The evaporation of seawater is more vigorous in the tropics than at higher, cooler latitudes. Tropical surface seawater should therefore be saltier. That expectation holds true in the North Pacific Ocean—less so in the North Atlantic. Carried northward by the North Atlantic Current, salty surface water in the North Atlantic encounters low atmospheric temperatures, whereupon it cools and becomes denser. Because the salty water is denser than the fresh water beneath, it sinks to the bottom. There, it forms a cold countervailing current that travels southward and spreads globally. Eventually, the action of winds and other processes return the water to the surface. The cycle, which takes about 2000 years to complete, begins again.

Several mechanisms have been proposed to explain why the North Atlantic is saltier than the North Pacific and, consequently, why the meridional overturning circulation (MOC) is far stronger in the Atlantic Basin than in the Pacific Basin. For example, the absence of high mountains on the East Coast of North America means that westerly winds can move water vapor from the Atlantic to the Pacific. High mountains on North Asia’s east coast block the corresponding winds over the Pacific. The North Pacific is rainier and its surface water is fresher, as a result.

Spencer Jones and Paola Cessi of the Scripps Institution of Oceanography at the University of California, San Diego, contend that the Atlantic’s narrower basin is responsible for its stronger MOC. They reached that conclusion after modeling the oceans as two vast baths of different widths and equal, constant depths. When the two model oceans were subjected to the same wind field and other initial conditions, the same boundary currents and gyres developed but with different relative strengths. In the narrow bath, the cold, southward current that corresponds to the Labrador Current (whose eddies appear in the accompanying image) was too weak to counter the northward current that corresponds to the North Atlantic Current. In the wide bath, the two currents were closer in strength, with the result that the wide bath lacked the means to convey salty water northward. (C. S. Jones, P. Cessi, J. Phys. Oceanogr., in press, doi:10.1175/JPO-D-17-0075.1.)