Introduction to special section: World Ocean Circulation Experiment: Southern Ocean results

King, B.A. (2001) Introduction to special section: World Ocean Circulation Experiment: Southern Ocean results. Journal of Geophysical Research, 106 (C2), p.2691.

Abstract

This special section of Journal of Geophysical Research contains a collection of papers on the Southern Ocean, presenting results that have emerged during the late phase of the World Ocean Circulation Experiment (WOCE). Previous special sections have included papers with Pacific results (World Ocean Circulation Experiment (WOCE), Journal of Geophysical Research, 103, 12,897-13,092, 1998) and South Atlantic results (WOCE South Atlantic Results, Journal of Geophysical Research, 104, 20,859-21,226, 1999). As before, this special section was stimulated by a WOCE basin workshop: the WOCE Southern Ocean workshop was held at the Antarctic CRC in Hobart, Tasmania, in July 1997. However, the correspondence between results presented at the workshop and papers that were subsequently submitted to the section is not as close as for the other workshops. Rather, this section represents a selection of Southern Ocean results that happened to be ready in the right time frame.
A first glance at the papers shows that a majority of them deal with in situ observations. The Pacific, South Atlantic, and Indian Basins had coherent measurement programs, rather concentrated in time. In contrast, the WOCE Southern Ocean observational program was less coordinated, even though the Southern Ocean was the subject of one of the WOCE Core Projects. Southern Ocean measurements were often made as part of cruises for which the focus was one of the Southern Hemisphere basins mentioned above. Data and results have therefore emerged at a more or less even rate throughout WOCE, which accounts in part for why such a snapshot selection like this one cannot be really representative.
The special section contains twelve papers. Of these, at least nine are principally concerned with observations. This balance between results from measurement programs and results from modeling efforts is probably representative of interest expressed at the Hobart workshop. So why might modeling be underrepresented in a collection of papers with a Southern Ocean focus? Certainly not because modelers are unaware of the significance of this basin in their efforts. I suggest it is in part because the current trend is for global rather than regional models. Where regional models are used, the aim is usually to run with very high resolution over a reasonably enclosed region; recently, the geographical focus of such studies has been elsewhere. Hence Southern Ocean modeling is often done within global general circulation models, and results are presented in the global context rather than in a specifically Southern Ocean one.
Another contrast is between the number of papers addressing the large-scale budget issues (transport and variability) and those addressing processes and other topics (water mass formation and sea ice): the transport and/or variability of the Antarctic Circumpolar Current alone accounts for over half the papers.
WOCE planners called for two sets of in situ observations. First, the global description of the ocean was to be based on the so-called one-time survey of the WOCE Hydrographic Programme (WHP). This was realized as a network of about 100 cruises covering the global ocean, of which about 20 could be considered as contributing significantly in the Southern Ocean. Second, it was recognized that it would be vital that some sections should be repeated several times in order to determine the representativeness of the sections occupied just once. Two sections have attracted particular attention in this repeat hydrography program: by international efforts the choke points at Drake Passage and Tasmania were occupied 10 and 6 times, respectively, with full-depth conductivity-temperature-depth sections. In addition, there have been time series data from moorings at both locations and expendable bathythermograph programs. The return on this investment is amply demonstrated by the fact that six out of the nine papers that depend on observations make use of the time series from repeat sections or moorings. The Southern Ocean has been a prime location for observing variability, and describing the variability there is recognized as being at least as important as efforts to refine estimates of the mean flow. The available time series measurements and the associated analyses also provide a valuable and timely resource when planning for the future. One of the WOCE goals is to find methods for determining long-term changes in ocean circulation. To this end the Climate Variability and Predictability program (CLIVAR) needs to develop a plan of sustained observations that is both practical and sufficient. The Southern Ocean time series initiated during WOCE provide a sound basis for further discussion, as well as a foundation for long-term observational programs.
One notable area that is underrepresented is that of transient tracers. Most of the WHP one-time cruises mentioned above will have produced transient tracer data, and yet there is just a single paper that includes tracer data. A possible explanation for this would seem to be the difficulty in combining tracer data from many different cruises with many different PIs collected over different years. The inventory of high-quality Southern Ocean tracer data has certainly increased significantly during WOCE, and it is reasonable to hope and expect that new syntheses of the tracer fields will emerge in the not too distant future from the consortia of investigators working with these data.
As guest editor for the special section, I thank Lewis Rothstein, the Scientific Editor of JGR-Oceans responsible for the section; Carol Gannon, his editorial assistant; associate guest editors Mark Warner and David Webb; and the many reviewers for their efforts and patience throughout.

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