to the south of Pt. Arena and deceleration to the north of Cape Mendocino. An important mechanism for floats, and water, to enter the ocean interior from the undercurrent is through the formation of submesoscale coherent vortices.