Large Anticyclone Research Articles

Marine heatwaves suppress ocean circulation and large vortices in the Gulf of Alaska

Large-scale anticyclonic vortices forming along the Gulf of Alaska continental slope serve as fertile ecosystems for marine life, significantly shaping the distribution of primary productivity, with 40–80% of the gulf’s open ocean surface chlorophyll-a concentrated in their cores. Between 2013 and 2023, Alaska experienced some of the largest and longest marine heatwaves ever recorded in the world’s oceans, persistently altering its ecosystem and fisheries. Here, using 30 years of satellite and reanalysis data, we find that the coastal upwelling atmospheric forcing conditions associated with the heatwaves have also significantly suppressed the Gulf of Alaska’s ocean circulation and the formation of large anticyclones. Climate model simulations spanning from 1850 to 2100 suggest that future changes in the Aleutian Low pressure system will lead to a 60% increase in upwelling extremes (>2 standard deviations), further weakening the ocean anticyclones. However, large uncertainties remain in the mechanisms controlling the Aleutian Low’s response to climate forcing in the models.

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.

Cover Photograph

Front cover: Altocumulus floccus, looking east from Randwick, near Stroud, on 10 July 2022 at 1901 utc. A large anticyclone was located over Britain and Ireland at the time, with local maximum temperatures in the high 20s Celsius. (© Sharon Andrews.)image

Jupiter's third largest and longest-lived oval: Color changes and dynamics

The transition region between the North Equatorial Band (NEBn) and North Tropical Zone (NTrZ) in Jupiter is home to convective storms, systems of cyclones and anticyclones and atmospheric waves. Zonal winds are weak but have a strong latitudinal shear allowing the formation of cyclones (typically dark) and anticyclones (typically white) that remain close in latitude. A large anticyclone formed in the year 2006 at planetographic latitude 19°N and persists since then after a complex dynamic history, being possibly the third longest-lived oval in the planet after Jupiter's Great Red Spot and oval BA. This anticyclone has experienced close interactions with other ovals, merging with another oval in February 2013; it has also experienced color changes, from white to red (September 2013) and back to white with an external red ring (2015–2016). The oval survived the effects of the closely located North Temperate Belt Disturbance, which occurred in October 2016 and fully covered the oval, rendering it unobservable for a short time. When it became visible again at its expected longitude from its previous longitudinal track, it reappeared as a white large oval keeping this color and the same morphology since 2017 at least until the onset of the new convective disturbance in Jupiter's North Temperate Belt in August 2020. Here we describe the historic evolution of the properties of this oval. We use JunoCam and Hubble Space Telescope (HST) images to measure its size obtaining a mean value of (10,500 ± 1000) x (5,800 ± 600) km2 and its internal rotation finding a value of -(2 ± 1)·10−5 s−1 for its mean relative vorticity. We also used HST and PlanetCam-UPV/EHU multi-wavelength observations to characterize its color changes and Junocam images to unveil its detailed structure. The color and the altitude-opacity indices show that the oval is higher and has redder clouds than its environment but has lower cloud tops than other large ovals like the GRS, and it is less red than the GRS and oval BA. We show that in spite of the dramatic environmental changes suffered by the oval during all these years, its main characteristics are stable in time and therefore must be related with the atmospheric dynamics below the observable cloud decks.

Observations and numerical modelling of a convective disturbance in a large-scale cyclone in Jupiter’s South Temperate Belt

Moist convective storms in Jupiter develop frequently and can trigger atmospheric activity of different scales, from localized storms to planetary-scale disturbances including convective activity confined inside a larger meteorological system. In February 2018 a series of convective storms erupted in Jupiter’s South Temperate Belt (STB) (planetocentric latitudes from −23° to −29.5°). This occurred inside an elongated cyclonic region known popularly as the STB Ghost, close to the large anticyclone Oval BA, resulting in the clouds from the storms being confined to the cyclone. The initial storms lasted only a few days but they generated abundant enduring turbulence. They also produced dark features, possibly partially devoid of clouds, that circulated around the cyclone over the first week. The subsequent activity developed over months and resulted in two main structures, one of them closely interacting with Oval BA and the other one being expelled to the west. Here we present a study of this meteorological activity based on daily observations provided by the amateur community, complemented by observations obtained from PlanetCam UPV/EHU at Calar Alto Observatory, the Hubble Space Telescope and by JunoCam on the Juno spacecraft. We also perform numerical simulations with the EPIC General Circulation Model to reproduce the phenomenology observed. The successful simulations require a complex interplay between the Ghost, the convective eruptions and Oval, and they demonstrate that water moist convection was the source of the initial storms. A simple scale comparison with other moist convective storms that can be observed in the planet in visible and methane absorption band images strongly suggests that most of these storms are powered by water condensation instead of ammonia.

Forcing of the Upper-Tropospheric Monsoon Anticyclones

Abstract During the boreal warm season (May–September), the circulation in the upper troposphere and lower stratosphere is dominated by two large anticyclones: the Asian monsoon anticyclone (AMA) and North American monsoon anticyclone (NAMA). The existence of the AMA has long been linked to Asian monsoon precipitation using the Matsuno–Gill framework, but the origin of the NAMA has not been clearly understood. Here the forcing mechanisms of the NAMA are investigated using a simplified dry general circulation model. The simulated anticyclones are in good agreement with observations when the model is forced by a zonally symmetric meridional temperature gradient plus a realistic geographical distribution of heating based on observed tropical and subtropical precipitation in the Northern Hemisphere. Model experiments show that the AMA and NAMA are largely independent of one another, and the NAMA is not a downstream response to the Asian monsoon. The primary forcing of the NAMA is precipitation in the longitude sector between 60° and 120°W, with the largest contribution coming from the subtropical latitudes within that sector. Experiments with idealized regional heating distributions reveal that the extratropical response to tropical and subtropical precipitation depends approximately linearly on the magnitude of the forcing but nonlinearly on its latitude. The AMA is stronger than the NAMA, primarily because precipitation in the subtropics over Asia is much heavier than at similar latitudes in the Western Hemisphere.

Multiscale Dynamics of the February 11-12, 2010, Deep South US Snowstorm Event

This study investigates the synoptic/mesoscale dynamics responsible for an unusually heavy southern US snowstorm that occurred on February 11-12, 2010, using reanalysis, observations, and numerical simulations. This record breaking snowfall event represents an example of multiple upper level and low-level jets (LLJs) and their accompanying baroclinic zones. The analysis reveals the following synoptic scale processes as significant contributors: (1) upper level jet splitting and merging, (2) advection of cold arctic air at low levels by a large anticyclone, and (3) an incoming upper level shortwave trough. In addition to the synoptic scale processes, the following mesoscale features played a major role in this snowstorm event: coexisting potential (convective) instability and conditional symmetric instability, terrain blocking, and a double LLJ development process. Sensitivity experiments including (1) limiting the orographic effects of elevated plateau in Texas and the Sierra Madre Mountains in Mexico by reducing the terrain height to 225 meters, (2) the microphysics/latent heating effects, and (3) surface fluxes on the development and intensity of the snowstorm were also conducted by turning these options off in the numerical model. Of all three experiments, the surface flux experiment displays the least amount of influence on the developing frozen precipitation bands.

Shallow water simulations of Saturn's giant storms at different latitudes

Shallow water simulations are used to present a unified study of three major storms on Saturn (nicknamed as Great White Spots, GWS) at different latitudes, polar (1960), equatorial (1990), and mid-latitude (2010) (Sánchez-Lavega, 2004; Sánchez-Lavega et al., 2011). In our model, the three GWS are initiated by introducing a Gaussian function pulse at the latitude of the observed phenomena with controlled horizontal size and amplitude. This function represents the convective source that has been observed to trigger the storm. A growing disturbance forms when the pulse reacts to ambient winds, expanding zonally along the latitude band of the considered domain. We then compare the modeled potential vorticity with the cloud field, adjusting the model parameters to visually get the closest aspect between simulations and observations. Simulations of the 2010 GWS (planetographic latitude ∼+40º, zonal velocity of the source ∼−30m s−1) indicate that the Coriolis forces and the wind profile structure shape the disturbance generating, as observed, a long region to the east of the convective source with a high speed peripheral anticyclonic circulation, and a long-lived anticyclonic compact vortex accompanied by strong zonal advection on the southern part of the storm forming a turbulent region. Simulations of the equatorial 1990 GWS (planetographic latitude +12º–+5º, zonal velocity of the source 365–400m s−1) show a different behavior because of the intense eastward jet, meridional shear at the equatorial region, and low latitude dynamics. A round shaped source forms as observed, with the rapid growth of a Kelvin-Helmholtz instability on the north side of the source due to advection and to the strong meridional wind shear, whereas at the storm latitude the disturbance grows and propagates eastward. The storm nucleus is the manifestation of a Rossby wave, while the eastward propagating planetary-scale disturbance is a gravity-Rossby wave trapped around the equator. The simulated 1960 GWS disturbance (planetographic latitude +56º, zonal velocity 4m s−1) formed a chain of periodic oval spots that mimic the few available observations of the phenomenon. For the mid and high latitude storms, simulations predict a strong injection of negative relative vorticity due to divergence of the upwelling storm material, which may produce large anticyclones on the anticyclonic side of the zonal profile, and a quick turbulent expansion on the background cyclonic regions. In general, simulations indicate that negative relative vorticity injected by storms determines the natural reaction to zonal winds at latitudes where Coriolis forces are dominant.

Dynamics of Saturn's polar regions

AbstractWe analyze data retrieved by the imaging science system onboard the Cassini spacecraft to study the horizontal velocity and vorticity fields of Saturn's polar regions (latitudes 60–90°N in June–December 2013 and 60–90°S in October 2006 and July–December 2008), including the northern region where the hexagonal wave is prominent. With the aid of an automated two‐dimensional correlation algorithm we determine two‐dimensional maps of zonal and meridional winds and deduce vorticity maps. We extract zonal averages of zonal winds, providing wind profiles that reach latitudes as high as 89.5° in the south and 89.9° in the north. Wind measurements cover the intense polar cyclonic vortices that reach similar peak velocities of 150 m s−1at ±88.5°. The hexagonal wave lies in the core of an intense eastward jet at planetocentric latitude 75.8°N with motions that become nonzonal at the hexagonal feature. In the south hemisphere the peak of the eastward jet is located at planetocentric latitude 70.4°S. A large anticyclone (the south polar spot, SPS), similar to the north polar spot (NPS) observed at the Voyager times (1980–1981), has been observed in images from April 2008 to January 2009 in the south polar region at latitude −66.1° close to the eastward jet. The SPS does not apparently excite a wave on the jet. We analyze the stability of the zonal jets, finding potential instabilities at the flanks of the eastward jets around 70°, and we measure the eddy wind components, suggesting momentum transfer from eddy motion to the westward jets closer to the poles.

The long‐term steady motion of Saturn's hexagon and the stability of its enclosed jet stream under seasonal changes

AbstractWe investigate the long‐term motion of Saturn's north pole hexagon and the structure of its associated eastward jet, using Cassini imaging science system and ground‐based images from 2008 to 2014. We show that both are persistent features that have survived the long polar night, the jet profile remaining essentially unchanged. During those years, the hexagon vertices showed a steady rotation period of 10 h 39 min 23.01 ± 0.01 s. The analysis of Voyager 1 and 2 (1980–1981) and Hubble Space Telescope and ground‐based (1990–1991) images shows a period shorter by 3.5 s due to the presence at the time of a large anticyclone. We interpret the hexagon as a manifestation of a vertically trapped Rossby wave on the polar jet and, because of their survival and unchanged properties under the strong seasonal variations in insolation, we propose that both hexagon and jet are deep‐rooted atmospheric features that could reveal the true rotation of the planet Saturn.

3D Modeling of interactions between Jupiter’s ammonia clouds and large anticyclones

The motions of Jupiter’s tropospheric jets and vortices are made visible by its outermost clouds, which are expected to be largely composed of ammonia ice. Several groups have demonstrated that much of this dynamics can be reproduced in the vorticity fields of high-resolution models that, surprisingly, do not contain any clouds. While this reductionist approach is valuable, it has natural limitations. Here we report on numerical simulations that use the EPIC Jupiter model with a realistic ammonia-cloud microphysics module, focusing on how observable ammonia clouds interact with the Great Red Spot (GRS) and Oval BA. Maps of column-integrated ammonia-cloud density in the model resemble visible-band images of Jupiter and potential-vorticity maps. On the other hand, vertical cross sections through the model vortices reveal considerable heterogeneity in cloud density values between pressure levels in the vicinity of large anticyclones, and interestingly, ammonia snow appears occasionally. Away from the vortices, the ammonia clouds form at the levels expected from traditional one-dimensional models, and inside the vortices, the clouds are elevated and thick, in agreement with Galileo NIMS observations. However, rather than gathering slowly into place as a result of Jupiter’s weak secondary circulation, the ammonia clouds instead form high and thick inside the large anticyclones as soon as the cloud microphysics module is enabled. This suggests that any weak secondary circulation that might be present in Jupiter’s anticyclones, such as may arise because of radiative damping of their temperature anomalies, may have little or no direct effect on the altitude or thickness of the ammonia clouds. Instead, clouds form at those locations because the top halves of large anticyclones must be cool for the vortex to be able to fit under the tropopause, which is a primary-circulation, thermal-wind-shear effect of the stratification, not a secondary-circulation thermal feature. A planetary-scale void of ammonia clouds persists in the model southward of -38° planetographic latitude, but may partially reflect the fact that we have not yet included a full complement of vortices, all condensable species or the underlying dry-convective forcing from Jupiter’s interior.

Colors of Jupiter's large anticyclones and the interaction of a Tropical Red Oval with the Great Red Spot in 2008

AbstractThe nature and mechanisms producing the chromophore agents that provide color to the upper clouds and hazes of the atmospheres of the giant planets are largely unknown. In recent times, the changes in red coloration that have occurred in large‐ and medium‐scale Jovian anticyclones have been particularly interesting. In late June and early July 2008, a particularly color intense tropical red oval interacted with the Great Red Spot (GRS) leading to the destruction of the tropical red oval and cloud dispersion. We present a detailed study of the tropical vortices, usually white but sometimes red, and a characterization of their color spectral signatures and dynamics. From the spectral reflectivity in methane bands we study their vertical cloud structure compared to that of the GRS and BA. Using two spectral indices we found a near correlation between anticyclones cloud top altitudes and red color. We present detailed observations of the interaction of the red oval with the GRS and model simulations of the phenomena that allow us to constrain the relative vertical extent of the vortices. We conclude that the vertical cloud structure, vertical extent, and dynamics of Jovian anticyclones are not the causes of their coloration. We propose that the red chromophore forms when background material (a compound or particles) is entrained by the vortex, transforming into red once inside the vortex due to internal conditions, exposure to ultraviolet radiation, or to the mixing of two chemical compounds that react inside the vortex, confined by a potential vorticity ring barrier.

Saturn’s visible lightning, its radio emissions, and the structure of the 2009–2011 lightning storms

Visible lightning on Saturn was first detected by the Cassini camera in 2009 at ∼35° South latitude. We report more lightning observations at ∼35° South later in 2009, and lightning in the 2010–2011 giant lightning storm at ∼35° North. The 2009 lightning is detected on the night side of Saturn in a broadband clear filter. The 2011 lightning is detected on the day side in blue wavelengths only. In other wavelengths the 2011 images lacked sensitivity to detect lightning, which leaves the lightning spectrum unknown.The prominent clouds at the west edge, or the “head” of the 2010–2011 storm periodically spawn large anticyclones, which drift off to the east with a longitude spacing of 10–15° (∼10,000km). The wavy boundary of the storm’s envelope drifts with the anticyclones. The relative vorticity of the anticyclones ranges up to −f/3, where f is the planetary vorticity. The lightning occurs in the diagonal gaps between the large anticyclones. The vorticity of the gaps is cyclonic, and the atmosphere there is clear down to level of the deep clouds. In these respects, the diagonal gaps resemble the jovian belts, which are the principal sites of jovian lightning.The size of the flash-illuminated cloud tops is similar to previous detections, with diameter ∼200km. This suggests that all lightning on Saturn is generated at similar depths, ∼125–250km below the cloud tops, probably in the water clouds. Optical energies of individual flashes for both southern storms and the giant storm range up to 8×109J, which is larger than the previous 2009 equinox estimate of 1.7×109J. Cassini radio measurements at 1–16MHz suggest that, assuming lightning radio emissions range up to 10GHz, lightning radio energies are of the same order of magnitude as the optical energies.Southern storms flash at a rate ∼1–2per minute. The 2011 storm flashes hundreds of times more often, ∼5times per second, and produces ∼1010W of optical power. Based on this power, the storm’s total convective power is of the order 1017W, which is uncertain by at least an order of magnitude, and probably is underestimated. This power is similar to Saturn’s global internal power radiated to space. It suggests that storms like the 2010–2011 giant storm are important players in Saturn’s cooling and thermal evolution.

Revised circulation scheme north of the Denmark Strait

The circulation and water mass transports north of the Denmark Strait are investigated using recently collected and historical in situ data along with an idealized numerical model and atmospheric reanalysis fields. Emphasis is placed on the pathways of dense water feeding the Denmark Strait Overflow Water plume as well as the upper-layer circulation of freshwater. It is found that the East Greenland Current (EGC) bifurcates at the northern end of the Blosseville Basin, some 450km upstream of the Denmark Strait, advecting overflow water and surface freshwater away from the boundary. This “separated EGC” flows southward adjacent to the previously identified North Icelandic Jet, indicating that approximately 70% of the Denmark Strait Overflow Water approaches the sill along the Iceland continental slope. Roughly a quarter of the freshwater transport of the EGC is diverted offshore via the bifurcation. Two hypotheses are examined to explain the existence of the separated EGC. The atmospheric fields demonstrate that flow distortion due to the orography of Greenland imparts significant vorticity into the ocean in this region. The negative wind stress curl, together with the closed bathymetric contours of the Blosseville Basin, is conducive for spinning up an anti-cyclonic gyre whose offshore branch could represent the separated EGC. An idealized numerical simulation suggests instead that the current is primarily eddy-forced. In particular, baroclinic instability of the model EGC spawns large anti-cyclones that migrate offshore and coalesce upon reaching the Iceland continental slope, resulting in the separated EGC. Regardless of the formation mechanism, the recently obtained shipboard data and historical hydrography both indicate that the separated EGC is a permanent feature of the circulation north of the Denmark Strait.

Assessing the Great Whirl, despite all the pirates

Each year, the powerful southwest monsoon ramps up in midsummer, bringing life‐giving rains to the Indian subcontinent. The monsoon winds also drive dramatic changes in the regional ocean currents, including a reversal in the circulation of the Arabian Sea, an energetic eddy field, and strong coastal upwelling. Off the east coast of Somalia, a large (300‐ to 550‐kilometer wide) anticyclone appears—known since 1876 as the Great Whirl—with surface currents as strong as 2.5 meters per second. The Great Whirl, while associated with the seasonal arrival of the southwest monsoon, is not caused entirely by it; the circulation of the Great Whirl starts a month before and persists for a month after the monsoon.

Tracer transport during the Arctic stratospheric final warming based on a 33‐year (1979‐2011) tracer equivalent latitude simulation

During the 2011 stratospheric final warming (SFW), a large anticyclone rapidly encompassed the pole, displacing the polar vortex and establishing strong summer easterlies. Tracer Equivalent Latitude (TrEL) maps indicate low latitude air was transported by the anticyclone into the summer polar vortex. MLS nitrous oxide was anomalously high throughout the following summer, confirming the TrEL results. A 33‐year (1979–2011) TrEL simulation at 850 K potential temperature reveals a number of similar low‐TrEL events, which are often, but not always, associated with Frozen‐In Anticyclone (FrIAC) formation. The summertime TrEL values are highly correlated with zonal wind speed in the polar stratosphere following the SFW, suggesting that strong post‐SFW circulation favors polar trapping of low‐TrEL air. The 2011 event, classified as a large‐scale FrIAC, was unusual in having the lowest TrEL values and the strongest easterly vortex within the past three decades.

Dynamics of the circulation in the Sea of Marmara: numerical modeling experiments and observations from the Turkish straits system experiment

During September 2008 and February 2009, the NR/V Alliance extensively sampled the waters of the Sea of Marmara within the framework of the Turkish Straits System (TSS) experiment coordinated by the NATO Undersea Research Centre. The observational effort provided an opportunity to set up realistic numerical experiments for modeling the observed variability of the Marmara Sea upper layer circulation at mesoscale resolution over the entire basin during the trial period, complementing relevant features and forcing factors revealed by numerical model results with information acquired from in situ and remote sensing datasets. Numerical model solutions from realistic runs using the Regional Ocean Modeling System (ROMS) produce a general circulation in the Sea of Marmara that is consistent with previous knowledge of the circulation drawn from past hydrographic measurements, with a westward meandering current associated with a recurrent large anticyclone. Additional idealized numerical experiments illuminate the role various dynamics play in determining the Sea of Marmara circulation and pycnocline structure. Both the wind curl and the strait flows are found to strongly influence the strength and location of the main mesoscale features. Large displacements of the pycnocline depth were observed during the sea trials. These displacements can be interpreted as storm-driven upwelling/downwelling dynamics associated with northeasterly winds; however, lateral advection associated with flow from the Straits also played a role in some displacements.

Keck adaptive optics images of Jupiter’s north polar cap and Northern Red Oval

We present observations at near-infrared wavelengths (1–5 μm) of Jupiter’s north polar region and Northern Red Oval (NN-LRS-1). The observations were taken with the near-infrared camera NIRC2 coupled to the adaptive optics system on the 10-m W.M. Keck Telescope on UT 21 August 2010. At 5-μm Jupiter’s disk reveals considerable structure, including small bright rings which appear to surround all small vortices. It is striking, though, that no such ring is seen around the Northern Red Oval. In de Pater et al. [2010a. Icarus 210, 742–762], we showed that such rings also exist around all small vortices in Jupiter’s southern hemisphere, and are absent around the Great Red Spot and Red Oval BA. We show here that the vertical structure and extent of the Northern Red Oval is very similar to that of Jupiter’s Red Oval BA. These new observations of the Northern Red Oval, therefore, support the idea of a dichotomy between small and large anticyclones, in which ovals larger than about two Rossby deformation radii do not have 5-μm bright rings. In de Pater et al. [2010a. Icarus 210, 742–762], we explained this difference in terms of the secondary circulations within the vortices. We further compare the brightness distribution of our new 5-μm images with previously published radio observations of Jupiter, highlighting the depletion of NH 3 gas over areas that are bright at 5 μm.

The mesoscale variability in the Caribbean Sea. Part II: Energy sources

The processes which drive the production and the growth of the strong mesoscale eddy field in the Caribbean Sea are examined using a general circulation model. Diagnostics of the simulations suggest that: (1) The mean currents in the Caribbean Sea are intrinsically unstable. The nature of the instability and its strength vary spatially due to strong differences of current structure among basins. (2) The greatest and most energetic eddies of the Caribbean Sea originate in the Venezuela Basin by mixed barotropic-baroclinic instability of an intense jet, formed with waters mostly from the surface return flow of the Meridional Overturning Circulation and the North Equatorial Current which converge and accelerate through the Grenada Passage. The vertical shear of this inflow is enhanced by an eastward undercurrent, which flows along the south American Coast between 100 and 250 m depth. The shallow eddies (less than 200 m depth) formed in the vicinity of the Grenada Passage get rapidly deeper (down to 1000 m depth) and stronger by their interaction with the deep interior flow of the Subtropical Gyre, which enters through passages north of St. Lucia. These main eastern Caribbean inflows merge and form the southern Caribbean Current, whose baroclinic instability is responsible for the westward growth and strengthening of these eddies from the Venezuela to the Colombia Basin. (3) Eddies of lesser strength are produced in other regions of the Caribbean Sea. Their generation and growth is also linked with instability of the local currents. First, cyclones are formed in the cyclonic shear of the northern Caribbean Current, but appear to be rapidly dissipated or absorbed by the large anticyclones coming from the southern Caribbean. Second, eddies in the Cayman Sea, which impact the Yucatan region, are locally produced and enhanced by barotropic instability of the deep Cayman Current. (4) The role of the North Brazil Current (NBC) rings is mostly to act as a finite perturbation for the instability of the mean flow. Their presence near the Lesser Antilles is ubiquitous and they appear to be linked with most of the Caribbean eddies. There are some evidences that the frequency at which they form near the Grenada Passage is influenced by the frequency at which the NBC rings impinge the Lesser Antilles. But large Caribbean eddies also form without a close influence of any ring, and comparison between simulations shows that mean eddy kinetic energy and eddy population in the Caribbean Sea are not substantially different in absence or presence of NBC rings: their presence is not a necessary condition for the generation and growth of the Caribbean eddies.

Eddy shedding and energy conversions in the East Australian Current

Mesoscale variability and eddy shedding in the Tasman Sea, particularly of the East Australian Current (EAC), is studied through the analysis of remotely sensed observations and outputs from a global ocean model. Previous observations of the western boundary current separation from the coast showed strong variability at periods ranging between 90 and 140 days. We show from satellite altimetric observations that rapid northward migration of the separation point of the EAC follows the formation of large eddies at periods of ∼100 days. After an eddy separation event the normally southward flowing current swiftly assumes a more zonal configuration near the separation latitude, with a cyclonic circulation developing inshore. The formation of large separation eddies is preceded by the southward propagation of sea level anomalies along the east Australian continental slope. From 25°S, sea level anomalies grow as they travel south, eventually being pinched off in the form of large anticyclones at ∼32°S, in the current retroflection area. Energy conversion terms in a global ocean model and in altimetric data suggest both barotropic and baroclinic instability may account for the growth of these anomalies as they propagate south. East of the main EAC jet there is evidence that eddies may be feeding potential energy back to the mean flow.