scholarly journals Observations of Submesoscale Variability and Frontal Subduction within the Mesoscale Eddy Field of the Tasman Sea

2020 ◽  
Vol 50 (5) ◽  
pp. 1509-1529
Author(s):  
Matthew Archer ◽  
Amandine Schaeffer ◽  
Shane Keating ◽  
Moninya Roughan ◽  
Ryan Holmes ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Mesoscale Eddy ◽  
Vertical Shear ◽  
High Oxygen Content ◽  
Ocean Fronts ◽  
Tasman Sea ◽  
Eddy Formation ◽  
Vertical Export ◽  
Anticyclonic Eddies ◽  
Insight Into

AbstractSubmesoscale lenses of water with anomalous hydrographic properties have previously been observed in the East Australian Current (EAC) system, embedded within the thermocline of mesoscale anticyclonic eddies. The waters within these lenses have high oxygen content and temperature–salinity properties that signify a surface origin. However, it is not known how these lenses form. This study presents field observations that provide insight into a possible generation mechanism via subduction at upper-ocean fronts. High-resolution hydrographic and velocity measurements of submesoscale activity were taken across a front between a mesoscale eddy dipole downstream of the EAC separation point. The front had O(1) Rossby number, strong vertical shear, and flow conducive to symmetric instability. Frontogenesis was measured in conjunction with subduction of an anticyclonic water parcel, indicative of intrathermocline eddy formation. Twenty-five years of satellite imagery reveals the existence of strong mesoscale strain coupled with strong temperature fronts in this region and indicates the conditions that led to frontal subduction observed here are a persistent feature. These processes impact the vertical export of tracers from the surface and dissipation of mesoscale kinetic energy, implicating their importance for understanding regional ocean circulation and biological productivity.

Sea ice protists in arctic marine sedaDNA records: origins, diversity, and vertical export of DNA from the sea surface to the seafloor 

2021 ◽  
Author(s):  
Sara Harðardóttir ◽  
Connie Lovejoy ◽  
Marit-Solveig Seidenkrantz ◽  
Sofia Ribeiro
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
Ancient Dna ◽  
Genetic Material ◽  
Amplicon Sequencing ◽  
Arctic Sea Ice ◽  
The Arctic ◽  
Gene Copy ◽  
Marine Biodiversity ◽  
Vertical Export

<p>Arctic sea ice is declining at an unprecedented pace as the Arctic Ocean heads towards ice-free summers within the next few decades. Because of the role of sea ice in the Earth System such as ocean circulation and ecosystem functioning, reconstructing its past variability is of great importance providing insight into past climate patterns and future climate scenarios. Today, much of our knowledge of past sea-ice variability derives from a relatively few microfossil and biogeochemical tracers, which have limitations, such as preservation biases and low taxonomic resolution. Marine sedimentary ancient DNA (marine <em>seda</em>DNA) has the potential to capture more of the arctic marine biodiversity compared to other approaches. However, little is known about how well past communities are represented in marine <em>seda</em>DNA. The transport and fate of DNA derived from sea-ice associated organisms, from surface waters to the seafloor and its eventual incorporation into marine sediment records is poorly understood.  Here, we present results from a study applying a combination of methods to examine modern and ancient DNA to material collected along the Northeast Greenland Shelf. We characterized the vertical export of genetic material by amplicon sequencing the hyper-variable V4 region of the 18S rDNA at three water depths, in surface sediments, and in a dated sediment core.  The amplicon sequencing approach, as currently applied, includes some limitations for quantitative reconstructions of past changes such as primer competition, PCR errors, and variation of gene copy numbers across different taxa. For these reasons we quantified amplicons from a single species, the circum-polar sea ice dinoflagellate <em>Polarella glacialis</em> in the marine <em>seda</em>DNA, using digital droplet PCR. The results will increase our understanding on the taphonomy of DNA in sea ice environments, how sedimentation differs among taxonomic groups, and provide indications to potentially useful marine <em>seda</em>DNA-based proxies for climate and environmental reconstructions.</p>

scholarly journals Assymmetric eddy populations in adjacent basins – a high resolution numerical study of the Tyrrhenian and Ligurian Seas

2012 ◽  
Vol 9 (6) ◽  
pp. 3521-3566 ◽  
Author(s):  
R. M. A. Caldeira ◽  
X. Couvelard ◽  
E. Casella ◽  
A. Vetrano
Keyword(s):  
High Resolution ◽  
Ocean Circulation ◽  
Numerical Solutions ◽  
Numerical Study ◽  
Mesoscale Eddy ◽  
Leeward Side ◽  
Boundary Current ◽  
High Concentration ◽  
Eddy Activity ◽  
Tyrrhenian Basin

Abstract. A high-resolution ocean circulation modelling system forced with a high-resolution numerical wind product was used to study the mesoscale and sub-mesoscale eddy population of the North-Western Mediterranean Sea, contrasting eddy-activity between the Tyrrhenian and Ligurian sub-basins. Numerical solutions reproduced some of the known regional dynamics, namely the occurrence and oceanic implications of Mistral events, the convective cell leeward of the Gulf of Lion, as well as the Balearic frontal system. Calculated transport across the Corsica Channel followed a similar trend, when compared to the transport computed from a moored current meter. The analysis of the results showed that surface eddy activity is mostly confined to the boundary-currents, whereas in the deeper layers most eddies are concentrated on the central-deeper part of the basins. The Liguro-Provençal basin shows a much higher concentration of intermediate and deep-water eddies, when compared to the Tyrrhenian basin. Sub-mesoscale surface eddies tend to merge and migrate vertically onto intermediate waters. Intense eddy activity in the boundary-current surrounding the Liguro-Provençal Gyre, concentrate high-productivity, manifested by higher concentrations of mean sea surface chlorophyll, in the central part of the gyre, defined herein as the Ligurian Productive Pool (LPP). On average, the Tyrrhenian was mostly oligotrophic except for a small productive vortice in the south-eastern (leeward) side of Corsica. The transport in the Tyrrhenian Gyre, and across the basin is one order of magnitude higher than the transport calculated for the Liguro-Provençal basin. A high concentration of eddies in the passage between the Balearic Archipelago and Sardinia suggests retention and longer residence times of nutrient rich water in the "Ligurian pool", compared to a "fast draining" Tyrrhenian basin. Previous studies support the cyclonic gyre circulation generated in the Liguro-Provençal basin but more studies are needed to address the surface and deep mesoscale activity of the Tyrrhenian basin.

scholarly journals Mesoscale eddy formation and evolution in the ice-covered ocean

1991 ◽  
Vol 15 ◽  
pp. 139-147 ◽  
Author(s):  
Motoyoshi Ikeda
Keyword(s):  
Deep Convection ◽  
Bottom Topography ◽  
Active Material ◽  
Mesoscale Eddy ◽  
Open Water ◽  
Ekman Pumping ◽  
Strong Current ◽  
Eddy Formation ◽  
Current Instability ◽  
Generation Mechanisms

Generation mechanisms of mesoscale eddies in the ice-covered ocean are studied by using numerical ice–ocean models and discussed with reference to previous papers. The three possible mechanisms of eddy generation, with sea ice as a passive tracer, are current instability, current-eddy interaction and current–bottom topography interaction. The current instability, categorized into barotropic and baroclinic instabilities, may be responsible for eddies near the ice edge associated with a strong current. An eddy can interact with a current, producing additional eddies, where stability of the current is again an important factor for eddy formation. Eddies over bottom topography on the continental shelf are explained by current–topography interaction; i.e. anticyclones are produced over banks. The particular mechanism that includes ice as an active material is an ice–ocean interaction; i.e. a wind stress is larger over the ice than on open water and induces Ekman pumping and suction, which produce dipole eddy motions in the non-uniformly ice-covered ocean. The eddies are suggested to be important for cross-shelf exchanges of ice and heat as well as determining locations of deep convection.

Interannual Variability of the North Pacific Subtropical Countercurrent and Its Associated Mesoscale Eddy Field

2010 ◽  
Vol 40 (1) ◽  
pp. 213-225 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen
Keyword(s):  
North Pacific ◽  
Southern Oscillation ◽  
North Pacific Ocean ◽  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Vertical Shear ◽  
North Equatorial Current ◽  
Altimeter Data ◽  
Subtropical Countercurrent ◽  
Eddy Field

Abstract Interannual changes in the mesoscale eddy field along the Subtropical Countercurrent (STCC) band of 18°–25°N in the western North Pacific Ocean are investigated with 16 yr of satellite altimeter data. Enhanced eddy activities were observed in 1996–98 and 2003–08, whereas the eddy activities were below average in 1993–95 and 1999–2002. Analysis of repeat hydrographic data along 137°E reveals that the vertical shear between the surface eastward-flowing STCC and the subsurface westward-flowing North Equatorial Current (NEC) was larger in the eddy-rich years than in the eddy-weak years. By adopting a 2½-layer reduced-gravity model, it is shown that the increased eddy kinetic energy level in 1996–98 and 2003–08 is because of enhanced baroclinic instability resulting from the larger vertical shear in the STCC–NEC’s background flow. The cause for the STCC–NEC’s interannually varying vertical shear can be sought in the forcing by surface Ekman temperature gradient convergence within the STCC band. Rather than El Niño–Southern Oscillation signals as previously hypothesized, interannual changes in this Ekman forcing field, and hence the STCC–NEC’s vertical shear, are more related to the negative western Pacific index signals.

scholarly journals Extraction of Revolving Channels of Drifters around Mesoscale Eddy Centers Based on Spatiotemporal Trajectory Clustering

2019 ◽  
Vol 36 (9) ◽  
pp. 1903-1916
Author(s):  
Chunyong Ma ◽  
Siqing Li ◽  
Yang Yang ◽  
Jie Yang ◽  
Ge Chen
Keyword(s):  
Clustering Algorithm ◽  
Spatial Clustering ◽  
Mesoscale Eddy ◽  
Mesoscale Eddies ◽  
Trajectory Clustering ◽  
Satellite Altimeter ◽  
Principal Mode ◽  
In Situ Data ◽  
Anticyclonic Eddies

The global oceanic transports of energy, plankton, and other tracers by mesoscale eddies can be estimated by combining satellite altimetry and in situ data. However, the revolving channels of particles entrained by mesoscale eddies, which could help explain the dynamic process of eddies entraining materials, are still unknown. In this study, satellite altimeter and drifter data from 1993 to 2016 are adopted, and the normalized trajectory clustering algorithm (N-TRACLUS) is proposed to extract the revolving channels of drifters. First, the trajectories of drifters are normalized and clustered by using the density-based spatial clustering of applications with noise (DBSCAN) algorithm. Next, the revolving channels of drifters around the eddy center are extracted. The ring or arc pattern in the middle of a normalized eddy appears when drifters are uninterruptedly entrained by eddies for more than 30 days. Moreover, the revolving channels of drifters in cyclonic eddies are relatively closer to the eddy center than those in anticyclonic eddies. These revolving channels suggest the principal mode of materials’ continuous motion processes that are inside eddies.

scholarly journals The Surface Circulation of the Caribbean Sea and the Gulf of Mexico as Inferred from Satellite Altimetry

2009 ◽  
Vol 39 (3) ◽  
pp. 640-657 ◽  
Author(s):  
Aida Alvera-Azcárate ◽  
Alexander Barth ◽  
Robert H. Weisberg
Keyword(s):  
Gulf Of Mexico ◽  
Time Scales ◽  
Satellite Altimetry ◽  
Southern Oscillation ◽  
Caribbean Sea ◽  
Vertical Shear ◽  
Loop Current ◽  
Surface Circulation ◽  
The Caribbean ◽  
Anticyclonic Eddies

Abstract The surface circulation of the Caribbean Sea and Gulf of Mexico is studied using 13 years of satellite altimetry data. Variability in the Caribbean Sea is evident over several time scales. At the annual scale, sea surface height (SSH) varies mainly by a seasonal steric effect. Interannually, a longer cycle affects the SSH slope across the current and hence the intensity of the Caribbean Current. This cycle is found to be related to changes in the wind intensity, the wind stress curl, and El Niño–Southern Oscillation. At shorter time scales, eddies and meanders are observed in the Caribbean Current, and their propagation speed is explained by baroclinic instabilities under the combined effect of vertical shear and the β effect. Then the Loop Current (LC) is considered, focusing on the anticyclonic eddies shed by it and the intrusion of the LC into the Gulf of Mexico through time. Twelve of the 21 anticyclonic eddies observed to detach from the LC are shed from July to September, suggesting a seasonality in the timing of these events. Also, a relation is found between the intrusion of the LC into the Gulf of Mexico and the size of the eddies shed from it: larger intrusions trigger smaller eddies. A series of extreme LC intrusions into the Gulf of Mexico, when the LC is observed as far as 92°W, are described. The analyses herein suggest that the frequency of such events has increased in recent years, with only one event occurring in 1993 versus three from 2002 to 2006. Transport through the Straits of Florida appears to decrease during these extreme intrusions.

Geochemical and Geophysical Evidence for Late Miocene Onset of Tasman Leakage

2020 ◽  
Author(s):  
Beth Christensen ◽  
David DeVleeschouwer ◽  
Jeroen Groeneveld ◽  
Jorijntje Henderiks ◽  
Gerald Auer ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Late Miocene ◽  
Return Flow ◽  
Intermediate Water ◽  
Isotope Data ◽  
The North ◽  
Tasman Sea ◽  
Intermediate Waters

<p>The recent documentation of the southern hemisphere “supergyre”, the coupled subtropical southern hemisphere gyres spanning the 3 ocean basins, leads to questions about its impact on Indian Ocean circulation. The Indonesian Throughflow (ITF) acts as a switchboard directing warm surface waters towards the Agulhas Current (AC) and return flow to the North Atlantic, but Tasman Leakage (TL) is another source of return flow, however, at intermediate water depths. Fed by a complex mixture of South Pacific (SP) western boundary current surface and intermediate waters, and Antarctic Intermediate Water (AAIW), today the topography forces it to flow in a westerly direction. The TL flows over the Broken Ridge towards Madagascar, joining the AC and ultimately Atlantic Meridional Circulation (AMOC).</p><p>Stable isotope data from 4 DSPD/ ODP Indian Ocean sites define the history of TL and constrain the timing of its onset to ~7 Ma.  A simple nannofossil- biostratigraphy age model applied to previously published benthic foraminiferal carbon isotope data ensures the 4 time-series (~11 – 2 Ma) are consistent. All 4 records (Sites 752 Broken Ridge, 590 Tasman Sea, 757 90 East Ridge, 751 Kerguelen Plateau) are similar from ~11 Ma to ~7 Ma, indicating the Tasman Sea intermediate water was sourced from the Southern Ocean (SO). A coeval shift at ~7 Ma at Sites 590 and 752 signals a SP contribution and the onset of TL. We do not observe TL at Sites 757 and 751 and so interpret the post-7 Ma divergence between the TL pair and the KP / 90E Ridge sites as a reflection of different intermediate water masses. The KP / 90E Ridge sites record a more fully SO signal, and these waters are constrained to the region west of the 90 East ridge.</p><p>The isotopic record of TL onset suggests important tectonic changes ~ 7 Ma: 1) opening of the Tasman Sea to the north and 2) Australia’s northward motion allowing westward flow around Tasmania. The former is supported by a change in sedimentation style on the Marion Plateau (ODP Site 1197). The latter is supported by unconformities on the South Australian Bight margin (Leg 182 Sites 1126 (784 m), 1134 (701 m), 1130 (488m) and coeval decreases in mud- sized sediments at the Broken Ridge sites, indicating winnowing associated with the onset of the TL. A divergence is also apparent between Broken Ridge and Mascarene Plateau Site 707 records at this time. These events, coupled with the temporal relationship between the onset of the TL and a change in the character of deposition in the Maldives indicate enhanced Indian Ocean circulation at intermediate depths coincident with the late Miocene global cooling. Combined, these observations suggest the Indian Ocean in general plays a larger role in the global ocean system than previously recognized, and intermediate waters in particular are a critical yet poorly understood component of AMOC.</p>

scholarly journals Large-scale drivers of Australian east coast cyclones since 1851

2016 ◽  
Vol 66 (2) ◽  
pp. 125
Author(s):  
Stuart A. Browning ◽  
Ian D. Goodwin
Keyword(s):  
19Th Century ◽  
Ocean Circulation ◽  
Large Scale ◽  
East Coast ◽  
Early Years ◽  
Winter Period ◽  
Storm Activity ◽  
Data Scarcity ◽  
Tasman Sea ◽  
The 19Th Century

Subtropical maritime low-pressure systems are one of the most complex and destructive storm types to impact Australia’s eastern seaboard. This family of storms, commonly referred to as East Coast Cyclones (ECC), is most active during the late autumn and early winter period when baroclinicity increases in the Tasman Sea region. ECC have proven challenging to forecast at both event and seasonal timescales. Storm activity datasets, objectively determined from reanalyses using cyclone detection algorithms, have improved understanding of the drivers of ECC over the era of satellite data coverage. In this study we attempt to extend these datasets back to 1851 using the Twentieth Century Reanalysis version 2c (20CRv2c). However, uncertainty in the 20CRv2c increases back through time due to observational data scarcity, and individual cyclones counts tend to be underestimated during the 19th century. An alternative approach is explored whereby storm activity is estimated from seasonal atmosphere-ocean circulation patterns. Seasonal ECC frequency over the 1955 to 2014 period is significantly correlated to regional sea-level pressure and sea surface temperature (SST) patterns. These patterns are used to downscale the 20CRv2c during early years when individual events are not well simulated. The stormiest periods since 1851 appear to have been 1870 to the early 1890s, and 1950 to the early 1970s. Total storm activity has been below the long-term average for most winters since 1976. Conditions conducive to frequent ECC events tend to occur during periods of relatively warm SST in the southwest Pacific typical of negative Interdecadal Pacific Oscillation (IPO-ve). Extratropical cyclogenesis is associated with negative Southern Annular Mode (SAM-ve) and blocking in the southern Tasman Sea. Subtropical cyclogenesis is associated with SAM+ve and blocking in the central Tasman Sea. While the downscaling approach shows some skill at estimating seasonal storm activity from the large-scale circulation, it cannot overcome data scarcity based uncertainties in the 19th century when the 20CRv2c is effectively unconstrained throughout most of the southern hemisphere. Storm frequency estimates during the 19th century are difficult to verify and should be interpreted cautiously and with reference to available documentary evidence.

scholarly journals Multivariate Sub-Regional Ocean Indicators in the Mediterranean Sea: From Event Detection to Climate Change Estimations

2021 ◽  
Vol 8 ◽  
Author(s):  
Mélanie Juza ◽  
Joaquín Tintoré
Keyword(s):  
Climate Change ◽  
Mediterranean Sea ◽  
Ocean Circulation ◽  
Heat Waves ◽  
Mesoscale Eddy ◽  
Value Added ◽  
Policy Decision ◽  
Balearic Islands ◽  
Wide Range ◽  
The Mediterranean

The increasing science and society requests for ocean monitoring from global to regional and local scales, the need for integration and convergence into a globally consistent ocean observing system as well as the need for improvement of access to information are now internationally recognized goals to progress toward the sustainable management of a healthy ocean. To respond to these challenges at regional level, the Balearic Islands Coastal Observing and Forecasting System (SOCIB) is developing a comprehensive set of ocean indicators in the Mediterranean Sea and around the Balearic Islands, key environments that are strongly affected by climate change and human pressure. This new SOCIB value-added product addresses the sub-regional ocean variability from daily (events) to interannual/decadal (climate) scales. A user-friendly interface has been implemented to monitor, visualize and communicate ocean information that is relevant for a wide range of sectors, applications and regional end-users. These sub-regional indicators allowed us to detect specific events in real time. Remarkable events and features identified include marine heat waves, atmospheric storm, extreme river discharge, mesoscale eddy, deep convection among others, all of them being oceanic phenomena that directly impact the ocean circulation and marine ecosystems. The long-term variations, in response to climate change, are also addressed highlighting and quantifying trends in physical and biogeochemical components of the ocean as well as sub-regional differences. At both (sub-) regional, national and international levels, a society-aligned science will have stronger impact on policy decision-makings and will support society to implement specific actions to address worldwide environmental challenges.

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