A wide variety of climate records have revealed the existence of sudden a r
ecurrent climatic changes. An important part of this variability might be r
elated to transitions between stable equilibrium states of the thermohaline
circulation. Here, we employ a box model of the ocean thermohaline circula
tion to show that in the presence of environmental fluctuations, a subthres
hold periodic perturbation in the fresh water fluxes can induce quasiperiod
ic transitions between the stable states of the thermohaline circulation Th
is enhanced response occurs for a wide range of frequencies, including the
Milankovic orbital forcing, and amplitudes. The mechanism that allows such
response of the system under small perturbations arise from a nonlinear coo
peration between the periodic perturbations and the fluctuations. Through t
his nonlinear mechanism, called stochastic resonance, significant climatic
variability may be originated due to small perturbations enhanced by enviro
nmental noise and dynamics.