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Interhemispheric Water Exchange in the Atlantic Ocean EDITED by G.J BOOK REVIEW Interhemispheric Water Exchange in the Atlantic Ocean EDITED BY G.J. GOÑI AND P. MALANOTTERIZZOLI REVIEWED BY ARNOLD L. GORDON atmospheric processes, both on the hori- of cold water within the deeper layers. zontal and vertical planes. Each tropical The resultant northward heat fl ux and ocean basin plays a unique role in the its variability in Ice Age (century to mil- climate system because of its particu- lennium) time scales has stimulated a lar geography. The Pacifi c, accounting great deal of “Conveyor Belt” (a term for about half of the equatorial ocean, I consider to be misleading) literature. hosts the powerful climate presence of El Within the western tropical Pacifi c there Niño and La Niña, with its near global is also northward fl ow in the upper ki- reach. The Asian Monsoon, imposing lometer of similar magnitude to that of strong seasonally reversing meridional the Atlantic; however, this northward winds across the equator, dominates the fl ow across the Pacifi c equator is bal- Indian Ocean. The narrow Atlantic trop- anced mainly within the horizontal fl ow ics, hemmed in by the jigsaw-puzzle fi t pattern mostly by export of warm North 524 pages, Elsevier Oceanographic Series of South America and Africa (making a Pacifi c water through the Indonesian #68, 2003, ISBN 0-444-51267-5, $179.00 believer in continental drift of a curious Seas, and hence does not impose as large school child) holds another special place a cross-equator heat fl ux as does the The tropical ocean spans 82 percent of in Earth’s climate system. While the At- Atlantic’s meridional overturning circu- Earth’s equatorial circumference (Indo- lantic has its own “El Niño” of sorts, and lation. nesian maritime continent included). plays a role in the African and American The interhemispheric exchange of the Here, where the atmosphere’s oppos- monsoons, its specifi c attribute is the warm water within the upper kilometer ing Hadley Cells meet to produce the interhemispheric water exchange across of the tropical Atlantic is the main theme rainy Intertropical Convergence Zone, the Atlantic’s tropical belt. of the excellent collection of research the ocean is characterized by a shallow The Atlantic is home of a vigorous papers or chapters composing the book intense thermocline, regional upwelling, meridional overturning (full depth, ver- Interhemispheric Water Exchange in the and zonally elongated circulation gyres, tical plane) circulation, a thermohaline Atlantic Ocean edited by G. J. Goñi and crisscrossed by Rossby and Kelvin Waves. circulation driven by North Atlantic The tropical ocean draws lots of atten- Deep Water formation that runs at a Arnold L. Gordon ([email protected] tion by the climate community as the hot clip of 15 to 20 Sv (Sv = 1 Sverdrup = 1 bia.edu) is Professor and Associate Director end-member of the global heat engine. million cubic meters per second), with Physical Oceanography, Lamont-Doherty There, excess heating is transferred pole- a northward fl ow of warm water in the Earth Observatory of Columbia University, ward by a variety of coupled oceanic and upper kilometer, and a southward fl ow Palisades, NY. Th is article has been published in Oceanography, Volume 17, Number 3, a quarterly journal of Th e Oceanography Society. Copyright 2003 by Th e Ocean- 84 Oceanography Vol.17, No.3, Sept. 2004 ography Society. All rights reserved. Reproduction of any portion of this article by photocopy machine, reposting, or other means without prior authori- zation of Th e Oceanography Society is strictly prohibited. Send all correspondence to: [email protected] or 5912 LeMay Road, Rockville, MD 20851-2326, USA. P. Malanotte-Rizzoli. In 19 chapters in- transfer is shown along the western higher-resolution OCCAM model (0.25° volving 55 authors, it presents us with boundary where the North Brazil Cur- horizontal grid), specifi cally to track an impressive array of results. The book rent passes into the northern hemisphere the Equatorial Undercurrent waters af- covers a wide range of observational and before retrofl ection (curling) into the ter they upwell along the equator. Some modeling topics involving cross-equato- ocean interior to feed the North Equa- of the upwelled water takes part in the rial transfer of water, zonal circulation, torial Counter Current and Equatorial shallow tropical-subtropical overturn- planetary waves, sea-air interaction, and Undercurrent. The retrofl ection shares ing cell, and part gets caught up in the meridional ocean overturning (mostly a trait common to other retrofl ection deeper meridional circulation cell associ- about the shallow tropical-subtropical of the world’s oceans, notably the Agul- ated with the North Atlantic Deep Water overturning circulation). Not all papers has retrofl ection—as large pools of one formation— about two-thirds is drawn address the main theme of interhemi- ocean’s water drift into a neighboring into the latter. The word “complicated” spheric water exchange, though all are ocean, each “pool” annualized contrib- is clearly evident in describing the tra- worthwhile contributions towards un- utes around 1 Sv of interocean transport. jectories. One wonders how the results derstanding Atlantic tropical oceanogra- At the northern tip of the North Brazil of models can be tested in the real world. phy. Of course, the book chapters have Current retrofl ection, South Atlantic I suppose if the model agrees with what gone through vigorous peer review, and upper layer water breaks away as ed- observations scientists have, then one are of uniformly high quality; however, I dies, or what are more often referred to must assume that it has a good chance of do have some concern that collections of as “rings,” and drifts into the northern being right for those attributes that can’t articles within books do not get as much hemisphere, contributing to the meridi- easily be observed—at least let’s hope so. attention (citations) as articles published onal overturning circulation. However, A powerful tool for studying ocean in standard, widely distributed journals. even the part of the North Brazil Cur- circulation and its variability is the satel- I hope this is not true as the Goñi and rent that “retrofl ects” into the interior lite altimeter. Chapters by Mayer, Baring- Malanotte-Rizzoli book deserves the at- appears to be lost to the South Atlantic’s er, and Goñi and by Buehner, Malanotte- tention of the ocean and climate science subtropical gyre, as there is no clear Rizzoli, Busalacchi, and Inui investigate communities. pathway back to the South Atlantic gyre ways to extend the usefulness of satellite- Strammer, Fischer, Brandt, and Schott (maybe some return within the surface altimeter-derived sea level variability by discuss the upper kilometer circulation Ekman Layer). incorporating in situ data or by its assim- and its variability. They present a useful Halliwell, Weisberg, and Mayer track ilation into a model to tell us more of the schematic of the seasonal surface circu- the circuitous three-dimensional La- full circulation. Vianna and Menezes in- lation in which fi ve bands of alternat- grangian paths of “synthetic” fl oats, vestigate the surface circulation variabil- ing zonal currents are shown between seeded in the South Atlantic subtropical ity from 1995 to 2000. They show that the South and North Equatorial Cur- gyre of a HYCOM model (30°S to 70°N), the eastward fl owing North Equatorial rents, including some that are mainly as they maneuver across the complicated, Counter Current draws its water from undercurrents. It is a pretty complicated seasonally variable equatorial and North two sources: (1) the southern side is sup- circulation pattern, accounting for the Brazil Current environment. Before en- plied by the North Brazil Current, which diffi culty in tracing the pathways that tering the Caribbean Sea, many fl oats introduces South Atlantic water; and (2) ultimately accomplish the interhemi- fi rst extend and upwell into the interior the northern side is composed of North spheric transfer of water. Schematics of of the northern tropics of the Atlantic Atlantic subtropical water injected by the the tropical Atlantic circulation included rather than pass quickly into the Carib- North Equatorial Current. Vianna and in many of the book’s contributions are bean along a western boundary route. Menezes contribute to the main theme rich in names and arrows, making it dif- But, as the authors point out, this may of the book by noting that the equatorial fi cult for the casual reader to know what be sensitive to the coarse 1.4° horizontal current exhibits quasi-standing waves elements of the circulation to focus on. resolution of the model. Hazeleger and that lead to northward transport in three The unambiguous interhemispheric de Vries investigate trajectories within a specifi c longitude windows along the Oceanography Vol.17, No.3, Sept. 2004 85 western tropical Atlantic. consider to be the central theme of the Experiment period. These eddies have a Interhemispheric exchange is also book: the North Brazil Current and its mean diameter of 390 km, transporting accomplished within the Antarctic In- spin-off rings, which entrap a central waters northward at 8 Sv annually, with a termediate Water layer at 800 to 1100 m core of South Atlantic surface and ther- total heat transport of 0.54 PW. The MI- depth, defi ning the base of the thermo- mocline water. The rings appear in a COM model results reported by Garraf- cline. Schmid, Garraffo, Johns (E. not variety of forms, differing in their verti- fo, W. Johns, Chassignet, and Goñi agree W.), and Garzoli investigate this with cal profi le; some are surface intensifi ed, remarkably well with the observations. observations and modeling (MICOM), others more apparent at depth.
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