ALBERT

Description: This paper studies the control of the Atlantic Warm Pool (AWP) on atmospheric moisture transport across the Central American isthmus as a potential feedback on rapid glacial climate fluctuations. Defined as a region of the Atlantic with surface temperatures above 28.5 °C, the modern AWP expands from the tropical Northwest Atlantic up to the Gulf of Mexico during boreal summer. Due to enhanced deep convection over these warm waters, changes in AWP area cause inverse changes in the strength of the Caribbean low level jet. This low level jet drives atmospheric moisture transport from the Atlantic across the Central American isthmus towards the Pacific. Changes in cross-isthmus moisture transport, potentially related to the AWP, may therefore have affected North Atlantic salinity and the partly density driven Atlantic Meridional Overturning Circulation (AMOC) during the glacial. Based on available proxy evidence we hypothesize that the AWP evolved independent of extratropical North Atlantic temperatures during most of the last glacial, except during periods of AMOC collapse when intense extratropical North Atlantic cooling may have limited eastward AWP expansion. We investigate the implications of this hypothesis for cross-isthmus moisture transport by simulating the coupled ocean-atmosphere response to AMOC collapse and the atmospheric sensitivity to additional variations in AWP area. Our simulations suggest that a decrease in AWP area may increase cross-isthmus moisture transport, whereas extratropical North Atlantic cooling beside a persistent AWP may decrease cross-isthmus moisture transport. Interpretation of these effects throughout an idealized Bond Cycle suggests a positive feedback of reduced cross-isthmus moisture transport in response to Greenland cooling prior to AMOC collapse. During AMOC collapse, when AWP expansion is proposed to have been inhibited, this positive feedback turns negative as enhanced cross-isthmus moisture transport may help AMOC recovery. Supported by reconstructed sea surface salinity changes, we propose that the AWP may have played a key role in the glacial climate by acting as a gatekeeper to regulate moisture transport across the Central American isthmus.

Description: We present a reconstruction of the change in climatic humidity around the Mediterranean between 3000–1000 yr BP. Using a range of proxy archives and model simulations we demonstrate that climate during this period was typified by a millennial-scale seesaw in climatic humidity between Spain and Israel on one side and the Central Mediterranean and Turkey on the other, similar to precipitation anomalies associated with the East Atlantic/West Russia pattern in current climate. We find that changes in the position and intensity of the jet stream indicated by our analysis correlate with millennial changes in North Atlantic sea surface temperature. A model simulation indicates the proxies of climatic humidity used in our analysis were unlikely to be influenced by climatic aridification caused by deforestation during the Roman Period. That finding is supported by an analysis of the distribution of archaeological sites in the Eastern Mediterranean which exhibits no evidence that human habitation distribution changed since ancient times as a result of climatic aridification. Therefore we conclude that changes in climatic humidity over the Mediterranean during the Roman Period were primarily caused by a modification of the jet stream linked to sea surface temperature change in the North Atlantic. Based on our findings, we propose that ocean-atmosphere coupling may have contributed to regulating Atlantic Meridional Overturning Circulation intensity during the period of analysis.

Description: Previous studies have proposed that potential vegetation in the Mediterranean maintained a wetter climate during the Roman Period until the initiation of large scale deforestation. The reduction in evapotranspirative fluxes associated with deforestation is suggested to have caused climatic aridification leading to the establishment of the present-day Mediterranean climate. There is also evidence to indicate that during the Roman Period Mediterranean climate was influenced by low frequency fluctuations in sea level pressure over the North Atlantic, termed here: the Centennial North Atlantic Oscillation (CNAO). In order to understand the importance of each of these mechanisms and disentangle their respective signals in the proxy record, we have employed an interdisciplinary approach that exploits a range of tools and data sources. An analysis of archaeological site distribution and historical texts demonstrate that climate did not increase in aridity since the Roman Period. Using an Earth system model of intermediate complexity prescribed with a reconstruction of ancient deforestation, we find that Mediterranean climate was insensitive to deforestation in the Late Holocene. A novel analysis of a composite of proxy indicators of climatic humidity depicts spatial and temporal patterns consistent with the CNAO. The link between the CNAO during the Roman Period and climatic humidity signals manifest in our composite analysis are demonstrated using a modelling approach. Finally, we present evidence indicating that fluctuations in the CNAO contributed to triggering a societal tipping point in the Eastern Mediterranean at the end of the Roman Period.

Description: Terrestrial vegetation influences climate by modifying the radiative-, momentum-, and hydrologic-balance. This paper contributes to the ongoing debate on the question whether positive biogeophysical feedbacks between vegetation and climate may lead to multiple equilibria in vegetation and climate and consequent abrupt regime shifts. Several modelling studies argue that vegetation-climate feedbacks at local to regional scales could be strong enough to establish multiple states in the climate system. An Earth Model of Intermediate Complexity, PlaSim, is used to investigate the resilience of the climate system to vegetation disturbance at regional to global scales. We hypothesize that by starting with two extreme initialisations of biomass, positive vegetation-climate feedbacks will keep the vegetation-atmosphere system within different attraction domains. Indeed, model integrations starting from different initial biomass distributions diverged to clearly distinct climate-vegetation states in terms of abiotic (precipitation and temperature) and biotic (biomass) variables. Moreover, we found that between these states there are several other steady states which depend on the scale of perturbation. From here global susceptibility maps were made showing regions of low and high resilience. The model results suggest that mainly the boreal and monsoon regions have low resiliences, i.e. instable biomass equilibria, with positive vegetation-climate feedbacks in which the biomass induced by a perturbation is further enforced. The perturbation did not only influence single vegetation-climate cell interactions but also caused changes in spatial patterns of atmospheric circulation due to neighbouring cells constituting in spatial vegetation-climate feedbacks. Large perturbations could trigger an abrupt shift of the system towards another steady state. Although the model setup used in our simulation is rather simple, our results stress that the coupling of feedbacks at multiple scales in vegetation-climate models is essential and urgent to understand the system dynamics for improved projections of ecosystem responses to anthropogenic changes in climate forcing.

Description: Terrestrial vegetation influences climate by modifying the radiative-, momentum-, and hydrologic-balance. This paper contributes to the ongoing debate on the question whether positive biogeophysical feedbacks between vegetation and climate may lead to multiple equilibria in vegetation and climate and consequent abrupt regime shifts. Several modelling studies argue that vegetation-climate feedbacks at local to regional scales could be strong enough to establish multiple states in the climate system. An Earth Model of Intermediate Complexity, PlaSim, is used to investigate the resilience of the climate system to vegetation disturbance at regional to global scales. We hypothesize that by starting with two extreme initialisations of biomass, positive vegetation-climate feedbacks will keep the vegetation-atmosphere system within different attraction domains. Indeed, model integrations starting from different initial biomass distributions diverged to clearly distinct climate-vegetation states in terms of abiotic (precipitation and temperature) and biotic (biomass) variables. Moreover, we found that between these states there are several other steady states which depend on the scale of perturbation. From here global susceptibility maps were made showing regions of low and high resilience. The model results suggest that mainly the boreal and monsoon regions have low resiliences, i.e. instable biomass equilibria, with positive vegetation-climate feedbacks in which the biomass induced by a perturbation is further enforced. The perturbation did not only influence single vegetation-climate cell interactions but also caused changes in spatial patterns of atmospheric circulation due to neighbouring cells constituting in spatial vegetation-climate feedbacks. Large perturbations could trigger an abrupt shift of the system towards another steady state. Although the model setup used in our simulation is rather simple, our results stress that the coupling of feedbacks at multiple scales in vegetation-climate models is essential and urgent to understand the system dynamics for improved projections of ecosystem responses to anthropogenic changes in climate forcing.