DOCSLIB.ORG

CPY Document

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
CPY Document L d tV) tí\'-J ~ () .:t i, N ~ ::cp 2- '\ h~ On the pathways of the return flow of the ""cot') - .,,_.,-2" meridional overturning circulation in the tropical Atlantic MBl/ WHOJ LIBRARY by Woods Hole, MA Manne Biologial Laboratory WOOs Hole Oçeaiiographk Markus Jochum Institutln ' MoSco, University of Kiel, 1997 Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY 'L. and the """"" t \;."....) WOODS HOLE OCEANOGRAPHIC INSTITUTION ..' June 2002 ~,' ; (92002 Markus Jochum All rights reserved The author hereby grants MIT and WHOI permission to reproduce paper and elec- tronic copies of this thesis in whole or in part and to distribute them publicly Signature of Author 0000000.......... 0 0.. 0.. 0 0 0 0.. oint Program in Physical Oceanography Massachusetts Institute of Technology and Woods Hole Oceanographic Institution "1 June 2002 Paola Malanotte- Rizzoli Thesis Supervisor Accepted by 000.. 0... 0.. 0.00... 0.. .. 0.. 0..... Carl Wunsch Chair, Joint Committee for Physical Oceanography Massachusetts Institute of Technology I. and Woods Hole Oceanographic Institution .--'-~ _.'.... ....1. y ,. l' _.' , , On the pathways of the return flow of the meridional overturning circulation in the tropical Atlantic. by Markus Jochum Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 15, 2002 ) Abstract A numerical model of the tropical Atlantic ocean is used to investigate the upper layer pathways of the Meridional Overturning Circulation (MOC) in the tropical Atlantic. The main focus of this thesis is on those parts of the tropical circulation that are thought to be important for the MOC return flow, but whose dynamics have not been understood yet. It is shown how the particular structure of the tropical gyre and the MOC act to inhibit the flow of North Atlantic water into the equatorial thermocline. As a result, the upper layers of the tropical Atlantic are mainly fed by water from the South Atlantic. The processes that carry the South Atlantic water across the trop- ical Atlantic into the North Atlantic as part of the MOC are described here, and three processes that were hitherto not understood are explained as follows: The North Brazil Current rings are created as the result of the reflection of Rossby waves at the South American coast. These Rossby waves are generated by the barotropic ally unstable North Equatorial Countercurrent. The deep structure of the rings can be explained by merger of the wave's anticyclones with the deeper in- termediate eddies that are generated as the intermediate western boundary current crosses the equator. The bands of strong zonal velocity in intermediate depths along the equator have hitherto been explained as intermediate currents. Here, an alternative interpretation of the observations is offered: The Eulerian mean flow along the equator is negli- ) gible and the observations are the signature of strong seasonal Rossby waves. The previous interpretation of the observations can then be explained as aliasing of the tropical wave field. The Tsuchyia Jets are driven by the Eliassen-Palm flux of the tropical instability waves. The equatorial current system with its strong shears is unstable and gen- erates tropical instability waves. These waves cause a poleward temperature flux which steepens the isotherms which in turn generates are geostrophically balanced zonal flow. In the eastern part of the basin this zonal flow feeds the southeastward flow of the equatorial gyre. Thesis supervisor: Paola Malanotte-Rizzoli Title: Professor, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology 3 Acknowledgments Looking back the last five years, I am stunned when I realize how many people it takes to create a thesis. Further reflection helps to blur the distinction between me as a creator of a thesis, and me as a saucepan that converts lots of love, ideas and money into a new idea. Each of these three ingredients are necessary, none of them is suffcient. Some people give a little bit of everything, some a lot of one ingredient, and of course there are also nonlinear interactions to be taken into account. This makes it almost impossible to acknowledge everybody in due manner (except for NASA and ONR ~ho generously funded me with their respective grants NAG5- 7194 and N00014-98-10881). Clearly, this thesis could not have been written without the love and support of my parents Ilona and Alois and my wife Elizabeth. Being Catholic, my family is of course more numerous than that and I am grateful to all of them for their support during the different stages of my life. My committee members Tony Busalacchi, Glenn Flierl, Breck Owens and Mike Spall are thanked for their seemingly endless doubts and their knowledgeable advice. This gratitude extends towards the student body of MIT and WHOI, and to Dave Fratantoni who let me see and touch a real North Brazil Current ring. My advisor Paola Malanotte-Rizzoli is more diffcult to place; depending on the time of the day and geographical location she makes a valuable contribution to each of the three aforementioned categories. This magic potion is probably the most im- portant ingredient of any, and especially my thesis. And then there is Raghu Murtugudde who told me always to mention him in the acknowledgments. I suspect not without reason. 4 From a certain Chinese encyclopedia: Animals are divided into: a) belonging to the Emperor b) embalmed c) tame d) sucking pigs e) sirens f) fabulous g) stray dogs h) included in the present classification i) frenzied j) innumerable k) drawn with a very fine camelhair brush l) et cetera m) having just broken the water pitcher n) that from a long way off look like flies. ) (Jorge Luis Borges) ') ~ 5 Contents 1 Introduction 8 2 The influence of the MOC 15 2.1 Introduction 15 2.2 The model and the experiments 16 2.3 The flow field . 19 2.4 The tropical-subtropical pathways. 23 3 Description of the tropical circulation 29 3.1 The model configuration . 29 3.2 Overview of the upper layer circulation in the tropical Atlantic . 37 3.3 Description of the upper layer mean flow in the model 46 3.3.1 The thermocline waters 54 3.3.2 The surface waters . 59 4 4.1North Introduction.... Brazil Current rings 61 61 4.2 The Ring Shedding 62 4.2.1 Rings without wind. 62 6 4.2.2 No rings with wind 65 4.2.3 The role of Rossby waves . 66 4.2.4 The unstable NECC ... 72 4.3 The vertical structure, seasonality, and transport of NBC rings . 78 5 The flow of AAIW along the equator 88 5.1 Introduction. 88 5.2 Observations 89 5.3 The AAIW in the model 90 5.4 Synthesis of theory and observations 97 6 On the dynamics of the Tsuchyia Jets 110 6.1 Introduction............... .110 6.2 The structure of the Tsuchyia Jets in the model 113 6.3 Wave-zonal flow interaction 117 ) 7 Summary 126 ) 7 Chapter i Introduction During the last five decades, oceanographers have made a considerable effort to es- timate and understand the global mean circulation of the world's oceans. Together, the conclusion of the World Ocean Circulation Experiment (WOCE), the system- atic use of satellites and the widespread availability of fast computers since the last decade enabled scientists to provide a coherent, albeit vague picture of the global circulation (Ganachaud and Wunsch (2001)). In addition to these developments, there were also advancements in analytical methodology that led to theories about the western boundary currents or the vertical stratification of the ocean (see Ped- losky (1996) for an overview). These developments made clear that in large parts of the ocean the variability of the flow is much stronger than the mean circulation. Therefore the understanding of the ocean's variability is indispensable for the in- terpretation of observations and the assessment of theories. Furthermore, the ocean forms the lower boundary of the atmosphere and wil directly affect its thermal structure and its water content. This implies that the ocean is not passively driven by the atmosphere, but instead the ocean and the atmosphere should be treated as a coupled system. A classical example of a coupled, highly variable ocean-atmosphere system is the equatorial Pacific. Its strong interannual variability CEl Nino - South- ern Oscilation (ENSO)') causes a world wide shift in weather patterns and can only be understood by taking into account atmospheric as well as oceanic dynamics. By now, the equatorial Pacific is a well observed system, and it was even possible in recent years to predict the phases of different ENSO cycles (Cane et al. (1986), Chen et al. (1999)). 8 Although one would expect that the dynamics that control the equatorial Pacific are the same in all equatorial oceans, scientists have been less successful in under- standing the climate fluctuations in the tropical Atlantic (see Seager at al. (2001) for a recent analysis) Not only are there very few observations away from the western boundary along the American coast, but matters are complicated further by the more complex dynamics of the Atlantic ocean. Apart from the seasonal cycle, there are two modes that dominate the tropical Atlantic SST variability. The equatorial mode, which is similar to ENSO, influences the equatorial SST at interannual frequencies (Carton and Huang (1994)). The inter- hemispheric (or dipole) mode, affects the gradient of the SST between the northern and southern tropics (Servain (1991)) and therefore influences the position of the ITCZ. It has been shown, that these modes are correlated with ENSO (Delecluse et al.
Load more

Recommended publications
  • Chapter 8 the Pacific Ocean After These Two Lengthy Excursions Into
    Chapter 8 The Pacific Ocean After these two lengthy excursions into polar oceanography we are now ready to test our understanding of ocean dynamics by looking at one of the three major ocean basins. The Pacific Ocean is not everyone's first choice for such an undertaking, mainly because the traditional industrialized nations border the Atlantic Ocean; and as science always follows economics and politics (Tomczak, 1980), the Atlantic Ocean has been investigated in far more detail than any other. However, if we want to take the summary of ocean dynamics and water mass structure developed in our first five chapters as a starting point, the Pacific Ocean is a much more logical candidate, since it comes closest to our hypothetical ocean which formed the basis of Figures 3.1 and 5.5. We therefore accept the lack of observational knowledge, particularly in the South Pacific Ocean, and see how our ideas of ocean dynamics can help us in interpreting what we know. Bottom topography The Pacific Ocean is the largest of all oceans. In the tropics it spans a zonal distance of 20,000 km from Malacca Strait to Panama. Its meridional extent between Bering Strait and Antarctica is over 15,000 km. With all its adjacent seas it covers an area of 178.106 km2 and represents 40% of the surface area of the world ocean, equivalent to the area of all continents. Without its Southern Ocean part the Pacific Ocean still covers 147.106 km2, about twice the area of the Indian Ocean. Fig. 8.1. The inter-oceanic ridge system of the world ocean (heavy line) and major secondary ridges.
    [Show full text]
  • Downloaded 09/25/21 10:39 PM UTC 1028 JOURNAL of PHYSICAL OCEANOGRAPHY VOLUME 33
    MAY 2003 SLOYAN ET AL. 1027 The Paci®c Cold Tongue: A Pathway for Interhemispheric Exchange* BERNADETTE M. SLOYAN,1 GREGORY C. JOHNSON, AND WILLIAM S. KESSLER NOAA/Paci®c Marine Environmental Laboratory, Seattle, Washington (Manuscript received 29 January 2002, in ®nal form 28 October 2002) ABSTRACT Mean meridional upper-ocean temperature, salinity, and zonal velocity sections across the Paci®c Ocean between 88S and 88N are combined with other oceanographic and air±sea ¯ux data in an inverse model. The tropical Paci®c Ocean can be divided into three regions with distinct circulation patterns: western (1438E± 1708W), central (1708±1258W), and eastern (1258W±eastern boundary). In the central and eastern Paci®c the downward limbs of the shallow tropical cells are 15(613) 3 106 m3 s 21 in the north and 20(611) 3 106 m3 s21 in the south. The Paci®c cold tongue in the eastern region results from diapycnal upwelling through all layers of the Equatorial Undercurrent, which preferentially exhausts the lightest (warmer) layers of the Equatorial Undercurrent [10(66) 3 106 m3 s21] between 1258 and 958W, allowing the denser (cooler) layers to upwell [9(64) 3 106 m3 s21] east of 958W and adjacent to the American coast. An interhemispheric exchange of 13(613) 3 106 m3 s 21 between the southern and northern Paci®c Ocean forms the Paci®c branch of the Paci®c± Indian interbasin exchange. Southern Hemisphere water enters the tropical Paci®c Ocean via the direct route at the western boundary and via an interior (basin) pathway. However, this water moves irreversibly into the North Paci®c by upwelling in the eastern equatorial Paci®c and air±sea transformation that drives poleward interior transport across 28N.
    [Show full text]
  • Equatorial Ocean Currents
    i KNAUSSTRIBUTE An Observer's View of the Equatorial Ocean Currents Robert H. Weisbeq~ University qf South Florida • St. Petersbm X, Florida USA Introduction The equator is a curious place. Despite high solar eastward surface current that flows halfway around the radiation that is relatively steady year-round, oceanog- Earth counter to the prevailing winds. raphers know to pack a sweater when sailing there. Curiosity notwithstanding, we study these equatori- Compared with the 28°C surface temperatures of the al ocean circulation features because they are adjacent tropical waters, the eastern halves of the equa- fundamental to the Earth's climate system and its vari- torial Pacific or Atlantic Oceans have surface ations. For instance, the equatorial cold tongue in the temperatures up to 10°C colder, particularly in boreal Atlantic (a band of relatively cold water centered on the summer months. The currents are equally odd. equator), itself a circulation induced feature, accounts Prevailing easterly winds over most of the tropical for a factor of about 1.5 increase in heat flux from the Pacific and Atlantic Oceans suggest westward flowing southern hemisphere to the northern hemisphere. In the currents. Yet, eastward currents oftentimes outweigh Pacific, the inter-annual variations in the cold tongue those flowing toward the west. Motivation for the first temperature are the ocean's part of the E1 Nifio- quantitative theory of the large scale, wind driven Southern Oscillation (ENSO) which affects climate and ocean circulation, in fact, came
    [Show full text]
  • Pliocene Stratigraphy and the Impact of Panama Uplift on Changes in Caribbean and Tropical East Pacific Upper Ocean Stratification (6 – 2.5 Ma)
    PLIOCENE STRATIGRAPHY AND THE IMPACT OF PANAMA UPLIFT ON CHANGES IN CARIBBEAN AND TROPICAL EAST PACIFIC UPPER OCEAN STRATIFICATION (6 – 2.5 MA) PLIOZÄNE STRATIGRAPHIE UND DER EINFLUSS DES PANAMA-SEEWEGES AUF DIE STRATIFIZIERUNG DER OBERFLÄCHEN-WASSERMASSEN IN DER KARIBIK UND IM TROPISCHEN OST-PAZIFIK (6 – 2.5 MA) Kumulative Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Silke Steph Kiel, 2005 PLIOCENE STRATIGRAPHY AND THE IMPACT OF PANAMA UPLIFT ON CHANGES IN CARIBBEAN AND TROPICAL EAST PACIFIC UPPER OCEAN STRATIFICATION (6 – 2.5 MA) PLIOZÄNE STRATIGRAPHIE UND DER EINFLUSS DES PANAMA-SEEWEGES AUF DIE STRATIFIZIERUNG DER OBERFLÄCHEN-WASSERMASSEN IN DER KARIBIK UND IM TROPISCHEN OST-PAZIFIK (6 – 2.5 MA) Kumulative Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Silke Steph Kiel, 2005 158 pages 52 figures 10 tables 8 appendices Referent: ________________________________________ Koreferent: ________________________________________ Tag der Disputation: ________________________________________ Zum Druck genehmigt, Kiel, den: ________________________________________ Der Dekan: ________________________________________ I ABSTRACT This thesis examines the closure history of the Central American Seaway (CAS) and its effect on changes in ocean circulation and climate during the time interval from ~6 – 2.5 Ma. It was accomplished within the DFG Research Unit "Impact of Gateways on Ocean Circulation, Climate and Evolution" at the University of Kiel. Proxy records from Ocean Drilling Program (ODP) Sites 999 and 1000 (Caribbean), and from ODP Sites 1237, 1239 and 1241 (low-latitude east Pacific) are developed and examined. In addition, previously established proxy data from Atlantic Sites 925/926 (Ceara Rise) and 1006 (western Great Bahama Bank) and from two east Pacific sites (851, 1236) are included for interpretations.
    [Show full text]
  • 36- 5. Rotating Flow the General Circulation of the Ocean
    -36- 5. Rotating Flow The general circulation of the ocean (and atmosphere) differs markedly from the flows we typically observe around us and from the solutions that we have discussed so far, because large scale geophysical flows are: (1) dominated by the effects of rotation and (2) are almost inviscid. In this chapter I will introduce a technique to help us decide what terms in the Navier-Stokes equations are important in a given situation, present additional basic concepts and finally apply all that we have learned to the problem of the surface circulation of the oceans. 5.1. Non-dimensionalization In the last chapter, we saw how approximate analyses of partial differential equa- tions can provide insight into the behavior of the solution. Scaling the equations with typ- ical values of the variables also has other uses, most of which begin with the basic con- cept of non-dimensionalization. But before I discuss non-dimensionalization, I want to first consider the effect of variable density on the Navier-Stokes equation. Let → + ρ → ρ + ρ p pH pˆ (z) ˆ ρ where pH and (z) satisfy the hydrostatic equation ∇ =−ρ pH (z)gzˆ If we subtract this equation from the Navier-Stokes equation Du (ρ(z) + ρˆ) =−∇(p + pˆ) − (ρ(z) + ρˆ)gzˆ + µ∇2u Dt H and divide by ρ(z) + ρˆ we obtain Du 1 ρˆ =− ∇pˆ − gzˆ + ν∇2u Dt ρ + ρˆ ρ + ρˆ Note that gravity enters only to the extent that the density deviates from the hydrostatic 1 1 − ρˆ situation. In must cases,ρ ˆ << ρ(z) and = .
    [Show full text]
  • The Reversal of the Wind-Driven Equatorial Undercurrent
    THE REVERSAL OF THE ilIND-DRIVllN EQUA'roRIAL UNDERCURRl!NT Philippe HISARD and Christian HmqN Centre ORS'roM E.P. 4 Noumea, New Caledonia The Equatorial Undercurrent in the Pacific Ocean, at 1700E, has been described as a double-cell structure (Hisard et al., 1970). The upper cell is located at the bottom of the homngeneous surface l~er (120 m thick), surrounded by the Equatorial Current; the lower cell is in the thermocline l~er (V max. at 220 m depth) and is connected to the eastward flow of the North l!qua.torial <bunterourrent. The flow of the lower cell is in geostro­ phic balance (Oolin et Rotschi, 1970), and permanent, unaltered by changes in the ,wind pattern; the upper cell flow does not appear to be in geostrnphic equilibrium and, moreover, it reverses westward when there is an eastward current, driven by a west wind, at the surface. So, it can be assumed that the upper cell of the Equatorial Undercurrent belongs to the wind-driven circulation, and that the lower cell belongs to the thermo-haline circulation. In the central Pacific, as the thermocline layer rises towards the east, these two mechanisms drive at about the same depth a one-cell equatorial undeI'­ current; however, it is possible through chemical considerations such as nutrient salts and oxygen contents, to outline the influence of each cell as far east as 1400W (Oolin et al., 1971). The reversal of the eastward-flowing upper cell of the equatorial under­ current at 1700E, whon an eastward surface current (driven by a west wind) substitutes for the Equatorial Current at the equator, leads us to question the generality of such a structure; from the theoretical point of view, most of the described models agree with the possibility of a reversal of the un­ derourrent under positive (eastward) Wind-stress; moreover, most of these models take into consideration an homogeneous ocean and are consistent with the observation of an undercurrent 11').
    [Show full text]
  • Journal of Marine Research, Sears Foundation for Marine Research
    The Journal of Marine Research is an online peer-reviewed journal that publishes original research on a broad array of topics in physical, biological, and chemical oceanography. In publication since 1937, it is one of the oldest journals in American marine science and occupies a unique niche within the ocean sciences, with a rich tradition and distinguished history as part of the Sears Foundation for Marine Research at Yale University. Past and current issues are available at journalofmarineresearch.org. Yale University provides access to these materials for educational and research purposes only. Copyright or other proprietary rights to content contained in this document may be held by individuals or entities other than, or in addition to, Yale University. You are solely responsible for determining the ownership of the copyright, and for obtaining permission for your intended use. Yale University makes no warranty that your distribution, reproduction, or other use of these materials will not infringe the rights of third parties. This work is licensed under the Creative Commons Attribution- NonCommercial-ShareAlike 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/ or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA. Journal of Marine Research, Sears Foundation for Marine Research, Yale University PO Box 208118, New Haven, CT 06520-8118 USA (203) 432-3154 fax (203) 432-5872 [email protected] www.journalofmarineresearch.org Further Measurements and Observations on the Cromwell Current 1 John A. Knauss Graduate School of Oceanog.-aphy Narragansett Marine Laboratory University of Rhode Island Kingston, Rhode Island ABSTRACT The Cromwell Current was observed in the eastern Pacific during the Scripps Institution of Oceanography SWAN SONG Expedition, 5 September-I December 1961, employing hydrographic stations and direct current measurements.
    [Show full text]
  • Chapter 5 EQUATORIAL CURRENTS 1 Phenomenology
    Chapter 5 EQUATORIAL CURRENTS 1 Phenomenology Tropical circulation is unique because of the vanishing of the Coriolis frequency f at the equator, with f ≈ βy in the vicinity of the equator at y = 0. This is called the equatorial β-plane approximation), and it has several dynamical consequences. One is that geostrophic balance is much less reliable a pri- ori as a large-scale dynamical approximation, although some of the most important zonal currents approximately remain geostrophic even at the equator (this requires that the meridional pressure gradient vanish there too). Another consequence is that the inviscid, adiabatic conservation of po- tential vorticity, q, constrains parcel displacements from crossing the equator for any substantial distance, since q must assume the sign of f away from the equator and this sign is different in the different hemispheres. This means that the mean circulation and most long-time parcel displace- ments are primarily zonal near the equation, but it also focuses our attention on locations (e.g., surface PBL, deep WBC) where mixing processes break the constraint of q conservation. A third consequence is that the equatorial band is an effective zonal waveguide for distinctive, zonally propagating Kelvin, Rossby, Yanai, and gravity wave modes. These waves can support east-west mass adjustment processes, including some essential behaviors for the most important global, cou- pled climate-variability mode on intermediate time scales, viz., El Nino˜ – Southern Oscillation (ENSO) (Philander, 1990). A fourth consequence is that the “linear” Ekman depth, √ hek ≈ τττ/f → ∞ as f → 0 ; i.e., the steady-state boundary-layer depth might reach all the way to the bottom.
    [Show full text]
  • A Theory of the Cromwell Current (The Equatorial Undercurrent) and of the Equatorial Upwelling an Interpretation in a Similarity to a Costal Circulation
    A Theory of the Cromwell Current (the Equatorial Undercurrent) and of the Equatorial Upwelling An Interpretation in a Similarity to •\ a Costal Circulation*•\ Kozo YOSHIDA** Abstract: A theory is given to account for the CROMWELL Current-the Equatorial Undercurrent. The equatorial upwelling is closely related to the Current, and a remark- able similarity between the equatorial circulation and the circulation in the coastal upwell- ing regions. The widths of the equatorial upwelling as well as of the CROMWELL Current the maximum speeds of the upwelling and of the CROMWELL Current, the depth of the core and the thickness of the CROMWELL Current, are all found to be explained quantita- tively. The rate of change in the CORIOLIS parameter with latitude and the mean vertical stability of the waters are essential to determine those length scales of the phenomena. The easterly wind stress over the equatorial region is responsible for the processes. 1. Introduction In 1952, Townsend CROMWELLet al. (CRom- WELL et al., 1954) discovered an excitingly strong current flowing eastward just below the west-going surface equatorial current. This was the current which had not been known to us before and has been established by later investigation. Particularly, recent direct measurements of the current by John KNAUSS (1958) have revealed striking features of the current. This subsurface current which had hitherto been entirely unknown and has now become convinced to exist is named the CROMWELLCurrent or the Equatorial Undercurrent. Fig. 1. Sigma-t. Northbound section along We are so curious about this current not 140•‹W longitude, 7•‹S to 8•‹N, Stations 17- only because it is new to us, having entirely 31, June 3-8, 1952 (Hugh M.
    [Show full text]
  • Salinity Intrusion and Convective Mixing in the Atlantic Equatorial Undercurrent
    ADVANCES IN SPANISH PHYSICAL Scientia Marina 76S1 OCEANOGRAPHY September 2012, 117-129, Barcelona (Spain) M. Espino, J. Font, J.L. Pelegrí, ISSN: 0214-8358 A. Sánchez-Arcilla (eds.) doi: 10.3989/scimar.03611.19B Salinity intrusion and convective mixing in the Atlantic Equatorial Undercurrent MARIONA CLARET, ROCÍO RODRÍGUEZ and JOSEP L. PELEGRÍ Departament d’Oceanografia Física, Institut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain. E-mail: [email protected] SUMMARY: This study investigates the advection of positive-salinity anomalies by the Equatorial Undercurrent (EUC) and their potential importance in inducing vertical convective mixing. For this purpose we use hydrographic and velocity observations taken in April 2010 along the western Atlantic equatorial ocean (32 to 43°W). The high-salinity EUC core is a few tens of metres thick and occupies the base of the surface mixed layer and the upper portion of the surface thermocline. It leads to high positive values of the vertical salinity gradient, which in many instances cause statically unstable conditions in otherwise well-stratified regions. The unstable regions result in vertical convection, hence favouring the occurrence of step-like features. We propose that this combination of horizontal advection and vertical-instability leads to a sequence of downward-convective events. As a result the EUC salinity is diffused down to a potential density of 26.43, or about 200 m deep. This mechanism is responsible for water-mass and salt downwelling in the equatorial Atlantic Ocean, with a potentially large influence on the tropical and subtropical cells. Keywords: Equatorial Undercurrent, high-salinity core, horizontal advection, convective mixing, step-like features.
    [Show full text]
  • The Cromwell Current on the East Side of the Galapagos Islands
    VOL. 78, NO. 33 3OURNAL OF GEOPHYSICAL RESEARCH NOVEMBER 20, 1973 TheCromwell Current on the East Side of the Galapagos Islands H•soN• P•: •NI• J. R. V. ZANEVELD School o] Oceanography,Oregon State University, Corvallis, Oregon 97331 Observationsmade during October and December1971 on the Yaloc 71 Cruise of Oregon State University indicate the presence of the Cromwell Current on the east side of the Galapagos Islands. Light scattering, particle size dist,ribution,nutrients, and standard hydrographic parameters were measured in water samples collected at !54 stations. The distribution of water properties shows that. the extension of the current to the east side of the islands derives primarily, from water flowing around the north side of the islands. This flow consistsof two branchesthat t6nd to merge into one at about 84øW. There was no clear indication of a branch around the south side of the islands within the area of the observations. What happens to the Cromwell Current as Islands [Knauss, 1966; Stevenson and Ta]t, it passes the Galapagos Islands has been a 1971]. White [1969] describedthe circulation topic of great interest ever since the discovery on •he basis of a network of stations extending of the Cromwell Current itself. Montgomery to the east of 91ø00'W; however, there were [1962], in a review of the Cromwell Current, few stations close to the islands. concludedthat the Cromwell Current is usually In the present paper, the Cromwell Current weak or absent on the east side of the Galapa- circulationpattern will be describedir• the vicin- gos Islands. Wyrtki [1966] said that the Crom- ity of the GalapagosIslands on the basisof the well Current disintegrateswest of the Galapa- observationsmade on RV Yaquina during the gos Islands and it splits into north and south Yaloc 71 cruise.
    [Show full text]
  • South Pacific Surface Circulation During the Austral Summer
    South Pacific Surface Circulation during the Austral Summer by Talbot Murray This paper summarises the characteristics of surface circulation of the South Pacific during austral summer months when a surface albacore fishery is most likely to develop. The emphasis is on the southwestern region which has been more thoroughly studied and demonstrated the potential for supporting surface albacore fisheries. The information is summarised from several sources and unless cited can be found in Pickard (1963), Tchernia (1980) and Heath (1985a). General Features of Circulation Surface circulation in the South Pacific is a wind driven gyre associated atmospherically with a permanent anticyclone centred near Easter Island. This anticyclone extends over most of the eastern Pacific and is continuous from about 10°S to 40°S. The intensity of this anticyclone weakens during the austral summer while in the Australia-New Guinea region a cyclonic system develops. The weakening anticyclone and its associated southeast Trade Winds result in a reduced flow in the westward flowing south Eguatorial Current during the summer. The cyclonic system over Australia and New Guinea largely affect circulation in the Coral and Tasman Seas. Circulation is also influenced by a complex series of bathymetric features including ridges, rises, seamount chains as well as numerous island platforms. The T.ajor features of the South Pacific gyre are the westward flowing South Equatorial Current, an eastward zonal flow norm or cne 2 West Wind Drift along the Subtropical Convergence, the southward flowing East Australian and East Auckland Currents and the northward flowing Peru Current. The isolation of the East Australian Current, a western boundary current, from the South Pacific gyre by the New Zealand shelf complicates the circulation of the Tasman Sea and New Zealand region.
    [Show full text]