Th e Agulhas Current

By Johann R.E. Lutjeharms, Springer, boundary currents, the Agulhas Current 2006, 330 pages, ISBN103540423923, system. Here, the subtropical gyres of Hardcover, 129.95 € the Indian and South Atlantic Oceans can, from time to time, join, linking REVIEW BY ARNOLD L. GORDON and blending their properties across the southern rim of Africa. The South The ocean is composed of interlock- Atlantic subtropical gyre, when linked ing regions, each with their own unique to its Indian cousin, is denied access to characteristics and advocates. All are the low-salinity subpolar water stream- special and worthy of study for their ing eastward along the circumpolar belt, own complex attributes, including their thus altering the freshwater budget or impacts upon local environmental con- salinity of the South Atlantic, a factor of presenting the Agulhas to the wider ditions. Some ocean regions, in the that has far-reaching effects on the deep- community. His encyclopedic knowledge wider community of oceanographers ocean overturning circulation associated nicely draws together the many research and climatologists, are viewed as more with the northern North Atlantic (Weijer studies devoted to the Agulhas Current important than others in that they in- et al., 1999; Gordon, 2001). However, system to produce an excellent, objective, fl uence the larger scale, even the global while large-scale consideration might and well-written analysis. While the top- ocean and its function in Earth’s climate draw one to these key regions, it is im- ics are mainly those associated with the system. Each region responds to fl uctua- portant to remember that these regions physical oceanography of the Agulhas, tions in the large-scale wind and buoy- have their own intrinsic values and com- issues of climate, biological, and geologi- ancy-forcing fi elds across a wide range plexites (and impacts to the larger scale cal topics are included. Most appreciated of time scales; a few feedback to these yet to be discovered). are the gray, boxed inserts that offer as- larger-scale fi elds, with far-a-fi eld rami- While books and collections of ar- pects of the rich history of the explor- fi cations. Regional regimes may be cen- ticles in journals are devoted to specifi c ers, ships, expeditions, and institutions ters of strong sea-air fl uxes or conduits regions, they often do not garner interest contributing to the opening up of the between larger ocean-circulation struc- beyond the specialists of those regions. Agulhas system to our consciousness. In tures, such as between circulation gyres The Agulhas system is different, or at these asides, we learn about the origin of or between ocean basins, affecting the least should be (Gordon, 2003). It is the the name “Agulhas” and of the term “ret- large-scale pattern of ocean temperature best example of a retrofl ection-type cir- rofl ection” as well as about Major James and salinity, ecosystems, and more ex- culation, a pattern shared by other re- Rennell, “the pre-eminent pioneer of re- otic seawater properties. It is speculated gions, such as the Brazil Current and the search on the Agulhas Current.” Another that the behavior of a few regions might North Brazil Current. Because of the role deals with a very pertinent question of be enough to fl ip Earth’s climate system of the retrofl ection pattern in the larger paleo-climate importance: “How old into another mode, perhaps one associ- scale, every oceanographer might want is the Agulhas Current?” Evidently, an ated with the shifts between glacial/inter- to learn more of the Agulhas Current “Agulhas Current” following pretty much glacial climate. system. Johann R.E. Lutjeharms’ book, the path it does today was established Among these strategic regions is one The Agulhas Current, offers such an op- fi ve million years ago. Lutjeharms adds that includes the strongest of the south- portunity. that “even though the leakage of Indian ern hemisphere subtropical western Lutjeharms does a remarkable job Ocean water is variable, over the past 450

54 Oceanography Vol. 20, No. 1 kyr [450,000 years] it has always been to read all of the literature about nearly the Mozambique Channel and from the present,” though bursts in its strengthen all oceanographic topics. I then grew up East Madagascar Current; then Chapters seem to be linked to the termination of during the explosion of publications, 4 and 5 take us to the jetlike boundary glacial ages (Peeters et al., 2004) and to so as to be able to absorb much of the current pressed up along the southeast the resumption of a strong Atlantic me- evolving information at a gentle rate. I and southern margin of South Africa ridional overturning circulation (Knorr pity the students of today: How can they (waving a bit as a Natal Pulse goes by). and Lohmann, 2003), a result recently possibly catch up and keep pace with the Chapters 6 and 7 discuss the energetic supported by Franzese et al. (2006). hundreds of oceanographic papers pub- loop of the Agulhas Retrofl ection and Lutjeharms notes that textbooks as lished monthly? For this reason, well- the Agulhas Return Current, respectively. late as the 1970s had just a few para- prepared review articles and books are As the name suggests, the Agulhas Re- graphs devoted to the Agulhas Cur- needed, such as Lutjeharms’s magnum turn Current returns most of the Agul- rent. The 2001 pdf version of Regional opus. Of course, students still need to has water back into the Indian Ocean— Oceanography: An Introduction by Mat- go to the originals to gather the details “most” but not all. The Agulhas Retro- thias Tomczak and J. Stuart Godfrey and nuances, but at least they can have fl ection is not complete, as some Agulhas (to download a copy, go to http://www. a construct of the body of knowledge water leaks, mostly within large rings of es.fl inders.edu.au/~mattom/regoc/index. to build upon. the Agulhas Current that entrap pools of html) has fi ve pages devoted to the Agul- The book is dedicated to the mem- Indian Ocean water. has system, but still nothing like the 238 ory of Günther Dietrich (1911–1972), Lutjeharms comes to the Agulhas pages of text and fi gures of Lutjeharms’s whose Ph.D. thesis in 1935 (and sub- story as an insider, one who has focused book. The bibliography, arranged in tra- sequent journal publication) may have his professional life on the pursuit of ditional alphabetical order, consists of rekindled interest in the Agulhas (de- “knowing” the Agulhas as a regional 24 pages. The listings referenced within ferred by World War II—one of the gray system. In this way, he presents a story the text and fi gures are identifi ed with a boxes deals with pre-World War II Ger- far richer in texture than just that of in- sequential numbering system (826 en- man research on the Agulhas Current). terocean exchange. In Chapter 8 Lutje- tries in total), which then are listed in It was Dietrich’s work that drew me to harms takes the well-deserved liberty of numerical order before the bibliography. the Agulhas in 1983, particularly to in- covering “The Greater Agulhas Current: There is a complete (34-page) name and vestigate the Agulhas Retrofl ection and Some Refl ections,” with a wonderful subject indexThe fi gures, largely drawn adjacent South Atlantic. This work even- opening paragraph: “In looking back from other publications, many from tually led me to the consideration of the over this historical development one is Lutjeharms, include useful schemat- larger-scale role of the Agulhas through invariably tempted to read the past by ics that faithfully bring out the system’s its leakage (of Indian Ocean water) into the agendas of the present. The prevail- form and complexities. Excellent satellite the Atlantic Ocean. ing interest in inter-ocean exchanges and views of sea-surface temperatures fur- There are eight chapters in Lutje- their effects on global ocean circulation ther imprint upon the mind the very ac- harms’ book, which, after a large-scale is fundamentally motivated by contem- tive and fascinating oceanography of the description of the current’s setting, are porary concerns about climate change Agulhas Current system. discussed and arranged as the Agulhas and climate variability. This was not Lutjeharms shows a histogram of water fl ows: from upstream to down- the case when the main momentum in the number of Agulhas publications by stream. Each chapter concludes with a Agulhas Current studies was initiated.” year. Their explosion in the mid-1970s section of a different name (e.g., per- That is, there is much more to a system has been sustained at a more-or-less spective, conclusions, synopsis, recapitu- than just the pieces that happen to be in constant rate of 10 per year since 1980. lation). In Chapter 3 we are introduced vogue. Aspects of the Agulhas system will I’m glad I came of age as an oceanogra- to the varied streams and eddies feeding no doubt be rediscovered as our overall pher in the 1960s when it was possible the northern Agulhas Current through appreciation of its complexities evolves,

Oceanography March 2007 55 R.F. Anderson. 2006. Reduced Agulhas leakage and as the author says, “This modern during the last glacial maximum inferred from incentive has helped in lifting investi- an integrated provenance and fl ux study. Earth and Planetary Science Letters 250:72–88. gations on the Agulhas Current from Gordon, A.L. 2001. Interocean exchange. Pp. a sphere of largely localised, parochial 303–314 in Ocean Circulation and Climate. interests to one where the Agulhas Cur- G.Siedler, J. Church, and J. Gould, eds, Aca- demic Press. rent system is instead seen in its more Gordon, A.L. 2003. The brawniest retrofl ection. comprehensive role as a conduit for in- Nature News and Views 421:904–905. Knorr, G., and G. Lohmann. 2003. Southern Ocean ter-ocean exchange of heat and salt. This origin for the resumption of Atlantic thermo- wider perspective has had some distinct haline circulation during deglaciation. Nature advantages, but may ultimately also have 424:532–536. Peeters, F.J.C., R. Acheson, G.-J.A. Brummer, a few drawbacks.” Interesting. W.P.M. deRuijter, R.R. Schneider, G.M. Professor Johann R.E. Lutjeharms has Ganssen, E. Ufkes, and D. Kroon. 2004. Vigor- produced a truly scholarly work, draw- ous exchange between the Indian and Atlantic oceans at the end of the past fi ve glacial peri- ing from a multitude of research papers, ods. Nature 430:661–665.Weijer, W., W.P.M. de many from his own pen, and presenting Ruijter, H.A. Dijkstra, and P.J. van Leeuwen. to us with clarity the complex oceanog- 1999. Impact of interbasin exchange on the At- lantic overturning circulation. Journal of Physi- raphy of the Agulhas Current system, cal Oceanography 29:2,266–2,284. including its rich history of explora- tion, from the early days to the current research endeavors. There is more to be done, for sure, and it will be built upon the solid foundation of work ac- complished by so many and skillfully reviewed within the pages of The Agulhas Current. Johann Lutjeharms indeed has met his objectives, as stated in the pref- ace: “…to bring together and synthesise what has been learnt…More than this it should be a user-friendly key to unlock- ing the wealth of information scattered throughout the primary literature. These are the aims of this book.” Johann: On target and well done!

ARNOLD L. GORDON (agordon@ldeo. columbia.edu) is Associate Director, Ocean and Climate Physics, and Professor, Earth and Environmental Sciences, Lamont- Doherty Earth Observatory of Columbia University, Palisades, NY, USA.

REFERENCES Franzese, A.M., S.R. Hemming, S.L. Goldstein, and

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