Loop Current Rings Research Articles

Low-frequency current variability observed at the shelfbreak in the northeastern Gulf of Mexico: May–October, 2004

High-resolution current measurements were made in the Northeastern Gulf of Mexico by the Naval Research Laboratory (NRL) as part of its Slope to Shelf Energetics and Exchange Dynamics (SEED) project. The major goal of SEED is to understand the mechanisms that transfer properties across the shelf slope. Fourteen acoustic Doppler current profilers (ADCPs) were deployed just west of the DeSoto Canyon on the shelf and down the slope from May to November, 2004 to measure nearly full water column current profiles. Currents were found more variable on the shelf than on the slope but in the mean strongly tended to follow bathymetry, particularly on the slope. During the SEED time period currents were driven by both local and remote winds, by cyclonic eddies associated with the Loop Current extension and Loop Current rings, by smaller eddies associated with the cyclonic eddies, by frontal meanders or streamers associated with the eddies, and by tropical storms. Currents were highly barotropic, accounting for more than 80% of the eddy kinetic energy (EKE). Current magnitudes generally increased from west to east, towards the DeSoto Canyon. Tropical storms had a relatively minor short-term effect upon mass transports. Cross-shelf transports were much smaller than the along-shelf transports. Onshore transports were stronger on the western side of the array while offshore transports were stronger on the eastern side of the array near the DeSoto Canyon. Offshore transports generally occurred during eastward flow periods, onshore transports during westward flow periods, and both during eddy periods. Mesoscale eddies also provided contributions to cross-shelf exchange. Large scale circulation features could be determined from the first two empirical-orthogonal function (EOF) modes which accounted for 83% of the variance and were strongly related to the integrated wind stress.

Loop Current Ring Shedding: The Formation of Cyclones and the Effect of Topography

Abstract The formation of cyclones in the vicinity of the Loop Current ring during the separation stage is analyzed in the frame of a high-resolution ECMWF daily wind-forced Miami Isopycnic Coordinate Ocean Model (MICOM) simulation. Mesoscale cyclones, observed in sea surface height maps in the vicinity of the Loop Current in a necking-down position, are found to contribute to the separation of the ring from the Loop Current as they grow between the Loop Current and the ring in the MICOM simulation. To understand the origin of the cyclones, the instability of the Loop Current idealized as an isolated vortex is studied. After noticing the cyclonic vorticity belt around the Loop Current, and based on the vertical distribution of potential vorticity in a Loop Current ring in the MICOM simulation, the linear stability of a shielded vortex is studied in the quasigeostrophic formalism. To simulate the effects of the planetary vorticity gradient and topography on the Loop Current, the nonlinear states of the idealized Loop Current are analyzed using the adiabatic MICOM code. Results from the analysis of the MICOM simulation show that cyclones are the products of a vortex rim instability. The modal analysis of the Loop Current instability reveals that mode 4 is the fastest-growing mode and that baroclinic (barotropic) instability is intensified in the deep (surface) layers. These results are confirmed by the analytical study of the idealized Loop Current. The nonlinear state shows that a Loop Current–like vortex is indeed a pentapole on an f plane. On the β plane, the northern cyclone is separated from the anticyclone by the β effect and both drift westward. When the topography of the Gulf of Mexico is taken into account—namely, the Campeche Bank, the southward slope north of the Loop Current, and the Florida shelf east of the Loop Current—several effects are observed: 1) the northern corner of the Campeche Bank erodes the Loop Current ring and its cyclones and interacts with the vortex’s most unstable mode, 2) the northern southward slope scatters the northern cyclone while the anticyclone remains coherent and propagates to the west, and 3) realistic westward propagation speeds are obtained in the presence of the northern Campeche shelf, which acts as a mirror effect on the Loop Current ring, as opposed to the Florida shelf, which tends to block the ring.

Deep flow variability in the vicinity of the Yucatan Straits from a high‐resolution numerical simulation

The deep flow variability in the vicinity of the Yucatan Channel between the Caribbean Sea and the Gulf of Mexico is examined within a high‐resolution numerical simulation of the North Atlantic Ocean. We focus on the 6 year integral cycle of Loop Current (LC) ring formation, and we study the flow regimes both in the Yucatan Channel and farther north in the LC. The circulation in the vicinity of the Yucatan Channel presents a high variability in the flow regimes and in the shedding period, in good agreement with earlier observations. The outflow (toward the Caribbean Sea) in the Yucatan Channel is shown to be controlled in part by a regular shift of the LC maximum velocity position, in phase with the transport variations. The outflow follows the inflow variations independently of the northward extension of the LC. Our analysis of the growth of the loop is also shown to be in good agreement with the ballooning process proposed by Pichevin and Nof (1997) and Nof and Pichevin (2001) to explain the LC ring formation. Moreover, at the end of each cycle of ring formation a sudden deepening of the LC deepest layer occurs together with an intensification of the transport and of the currents in the deep layers beneath the LC. This process is shown to be connected with the growth of cyclones in the deep layers. They result from the bottom‐intensified instability of the LC ring and contribute to its separation from the LC.

Numerical simulation of the Gulf Stream System: The Loop Current and the deep circulation

The Loop Current and the deep circulation in the Gulf of Mexico are numerically investigated by a primitive equation, sigma coordinate ocean model with realistic surface fluxes obtained from an atmospheric forecast model. A deep cyclonic circulation, bounded by the deep basin in the eastern Gulf of Mexico, is spun up by the Loop Current; the deep cyclonic circulation is coincident with a southward current of the Loop Current eastern limb and weakens after Loop Current ring separation and cessation of the southward current. The anticyclonic, semienclosed Loop Current also induces anticyclonic lower layer columnar eddies in the eastern gulf. These lower layer eddies decouple from the upper layer Loop Current. The westward translation speed of a Loop Current ring is about 2.16–5.18 km d−1; the lower layer eddies have a higher speed and lead the rings into the central gulf. The time‐averaged surface circulation of the Gulf of Mexico basin is anticyclonic, mainly because of the transport of anticyclonic vorticity by Loop Current rings in the surface layer an average lower layer cyclonic circulation occurs along the continental slope of the basin.

The Loop Current and adjacent rings delineated by Lagrangian analysis of the near-surface flow

Evolution of the Loop Current and the adjacent mesoscale rings was investigated over a 20-day period, starting June 1, 1998. Model sea-surface height and near-surface velocity fields from the Colorado University Princeton Ocean Model of the Gulf of Mexico were used along with drifter data in the study. A Lagrangian analysis provided detailed information about ring interaction and revealed features that were not apparent in the Eulerian picture. During the observation period, a new ring was formed in the meander of the Loop Current and a large anticyclonic ring to the west was cleaved by a cyclonic eddy. The coherent structures were identified in the Lagrangian framework by means of material curves (manifolds) that act as their boundaries. Concurrent drifter trajectories were in qualitative agreement with the results of the Lagrangian analysis.

Can general circulation models be assessed and their output enhanced with drifter data?

Drifter data from the Gulf of Mexico are used to assess and enhance the output of a primitive equation general circulation model. The analysis is made in a 450 km × 450 km open subdomain encompassing a Loop Current ring. The model velocity field is compared with position data from four drifters at the drogue depth of 50 m using geometrical orthogonal functions (GOF). An Eulerian velocity field is reconstructed from the model velocity field and drifter velocities. This reconstructed velocity improves 8‐day numerical trajectories relative to the model field by at least an order of magnitude, as quantified by two Lagrangian error metrics referenced to the real drifter paths. An Eulerian metric that compares the two fields, however, does not exceed 7% for the 20‐day assessment period. Thus the drifter data may be reproduced with modest impact on the model velocity. Enhancement of the model velocity field is determined by two tests: the ability of the GOF velocity field to (1) improve the forecast of drifter positions using only a posteriori data and (2) improve the forecast of withheld drifter data. Using a posteriori data, the 20‐day temporal mean of the position error is improved for all drifters by 87–89% for 6‐hour and 26–38% for 30‐hour forecasts. For 6 days, a withheld drifter is 35–40 km from a drifter whose velocity is used in the reconstructed velocity field. The temporal mean of the position error during this period is improved by 20% for 6‐hour and 26% for 30‐hour forecasts.

Loop Current rings and the deep circulation in the Gulf of Mexico

A detailed examination of the three‐dimensional circulation associated with Loop Current rings in the Gulf of Mexico has been made using the Modular Ocean Model with 15 levels in the vertical and ⅛° horizontal grid spacing. Monthly mean wind stress and a seasonal boundary condition on temperature and salinity fields are applied at the surface. The vertical shear and geostrophic transport of the Yucatan Current also vary seasonally. The upper layer seasonal circulation, and the formation, structure, and westward migration of Loop Current rings in the model compare favorably with observations. An anticyclone‐cyclone pair develops in the deep layer beneath the Loop Current as a ring is forming in the eastern Gulf. The model circulation beneath the Loop Current reverses from anticyclonic to cyclonic because the anticyclone leads the pair in the deep layer as the ring separates and migrates westward in the surface layer. The deep anticyclone‐cyclone pairs are guided by the bottom bathymetry as they migrate westward in tandem with the rings in the surface layer. The deep anticyclone decays more quickly than the cyclone in the western Gulf, and the cyclone dominates the deep circulation beneath the ring. Cyclonic flow in the deep Gulf of Mexico has been observed using current meters, inferred from hydrographic data, and produced in other numerical and theoretical models. Although the formation and westward migration of deep anticyclone‐cyclone pairs in conjunction with rings has not been directly observed, the results of this modeling study agree with other published reports on the deep circulation in the Gulf of Mexico.

Ring‐slope interactions and the formation of the western boundary current in the Gulf of Mexico

Hydrographic data from the Gulf of Mexico (gulf) provide evidence that a western boundary current was set up by the interaction of an anticyclonic Loop Current (LC) ring with the continental margin of the western gulf during March‐August 1985. The March 1985 geostrophic circulation reveals a remnant anticyclonic ring colliding with the slope. During this collision, two cyclonic rings were shed as the anticyclone transferred vorticity to the surrounding slope water. During July‐August 1985, the ring triad weakened and evolved into a ∼900‐km‐long, north flowing, along‐slope, western boundary current and cyclonic‐anticyclonic ring pairs distributed throughout the central and western gulf. This western boundary current attained maximum northward flow speeds of 25 cm s−1 and an 8.3‐Sv mass transport between 94°–96°W at 25°N. Our March‐August 1985 observations reveal that the residence time and decay period of LC anticyclones in the western gulf may exceed 150 days. Within this time period the western gulf's cyclonic‐anticyclonic vorticity field decayed ∼50%. Thus the western boundary current's evolutionary period, from its gestation to its absolute decay, is estimated to be of the order of 300 days. Although the presence of a western boundary current in the gulf has been attributed to the annual wind stress curl cycle [Sturges, 1993], our analyses of the western gulf March and July‐August 1985 ring‐driven geostrophic circulation and corresponding (January, February and May, June 1985) monthly mean synoptic wind stress curl distributions reveal that these constitute competing forcing mechanisms for the gulf's regional circulation. However, when very strong local forcing such as large eddies are present, the wind‐driven background circulation is overwhelmed by such eddy forcing.

Strange encounters of eddies with walls

The impingement of a westward drifting lens-like anticyclonic ring on a meridional boundary is examined analytically and numerically using an (inviscid) layer-and-a-half model. A nonlinear analytical solution is constructed with the aid of a slowly varying perturbation expansion and numerical simulations are performed with the Bleck and Boudra model. It is found that, in contrast to conventional wisdom which implies that an anticyclonic ring encountering a wall moves poleward due to the image effect, the ring stays roughly in a e xed latitude. As it slowly moves into the wall due to b , it gradually leaks e uid toward the equator until it loses all of its mass. As expected, different eddies lose their mass in different rates. The (nondimensional) central thickness of an intense zero potential vorticity ring decays as 1/[1 1 b (2g8Hi) 1/2 t/18f ] 2 where Hi is the initial central thickness, t is time, g8 the ‘ ‘ reduced gravity,’ ’f the Coriolis parameter, andb is the familiar variation of the Coriolis parameter with latitude. Due to the gradually decreasing depth, the ring’ s westward drift into the wall also gradually slows down; i.e., the ring advances toward the wall and loses mass like a gradually peeling onion. The ‘ ‘ peeling’ ’ rate is 2 b Rd /9 [where Rd is the instantaneous Rossby radius based on the instantaneous maximum thickness] implying that the eddy migrates toward the wall at a rate that is one-third of the free open ocean rate. Numerical simulations are in excellent agreement with the above solution.As expected, they show that frictional forces decrease the intensity of the leakage and, hence, slow down the draining process. Unfortunately, no analytical solution can be found for weak or moderately strong rings because, for such lenses, there is no known analytical solution even for the nonmigrating state (i.e., rings on an f-plane). Numerical simulations show, however, that such weak or moderately strong rings display (qualitatively) similar characteristics to those of intense zero potential vorticity rings. It is argued that rings such as warm-core Gulf Stream rings, Loop Current rings,Agulhas rings, as well as other eddies (e.g., Meddies) may all experience similar encounters. As the walls that they encounter are subsurface (e.g., the continental rise, the Walvis Ridge and the mid-Atlantic Ridge), the leakages are not expected to be easily detected.

Ecological Implications of Hydrography and Circulation to the Flower Garden Banks, Northwest Gulf of Mexico

A synthesis of historic and new physical oceanographic observations on the Louisiana-Texas shelf helps change the circulation paradigm around the Flower Garden Banks. The location of these banks near the shelf edge shields them from the direct influence of the Mississippi and Atchafalaya Rivers and summer hypoxic episodes but places them under direct influence of deepwater physical processes. These deepwater processes control temperature and salinity within ranges that are adequate for coral growth; however, summer temperatures can exceed thermal tolerance limits as evidenced by coral bleaching episodes. Currents near the Banks have longshore and significant offshore components. Loop Current (LC) rings and companion eddies (anticyclones and cyclones) with spatial scales of 30-150 km and residence times ~ 6 mo over the slope are the main driving force of shelf edge currents. This active eddy field induces significant and frequent cross-shelf water exchanges with the deep Gulf, which help regulate temperature, salinity, and larvae dispersal. The eddies also induce oxygen and nutrient enrichments near the mixed layer by sinking and rising water parcels that can help the reef biota. Four links between the Flower Garden Banks and the rest of the Gulf of Mexico are identified: the shelf edge current, LC rings and associated eddies, the mean westward surface wind drift in the Gulf, and alongshelf edge intrusions from the northeastern Gulf of Mexico. The coral reefs of the Flower Garden Banks can function as repositories and sources of reef biota.

Winter water mass distributions in the western Gulf of Mexico affected by a colliding anticyclonic ring

The winter water mass distributions in the western Gulf of Mexico, affected by the collision of a Loop Current anticyclonic ring, during January 1984 are analyzed. Two principal modes of Gulf Common Water (GCW) formation, arising from the dilution of the Caribbean Subtropical Underwater (SUW), are identified. Within the western gulf continental slope to the east of Tamiahua, the GCW is formed by the collision of anticyclonic rings. During these collision events, the SUW, entrapped at the core (200 m depth) of these features, is diluted by low salinity (36.1≤S≤36.3‰) water from the uppermost layer of the main thermocline. The end product of this mixture is GCW, which is further diluted by low salinity coastal water within the western gulf continental shelf. The second GCW formation mode is associated to the northerly wind stress which propagates over the western gulf during winter. During January, 1984, this wind stress gave rise to a 175 m mixed layer. This convective mixing destroyed the static stability of the summer thermocline and allowed for the partial dilution of the SUW with low salinity (S≤36.3‰) water from the western gulf continental shelf. Within the western gulf's upper 2000 m, the following water masses were identified to be present: GCW, SUW, Tropical Atlantic Central Water and associated dissolved oxygen minimum stratum, Antarctic Intermediate Water remnant, a mixture of the Caribbean Intermediate Water and the upper portion of North Atlantic Deep Water (NADW), and the NADW itself. The topographic distribution of these water masses' strata was dictated by the cyclonic-anticyclonic baroclinic circulation that evolved from the anticyclone's collision to the east of Tamiahua. Between the cyclonic and anticyclonic domains, the maximum pressure differential of these water masses' core occurrences was 150 to 280 dbar. The topographic transition zone defined by these strata occurred between the cyclonic and anticyclonic domains and coincided unambiguously with the anticyclone's collision zone. Within the continental shelf, we identified low temperature (12°C) and low salinity (31‰) coastal waters contributed by river runoff. Driven by the northerly wind stress, these coastal waters were advected toward the south hugging the coastline. The coastal and continental shelf waters demarcated a sea surface temperature, salinity, and dissolved oxygen discontinuity region that coincided with the horizontal baroclinic flow transition zone associated to the anticyclone's collision.

The annual cycle of the western boundary current in the Gulf of Mexico

It has been known for decades that the mean circulation in the western Gulf of Mexico is a large anticyclone. This flow is driven both by wind stress curl and detached rings from the Loop Current. This paper is an attempt to examine their relative importance by focusing on the annual cycle of the observed flow. A new compilation of ship drift data shows that the flow along the western boundary is strongest in July and weakest in October. It appears physically plausible that the annual variation of this western boundary current is driven by the annual variation in wind curl augmented by Ekman pumping over the western gulf. Because the maximum in wind curl is so close to the western boundary, the phase delay between the annual cycles of curl and current is only approximately 1 month. The magnitudes of the curl and resulting current seem physically plausible. The response is baroclinic and penetrates to ∼1000 m, the bottom of the main thermocline. The change in velocity at the sea surface is consistent with the cross‐stream pressure gradient as determined by the changes in sea level at the coast and dynamic height changes offshore. The annual cycle is relatively large; at the annual minimum the flow is only approximately 25% of the maximum. The data suggest that this flow could be driven by wind curl, but the uncertainty is high. Consequently, the contribution of Loop Current rings turns out to be relatively small. The reason for this is that when rings reach the western wall, the steep bottom topography intersects the sloping fluid contours of the rings. The rings decay slowly by normal deepwater processes until they reach the edge of the continental shelf, and then they decay more rapidly by loss of fluid. Once the rings have reached the western boundary, the decay time scale is found to be only ∼ 70 days.

Correction [to “Observed and simulated kinematic properties of loop current rings” by A. D. Kirwan, Jr., et al.

Correction [to “Observed and simulated kinematic properties of loop current rings” by A. D. Kirwan, Jr., et al.

Collision of a loop current anticyclonic ring against the continental shelf slope of the western Gulf of Mexico

A Loop Current anticyclonic ring ∼330 km in diameter and extending to a depth of >1500 m was observed to collide in January of 1984 against the continental shelf slope of the western Gulf of Mexico between 21.5° and 23°N. The collision occurred precisely at the time we conducted our Argos 84−1 hydrographic cruise in the western gulf (26°00′ to 19°20′N) aboard the R/V Justo Sierra. The Caribbean Subtropical Underwater (SUW) was used as a tracer to identify the Loop Current anticyclonic ring within the western gulf. The collision was identified from temperature and salinity distributions and from the dynamic topography distribution relative to 500 m. The ring's collision zone was identified by the presence of a horizontal baroclinic flow divergence, to the east of Tamiahua, that divides the surface circulation into northward and southward baroclinic currents parallel to the western gulf's continental shelf break, with speeds of 85 and 32 cm s−1, respectively. Horizontal divergence and vertical convergence (ring asymmetries) resulted at the focus of the anticyclonic ring's collision and originated the alongshore self advection and northward translation of the colliding anticyclone. Upon colliding the anticyclonic ring shed approximately one third of its volume (∼2 × 104 km3), mass, and transferred angular momentum to the south flanking water mass, thus generating a cyclonic ring to the south of the collision zone. The observed alongshelf southward current results from mass conservation and volume continuity requirements associated with the anticyclonic ring's volume shedding and most probably constitutes the colliding ring's potential vorticity conservation mechanism. The weakening of the anticyclonic ring's relative vorticity due to the collision is most likely made up by gain of vorticity from lateral shear in the northward and southward current jets parallel to the continental shelf break. The core of both the anticyclonic and cyclonic rings had typical SUW salinity (>36.5‰) and temperature (∼22.5°C) values. The rings were separated by a 5 × 104 km2 divergence zone occupied by Gulf Common Water (GCW). The SUW was absent within the collision zone to the east of Tampico (22.3°N, 97.8°W). The GCW within this divergence zone resulted from the convective mixing and dilution of the SUW with less saline (36.1 ≤ S ≤ 36.3‰) water from the uppermost layer of the thermocline. Hence the collision of Loop Current anticyclones against the western continental shelf slope of the gulf constitutes a primary mechanism by which 30 Sv of SUW are converted to GCW in the Gulf of Mexico. On the other hand, the coastal and continental shelf water temperature and salinity distributions that resulted from the ring's collision indicate that the offshore GCW mass intrudes the continental shelf to the east of Tamiahua and is diluted by low‐salinity coastal water within the western continental shelf. This GCW mass intrusion most probably constitutes a principal and efficient exchange mechanism between the western gulf's continental shelf and offshore waters.

Ring evolution in general circulation models from path analysis

The thesis explored here is the use of path data for comparing primitive equation models with observations. Such comparisons naturally focus on the inversion of path data for fundamental kinematic characteristics and subsequent descriptive interpretations of both models and observations. The analysis here focuses on uncertainties that may affect this interpretation. These include random position errors and numerical noise introduced by the nonlinear inversion ansatz as well as dynamical processes that introduce both low‐ and high‐frequency variability in the kinematic fields. After each of these effects is assessed, the inversion ansatz is applied to paths generated from an eddy‐resolving general circulation model of the Gulf of Mexico. Model Loop Current ring centers determined from different paths are found to be consistent and in good agreement with observations in the abyssal gulf. In addition, these centers are compared with the model surface and interface pressure anomaly centers. The simulated paths showed ejection of mass from the Loop Current ring as it interacted with the western slope. Observed and simulated ring kinematics inferred from appropriate paths are also made and compared with observations for the slope region. The paper concludes with a discussion of the potential for using simulated and observed path data for extracting flow physics and quantifying comparisons between models and observations.

Observed and simulated kinematic properties of Loop Current rings

Two rings, shed by the Loop Current in 1980 and 1982, were observed for several months by satellite‐tracked drifters to migrate across the Gulf of Mexico. The drifter path data have been inverted to obtain estimates of the paths of the centers of the two rings, ring shape, and the swirl velocities. Three drifters were deployed in the 1980 ring, and the analysis of that data set establishes the variability of the above kinematic estimates for one ring. A comparison of the analysis of data from both rings provides some idea on inter‐ring variability. Both rings impacted the Mexican continental slope at about 22.8°N, 95.5°W. After a brief adjustment period, both rings reestablished and maintained a vortex character for several months in the slope region while migrating slowly to the north. The paths of the centers of the two rings along the slope are virtually identical. The same analysis routine was applied to some simulated drifter data obtained from the Hurlburt and Thompson (1980) Gulf of Mexico primitive equation model. In the midgulf, the agreement between the observed rings and the simulated ring is good, although the former showed stronger interaction with the continental slope topography and/or circulation than was seen in the latter. Along the slope, the model ring kinematic characteristics were in extraordinary agreement with the observations.

Genesis of a Gulf of Mexico ring as determined from kinematic analyses

The kinematics of the Loop Current are studied using trajectories of drifters in the Gulf of Mexico during mid‐June through September 1985. One of the drifters was in the Loop Current proper, while other drifters were in two recently shed Loop Current rings. The drifter in the Loop Current showed strong anticyclonic motion during the study period. This Loop Current anticyclone first began off the northwest coast of Cuba. It rapidly moved northward into the Gulf of Mexico as a ring pinched off from the Loop Current. Analysis of the Loop Current drifter motion showed that the anticyclone became an integral part of the Loop Current, taking on many of the characteristics of the most recently shed ring. The results of the analysis suggest a process by which Loop Current rings can be generated. Apparently, this mechanism can cause the Loop Current to become reconfigured in 2–3 months for beginning the process of ring separation.

Five‐year climatological survey of the Gulf Stream system and its associated rings

A 5‐year climatological survey of the Gulf Stream System's landward surface edge defines the mean, standard deviation, extreme limits, annual and interannual variabilities, and frequency distributions along 0.5° longitudinal transects from 91° to 44°W. The climatology covers the Loop Current in the Gulf of Mexico and the Gulf Stream in the northwest Atlantic Ocean. Autocorrelations and power spectra are derived for longitudinal transects between 79.5° and 57.5°W. The study also investigates the observed warm‐core ring and cold‐core ring movements and their areal extent. The observed data are compiled from the Oceanographic Analysis charts (produced by the National Oceanic and Atmospheric Administration) which utilize infrared satellite imagery and in situ observations. East of Cape Hatteras the Gulf Stream System Landward Surface Edge exhibits both interannual and annual signals. The interannual variability is equal to the annual variability. The mean monthly Gulf Stream position for the longitudinal band 70°–44°W shifts from farthest north in September to farthest south in February. The magnitude of the annual shift is largest near the Grand Banks. Frequency distributions of the Gulf Stream System position are quasi‐normal nearly everywhere. Autocorrelation plots from the 5‐year time series of Gulf Stream positions at longitudinal transects 79.5°–57.5°W exhibit three well‐defined e‐folding time regimes with a maximum time of 10 weeks at 72°W. Power spectrums from 79.5° to 57.5°W are red spectrums having dominant peaks of 1‐year period or greater. The largest total spectra energies are found at 61°W. The Loop Current sheds rings at two apparent spatial scales which are labeled major (307‐km mean) and minor (185‐km mean) warm‐core rings. Annually, the Loop Current sheds about one major warm‐core ring which translates into the western basin of the Gulf of Mexico and one minor warm‐core ring which is usually reabsorbed by the Loop Current. The major Loop Current warm‐core ring sheddings have no seasonal preference. Loop Current warm‐core rings have a mean velocity of 2.4 km/d west‐southwest. An average of 22 Gulf Stream warm‐core rings are formed and absorbed annually between 75° and 44°W. The largest frequency of warm‐core ring formations is seen near the New England Seamounts, while the largest frequency of warm‐core ring absorptions is found near Cape Hatteras. Gulf Stream warm‐core rings can absorb each other as occurred in 14% of the absorption cases with the highest frequency just upstream of the New England Seamounts. The Gulf Stream warm‐core rings have a surface diameter decay rate of − 0.026 per week (Loop Current warm‐core rings are similar). The Gulf Stream warm‐core rings have a mean velocity of 2.4 km/d west‐southwest. Cold‐core rings have a mean cool‐core surface diameter of 105 km and a mean velocity of 1 km/d southwest. More definitive conclusions on cold‐core rings are not possible, as ring identification and tracking by satellite infrared imagery is tentative.

Some observations of ring topography and ring‐ring interactions in the Gulf of Mexico

The movement and interactions of Loop Current rings are studied using trajectory and sea surface temperature (SST) data from the Gulf of Mexico. The movements of three rings as determiend by drifters seeded in the rings are compared. The data show the rings moving west through the deepest portion of the Gulf and then northward after impacting with the Mexican coast. The trajectory data indicate a ring life span of at least 9–13 months, with two of the rings persisting off the Mexican coast for at least 3–5 months. The trajectory of a fourth drifter, not seeded in a ring, still shows strong anticyclonic motion. The SST data for the period that this last drifter was deployed show the interaction of three anticyclones in the western Gulf. The drifter circles all three anticyclones and shows, along with the SST data, the coalescing of two of the anticyclones.