Figure 13.3 The meridional-overturning circulation is part of a non-linear system. The circulation has two stable states near 2 and 4. The switching of north Atlantic from a warm, salty regime to a cold, fresh regime and back has hysteresis. This means that as the warm salty ocean in an initial state 1 freshens, and becomes more fresh than 2 it quickly switches to a cold, fresh state 3. When the area again becomes salty, it must move past state 4 before it can switch back to 1.