Climate studies are motivated by the curiosity we all have about the weather, and our desire to predict it in order to make economic projections. The general public tends to focus on local weather—and thus generally on weather over land rather than over the ocean—as well as on fairly short-scale climate variations, several years rather than decades to millennia. For these reasons it is not always appreciated outside the scientific community that the ocean is an essential component of the coupled climate system. Understanding and modeling the ocean and its coupling to the atmosphere, land, and biosphere is vital. In the realm of fisheries, climate variations in the ocean itself can be seen to have an economic impact; recognizing this, coastal states have supported oceanography.

Approaches to studying the ocean's role in climate can be divided into two types: understanding and modeling the ocean as part of the fully coupled climate system, and observing, quantifying, and modeling the dynamics of the ocean itself. Clearly there is great overlap, and a full grasp of the latter is necessary for progress in the former. The ocean's most obvious and direct importance to land-based climate variations lies in the fact that it sets the surface temperature that forces the atmosphere over three-quarters of the planet; distribution of sea ice is important also, since it affects the planetary albedo and the amount of ocean/ atmosphere heat exchange. Predicting the surface temperature of the ocean and the extent of sea ice is not a simple exercise, however: It involves atmospheric forcing, lateral circulation, and vertical overturning. The last is affected by the salinity distribution, and salinity depends on factors that are similar to those influencing the ocean temperature.

Because of its great thermal inertia relative to that of the atmosphere, the ocean has a significant effect on climate. The two most commonly mentioned climate phenomena in which the ocean's role is important are El Niño, which is purely natural variability, and global warming, which is partly anthropogenic. Much progress has been made in observing and modeling both the oceanic and the atmospheric components of El Niño; other obvious climate effects are also strongly tied to the ocean, such as the higher temperatures in northern Europe relative to those of northeastern Canada. The papers in this volume discuss a decadal climate oscillation in the North Atlantic involving both the atmosphere and ocean, and a longer-period oscillation in the North Pacific, neither of which is firmly tied to El Niño. Our understanding of the ocean's role in much longer-scale variations, such as glaciations, has also improved greatly through the use of proxy records and coupled ocean-atmosphere modeling, both of which are represented in this volume. Thus our vocabulary of climate variations, even if limited to those we can quantify today, is already much broader than those that have drawn the most public attention.

In order to dissect the relative roles of the ocean and atmosphere in climate, it is necessary to both observe and model. Modeling is particularly important because of the relative lack of long-term observations. The merchant marine data set is the most comprehensive in time and space, since ship observations have been reported and archived for many years. This data set is limited to the sea surface, however, and is good only in regions of high merchant marine activity. The time series of water-column ocean

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