DOCSLIB.ORG

A Model of the Seasonal Circulation in the Arabian Sea Forced By

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
A Model of the Seasonal Circulation in the Arabian Sea Forced By Pmg. Oce4nol'. VoL 14, pp. 353-385, 1985. 0079-66il/8S $0.00 + .sO Printed in Great Britain. AU ~ts reerved. Copyright e 1985 hzalmon-Preu Ltd. A Model of the SeasonalCirculation in the Arabian Sea Forced by Observed Winds MARK E. LUTHER and JAMES J. O'BRIEN Me.,~ale Air-Sea Interaction Group, The Florida State Univemry, TalJahal.fee,Florida 32306, au. Abstract - Re.hs of a numericalmodel of the wind driven seasonalcirculation in the Arabian Seaare presented,with partjcular emphasison the ocean'sresponse to the mon~on winds. The model equationsare the fully nonlinearreduced gravity transportequations in spherical coordinates.The model re~lution is 1/8° in the east-westdirection and 1/4° in the north- south direction. The model basin geometrycorresponds as clo~ly as possibleto that of the north-west Indian Oceanfrom 400E to 7JoE and from 100Sto 25°N, and includesthe gulfs of Aden and Oman,and the islandsof Socotraand the Seychelles.The southernboundary and a portion of the easternboundary, from the equatorto 60S,representing the openingbetween the Maldivesand the ChagosArchipelago, are open boundaries.At other boundaries,the no- slip condition is applied.The wind stressdata used to force the modelcomes from the NOAA NationalClimate Center'sTD-9757 Global Marine Sumsdata, which consistsof monthly mean winds compiled on 1° squaresfrom over 60 yearsof ship ob~rvations. Thesedata are inter- polatedin time using the meanand Iust lIVe Fourier harmonicsat eachpoint, and then inter- polatedlinearly to the model grid. The model equationsare integratedin time using centered finite differencesin time and space(a leap-frogscheme), with lateral friction treated by a Dufort-Frankel scheme. After a one year spin up, the model settlesinto a regular periodic seasonalcycle, even though the ~lution to the model equationsis locally highly nonlinear,with largenonlinear eddiesdeveklping in the samelocation at the sametime of year from one year to the next. The developmentof the model SomaliCurrent systemwith the onset of the (northern hemi- sphere)summer mon~on is consistentwith the avajIableob~rvations in the region.The model reproducesmany of the observedfeatures in this region, such as the two-gyre circulation pattern, and the timing and movementof thesefeatures corresponds well with their real world counterparts.The model also showsan eastwardjet forming in late Juneto early July at 1JoN, just to the east of Socotra. This jet is fed by flow comingout of the great whirl. The break down of the two..gyrepattern occursin mid to late August.when the southerngyre breaksup into severalsmaner eddies, the northem-mostof which coalesceswith the greatwhirl. Numer- ous small cycklnic eddiesdevelop aklng the Arabian coast, from the Gulf of Omaninto the Gulf of Aden, in early to mid August,and persistwell into the winter monsoon.The model shows that it is possibleto simulatevery complicatedflows, if one has sufficient wind data, usingfairly simplemodels with a realisticbasin geometry. 1. INTRODUCTION THE NORTHWESTIndian Ocean,the Arabian Sea,is unique amongthe world's oceansin that the winds over the ocean basin reversesemiannually, blowing from the southwest during the northern SUJJUDer,and from the northeast during the northern winter (the southwest and northeast monsoons).The periodic reversalsin the winds drive correspondingreversals in the currents of the upper ocean.These seasonalreversals of the-wmd and surfacecirCulation over such a vast area are remarkablewhen comparedto the seasonalsignals in the Atlantic or Pacific Oceans.The Somali Current has surfacevelocities and masstransports that exceedthose of the Gulf Stream but that change direction every six months. There are numerous eddies in the Arabian and Somali basins that recur on a seasonalbasis. The Indian Oceanis thus an ideal place in which to study the time-dependentnature of the responseof the ocean to changing winds. 353 354 M. E. LUTHER and,], J. O'BRIEN The currents in the Arabian Seabasins evolve rapidly into a very complexpattern of eddies with the onset of the southwestmQnsoon. Associated with theseeddies are very strong hori- zontal temperaturegradients and current shears.BRUCE (1973, 1979, 1983) and BROWN, BRUCE and EVANS (1980) have descn'bed.theeddies associatedwith the Somali Current systemin some detail. Recent attention has focused on the two gyre nature of the Somali Current onset. SWALLOWand FIEUX (1982) have reviewedthe availablehistorical daU and found that the two gyre circulation pattern, with one clockwisegyre situatedsouth of SoNand another between SoN and lOoN, is usually present in June. It is the northern gyre that was dubbed the 'great whirl' by FINDLAY (1866). More recent observations,especially those during the 1979 Indian OceanExperiment, have revealedmore details of the development of the two gyre system. SWALLOW, MOLINARI, BRUCE, BROWN and EVANS (1983) havedescribed the reversalof' the Somali Current with onsetof the southwestmonsoon during 1979 uSingtemperature, salinity and current measurements.These observations are ~arized inSCHOTT(1983). , Very little attention has been paid to the northeastmonSOon, from Decemberto January, primarily becauseit is so much weakerthan the southwestmonsoon and thereforeless dramatic.. BRUCE (1983) describesthe long term monthly meanwind stresspatterns obtained from ship observations,for both monsoons,and relatesthese to the observedocean circulation patterns. Thesesame wind data for the Indian Oceanare describedin an atlasby HASTENRATH and LAMB (1979). The numerous eddies in the northern Arabian Sea have been observedby CAGLE and WHRITNER (1981) using satellite infrared imagery(Fig. 1). They find that the eddiesappear to be controlled both by the wind forcing and by topographic features associatedwith the coastlinegeometry. The eddies persistfor long periods of time.and appearto strengthenand weakenwithout significant advection. We have developed a wind-driven numerical model to investjgate the seasonalcycle of currents in the Arabian Sea. The nonlinear model includes a realistic baSingeometry and is forced by observedmean monthly winds on a high resolution mesh in sphericalcoordinates. There are several earlier models of the Indian Ocean.COX (1970) developeda multi-layer model that was driven by idealizedseasonal winds and temperature.The model was integrated for two centuriesto arrive at a steadyannual signal. The slow responsetime of the very deepest layers in the model necessitatedthe very long integration. HURLBURT and THOMPSON (1976) were the first to demonstratethe ~y nonlinearnature of the eddiesthat form in the Somali Current during the onset of the southwestmonsoon. COX (1976) found that local forcing by the wind dominatesduring the onset of the monsoon,but that remote forcing by equatorially trapped wavesmay alter the flow at later times. COX (1979) found that the move- ment of the eddies was strongly influenced by the basin geometry. UN and HURLBURT (1981) showed that the spin-up of the Somali Current system could be effectively modeled using only the fIrst baroclinic mode. They also demonstratedthat the temporal and spatial distribution of the wind stressis crucial in determining the location and movement of the eddiesin the Somali Current. ANDERSONand MOORE(1979) consideredthe Somali Current as a free inertial cross-equatorialjet, forced by the southernhemisphere trades and fed by the South Equatorial Current (SEC).and the EastAfrican CoastalCurrent (EACC). All previousmodels have consideredonly the responseof the Indian Oceanto very idealized wind forcing, consideringprimarily the responseof the westernboundary region to the sudden onset of the southwest monsoon. No models to date have looked at the eddiesfound in the northern Arabian Sea.The model we presenthere examinesthe responseof the Arabian Seato A modelof' the ~naI ckcuJationin the ArabianSea 155 FIG. 1. Interpretation of infrared Imageof the coastal resion south of the Gulf of Oman. Strong fronts are indicated by ~Iid lines, weak fronts by dashedlines. A strong&w is indi- cated, ~clated with a major feature which propagatesfrom ~e upwellingoff the south tip of the Island of Madrah. A cokt cycbnk: eddy is iodated to the north of this tounge-Uke feature.A sequenceof warm and cokl eddiesIa indicatedin progrellbn down the coastof the Arabian Peninsula,and major features~clated with points of land are repeatedalong the coast. Image was taken on 26 October 1980 by the NOAA-6 satellite. From CAGLE and WHRITNER (1981). act.ualobserved winds over the entire seasonalcycle. Other models of the onset of the Somali Current have consideredonly the spin-up of an oceanwhich is initially at rest in responseto an mpulsively applied, constant wind stress.As we will show, the put ~agJn's circulation influences die onset of both the (northern hemisphere)RImmer and winter Somali Current, so that to accuratelymodel either, we must model the entire seasonalcycle. We pre~nt here the results of one particular model simulation. and compare theseremIts with recent ob~rvations. We focus our attention on the westernboundary region,particularly the Somali Current regime, becausethat is where the most prominent variations occur and where the great majority of ob~rvations are made. This is not to say that currents in other areasof the Arabian Sea are not important, but the ob~rvations there are very lhnited. We prefer to concentrate here on a de-=ription of the model and to emphasizethe agreement between the model reRllts and observations.We will leave a detailed analy.sisof the
Load more

Recommended publications
  • 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]].
    [Show full text]
  • Somali Fisheries
    www.securefisheries.org SECURING SOMALI FISHERIES Sarah M. Glaser Paige M. Roberts Robert H. Mazurek Kaija J. Hurlburt Liza Kane-Hartnett Securing Somali Fisheries | i SECURING SOMALI FISHERIES Sarah M. Glaser Paige M. Roberts Robert H. Mazurek Kaija J. Hurlburt Liza Kane-Hartnett Contributors: Ashley Wilson, Timothy Davies, and Robert Arthur (MRAG, London) Graphics: Timothy Schommer and Andrea Jovanovic Please send comments and questions to: Sarah M. Glaser, PhD Research Associate, Secure Fisheries One Earth Future Foundation +1 720 214 4425 [email protected] Please cite this document as: Glaser SM, Roberts PM, Mazurek RH, Hurlburt KJ, and Kane-Hartnett L (2015) Securing Somali Fisheries. Denver, CO: One Earth Future Foundation. DOI: 10.18289/OEF.2015.001 Secure Fisheries is a program of the One Earth Future Foundation Cover Photo: Shakila Sadik Hashim at Alla Aamin fishing company in Berbera, Jean-Pierre Larroque. ii | Securing Somali Fisheries TABLE OF CONTENTS LIST OF FIGURES, TABLES, BOXES ............................................................................................. iii FOUNDER’S LETTER .................................................................................................................... v ACKNOWLEDGEMENTS ............................................................................................................. vi DEDICATION ............................................................................................................................ vii EXECUTIVE SUMMARY (Somali) ............................................................................................
    [Show full text]
  • Structure and Transport of the East African Coastal Current
    See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/248793481 Structure and transport of the East African Coastal Current Article in Journal of Geophysical Research Atmospheres · January 1991 DOI: 10.1029/91JC01942 CITATIONS READS 48 209 3 authors, including: Michele Fieux Pierre and Marie Curie University - Paris 6 56 PUBLICATIONS 1,894 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: It is in a book: " L'océan à découvert " , 321 p. , sept 2017, CNRS Editions View project All content following this page was uploaded by Michele Fieux on 09 June 2018. The user has requested enhancement of the downloaded file. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. C12, PAGES 22,245-22,257, DECEMBER 15, 1991 Structure and Transport of the East African Coastal Current JOHN C. SWALLOW Drakewalls, Gunnislake, Cornwall, England FRIEDRICH SCHOTT lnstitut fiir Meereskunde an der Universitiit Kiel, Kiel, Germany MICH•,LE FIEUX Laboratoire d'Oc•anographie Dynamique et de Climatologie, Universit• Paris VI, Paris The East African Coastal Current (EACC) runs northward throughout the year between latitudes 11øSand 3øS, with surfacespeeds exceeding 1 m s-1 in northernsummer. Mean transport from five sectionsnear 4ø-5øS is 19.9Sv (1 Sv -- 106 m3 s-1) northwardin theupper 500 dbar, out to 120km offshore. Below that, between 500 and 1000 dbar, there appears to be a weak variable transport of the order of 1 Sv. Comparing transports in the EACC with those in the boundary current north of Madagascar, it seems that most of the water in the upper 300 dbar of the northern branch of the South Equatorial Current goes into the EACC.
    [Show full text]
  • 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.
    [Show full text]
  • Atlantic Ocean Equatorial Currents
    188 ATLANTIC OCEAN EQUATORIAL CURRENTS ATLANTIC OCEAN EQUATORIAL CURRENTS S. G. Philander, Princeton University, Princeton, Centered on the equator, and below the westward NJ, USA surface Sow, is an intense eastward jet known as the Equatorial Undercurrent which amounts to a Copyright ^ 2001 Academic Press narrow ribbon that precisely marks the location of doi:10.1006/rwos.2001.0361 the equator. The undercurrent attains speeds on the order of 1 m s\1 has a half-width of approximately Introduction 100 km; its core, in the thermocline, is at a depth of approximately 100 m in the west, and shoals to- The circulations of the tropical Atlantic and PaciRc wards the east. The current exists because the west- Oceans have much in common because similar trade ward trade winds, in addition to driving divergent winds, with similar seasonal Suctuations, prevail westward surface Sow (upwelling is most intense at over both oceans. The salient features of these circu- the equator), also maintain an eastward pressure lations are alternating bands of eastward- and west- force by piling up the warm surface waters in the ward-Sowing currents in the surface layers (see western side of the ocean basin. That pressure force Figure 1). Fluctuations of the currents in the two is associated with equatorward Sow in the thermo- oceans have similarities not only on seasonal but cline because of the Coriolis force. At the equator, even on interannual timescales; the Atlantic has where the Coriolis force vanishes, the pressure force a phenomenon that is the counterpart of El Ninoin is the source of momentum for the eastward Equa- the PaciRc.
    [Show full text]
  • A Numerical Investigation of the Somali Current During the Southwest Monsoon
    Journal of Marine Research, 46. 25-58, 1988 A numerical investigation of the Somali Current during the Southwest Monsoon by Julian P. McCreary, Jr.' and Pijush K. Kundu1 ABSTRACT The dynamics of the Somali Current system during the Southwest Monsoon are investigated using a 21/2-layer numerical model that includes entrainment of cool water into the upper layer. Entrainment cools the upper layer, provides interfacial drag, and prevents the interface from surfacing in regions of strong coastal upwelling. Solutions are forced by a variety of wind stress fields in ocean basins with western boundaries oriented either meridionally or at a 45° angle. Solutions forced by southern hemisphere easterlies develop a strong coastal current south of the equator. When the western boundary is slanted, this current bends offshore at the equator and meanders back into the ocean interior. No cold wedge forms on the Somali Coast. These solutions suggest that the southern hemisphere trades are not an important forcing mechanism of the Somali Current circulation. Solutions forced by northward alongshore winds differ considerably depending on the orientation of the western boundary and the horizontal structure of the wind. When the boundary is meridional and the wind is uniform (a curl-free wind field), solutions continuously shed eddies which propagate northward along the coast and weaken. When the boundary is meridional and the wind weakens offshore, they reach a completely steady, eddy-free state with no coastal upwelling. If the boundary is slanted and the wind does not vary alongshore, solutions reach a steady state that now contains stationary gyres and cold wedges.
    [Show full text]
  • Chapter 11 the Indian Ocean We Now Turn to the Indian Ocean, Which Is
    Chapter 11 The Indian Ocean We now turn to the Indian Ocean, which is in several respects very different from the Pacific Ocean. The most striking difference is the seasonal reversal of the monsoon winds and its effects on the ocean currents in the northern hemisphere. The abs ence of a temperate and polar region north of the equator is another peculiarity with far-reaching consequences for the circulation and hydrology. None of the leading oceanographic research nations shares its coastlines with the Indian Ocean. Few research vessels entered it, fewer still spent much time in it. The Indian Ocean is the only ocean where due to lack of data the truly magnificent textbook of Sverdrup et al. (1942) missed a major water mass - the Australasian Mediterranean Water - completely. The situation did not change until only thirty years ago, when over 40 research vessels from 25 nations participated in the International Indian Ocean Expedition (IIOE) during 1962 - 1965. Its data were compiled and interpreted in an atlas (Wyrtki, 1971, reprinted 1988) which remains the major reference for Indian Ocean research. Nevertheless, important ideas did not exist or were not clearly expressed when the atlas was prepared, and the hydrography of the Indian Ocean still requires much study before a clear picture will emerge. Long-term current meter moorings were not deployed until two decades ago, notably during the INDEX campaign of 1976 - 1979; until then, the study of Indian Ocean dynamics was restricted to the analysis of ship drift data and did not reach below the surface layer. Bottom topography The Indian Ocean is the smallest of all oceans (including the Southern Ocean).
    [Show full text]
  • The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing Verena Hormann, SIO/UCSD [email protected] Lisa M
    The response of the surface circulation of the Arabian Sea to monsoonal forcing Verena Hormann, SIO/UCSD [email protected] Lisa M. Beal, Rick Lumpkin, Gregory R. Foltz, Luca Centurioni DBCP-29 Workshop, 23 Sep. 2013, Paris, France Monsoon circulation of the Indian Ocean § Circulation of the Arabian Sea switches direction annually under the influence of strong monsoon winds § Upwelling wedges associated with eddy structures in the western Arabian Sea connected to changes in the wind field [Vecchi et al., 2004] Schott et al., 2009 Indian monsoon rainfall § Horizontal advection, coastal upwelling, wind-driven mixing, and Ekman pumping all affect sea surface temperature (SST) in the Arabian Sea [Lee et al., 2000] § Beside the winds, SST feeds back on rainfall over India Izumo et al., 2008 Data § Large amounts of data required to resolve the evolution of the flow in a region with no steady circulation § Monthly-mean scatterometer winds (QuickSCAT) § Monthly-mean circulation from a global drifter climatology of absolute near-surface currents [Lumpkin and Jonson, 2013] § Weekly maps of geostrophic flow from a drifter-altimeter synthesis [Lumpkin and Garzoli,2011; Hormann et al., 2012] § Satellite altimeter data from AVISO [Le Traon et al., 1998] Drifter array in the western Arabian Sea (left)and correlation with currents from altimetry (right) Monsoons as static circulations § Seasonally-averaged circulations can show currents and connections that do not actually exist NMC SC § Northeast Monsoon Current appears to SECC feed into southward Somali Current § Northward Somali Current appears to feed into Southwest Monsoon Current § South Equatorial Counter Current appears to be lacking during the southwest monsoon GW Magenta arrows are schematics of identified SC SMC currents after Schott and McCreary [2001].
    [Show full text]
  • Western Boundary Upwelling Dynamics Off Oman
    Ocean Dynamics DOI 10.1007/s10236-017-1044-5 Western boundary upwelling dynamics off Oman Clement´ Vic1,2 · Xavier Capet3 · Guillaume Roullet1 · Xavier Carton1 Received: 19 May 2016 / Accepted: 2 March 2017 © The Author(s) 2017. This article is published with open access at Springerlink.com Abstract Despite its climatic and ecosystemic significance, Keywords Oman · Upwelling · Arabian sea · ROMS · the coastal upwelling that takes place off Oman is not well Rossby waves · Eddy kinetic energy understood. A primitive-equation, regional model forced by climatological wind stress is used to investigate its dynamics and to compare it with the better-known Eastern Bound- 1 Introduction ary Upwellings (EBUs). The solution compares favorably with existing observations, simulating well the seasonal Coastal upwelling can occur wherever the wind has an cycles of thermal structure, surface circulation (mean and alongshore component that drives offshore Ekman drift. turbulent), and sea-surface temperature (SST). There is a Regions of sustained upwelling exist on both eastern- 1.5-month lag between the maximum of the upwelling- and western-ocean boundaries. The four major Eastern- favorable wind-stress-curl forcing and the oceanic response Boundary Upwellings (EBUs) occur along the west coasts (minima in sea-surface height and SST), which we attribute of North and South America and North and South Africa. to onshore-propagating Rossby waves. A southwestward- Western-boundary upwelling regions include southeastern flowing undercurrent (opposite to the direction of the near- Australia (Marchesiello et al. 2000), Brasil (Azevedo et al. surface flow) is also simulated with a core depth of 1000 2012), and eastern Africa/northern Arabian seas (McCreary m, much deeper than found in EBUs (150–200 m).
    [Show full text]
  • Western Boundary Currents
    ISBN: 978-0-12-391851-2 "Ocean Circulation and Climate, 2nd Ed. A 21st century perspective" (Eds.): Siedler, G., Griffies, S., Gould, J. and Church, J. Academic Press, 2013 Chapter 13 Western Boundary Currents Shiro Imawaki * Amy S. Bower† Lisa Beal‡ Bo Qiu§ *Japan Agency for Marine–Earth Science and Technology, Yokohama, Japan †Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA ‡Rosenstiel School of Marine and Atmospheric Science, University of Mi ami, Miami, Florida, USA §School of Ocean and Earth Science and Technology, University of Hawaii , Honolulu, Hawaii, USA 1 Abstract Strong, persistent currents along the western boundaries of the world’s major ocean basins are called "western boundary currents" (WBCs). This chapter describes the structure and dynamics of WBCs, their roles in basin-scale circulation, regional variability, and their influence on atmosphere and climate. WBCs are largely a manifestation of wind-driven circulation; they compensate the meridional Sverdrup transport induced by the winds over the ocean interior. Some WBCs also play a role in the global thermohaline circulation, through inter-gyre and inter-basin water exchanges. After separation from the boundary, most WBCs have zonal extensions, which exhibit high eddy kinetic energy due to flow instabilities, and large surface fluxes of heat and carbon dioxide. The WBCs described here in detail are the Gulf Stream, Brazil and Malvinas Currents in the Atlantic, the Somali and Agulhas Currents in the Indian, and the Kuroshio and East Australian Current in the Pacific Ocean. Keywords Western boundary current, Gulf Stream, Brazil Current, Agulhas Current, Kuroshio, East Australian Current, Subtropical gyre, Wind-driven circulation, Thermohaline circulation, Recirculation, Transport 2 1.
    [Show full text]
  • Spreading of the Indonesian Throughflow in the Indian Ocean*
    772 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 34 Spreading of the Indonesian Through¯ow in the Indian Ocean* QIAN SONG,ARNOLD L. GORDON, AND MARTIN VISBECK Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York (Manuscript received 27 March 2003, in ®nal form 19 August 2003) ABSTRACT The Indonesian Through¯ow (ITF) spreading pathways and time scales in the Indian Ocean are investigated using both observational data and two numerical tracer experiments, one being a three-dimensional Lagrangian trajectory experiment and the other a transit-time probability density function (PDF) tracer experiment, in an ocean general circulation model. The model climatology is in agreement with observations and other model results except that speeds of boundary currents are lower. Upon reaching the western boundary within the South Equatorial Current (SEC), the trajectories of the ITF tracers within the thermocline exhibit bifurcation. The Lagrangian trajectory experiment shows that at the western boundary about 38%65% thermocline ITF water ¯ows southward to join the Agulhas Current, consequently exiting the Indian Ocean, and the rest, about 62%65%, ¯ows northward to the north of SEC. In boreal summer, ITF water penetrates into the Northern Hemisphere within the Somali Current. The primary spreading pathway of the thermocline ITF water north of SEC is upwelling to the surface layer with subsequent advection southward within the surface Ekman layer toward the southern Indian Ocean subtropics. There it is subducted and advected northward in the upper thermocline to rejoin the SEC. Both the observations and the trajectory experiment suggest that the upwelling occurs mainly along the coast of Somalia during boreal summer and in the open ocean within a cyclonic gyre in the Tropics south of the equator throughout the year.
    [Show full text]
  • Variability of Western Indian Ocean Currents 83
    Western Indian Ocean J. Mar.VARIABILITY Sci. Vol. 1,OF No. WESTERN 1, pp. 81–90,INDIAN 2002 OCEAN CURRENTS 81 © 2002 WIOMSA VariabilityofWesternIndianOceanCurrents PeterN.Benny Department of Physical Oceanography, Cochin University of Science & Technology, Cochin, India –682016 E-mail: [email protected] Key words: western Indian Ocean, currents, circulation, Somali Current, dynamic topography, variability Abstract—In the study reported, an attempt was made to understand the intra-annual variability of the western Indian Ocean circulation by estimating the monthly dynamic topography with respect to 400db. The major currents in the western Indian Ocean are clearly depicted in the topography. Among the currents, the Somali Current exhibits strong annual variability. Eddy circulation is prominent in the northern part of the Somali Current during the southwest monsoon period. Seasonal variability is also noticed in the North Equatorial Current. Slight spatial and temporal changes are noticed in the South Equatorial Current and Equatorial Counter Current. The Equatorial Jet flow occurs in the monsoon transition periods of May and November between the equator and 3° South. INTRODUCTION Equatorial Current, which was directed towards the west during the northeast monsoon, reverses The western Indian Ocean receives special its directions towards the east. This combines with attention from oceanographers and meteorologists, the eastward-flowing Equatorial Counter Current as it exhibits more dramatic seasonal variation than and the whole eastward flow from 7 to 15 °N is the rest of the Indian Ocean and is an important called the monsoon current. One component of the region of air–sea interaction. Intense upwelling South Equatorial Current turns to north and occurs during the south-west monsoon season in supplies the Somali Current up the east coast of the Somali area (Bruce, 1974; Packard, 1985).
    [Show full text]