Description: Previous studies on surface temperature reconstructions for the last 2000 years (2 k) revealed a long-term cooling trend for the last millennium in comparison to the previous millennium. However, knowledge on the decadal- to centennial-scale variability in sea surface temperature and the underlying governing mechanisms throughout the period is limited. We reconstructed high-resolution continuous sea surface temperature changes over the last 2 k in the northwest Pacific margin based on the alkenone unsaturation index. Our alkenone temperature record revealed enhanced and more rapidly changing climate variability during the last millennium (approximately 1200–1850 Common Era) than during the previous millennium. Cold and hot extremes also occurred more frequently during the last millennium. The enhanced and rapidly changing climate variability appears to be associated with frequent volcanic eruptions and grand solar minima. The reconstructed surface temperature variability tends to be associated with variations in the East Asia summer monsoon and the Pacific Decadal Oscillation, implying that these variations are also enhanced in the last millennium than in the previous millennium.

Description: Highlights: • Climate model sensitivity experiments are performed using state-of-the-art ice sheet and freshwater reconstructions • Declining Northern Hemisphere ice sheets increase the sensitivity of the AMOC to North Atlantic meltwater discharge • Deglacial rise in atmospheric CO2 concentration decreases the sensitivity of the AMOC to North Atlantic meltwater discharge • Both effects provide a complementary perspective to existing explanations for abrupt AMOC transitions Abstract: The last deglaciation was characterized by a sequence of abrupt climate events thought to be linked to rapid changes in Atlantic meridional overturning circulation (AMOC). The sequence includes a weakening of the AMOC after the Last Glacial Maximum (LGM) during Heinrich Stadial 1 (HS1), which ends with an abrupt AMOC amplification at the transition to the Bølling/Allerød (B/A). This transition occurs despite persistent deglacial meltwater fluxes that counteract vigorous North Atlantic deep-water formation. Using the Earth system model COSMOS with a range of deglacial boundary conditions and reconstructed deglacial meltwater fluxes, we show that deglacial CO2 rise and ice sheet decline modulate the sensitivity of the AMOC to these fluxes. While declining ice sheets increase the sensitivity, increasing atmospheric CO2 levels tend to counteract this effect. Therefore, the occurrence of a weaker HS1 AMOC and an abrupt AMOC increase in the presence of meltwater, might be explained by these effects, as an alternative to or in combination with changes in the magnitude or routing of meltwater discharge.

Description: Previous modelling efforts have investigated climate responses to different Milankovitch forcing during Marine Isotope Stage (MIS) 13. During this time the climate has been highly variable at atmospheric CO2 concentrations of ~240 ppm. As yet, ice sheet-climate feedbacks were missing in previous studies. Therefore we use the state-of-the-art coupled climate-ice sheet model, AWI-ESM-1.2, to investigate the MIS-13 climate and corresponding Northern Hemisphere ice sheet (NHIS) evolution by performing simulations under three different astronomical configurations representing 495, 506 and 517 kyr BP. The simulated excess ice compared to present-day is mainly over the Cordillera, Arctic islands and Tibet. The global mean surface air temperature for the MIS-13 experiments have the same magnitude. At 506 kyr BP with boreal summer at perihelion, the Northern Hemisphere continents are warmer during summer than the other experiments, which could potentially inhibit the development of the ice sheets. The Cordilleran Ice Sheet is found to be especially sensitive to orbital (precession) forcing, at an intermediate CO2 level. This is probably due to its high elevation where the freezing point could be easily maintained. The other ice sheets over northeast America and Eurasia, however, are absent in our simulations. We propose that the alpine-based Cordilleran Ice Sheet is more sensitive and easier to build up than other NHISs in response to the astronomical controlled summer insolation. Dynamic surges are simulated for the Cordilleran Ice Sheet under fixed low orbital forcing. These surges due to internal ice sheet-climate feedbacks could potentially be the mechanism for the millennial scale H-like events.

Description: Highlights: • Multi-centennial oscillation with 100–200 years periods is evident in proxy data and model simulations during the Holocene. • Multi-centennial oscillation is a global signal and is more significant in the Northern Hemisphere high latitudes. • None of the external forcings is found to be the sole driver of the multi-centennial variability. • It indicates the multi-centennial oscillation may be due to potential internal drivers and essential feedbacks. Abstract: Variability on centennial to multi-centennial timescales is mentioned as a feature in reconstructions of the Holocene climate. As more long transient model simulations with complex climate models become available and efforts have been made to compile large proxy databases, there is now a unique opportunity to study multi-centennial variability with greater detail and a large amount of data than earlier. This paper presents a spectral analysis of transient Holocene simulations from 9 models and 120 proxy records to find the common signals related to oscillation periods and geographic dependencies and discuss the implications for the potential driving mechanisms. Multi-centennial variability is significant in most proxy records, with the dominant oscillation periods around 120–130 years and an average of 240 years. Spectra of model-based global mean temperature (GMT) agree well with proxy evidence with significant multi-centennial variability in all simulations with the dominant oscillation periods around 120–150 years. It indicates a comparatively good agreement between model and proxy data. A lack of latitudinal dependencies in terms of oscillation period is found in both the model and proxy data. However, all model simulations have the highest spectral density distributed over the Northern hemisphere high latitudes, which could indicate a particular variability sensitivity or potential driving mechanisms in this region. Five models also have differentiated forcings simulations with various combinations of forcing agents. Significant multi-centennial variability with oscillation periods between 100 and 200 years is found in all forcing scenarios, including those with only orbital forcing. The different forcings induce some variability in the system. Yet, none appear to be the predominant driver based on the spectral analysis. Solar irradiance has long been hypothesized to be a primary driver of multi-centennial variability. However, all the simulations without this forcing have shown significant multi-centennial variability. The results then indicate that internal mechanisms operate on multi-centennial timescales, and the North Atlantic-Arctic is a region of interest for this aspect.

Description: Highlights: • Temperature anomalies for the Mid-Holocene compared to preindustrial are significantly different in the low- and high-resolution versions of the atmospheric model ECHAM5 • For summer, shortwave cloud radiative forcing emerges as an important factor. • For boreal winter, differences are mainly related to circulation changes. • Anomaly differences are regionally as large as the mid-Holocene minus preindustrial temperature signals. Abstract: This study evaluates the dependence of simulated surface air temperatures on model resolution and orography for the mid-Holocene. Sensitivity experiments with the atmospheric general circulation model ECHAM5 are performed with low (∼3.75°, 19 vertical levels) and high (∼1.1°, 31 vertical levels) resolution. Results are compared to the respective preindustrial runs. It is found that the large-scale temperature anomalies for the mid-Holocene (compared to preindustrial) are significantly different in the low- and high-resolution versions. For boreal winter, differences are mainly related to circulation changes caused by the response to thermal forcing in conjunction with orographic resolution. For summer, shortwave cloud radiative forcing emerges as an important factor. The anomaly differences (low minus high resolution version) in the Northern Hemisphere are regionally as large as the anomalous mid-Holocene temperature signals. Furthermore, they depend on the applied surface boundary conditions. We conclude that the resolution matters for the Northern Hemisphere response in mid-Holocene simulations, which should be taken into account in model-model and data-model comparisons.

Description: The global climate has been gradually cooling over the Cenozoic and is punctuated by the intensification of Northern Hemisphere Glaciation (NHG) from the latest Pliocene to earliest Pleistocene (∼3.1–2.5 millions of years ago, Ma). A decline of atmospheric CO2 is supposed as a prerequisite for the NHG, but the associated carbon-cycle processes remain elusive. Here we combine foraminiferal records of neodymium isotope and boron-calcium ratio, and simulations of an Earth system model, to investigate changes in the water-mass composition and carbonate-ion concentration of the deep Pacific Ocean during the NHG. Our proxy records have revealed a significant expansion of southern-sourced waters with increased respired carbon storage into the deep Pacific during the NHG. These changes may be explained by strengthened deep-water formation and biological-pump efficiency in the Southern Ocean due to Antarctic sea-ice growth, as suggested by our model experiments and evidence from the Sub-Antarctic region. These results provide key clues for quantifying the role of the dissolved inorganic carbon content of deep Pacific waters in modulating atmospheric CO2 during the NHG.

Description: The last deglaciation was characterized by a sequence of abrupt climate events thought to be linked to rapid changes in Atlantic meridional overturning circulation (AMOC). The sequence includes a weakening of the AMOC after the Last Glacial Maximum (LGM) during Heinrich Stadial 1 (HS1), which ends with an abrupt AMOC amplification at the transition to the Bølling/Allerød (B/A). This transition occurs despite persistent deglacial meltwater fluxes that counteract vigorous North Atlantic deep-water formation. Using the Earth system model COSMOS with a range of deglacial boundary conditions and reconstructed deglacial meltwater fluxes, we show that deglacial CO2 rise and ice sheet decline modulate the sensitivity of the AMOC to these fluxes. While declining ice sheets increase the sensitivity, increasing atmospheric CO2 levels tend to counteract this effect. Therefore, the occurrence of a weaker HS1 AMOC and an abrupt AMOC increase in the presence of meltwater, might be explained by these effects, as an alternative to or in combination with changes in the magnitude or routing of meltwater discharge.

Description: The increase in summer monsoon precipitation over western Africa during the last interglacial (LIG) relative to the pre-industrial (PI) is well documented, but it is uncertain whether this increase is due to larger rainfall rate alone, an extension of the summer monsoon season or a combination of the two. Due to different orbital config- uration, the boreal summer of the LIG was warmer but shorter than the PI, potentially influencing the summer monsoon duration. In this study, we employ a newly developed isotope-enabled climate model, AWI-ESM-wiso to investigate the intensity and length of the West African Summer Monsoon (WASM) for both LIG and PI time periods. Our model results indicate that, despite an intensification in summer insolation and an enhanced hydro-logical cycle, WASM season in the LIG is 9 days shorter compared to the PI. During the LIG, increased insolation in late spring and early summer strengthens the Saharan heat low (SHL) and its associated sub-systems, facilitating a faster accumulation of potential instability and an earlier WASM onset. However, a substantial earlier withdrawal of the WASM is also detected, driven by an earlier southward shift of insolation maximum. More- over, our findings are further supported by models participating in the 4th phase of the Paleoclimate Modelling Intercomparison Project (PMIP4).