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Indian - Atlantic Interocean Exchange: Variability and Controls

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Indian - Atlantic Interocean Exchange: Variability and Controls INDIAN - ATLANTIC INTEROCEAN EXCHANGE: VARIABILITY AND CONTROLS. UITWISSELING TUSSEN DE INDISCHE EN ATLANTISCHE OCEAAN: VARIABILITEIT EN FORCERING. (met een samenvatting in het Nederlands) PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de Rector Magnificus, prof. dr. W.H. Gispen, ingevolge het besluit van het College voor Promoties in het openbaar te verdedigen op woensdag 15 mei 2002, om kwart voor e´en´ ’s middags. DOOR MATHIJS WILHELMUS SCHOUTEN geboren op 3 juli 1975 te ’s Gravenhage ii Promotor: Prof. dr. W.P.M. de Ruijter Co-promotor: Dr. P.J. van Leeuwen Faculteit Natuur- en Sterrenkunde, Universiteit Utrecht Dit proefschrift werd mede mogelijk gemaakt door financiele¨ steun van de Nederlandse or- ganisatie voor Wetenschappelijk Onderzoek (NWO), in het kader van het tweede nationaal Onderzoeks Programma (NOP II, contract 013001237-10. ISBN 90-393-3028-X Contents 1 Introduction 1 2 Tools and techniques 11 2.1 Satellite altimetry and oceanic variability . 11 2.2 Statistical tools . 14 2.3 An example . 17 3 Translation, decay and splitting of Agulhas rings in the Atlantic Ocean 21 3.1 Introduction . 21 3.2 Data and Methods . 23 3.3 Agulhas Rings . 25 3.3.1 Ring Shedding . 25 3.3.2 Ring Paths . 25 3.3.3 Ring Decay . 28 3.4 Ring Splitting . 30 3.4.1 Vema Seamount . 30 3.4.2 Other Splitting Events . 33 3.5 Dissipating Rings . 34 3.6 Discussion and Conclusions . 36 4 Upstream control of Agulhas Ring Shedding 39 4.1 Introduction . 39 4.2 Ring Shedding and the Retroflection . 42 4.3 The Retroflection: Connection to Upstream Regions . 46 4.3.1 SSH Observations . 46 4.3.2 Natal Pulse Generation . 49 4.4 Connection to the Agulhas sources . 50 4.4.1 The Mozambique Channel . 50 4.4.2 The East Madagascar current . 50 4.4.3 MSSA Results . 52 4.5 Connection to the central and eastern Indian Ocean . 53 4.6 Summary and Discussion . 56 iv CONTENTS 5 A teleconnection between the equatorial and southern Indian Ocean 59 5.1 Introduction . 59 5.2 The possible equatorial origin of a four per year cycle . 61 5.3 Propagation of the signal to the subtropical region . 67 5.3.1 The coastal Kelvin wave . 67 5.3.2 Rossby waves across the subtropical gyre . 70 5.4 Impact on the upstream Agulhas and interbasin exchange . 74 5.4.1 Connection to the Mozambique Channel . 74 5.4.2 Connection to Agulhas ring shedding . 78 5.5 Summary and discussion . 78 5.5.1 The semi-annual cycle at the equator . 80 5.5.2 Interbasin exchange . 82 6 Modification of Rossby waves across the subtropical gyre 85 6.1 Introduction . 85 6.2 Rossby waves in the subtropical gyre . 86 6.3 Why are there two separate bands of variability? . 88 6.4 The influence of topography . 94 6.5 Summary and discussion . 97 7 Variability in the Mozambique Channel 101 7.1 Eddies in the Mozambique Channel . 101 7.2 Southward reduction of the dominant timescale . 106 7.3 Dominant variability around Madagascar . 108 7.4 Discussion . 112 Bibliography 113 Samenvatting 123 Dankwoord 126 Curriculum Vitae 127 Publications 128 Chapter 1 Introduction The incoming part of the radiation budget of the earth is characterized by strong solar input of heat at low latitudes. The outgoing radiation is more evenly distributed over the earth, implying that there must be considerable transport from low to higher latitudes. This transport is accomplished both by the atmosphere and the oceans, in about equal parts (Peixoto and Oort, 1992). The ocean-component is achieved mainly by the wind-driven ocean circulation, but in some basins the thermohaline circulation has a considerable impact too, for instance in the climatic conditions of Northern Europe. Large scale wind driven circulation The global ocean circulation is driven by winds and fluxes of heat and fresh water. To first order, the location and shape of the large scale surface currents are determined by the shape of the wind field driving it: the curl of the wind stress is the primary source of vorticity for the gyre circulation, by the so-called Sverdrup balance (Sverdrup, 1947). The average winds over the years 1990-2001 are plotted in Fig. 1.1 for the Atlantic and Indian Ocean basins. The meridional extent of the gyres is confined to the regions between two (more or less zonal) lines of zero wind stress curl, where the vorticity input changes sign. Between those lines (the grey lines in Fig. 1.1), large and more or less closed current systems are formed: the wind-driven gyres. The gyres are far from symmetric: the rotation of the earth results in a strong intensification along the western boundaries. The equatorward transport takes place over almost the entire gyre, whereas the poleward flow is established in very narrow bands along the western boundaries. The mean surface velocities of the Atlantic and Indian Ocean basins, determined from satellite tracked surface drifters (Hansen and Poulain, 1996), are plotted in Fig. 1. The western boundary currents that form the western limbs of the gyres are the strongest permanent currents of the World Oceans. Among them are the Gulf Stream and Kuroshio currents on the northern hemisphere, and the Agulhas and Brazil currents in the southern hemisphere. These western boundary currents all carry large amounts of heat from low latitudes into the midlatitudes, thus having strong impact on local and global climatolog- ical conditions. Also, they show strong variability on the shorter timescales: meandering and eddy formation are characteristic to all of them. 2 Introduction 60ÊN 30ÊN 10 m/s 0Ê 30ÊS 60ÊS 90ÊW 60ÊW 30ÊW 0Ê 30ÊE 60ÊE 90ÊE 120ÊE Figure 1.1: Mean winds in the American National Centers for Environmental Prediction and Atmo- spheric Research (NCEP/NCAR) reanalysis for 1990-2000 (Kalnay et al., 1996). The lines of zero wind stress curl have been added in grey. 60ÊN 30ÊN 25 cm/s 0Ê 30ÊS 60ÊS 90ÊW 60ÊW 30ÊW 0Ê 30ÊE 60ÊE 90ÊE 120ÊE Figure 1.2: Mean surface velocities as measured by satellite tracked surface drifters (Hansen and Poulain, 1996) . The region around South Africa is a region of unusual current characteristics. The line of zero wind stress curl is located roughly at 45◦S (Fig. 1.1), the African continental shelf ends at 37◦S. Therefore, a gap of about 1000 km exists between the southern tip of the continent 3 and the ’natural’ southern extremum of the subtropical gyres of the South Atlantic and South Indian Oceans. Application of Sverdrup theory alone would result in the formation of an Atlantic-Indian Ocean ’Supergyre’ with the western boundary current of the South Indian Ocean turning westward at the end of the African continent, only to return its Indian Ocean waters via a loop across the south Atlantic (De Ruijter, 1982). This is not what is observed. Most of the Agulhas current retroflects back into the Indian Oceanalmost immediately after leaving the African continent. About 10% of its waters are leaking into the South Atlantic Ocean, but this still provides a crucial connection in the global overturning circulation of the ocean (Gordon, 1985). The retroflection of the Agulhas current has been noticed and described long ago (Ren- nell, 1832) (see Lutjeharms et al. (1992) for a historical overview), but we are only recently coming towards a dynamical understanding. The ’gap’ between the continent and the region of zero wind stress curl is bridged by a delicate interplay of inertial overshoot, advection of planetary vorticity (β-effect) (De Ruijter and Boudra, 1985; Boudra and de Ruijter, 1986), bottom topography (Matano, 1996) and continental geometry (Ou and de Ruijter, 1986). Re- cent numerical studies (Dijkstra and de Ruijter, 2001b) indicate that the seemingly realistic retroflections obtained by several large scale general circulation models (GCM’s) are due to their unrealistically high eddy viscosities. Viscous boundary layers in these models may artificially enlarge the African continent beyond its true size. Thermohaline circulation The wind is not the only driving force of the ocean currents. Part of the motion is forced by fluxes of heat and fresh water. These buoyancy fluxes can be provided through the ocean surface by the nett effect of incoming and outgoing radiation for heat, and by the nett effect of precipitation and evaporation for fresh water. They can also be established by direct input of water from other ocean basins, with different temperature and salinity characteristics. The Atlantic ocean circulation is thus forced by differential heating, a nett surface loss of freshwa- ter, by inflow of relatively warm and saline water from the Mediterranean, and by interocean exchanges at its northern and southern extrema. The Bering Strait throughflow provides the Northern Atlantic with cold but relatively fresh water; water entering the Atlantic via Drake Passage south of South America is also relatively cold and fresh, whereas the inflow of Indian Ocean water around South Africa provides the South Atlantic with warm and saline waters. At deeper levels, the inflow into the Atlantic ocean is balanced by an outflow of North Atlantic Deep Water (NADW). Another important link between two oceans is the Indonesian throughflow, where warm but relatively fresh Pacific water enters the Indian ocean. Together these surface and inter-ocean fluxes constitute the overturning circulation of the ocean. A salient feature of this circulation is the nett equatorward transport of heat in the South Atlantic Ocean. In the northern part of the Atlantic NADW is formed by convection when the saline water is cooled at high latitudes. It flows southward along the western bound- ary of the Atlantic, into the Antarctic Circumpolar current (ACC) region where it is spread throughout the oceans.
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