Part One General Ocean Circulation I Deep Circulation of the World Ocean Bruce A Warren 1 1 Introduction Historically the deep circulation of the ocean has been viewed from the perspective of property fields mainly the distributions of temperature salinity density and dissolved oxgyen concentration The practical reason for not considering velocity measurements as well of course was a technical incapacity for making them until very recently On the whole this was probably not a bad thing not merely because the property distributions are as interesting in themselves as the motion field but also because the scalar fields are so much more stable than the velocity vectors allowing spot measurements from different areas even years apart to be combined into coherent pictures that tell a good deal about general patterns of deep flow albeit indirectly The slight differences between corresponding hydrographic sections in the atlases by Fuglister 1960 and by Wust and Defant 1936 when compared with the fluctuations much larger than the means of deep velocities observed by the MODE Group 1978 for example demonstrate how much easier it is to obtain statistically significant information pertinent to the overall global deep circulation from water property data than from current measurements On the other hand the information gained from the property fields allows only a limited view of the deep motions at very best some kind of long term average Although oceanographers have usually been mindful of variability in the deep flow even if only to accomplish eddy mixing it seems extremely unlikely that anyone imagined the highly energetic low frequency mesoscale motions that current records have revealed Instead because of the stability of the property fields it was stationarity rather than variability that was emphasized however implicitly in the circulation pictures derived from them That stability also was surely the basis for the conceptual structure of water types and masses that has been so enormously useful in summarizing and comprehending the temperature salinity structure of the ocean and in identifying features in the property fields that can be exploited as tracers for the flow Without velocity information though such descriptions of oceans have sometimes degenerated into taxonomic sterility naming something doesn t explain it and perhaps sometimes there has been too elemental a character ascribed to water masses as if they were truly building blocks rather than names for features leading to pictures of the ocean more suggestive of rigid geological strata than of the real motion field that forms the distributions Plainly there cannot be a satisfying description of the deep ocean circulation that does not meld station data with current records It does not seem to me though that such a description is yet possible Far too 6 Bruce A Warren few current records have been obtained to describe the deep low frequency motions in a global sense it is only in the western North Atlantic that one can even contemplate making a basin wide description Moreover we simply have not learned how to combine the stable station data with the fluctuating velocity records to tell a story that is both consistent and informative For example Reid Nowlin and Patzert 1977 reported a record Cato 2 from a current meter moored on the South American continental slope in the core of the North Atlantic Deep Water for 2 weeks the daily averaged velocity vectors were directed southwestward parallel to the isobaths as one would have expected in this particular deep western boundary current but then the flow abruptly changed direction and went eastward for nearly 2 weeks Nevertheless with due regard for different density and accuracy of observations the high salinity core of the current looked very much as had been depicted by Fuglister 1960 and Wuiistand Defant 1936 How are we to approach these two different sets of data to reconcile the variability of the one to the steadiness of the other and to learn something significant from their combination about that boundary current Finally it is not at all clear what effect the lowfrequency velocity fluctuations have on the long term mean flow There is enough theoretical reason e g Rhines 1977 to suspect that their role in its dynamics may be substantial but measurements of deep Reynolds stresses are meager In fact values reported by Schmitz 1977 from the Sargasso Sea well south of the Gulf Stream actually favor a negligible contribution to the vorticity balance there but those measurements are far too few to give a general characterization of the deep open ocean Consequently although I recognize its incompleteness the following account of the deep circulation of the world ocean is undertaken mainly from the traditional perspective of hydrographic station data with reference to current measurements only where they seem helpful in estimating velocities and transports of the prominent currents The emphasis is on mean thermohaline circulation What has been learned about the low frequency motions is described in detail by Wunsch in chapter 11 of this volume In section 1 2 I have attempted a historical review of what seem to me to be the important events and dates in the development of ideas about the deep circulation from the first deep temperature measurements through Sverdrup s comprehensive synthesis in chapter XV of The Oceans Sverdrup Johnson and Fleming 1942 In section 1 3 I have discussed the dynamical ideas of Stommel and his colleagues that led to the overthrow of a substantial part of Sverdrup s picture and its revision in contemporary thinking with dynamically consistent models of circulation Section 1 4 is an account of the sinking processes that supply water to the deep ocean from the surface layer Section 1 5 is a consideration of how well the kinds of deepcirculation patterns envisioned in dynamical theory stand up to observation it is necessarily mainly a digest of the evidence for deep western boundary currents in the world ocean Finally in section 1 6 I have speculated about some fundamental aspects of the deep circulation that seem to me to be not very well under stood at this time The focus throughout is more on the circulation of deep water than on the complementary problem of its properties because the subject of deep water characteristics has been treated in detail by Worthington in chapter 2 of this volume There is of course some overlap with that chapter as well as
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