The Agulhas Current 2006, 330 Pages, ISBN103540423923, System
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Fronts in the World Ocean's Large Marine Ecosystems. ICES CM 2007
- 1 - This paper can be freely cited without prior reference to the authors International Council ICES CM 2007/D:21 for the Exploration Theme Session D: Comparative Marine Ecosystem of the Sea (ICES) Structure and Function: Descriptors and Characteristics Fronts in the World Ocean’s Large Marine Ecosystems Igor M. Belkin and Peter C. Cornillon Abstract. Oceanic fronts shape marine ecosystems; therefore front mapping and characterization is one of the most important aspects of physical oceanography. Here we report on the first effort to map and describe all major fronts in the World Ocean’s Large Marine Ecosystems (LMEs). Apart from a geographical review, these fronts are classified according to their origin and physical mechanisms that maintain them. This first-ever zero-order pattern of the LME fronts is based on a unique global frontal data base assembled at the University of Rhode Island. Thermal fronts were automatically derived from 12 years (1985-1996) of twice-daily satellite 9-km resolution global AVHRR SST fields with the Cayula-Cornillon front detection algorithm. These frontal maps serve as guidance in using hydrographic data to explore subsurface thermohaline fronts, whose surface thermal signatures have been mapped from space. Our most recent study of chlorophyll fronts in the Northwest Atlantic from high-resolution 1-km data (Belkin and O’Reilly, 2007) revealed a close spatial association between chlorophyll fronts and SST fronts, suggesting causative links between these two types of fronts. Keywords: Fronts; Large Marine Ecosystems; World Ocean; sea surface temperature. Igor M. Belkin: Graduate School of Oceanography, University of Rhode Island, 215 South Ferry Road, Narragansett, Rhode Island 02882, USA [tel.: +1 401 874 6533, fax: +1 874 6728, email: [email protected]]. -
Storm Waves Focusing and Steepening in the Agulhas Current: Satellite Observations and Modeling T ⁎ Y
Remote Sensing of Environment 216 (2018) 561–571 Contents lists available at ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse Storm waves focusing and steepening in the Agulhas current: Satellite observations and modeling T ⁎ Y. Quilfena, , M. Yurovskayab,c, B. Chaprona,c, F. Ardhuina a IFREMER, Univ. Brest, CNRS, IRD, Laboratoire d'Océanographie Physique et Spatiale (LOPS), Brest, France b Marine Hydrophysical Institute RAS, Sebastopol, Russia c Russian State Hydrometeorological University, Saint Petersburg, Russia ARTICLE INFO ABSTRACT Keywords: Strong ocean currents can modify the height and shape of ocean waves, possibly causing extreme sea states in Extreme waves particular conditions. The risk of extreme waves is a known hazard in the shipping routes crossing some of the Wave-current interactions main current systems. Modeling surface current interactions in standard wave numerical models is an active area Satellite altimeter of research that benefits from the increased availability and accuracy of satellite observations. We report a SAR typical case of a swell system propagating in the Agulhas current, using wind and sea state measurements from several satellites, jointly with state of the art analytical and numerical modeling of wave-current interactions. In particular, Synthetic Aperture Radar and altimeter measurements are used to show the evolution of the swell train and resulting local extreme waves. A ray tracing analysis shows that the significant wave height variability at scales < ~100 km is well associated with the current vorticity patterns. Predictions of the WAVEWATCH III numerical model in a version that accounts for wave-current interactions are consistent with observations, al- though their effects are still under-predicted in the present configuration. -
Johann R.E. Lutjeharms the Agulhas Current
330 Subject index Johann R.E. Lutjeharms The Agulhas Current 330 SubjectJ.R.E. index Lutjeharms The Agulhas Current with 187 figures, 8 in colour 123 330 Subject index Professor Johann R.E. Lutjeharms Department of Oceanography University of Cape Town Rondebosch 7700 South Africa ISBN-10 3-540-42392-3 Springer Berlin Heidelberg New York ISBN-13 978-3-540-42392-8 Springer Berlin Heidelberg New York Library of Congress Control Number: 2006926927 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable to prosecution under the German Copyright Law. Springer is a part of Springer Science+Business Media springer.com © Springer-Verlag Berlin Heidelberg 2006 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Camera-ready by Marja Wren-Sargent, ADU, University of Cape Town Figures: Anne Westoby, Cape Town Satellite images: Tarron Lamont and Christo Whittle, MRSU, University of Cape Town Technical support: Rene Navarro, ADU, University of Cape Town Cover design: E. -
Lecture 4: OCEANS (Outline)
LectureLecture 44 :: OCEANSOCEANS (Outline)(Outline) Basic Structures and Dynamics Ekman transport Geostrophic currents Surface Ocean Circulation Subtropicl gyre Boundary current Deep Ocean Circulation Thermohaline conveyor belt ESS200A Prof. Jin -Yi Yu BasicBasic OceanOcean StructuresStructures Warm up by sunlight! Upper Ocean (~100 m) Shallow, warm upper layer where light is abundant and where most marine life can be found. Deep Ocean Cold, dark, deep ocean where plenty supplies of nutrients and carbon exist. ESS200A No sunlight! Prof. Jin -Yi Yu BasicBasic OceanOcean CurrentCurrent SystemsSystems Upper Ocean surface circulation Deep Ocean deep ocean circulation ESS200A (from “Is The Temperature Rising?”) Prof. Jin -Yi Yu TheThe StateState ofof OceansOceans Temperature warm on the upper ocean, cold in the deeper ocean. Salinity variations determined by evaporation, precipitation, sea-ice formation and melt, and river runoff. Density small in the upper ocean, large in the deeper ocean. ESS200A Prof. Jin -Yi Yu PotentialPotential TemperatureTemperature Potential temperature is very close to temperature in the ocean. The average temperature of the world ocean is about 3.6°C. ESS200A (from Global Physical Climatology ) Prof. Jin -Yi Yu SalinitySalinity E < P Sea-ice formation and melting E > P Salinity is the mass of dissolved salts in a kilogram of seawater. Unit: ‰ (part per thousand; per mil). The average salinity of the world ocean is 34.7‰. Four major factors that affect salinity: evaporation, precipitation, inflow of river water, and sea-ice formation and melting. (from Global Physical Climatology ) ESS200A Prof. Jin -Yi Yu Low density due to absorption of solar energy near the surface. DensityDensity Seawater is almost incompressible, so the density of seawater is always very close to 1000 kg/m 3. -
Global Ocean Surface Velocities from Drifters: Mean, Variance, El Nino–Southern~ Oscillation Response, and Seasonal Cycle Rick Lumpkin1 and Gregory C
JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 2992–3006, doi:10.1002/jgrc.20210, 2013 Global ocean surface velocities from drifters: Mean, variance, El Nino–Southern~ Oscillation response, and seasonal cycle Rick Lumpkin1 and Gregory C. Johnson2 Received 24 September 2012; revised 18 April 2013; accepted 19 April 2013; published 14 June 2013. [1] Global near-surface currents are calculated from satellite-tracked drogued drifter velocities on a 0.5 Â 0.5 latitude-longitude grid using a new methodology. Data used at each grid point lie within a centered bin of set area with a shape defined by the variance ellipse of current fluctuations within that bin. The time-mean current, its annual harmonic, semiannual harmonic, correlation with the Southern Oscillation Index (SOI), spatial gradients, and residuals are estimated along with formal error bars for each component. The time-mean field resolves the major surface current systems of the world. The magnitude of the variance reveals enhanced eddy kinetic energy in the western boundary current systems, in equatorial regions, and along the Antarctic Circumpolar Current, as well as three large ‘‘eddy deserts,’’ two in the Pacific and one in the Atlantic. The SOI component is largest in the western and central tropical Pacific, but can also be seen in the Indian Ocean. Seasonal variations reveal details such as the gyre-scale shifts in the convergence centers of the subtropical gyres, and the seasonal evolution of tropical currents and eddies in the western tropical Pacific Ocean. The results of this study are available as a monthly climatology. Citation: Lumpkin, R., and G. -
Why Are There Agulhas Rings?
APRIL 1999 PICHEVIN ET AL. 693 Why Are There Agulhas Rings? THIERRY PICHEVIN SHOM/CMO, Brest, France DORON NOF Department of Oceanography and the Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, Florida JOHANN LUTJEHARMS Department of Oceanography, University of Cape Town, Rondebosch, South Africa (Manuscript received 31 March 1997, in ®nal form 7 May 1998) ABSTRACT The recently proposed analytical theory of Nof and Pichevin describing the intimate relationship between retro¯ecting currents and the production of rings is examined numerically and applied to the Agulhas Current. Using a reduced-gravity 1½-layer primitive equation model of the Bleck and Boudra type the authors show that, as the theory suggests, the generation of rings from a retro¯ecting current is inevitable. The generation of rings is not due to an instability associated with the breakdown of a known steady solution but rather is due to the zonal momentum ¯ux (i.e., ¯ow force) of the Agulhas jet that curves back on itself. Numerical experiments demonstrate that, to compensate for this ¯ow force, several rings are produced each year. Since the slowly drifting rings need to balance the entire ¯ow force of the retro¯ecting jet, their length scale is considerably larger than the Rossby radius; that is, their scale is greater than that of their classical counterparts produced by instability. Recent observations suggest a correlation between the so-called ``Natal Pulse'' and the production of Agulhas rings. As a by-product of the more general retro¯ection experiments, the pulse issue is also examined numerically using two different representations for the pulses. -
Indian Ocean Crossing Swells: New Insights from “Fireworks” Perspective Using Envisat Advanced Synthetic Aperture Radar
remote sensing Communication Indian Ocean Crossing Swells: New Insights from “Fireworks” Perspective Using Envisat Advanced Synthetic Aperture Radar He Wang 1,2,* , Alexis Mouche 2 , Romain Husson 3 and Bertrand Chapron 2 1 National Ocean Technology Center, Ministry of Natural Resources, Tianjin 300112, China 2 Laboratoire d’Océanographie Physique Spatiale, Centre de Brest, Ifremer, 29280 Plouzané, France; [email protected] (A.M.); [email protected] (B.C.) 3 Collecte Localisation Satellites, 29280 Plouzané, France; [email protected] * Correspondence: [email protected]; Tel.: +86-22-2753-6513 Abstract: Synthetic Aperture Radar (SAR) in wave mode is a powerful sensor for monitoring the swells propagating across ocean basins. Here, we investigate crossing swells in the Indian Ocean using 10-years Envisat SAR wave mode archive spanning from December 2003 to April 2012. Taking the benefit of the unique “fireworks” analysis on SAR observations, we reconstruct the origins and propagating routes that are associated with crossing swell pools in the Indian Ocean. Besides, three different crossing swell mechanisms are discriminated from space by the comparative analysis between results from “fireworks” and original SAR data: (1) in the mid-ocean basin of the Indian Ocean, two remote southern swells form the crossing swell; (2) wave-current interaction; and, (3) co- existence of remote Southern swell and shamal swell contribute to the crossing swells in the Agulhas Current region and the Arabian Sea. Keywords: crossing swells; synthetic aperture radar; wave mode; fireworks analysis; Indian Ocean Citation: Wang, H.; Mouche, A.; Husson, R.; Chapron, B. Indian Ocean Crossing Swells: New Insights from “Fireworks” Perspective Using 1. -
Upper-Level Circulation in the South Atlantic Ocean
Prog. Oceanog. Vol. 26, pp. 1-73, 1991. 0079 - 6611/91 $0.00 + .50 Printed in Great Britain. All fights reserved. © 1991 Pergamon Press pie Upper-level circulation in the South Atlantic Ocean RAY G. P~-rwtSON and LOTHAR Sa~AMMA lnstitut fiir Meereskunde an der Universitiit Kiel, Diisternbrooker Weg 20, 2300 Kiel 1, F.R.G. Abstract - In this paper we present a literature survey of the South Atlantic's climate and its oceanic upper-layer circulation and meridional beat transport. The opening section deals with climate and is focused upon those elements having greatest oceanic relevance, i.e., distributions of atmospheric sea level pressure, the wind fields they produce, and the net surface energy fluxes. The various geostrophic currents comprising the upper-level general circulation are then reviewed in a manner organized around the subtropical gyre, beginning off southern Africa with the Agulhas Current Retroflection and then progressing to the Benguela Current, the equatorial current system and circulation in the Angola Basin, the large-scale variability and interannual warmings at low latitudes, the Brazil Current, the South Atlantic Cmrent, and finally to the Antarctic Circumpolar Current system in which the Falkland (Malvinas) Current is included. A summary of estimates of the meridional heat transport at various latitudes in the South Atlantic ends the survey. CONTENTS 1. Introduction 2 2. Climatic Elements 2 3. Subtropical and Equatorial Circulation 11 3.1. Agulhas Current Retroflection 11 3.2. Benguela Cmrent 16 3.3. Equatorial Cttrrents 18 3.3.1. Components of the system 18 3.3.2. Angola Basin circulation 26 3.3.3. -
Hydrodynamic Changes in the Agulhas Current and Associated Changes in the Indian and Atlantic Ocean
Hydrodynamic changes in the Agulhas Current and associated changes in the Indian and Atlantic Ocean Mathieu Rouault Department of Oceanography, Mare Institute, University of Cape Town Nansen-Tutu Center for Marine Environment, South Africa Contributions from Pierrick Penven, Benjamin Pohl, Bjorn Backeberg Sea surface temperature estimated by AVHRR aboard NOAA (1x1 km resolution) Funding from WRC, ACCESS, Nansen-Tutu Center Mean AVHRR Pathfinder 4x4 km sea surface temperature and merged altimetry derived geostrophic currents Sequence of weekly mean TMI TRMM sea surface temperature showing an unusual early retroflection of the Agulhas Current at a position more eastward and northwards than normal. Warm Agulhas water eventually re-enter the current. Data is shown each week from the last week of January 2001 to the first week of March 2001 (Rouault and Lutjeharms, 2003) Siedler G, M Rouault, A Biastoch, B Backeberg, C J.C. Reason, and J. R. E. Lutjeharms 2009 Modes of the southern extension of the East Madagascar Current, JGR Ocean Altimetry derived geostrophic currents averaged over five years from August 2001 to May 2006 showing the newly documented South Indian Counter Current and the related retroflection South of Madagascar . The magenta dots indicate the positions of the WOCE stations used for transport calculations (Siedler et al, 2009) Change in sea surface temperature from 1985 to 2007 in C per 10-year using 4 km resolution AVHRR only. Superimposed is the mean ocean current (yellow to red: warming, green blue: cooling) Net heat budget -
Wind Changes Above Warm Agulhas Current Eddies
Ocean Sci., 12, 495–506, 2016 www.ocean-sci.net/12/495/2016/ doi:10.5194/os-12-495-2016 © Author(s) 2016. CC Attribution 3.0 License. Wind changes above warm Agulhas Current eddies M. Rouault1,2, P. Verley3,6, and B. Backeberg2,4,5 1Department of Oceanography, Mare Institute, University of Cape Town, South Africa 2Nansen-Tutu Center for Marine Environmental Research, University of Cape Town, South Africa 3ICEMASA, IRD, Cape Town, South Africa 4Nansen Environmental and Remote Sensing Centre, Bergen, Norway 5NRE, CSIR, Stellenbosch, South Africa 6IRD, UMR AMAP, Montpellier, France Correspondence to: M. Rouault ([email protected]) Received: 8 September 2014 – Published in Ocean Sci. Discuss.: 21 October 2014 Revised: 7 March 2016 – Accepted: 9 March 2016 – Published: 5 April 2016 Abstract. Sea surface temperature (SST) estimated from higher wind speeds and occurred during a cold front associ- the Advanced Microwave Scanning Radiometer E onboard ated with intense cyclonic low-pressure systems, suggesting the Aqua satellite and altimetry-derived sea level anomalies certain synoptic conditions need to be met to allow for the de- are used south of the Agulhas Current to identify warm- velopment of wind speed anomalies over warm-core ocean core mesoscale eddies presenting a distinct SST perturbation eddies. In many cases, change in wind speed above eddies greater than to 1 ◦C to the surrounding ocean. The analysis was masked by a large-scale synoptic wind speed decelera- of twice daily instantaneous charts of equivalent stability- tion/acceleration affecting parts of the eddies. neutral wind speed estimates from the SeaWinds scatterom- eter onboard the QuikScat satellite collocated with SST for six identified eddies shows stronger wind speed above the warm eddies than the surrounding water in all wind direc- 1 Introduction tions, if averaged over the lifespan of the eddies, as was found in previous studies. -
Seasonal Phasing of Agulhas Current Transport Tied to a Baroclinic
Journal of Geophysical Research: Oceans RESEARCH ARTICLE Seasonal Phasing of Agulhas Current Transport Tied 10.1029/2018JC014319 to a Baroclinic Adjustment of Near-Field Winds Key Points: • A baroclinic adjustment to Indian Katherine Hutchinson1,2 , Lisa M. Beal3 , Pierrick Penven4 , Isabelle Ansorge1, and Ocean winds can explain the 1,2 Agulhas Current seasonal phasing, Juliet Hermes and a barotropic adjustment cannot 1 2 • Seasonal phasing is found to be Department of Oceanography, University of Cape Town, Cape Town, South Africa, South African Environmental 3 highly sensitive to reduced gravity Observations Network, SAEON Egagasini Node, Cape Town, South Africa, Rosenstiel School of Marine and Atmospheric values, which modify adjustment Science, University of Miami, Miami, FL, USA, 4Laboratoire d’Oceanographie Physique et Spatiale, University of Brest, times to wind forcing CNRS, IRD, Ifremer, IUEM, Brest, France • Near-field winds have a dominant influence on the seasonal cycle of the Agulhas Current as remote signals die out while crossing the basin Abstract The Agulhas Current plays a significant role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain unclear, with discrepancies between ocean models and observations. Observations from moorings across the Correspondence to: K. Hutchinson, current and a 22-year proxy of Agulhas Current volume transport reveal that the current is over 25% [email protected] stronger in austral summer than in winter. We hypothesize that winds over the Southern Indian Ocean play a critical role in determining this seasonal phasing through barotropic and first baroclinic mode adjustments Citation: and communication to the western boundary via Rossby waves. -
Investigating the Global Impacts of the Agulhas Current
Eos, Vol. 91, No. 12, 23 March 2010 VOLUME 91 NUMBER 12 23 MARCH 2010 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 109–116 responses with potential consequences Investigating the Global Impacts for the Atlantic MOC [Biastoch et al., 2009; Knorr and Lohmann, 2003]. While the signif- icance of these mechanisms is increasingly of the Agulhas Current recognized, their dynamics and sensitiv- ity are not well understood. The key ques- PAGES 109–110 The Agulhas Current infl uences climate tions that arise are as follows: (1) How does in two ways. First, the warm tropical waters the Agulhas Current react to shifts in wind The Agulhas Current is the major west- carried by the current stimulate convec- fi elds and regional ocean fronts? (2) How ern boundary current of the Southern Hemi- tion of the overlying atmosphere with direct do such changes affect the Agulhas leak- sphere [Lutjeharms, 2006] and a key com- consequences for regional weather sys- age into the Atlantic? (3) Does the leak- ponent of the global ocean “conveyor” tems [Reason and Jagadheesha, 2005]. Sec- age indeed perturb the Atlantic MOC? To circulation controlling the return fl ow to the ond, the shedding of Agulhas rings south answer these questions, paleoceanographic Atlantic Ocean [Gordon, 1986]. As such, it of Africa causes buoyancy anomalies in reconstructions and numerical modeling is increasingly recognized as a key player in the South Atlantic that stimulate dynamical are essential tools. ocean thermohaline circulation, with impor- tance for the meridional overturning circula- tion (MOC) of the Atlantic Ocean. Unusual dynamics pervade the motion of this warm- water current—as it moves west around the southern tip of Africa, it is retro- fl ected back east by the Antarctic Circum- polar Current.