Modeling the effect of freshwater pulses on the early Holocene climate: The influence of high‐frequency climate variability

Abstract

The effect of freshwater pulses on the early Holocene climate is investigated with a global coupled atmosphere‐sea ice‐ocean model. In the model an early Holocene equilibrium climate state is perturbed by releasing a fixed amount of freshwater (4.67 × 1014 m3) into the Labrador Sea at three different constant rates: 1.5 Sv (1 Sv = 1 × 106 m3 s−1) in 10 years, 0.75 Sv in 20 years, and 0.3 Sv in 50 years. For each rate, five ensemble experiments have been performed, varying in initial conditions. The freshwater pulses produce a weakening of the thermohaline circulation. The perturbed state is in agreement with proxy evidence for the 8.2 ka event. Two types of recovery of the thermohaline circulation occurred, differing in time‐scale: (1) ≤200 years and (2) >200 years. In the experiments with 10 year and 20 year pulses, both types of recovery were observed. This suggests that the model response is unpredictable in the range of parameters studied here. It is hypothesized that the unpredictability is associated with annual‐to‐decadal climate variability. Our results demonstrate that several types of recovery may exist with the same kind of perturbation. The interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere–sea ice–ocean system.