GM01073_CH06.qxd

9/8/07

5:24 PM

Page 53

The Past and Future Ocean Circulation From a Contemporary Perspective
Carl Wunsch
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 very simplified 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 turbulence and 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 and future 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 modern climate 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 of