A Heuristic Model of Dansgaard–Oeschger Cycles. Part I: Description, Results, and Sensitivity Studies

Abstract

A simple model for studying the Dansgaard-Oeschger (D-O) cycles of the last glacial period is presented, based on the T. Dokken et al. hypothesis for D-O cycles. The model is a column model representing the Nordic seas and is composed of ocean boxes stacked below a one-layer sea ice model with an energy-balance atmosphere; no changes in the large-scale ocean overturning circulation are invoked. Parameterizations are included for latent heat polynyas and sea ice export from the column. The resulting heuristic model was found to cycle between stadial and interstadial states at times scales similar to those seen in the proxy observational data, with the presence or absence of perennial sea ice in the Nordic seas being the defining characteristic for each of these states. The major discrepancy between the modeled oscillations and the proxy record is in the length of the interstadial phase, which is shorter than that observed. The modeled oscillations were found to be robust to parameter changes, including those related to the ocean heat flux convergence (OHFC) into the column. Production of polynya ice was found to be an essential ingredient for such sustained oscillatory behavior. A simple parameterization of natural variability in the OHFC enhances the robustness of the modeled oscillations. The authors conclude by discussing the implications of such a hypothesis for the state of the Nordic seas today and its state during the Last Glacial Maximum and contrasting the model to other hypotheses that invoke large-scale changes in the Atlantic meridional overturning circulation for explaining millennial-scale variability in the climate system. An extensive time-scale analysis will be presented in the future.

Thermohaline instability and the formation of glacial North Atlantic super polynyas at the onset of Dansgaard‐Oeschger warming events

“…Recently, Singh et al . [] employed a simple heuristic column model of MIS3 Nordic Sea conditions and found that a parameterization of latent heat polynyas in their model was critical in their development of a heuristic understanding of the D‐O oscillation. The super polynya that marks the onset of transition to stadial conditions in our model is, however, a sensible heat polynya as we discuss in what follows.…”

The southern Norwegian Sea during the last 45 ka: hydrographical reorganizations under changing ice‐sheet dynamics

“…Such a scheme has been supported by model simulations (e.g. Li et al ., ; Petersen et al ., ; Singh et al ., ) as well as by benthic and planktonic isotopic measurements (Dokken and Jansen, ; Dokken et al ., ). However, Eynaud et al .…”

Dansgaard‐Oeschger oscillations predicted in a comprehensive model of glacial climate: A “kicked” salt oscillator in the Atlantic

“…An alternative hypothesis involves the idea of “stochastic resonance” [ Alley et al ., ; Ganopolski and Rahmstorf , ] in which a small‐amplitude harmonic excitation of the Atlantic meridional overturning circulation (AMOC) is amplified in the presence of noise. An even more recent model is one which does not specifically invoke oscillatory variability of the AMOC at all [ Singh et al ., ]. A further suggested mechanism is that involving a “salt oscillator” [ Broecker et al ., ; Birchfield and Broecker , ], but this suggestion was advanced on qualitative grounds or in terms of a box model representation which specifically envisioned freshwater forcing as being required.…”