Figure 1.

The ocean–atmosphere system is a nonlinear physical system that can exhibit hysteresis behavior (13). The upper branch of the hysteresis is characterized by warm North Atlantic sea surface temperatures (SST), the lower branch by cold sea surface temperatures. A given perturbation (indicated by the blue and red arrows) in the freshwater balance of the North Atlantic (precipitation + runoff − evaporation) causes transitions from an initial state 1 to states 2 and/or 3. Three structurally different responses are possible depending on whether threshold values (dashed line) are crossed: (a) Linear, reversible response. (b) Nonlinear, reversible response. (c) Nonlinear, irreversible response.