observations reported in this volume are representative of what is available globally; at best they are repeated measurements of temperature and salinity through the water column, collected at regular intervals and at the same location over decades. Such time series are very restricted spatially; they are usually localized to the coastal waters of a state or country that depends on fisheries. The time series that are more widely spatially distributed, such as those from the defunct weather ships, tend to be single points separated by a long distance from the next station. The upper water column has been better measured since the 1970s using ships of opportunity, but only for temperature and not salinity. (Relatively little from this extensive data set is reported in this volume.) Proxy records from sediment cores relate primarily to much longer time scales than the decade-to-century ones that are the focus of this volume, and consist of very few data points. Thus we are operating observationally from an extremely limited data set. In the last few years, attention has been turned to extracting as much information as possible from the data we do have, and progress has been remarkable under the circumstances. Many fine examples of this work are presented in this chapter.
Ocean modeling relevant to climate has progressed greatly in the last decade, due to the growth of computing power, the availability of a community ocean model, and an increased focus on these problems by a small community of modelers. Enormous advances have been made in understanding the El Niño problem in the tropical Pacific. Several modeling-related papers in this volume present new insights into the role of the lateral and overturning modes of ocean circulation in producing climate oscillations.
The current public concern about global warming and other issues related to climate and its prediction has resulted in international plans for greatly enhanced ocean observations in space and time. Major efforts include the placement of monitoring equipment in the tropical Pacific for El Niño prediction, a global ocean-circulation observational experiment designed to enhance our knowledge of the circulation as it exists today and provide better data for modeling, and establishment of global monitoring. Global monitoring should be focused on time series of ocean properties that affect climate and/or reflect climate change; these include temperature and velocity, and, as we are recognizing more and more, salinity. For this monitoring to be effective, measurements must be made in the upper ocean worldwide; at locations likely to be indicative of climate, they should be made throughout the relevant portion of the water column. Perhaps even more important than location, however, will be some assurance that a time series will be maintained indefinitely. If these two conditions can be met, monitoring should be begun as soon as possible. It is therefore critical that as much knowledge as possible be synthesized from currently available data and models, to ensure that the ocean observing systems will be both efficient and comprehensive. The papers in this chapter, which reflect the increasing attention being paid to climate issues by the oceanographic research community, represent a large advance in our knowledge of ocean variability on decade-to-century time scales.