Anticyclonic and cyclonic eddies of subtropical origin in the subantarctic zone south of Africa (original) (raw)
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Eddies and dipoles around South Madagascar: formation, pathways and large-scale impact
Deep-sea Research Part I-oceanographic Research Papers, 2004
During a hydrographic cruise in March 2001, we encountered a dipole-like vortex structure directly southwest of Madagascar. The cruise formed part of the Dutch-South African Agulhas Current Sources Experiment (ACSEX). Direct current observations with a lowered ADCP showed that the multipole involved a deep-reaching central jet, with over 20 cm/s speeds still at 2000 m depth, with two contra-rotating eddies on either side. Both eddies had a size of about 250 km. A subsequent analysis of the combined TOPEX-POSEIDON/ERS altimeter dataset revealed the regular formation of dipoles around south Madagascar, where the East Madagascar Current (EMC) separates from the shelf. These periods of enhanced dipole formation coincided with the negative phases of the Indian Ocean Dipole and El Nin˜o cycles, suggesting a connection between these climate modes and the interannual variability in the periods of dipole train formation.
Introduction to the “Inter-ocean exchange around southern Africa”
Deep Sea Research Part II: Topical Studies in Oceanography, 2003
This issue of Deep-Sea Research II contains 12 papers that describe results from field experiments and modeling studies concerning the Indian-Atlantic exchange of water around the Cape of Good Hope. A central theme is the important role of the mesoscale features, such as eddies and filaments, in the leakage of Indian water into the Atlantic. The papers describe new direct-velocity measurements and model simulations, which reveal information about circulation patterns and physical processes that control the inter-ocean exchange. For the first time a large array of subsurface acoustic floats was tracked at the intermediate water level. The float trajectories reveal that the Cape Basin off Cape Town is virtually filled with Agulhas Rings and cyclonic eddies, which interact intensely with each other, change shape, bifurcate, and merge. These energetic eddies dominate the velocity field in the source region of the Benguela Current; they stir the various source waters and advect the blended product westward over the Walvis Ridge. These seminal results have important and lasting implications for our conceptions of the processes involved in interocean exchanges around southern Africa. r
Generation of cyclonic eddies by the Agulhas Current in the Lee of the Agulhas Bank
Geophysical Research Letters, 2001
Anti-cyclonic rings are shed from the Agulhas Current at its retroflection. They subsequently drift off into the South Atlantic. Smaller, cyclonic eddies have also been observed in this region. The origin of these latter eddies has remained unknown. We present model results that indicate that the configuration of the southern Agulhas Current and the poleward termination of the continental shelf of Africa allows shedding of cyclonic lee eddies by a flow detachment process. Hydrographic data, thermal infrared satellite images and altimetric observations are furnished that show that this model simulation is consistent with the characteristics and the behaviour of cyclonic eddies in the region.
Hydrodynamics between Africa and Antarctica during Austral Summer 2008
Journal of Marine Systems, 2010
Under the International Polar Year endorsed project (IPY#70), the southwest Indian Ocean sector of the Southern Ocean was surveyed during the austral summer of 2008 by deploying expendable CTD probes along the ship tracks : Cape Town-India Bay, Antarctica (Track-1) and Prydz Bay, Antarctica-Cape Town (Track-2). The meteorological data revealed that the unstable marine atmosphere boundary layer (MABL) facilitated a turbulent heat loss of 45 Wm −2 on average, in conditions of variable wind intensity north of 43°S along Track-1; south of 63°S and north of 51°S the ocean was conducive to higher turbulent heat loss amounting to 95 Wm − 2 (on average) along Track-2. Surface imprints of hydrological fronts were determined by using surface gradient and subsurface temperature and salinity indicators. The core of Agulhas Current was identified between 36.5°and 37.5°S along Track-2, while the Agulhas Retroflection (AR) Front was located at 39.7°S south of Cape Town. The Subtropical (STF), Subantarctic (SAF) and Polar Fronts (PF) exhibited double frontal structures, whose meridional meandering is governed by bottom topography and modulated by planetary vorticity. A large southward deviation in the position of southern PF by 3.5°l atitude on Track-1 was observed. Northern and southern SAF and southern STF meander by 2°-3.5°n orthward; their merger just north of Crozet Island facilitate an enhanced baroclinic transport of 12 Sv in the upper 1000 m. Three anticyclones that detach from the AR transport 17 Sv into the southeast Atlantic. The baroclinic transport contributed by AC and its retroflection across Track-2 amounted to 17.6 Sv. More than 50% of the ACC transport was confined to the 100-500 m depth layer. Water masses have been identified and their zonal extent quantified along the tracks. Strong convective cooling is responsible for the production of Subtropical Mode Water in the eastern Crozet Basin, which was detected near 43.5°and 41.5°S along Track-1 and-2, respectively. The Circumpolar Deep Water was encountered between 600 and 900 m at 51°S on Track-1 and spanned 54°-60°S on Track-2. We also offer an example of the modification of the MABL through strong air-sea interaction in the vicinity of South Africa by employing satellite-derived surface winds and heat fluxes.
Eddies in the southern Mozambique Channel
Deep Sea Research Part II: Topical Studies in Oceanography, 2004
The Agulhas Current system contains one of the world's strongest western boundary currents, and plays an important part in the warm water path of the global thermohaline circulation. However, there have been few surveys of the source regions of the Agulhas Current, and thus little in situ measurement of their variability. Utilizing the more than 5-year record of SeaWiFS data, we examine the eddy activity present in the southern portion of the Mozambique Channel. The two sources of Agulhas input from the central Indian Ocean (southward flow through the Mozambique Channel and westward flow around the southern limit of Madagascar) both show great temporal variability, with no clear seasonal signal.
Anticyclonic eddies in the northwestern Black Sea
Journal of Marine Systems, 2002
National Oceanic and Atmospheric Administration (NOAA) Advanced Very-High Resolution Radiometer (AVHRR) imagery (1993, 1998), along with attendant daily meteorological information from seaports and available hydrographic information from different years, was used to investigate the structure and evolution of mesoscale anticyclonic eddies in the northwestern Black Sea, and their role in shelf/deep basin water exchange. In the summer of 1993, two anticyclonic eddies with diameters of 90 and 55 km coexisted without coalescence for 1.5 months over a wide and relatively gentle part of the northwestern continental slope. The directions of the eddies' movements inside this zone (speed of movement up to 16 cm/s) were likely determined by the interaction between eddies themselves, and by the Rim Current meandering and forcing. For June -August of 1998, three such eddies have been traced in the sea surface temperature (SST, AVHRR) and chlorophyll a (SeaWiFS) fields. The largest anticyclone with a diameter of 90 km moved during 3 months southwestward from the wide slope region west of Sevastopol to the area of narrower slope southeast of Cape Kaliakra with a mean speed of about 3 cm/s. Together with nonstationary associated elements (cyclones at the eddies' peripheries, entrained and ejected jets), anticyclonic eddies determine water exchange processes in a large area of the western Black Sea between 43 -45jN and 29 -33jE. They transport chlorophyll-rich coastal waters to the deep basin, westerly winds being favorable to the process. D
Climate of the Past
Due to the high sensitivity of southern Africa to climate change, a reliable understanding of its hydrological system is crucial. Recent studies of the regional climatic system have revealed a highly complex interplay of forcing factors on precipitation regimes. This includes the influence of the tropical easterlies, the strength of the southern hemispheric westerlies as well as sea surface temperatures along the coast of the subcontinent. However, very few marine records have been available in order to study the coupling of marine and atmospheric circulation systems. Here we present results from a marine sediment core, recovered in shallow waters off the Gouritz River mouth on the south coast of South Africa. Core GeoB18308-1 allows a closer view of the last ∼ 4 kyr. Climate sensitive organic proxies, like the distribution and isotopic composition of plant-wax lipids as well as indicators for sea surface temperatures and soil input, give information on oceanographic and hydrologi...
Journal of Geophysical Research: Oceans, 2020
The presence of a persistent surface anticyclone centered at approximately 55°N, 12°W in the Rockall Trough, northeast North Atlantic, has been previously noted in satellite altimetry data. Here, we show that this surface anticyclone is the imprint of a deep, persistent, non-stationary anticyclonic vortex. Using wintertime 2007 and 2011 ship-board data, we describe the anticyclone's vertical structure for the first time and find that the anticyclone core is partly made of warm and salty Mediterranean Overflow Water. The anticyclone has a radius of~40 km, it stretches down to 2,000 m, with a velocity maximum around 500 m. To analyze the anticyclone's generating mechanism, we use a mesoscale-resolving (~2 km) simulation, which produces a realistic pattern of the Rockall Trough anticyclone. The simulation indicates that the anticyclone is locally formed and sustained by two types of processes: wintertime convection and merger with anticyclonic vortices shed from the slope current flowing poleward along the eastern Rockall Trough slope. Intense negative vorticity filaments are generated along the Rockall Trough southeastern slope, and they encapsulate Mediterranean Overflow Water as they detach and grow into anticyclonic vortices. These Mediterranean Overflow Water-rich vortices are advected into the trough, consequently merging with the Rockall Trough anticyclone and sustaining it. We suggest that the Rockall Trough anticyclone impacts regional intermediate water masses modifications, heat and salt budgets locally, and further afield into the neighboring subpolar northeast North Atlantic. Plain Language Summary Water masses of different origins converge in the Rockall Trough, a deep bathymetric depression in the northeast North Atlantic, and undergo transformations with direct implications for the inflow of warm water into the Nordic Seas. We use in situ observations to document, for the first time, the vertical structure of a subsurface anticyclone, which is a clockwise oceanic vortex in the trough. We show that the anticyclone has a radius of~40 km, extends down to 2,000 m, with a velocity maximum at 500 m depth. Its core is made of warm and salty Mediterranean water. We use outputs from a high-resolution (~2 km) realistic simulation to study the mechanisms driving the anticyclone. We show that the anticyclone is impacted predominantly by two different processes. One is the wintertime convection, which mixes waters from the surface down to 1,000 m inside the anticyclone. The other is the merger with smaller vortices that pinch off the slope current flowing northward along the Porcupine Bank, south of the Rockall Trough, and feed the anticyclone with water masses of Mediterranean origin. We showcase the potential impact of the anticyclone on the regional and nearby northeast North Atlantic heat and salt distributions.