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Peter D. Ditlevsen and Ove D. Ditlevsen

Abstract

The rapid climate shifts observed in the glacial climate are analyzed. The transitions into the warm interstadial states, the onsets, are easily identifiable in the record. The distribution of waiting times between consecutive onsets is well fitted, assuming that the remaining residence time in each state is independent of the past. This implies that it has a simple no-memory exponential waiting time distribution, but with the mean waiting time depending on the climate state. The mean waiting time from one onset to the next is around 2400 yr. The most likely (maximum likelihood) distribution derived solely from the onset sequence is rather insensitive to the mean waiting time in the warm interstadials in the range of 400–1200 yr. When extending the analysis to include the transitions from the warm interstadials to the cold stadials observed with a larger uncertainty, the distributions in the two states are well fitted to exponential distributions, with mean waiting times of around 800 yr in the warm state and around 1600 yr in the cold state. This observation is an important piece in the climate puzzle, because the fact that the climate is a no-memory process indicates that the transitions are noise induced and the mean residence time in one state indicates how stable that climate state is to perturbations. The possibility of a hidden periodic driver is also investigated. The existence of such a driver cannot be ruled out by the relatively sparse data series (containing only 21 onsets). However, because the record is fitted just as well by the much simpler random model, this should be preferred from the point of view of simplicity.

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Ivana Cvijanovic, Peter L. Langen, Eigil Kaas, and Peter D. Ditlevsen

Abstract

In this study, southward intertropical convergence zone (ITCZ) shifts are investigated in three different scenarios: Northern Hemispheric cooling, Southern Hemispheric warming, and a bipolar seesaw-like forcing that combines the latter two. The experiments demonstrate the mutual effects that northern- and southern-high-latitude forcings exert on tropical precipitation, suggesting a time-scale-dependent dominance of northern versus southern forcings. In accordance with this, two-phase tropical precipitation shifts are suggested, involving a fast component dominated by the high-northern-latitude forcing and a slower component due to the southern-high-latitude forcing. The results may thus be useful for the future understanding and interpretation of high-resolution tropical paleoprecipitation proxies and their relation to high-latitude records (e.g., ice core data). The experiments also show that Southern Ocean warming has a global impact, affecting both the tropics and northern extratropics, as seen in a southward ITCZ shift and mid- and high-latitude North Atlantic surface temperature and wind changes. In terms of dynamical considerations, the tropical circulation response to high-latitude forcing is found to be nonlinear: the atmospheric heat transport and Hadley cell anomalies differ significantly (in magnitude) when comparing the warming and cooling experiments. These are related to different interhemispheric temperature gradients that are altered mainly by nonlinearities in water vapor response. Decomposition of the top-of-the-atmosphere flux response into atmospheric feedback effects shows the dominance of water vapor and cloud feedbacks in the tropics, with the longwave cloud feedback effect governing the overall cloud response.

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Peter D. Ditlevsen, Mikkel S. Kristensen, and Katrine K. Andersen

Abstract

By comparing the high-resolution isotopic records from the Greenland Ice Core Project (GRIP) and the North Greenland Ice Core Project (NGRIP) ice cores, the common climate signal in the records has been approximately separated from local noise. From this, an objective criterion for defining Dansgaard–Oeschger (DO) events is achieved. The analysis identifies several additional short-lasting events, increasing the total number of DO events to 27 in the period 12–90 kyr before present (BP). The quasi-regular occurrence of the DO events could indicate a stochastic or coherent resonance mechanism governing their origin. From the distribution of waiting times, a statistical upper bound on the strength of a possible periodic forcing is obtained. This finding indicates that the climate shifts are purely noise driven with no underlying periodicity.

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