摘要：In much the same way that a sharp contact between warm and cold air masses on a weather map suggests the presence of strong winds, the horizontal differences in seawater density (determined by both temperature and salinity) can tell us about the strength of the ocean currents. Specifically, because large-scale ocean flows are generally in geostrophic and hydrostatic balance, the vertical differences (shear) in horizontal velocities can be determined from the horizontal density gradients. This approach, based on detailed density measurements, has underpinned most of the efforts to quantify elements of the modern ocean circulation over the last century. While closely spaced density measurements are necessary to construct a detailed picture of ocean currents, the meridional circulation integrated zonally across a given basin can be determined from density measurements at the ocean margins (Cunningham et al., 2007; Hirschi and Marotzke, 2007; Marotzke et al., 1999). This has allowed, for example, continuous monitoring of the modern Atlantic Meridional Overturning Circulation (AMOC) at 26°N (Cunningham et al., 2007). Lynch-Stieglitz et al. (1999a; 1999b) showed how the oxygen isotopic composition (δ18O) of benthic foraminifers from sediments could be used to estimate water density#, and hence reconstruct the horizontal flow in the upper ocean for times in the past.
