The Past and Future Ocean Circulation From a Contemporary Perspective
Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
Meridional overturning of the ocean, particularly the North Atlantic, is commonly invoked as the “trigger” and major cause of global climate change in a series of stories based upon a verysimplified view of the circulation (a “conveyor belt”). Observational and computational progress in physical oceanography, however, over the last 30 years has rendered obsolete the old idea that the fluid ocean is a slowly changing, passive, almost geological system. Instead, it is a dynamically active, essentially turbulent fluid, in which large-scale tracer patterns arise from active turbulenceand do not necessarily imply domination of the physics and climate system by large-scale flow fields. To the contrary, oceanic kinetic energy is dominated by the time- and space-varying components. The complexity of the resulting fluid pathways is an essential part of any zero-order description of the system. Thus general circulation models are the essential tool for understanding past, present andfuture climate states. Quantification of the likely major errors in using oversimplified models with inadequate turbulence closures and undersampled data becomes the main issue. Determining the past and future circulations is not easy, but hiding the difficulties is not a viable option.
1. INTRODUCTION Physical, chemical, and circulation properties of the ocean are important elements of modernclimate and its variability. Thus it is no surprise that these same properties are of intense interest to anyone attempting to understand the climate of the past or of the future. Meridional overturning rates, in particular, have been the focus of much of the discussion although it is important not to overemphasize a single, somewhat arbitrary, component of oceanic transport mechanisms. A reader ofthe literatures on past and future ocean circulations, and of that concerning the contemporary ocean, could, however, infer that the sciences are discordant and not obviously converging. Much about the modern system remains inadequately described and understood, but a great deal is known, and in
Ocean Circulation: Mechanisms and Impacts Geophysical Monograph Series 173 Copyright 2007 by theAmerican Geophysical Union 10.1029/173GM06 53
particular, perception of the very nature of the ocean circulation has changed from the classical view. Unfortunately, this new understanding is sufficiently recent that little of it has penetrated the textbook literature or more generally into discussions of past and future climates. A central purpose of this Chapter, therefore, is to briefly sketch whymuch of the interpretation of past (particularly) and future ocean circulation states is viewed with skepticism bordering on disbelief in a large part of the oceanographic community. As with the more general problem of climate, determining the meridional overturning circulation of the ocean can be divided conveniently into three major aspects, in approximate ascending order of difficulty: itscharacteristics (1) today; (2) in the past; (3) in the future. Acknowledging that the ocean circulation is a problem in fluid dynamics, the history of that subject shows that the combination of theory with observations (experiments) is essential. Without observations, theories rapidly become irrelevant or go badly awry; without theory, observations are uninterpretable. Thus for the contemporaryocean, we have substantial, if still incomplete, observations. For the ocean of the past, there are fragmentary
PAST AND FUTURE OCEAN CIRCULATION
data, and for the ocean of the future we have, by definition, no data at all. To keep the structure of this paper simple, I will focus primarily on (1) and (2), acknowledging that (3) is very...