The past provides evidence of abrupt climate shifts and changes in the frequency of climate and weather extremes. We explore the nonlinear response to orbital forcing and then consider climate millennial variability down to daily weather events. Orbital changes are translated into regional responses in temperature, where the precessional response is related to nonlinearities and seasonal biases in the system. We question regularities found in climate events by analyzing the distribution of interevent waiting times. Periodicities of about 900 and 1,150 yr are found in ice cores besides the prominent 1,500 yr cycle. However, the variability remains indistinguishable from a random process, suggesting that centennial-to-millennial variability is stochastic in nature. New numerical techniques are developed allowing for a high resolution in the dynamically relevant regions like coasts, major upwelling regions, and high latitudes. Using this model, we find a strong sensitivity of the Atlantic meridional overturning circulation depending on where the deglacial meltwater is injected into. Meltwater into the Mississippi and near Labrador hardly affect the large-scale ocean circulation, whereas subpolar hosing mimicking icebergs yields a quasi shutdown. The same multiscale approach is applied to radiocarbon simulations enabling a dynamical interpretation of marine sediment cores. Finally, abrupt climate events also have counterparts in the recent climate records, revealing a close link between climate variability, the statistics of North Atlantic weather patterns, and extreme events. Plain Language Summary Predicting the future spread of possible climates, the risk of climate extremes and the risk of rapid transitions is of high socioeconomic relevance. The past provides evidence of abrupt climate change and the frequency of extremes. This allows to separate anthropogenic signals from natural climate variability. Earth system models applied both to past and future scenarios will enhance our ability to detect regime shifts which are necessary to potentially predict climate extremes and transitions. We consider the response of the system to regular orbital forcing and then focus on shorter time scales down to weather. The appearance of precession is linked to nonlinear responses of the climate system to external orbital forcing. Furthermore, we find that centennial-to-millennial variability is stochastic in nature. We also discuss recent developments of climate models with superior resolution in typical retrieval regions of paleoclimate records, such as continental margins and coasts. Using this model, we find a strong sensitivity of the Atlantic meridional overturning circulation depending on where the deglacial meltwater is injected into. Meltwater into the Mississippi and near Labrador hardly affect the large-scale ocean circulation, whereas subpolar hosing related to icebergs yields a quasi shutdown. Our multiscale approach is applied to radiocarbon simulations enabling a dynamical interpretation of marine sediment cores....

<p>The Earth’s climate is characterized by many modes of variability. On millennial timescales, decaying Northern Hemisphere ice sheets during the last deglaciation affect the high latitude hydrological balance in the North Atlantic and therefore the ocean circulation after the Last Glacial Maximum. Global sea-level reconstructions indicate marked abrupt changes within several hundred years. Using a multi-scale climate model with a high resolution near the coast, we find a strong sensitivity of the ocean circulation depending on where the deglacial meltwater is injected. Meltwater injections via the Mississippi and near Labrador hardly affect the AMOC. The reduced sensitivity of the overturning circulation against freshwater perturbations following the Mississippi route provides a consistent representation of the deglacial climate evolution. A subpolar North Atlantic Ocean freshening, mimicking a transport of water by icebergs, yields, on the other hand, a quasi-shutdown. We can conclude that millennial climate variability depends on the spatio-temporal structure and their representation in models.</p><p>Millennial DO-like variability is seen in a handful of model simulations, including even some pre-industrial simulations. As a mechanism, the subsurface is warmed by the downward mixing of the warmer overlying water during an AMOC weak state, until the surface became denser than at mid-depth and deep ventilation is initiated. In recent model developments, the large oscillations in the Labrador Sea mixing were reduced. However, it might be that the centennial-to-millennial oscillations are required to explain climate variability as expressed e.g. by the Little Ice age and the Medieval Warm Event during the last 1000 years. It could be that a de-tuning of the models is necessary in order to exhibit irregular oscillations on centennial-to-millennial time-scales. Although a systematical analysis of the mechanisms leading to centennial-to-millennial variability remains open, numerical experiments suggest that at least in the Labrador Sea and other sensitive areas the high resolution can play an important role in realistically simulating the variability in the mixed layer depth affecting AMOC. One can question regularities found in DO-events occurrence and statistically analyzed the distribution of inter-event waiting times. To estimate the statistical significance of detected event patterns, we construct a simple stochastic process in which events are triggered each time a threshold criterion is fulfilled. For a given time interval each event occurs therefore stochastically independent of another meaning that the probability of one abrupt warming does not affect the probability distribution of any other warming events in that interval. Additionally, novel periodicities of ∼900 and ∼1150 yrs in the NGRIP record are reported besides the prominent 1500 yrs cycle but demonstrate that although a high periodicity reflected in a high Rayleigh R can be found in the data it remains indistinguishable from a simple stationary random Poisson process. These are quite interesting findings as ∼1500 and ∼900 yrs periods are visible throughout the Holocene. The understanding of such low-frequency variability is crucial to allow a separation of anthropogenic signals from natural variability. </p>