The present-day ocean observing system includes global observations of velocity made at the sea surface by the Global Drifter Program (Dohan et al., 2010), and at 1000 m depth by the Argo Program (Freeland et al., 2010). In addition, Argo observes the geostrophic shear between 2000 m and the sea surface. These two recently implemented observing systems, if sustained, will continue to document the large-spatial scale, long-time-scale variability of circulation in the upper ocean. The drifter program achieved its target of 1250 drifters in 2005, and Argo its target of 3000 floats in 2007.
Historically, global measurements of ocean circulation are much sparser, so estimates of decadal and longer-term changes in circulation are very limited. Since 1992, high-precision satellite altimetry has measured the time variations in sea surface height (SSH), whose horizontal gradients are proportional to the surface geostrophic velocity. In addition, a single global top-to-bottom hydrographic survey was carried out by the World Ocean Circulation Experiment (WOCE), mostly during 1991–1997, measuring geostrophic shear as well as velocity from middepth floats and from lowered acoustic Doppler current profilers. A subset of WOCE and pre-WOCE transects is being repeated at 5- to 10-year intervals (Hood et al., 2010).
Ocean circulation studies in relation to climate have focused on variability in the wind-driven gyres (Section 3.6.2) and changes in the meridional overturning circulations (MOCs, Sections 3.6.3 and 3.6.4) influenced by buoyancy loss and water-mass formation as well as wind forcing. The MOCs are responsible for much of the ocean’s capacity to carry excess heat from the tropics to middle latitudes, and also are important in the ocean’s sequestration of carbon. The connections between ocean basins (Section 3.6.5) have also been subject to study because of the significance of inter-basin exchanges in wind-driven and thermohaline variability, and also because these can be logistically advantageous regions for measurement (“chokepoints”). An assessment is now possible of the recent mean and the changes in global geostrophic circulation over the previous decade (Figure 3.10, and discussion in Section 3.6.2). In general, changes in the slope of SSH across ocean basins indicate changes in the major gyres and the interior component of MOCs. Changes occurring in high gradient regions such as the Antarctic Circumpolar Current (ACC) may indicate shifts in the location of those currents. In the following, the best-studied and most significant aspects of circulation variability and change are assessed including wind-driven circulation in the Pacific, the Atlantic and Antarctic MOCs, and selected interbasin exchanges.