Description: The influences of North Atlantic biases on multiyear predictability of unforced surface air temperature (SAT) variability are examined in the Kiel Climate Model (KCM). By employing a freshwater flux correction over the North Atlantic to the model, which strongly alleviates both North Atlantic sea surface salinity (SSS) and sea surface temperature (SST) biases, the freshwater flux-corrected integration depicts significantly enhanced multiyear SAT predictability in the North Atlantic sector in comparison to the uncorrected one. The enhanced SAT predictability in the corrected integration is due to a stronger and more variable Atlantic Meridional Overturning Circulation (AMOC) and its enhanced influence on North Atlantic SST. Results obtained from preindustrial control integrations of models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) support the findings obtained from the KCM: models with large North Atlantic biases tend to have a weak AMOC influence on SAT and exhibit a smaller SAT predictability over the North Atlantic sector.

Description: Highlights: • We compare proxy moisture records in Northeast Asia with the results from a transient simulation. • An east–west antiphasing of summer precipitation in Northeast Asia during the Holocene is found. • The East Asian summer monsoon circulation and mid-latitude westerlies caused the zonal precipitation contrast. Abstract: The East Asian summer monsoon (EASM) is a complex system that brings precipitation to East Asia showing considerable spatiotemporal variations. This study explored the zonal differences of summer precipitation in Northeast Asia at orbital timescales during the Holocene by comparing proxy records with simulation results. At orbital timescales, there was generally an east–west antiphasing of summer precipitation in Northeast Asia during the Holocene. Model–proxy comparison revealed that the driest interval occurred during the late Holocene in western Northeast Asia and during the early to middle Holocene in eastern Northeast Asia. Changes of summer precipitation in western Northeast Asia were mainly influenced by precession-driven EASM circulation. On the one hand, a weaker EASM circulation during the late Holocene weakened water vapor transport from the North Pacific Ocean to Northeast Asia, and on the other hand it was associated with anomalous downward motions in western Northeast Asia. Both factors were in favor of a reduction of summer precipitation in western Northeast Asia during the late Holocene. In contrast, anomalous downward motions prevailed in eastern Northeast Asia during the early to middle Holocene, which were probably related to stronger western Pacific subtropical high and weaker westerlies. The effect of the anomalous downward motions overwhelmed the enhanced water vapor transport, leading to a dry climate in this area from the early to middle Holocene. This study suggests that special care should be taken when discussing the meridional shift of the Holocene climatic optimum in the EASM region due to the zonal precipitation contrast.

Description: Identifying the relationships between moisture changes in arid central Asia and those in East Asia may help us understand the interplay between the westerlies and the Asian summer monsoon. We combined proxy moisture records with the results from a transient simulation forced by changes in orbital parameters to analyse their relationships during the Holocene (9.5–0 ka BP). The proxy records and simulation results generally agree with a relatively dry early Holocene, the wettest period in the middle Holocene, and a dry late Holocene in East Asia. These periods were not solely controlled by precession-driven East Asian summer monsoon variability, but were significantly influenced by precipitation during the other seasons and by evaporation. However, different proxy records show contrasting results for moisture changes in arid central Asia during the Holocene. To study this, we analysed the climatic signals of the competing proxy records by comparing these proxy records with simulation results. We found that speleothem δ18O was significantly influenced by water vapour sources and evaporation rather than by the amount of precipitation. Thus, the model data reveals a persistent wetting trend throughout the Holocene that was out-of-phase with the trend in East Asia. The wetting trend in arid central Asia was caused by precipitation that increased faster than evaporation during the Holocene. The enhanced water vapour input from South Asia and the Middle East was the main cause of the increase in precipitation in arid central Asia, which in turn gave rise to the intensification of evaporation.

Description: Internal multi-centennial variability of open ocean deep convection in the Atlantic sector of the Southern Ocean impacts the strength of the Atlantic Meridional Overturning Circulation (AMOC) in the Kiel Climate Model. The northward extent of Antarctic Bottom Water (AABW) strongly depends on the state of Weddell Sea deep convection. The retreat of AABW results in an enhanced meridional density gradient that drives an increase in the strength and vertical extent of the North Atlantic Deep Water (NADW) cell. This shows, for instance, as a peak in AMOC strength at 30°N about a century after Weddell Sea deep convection has ceased. The stronger southward flow of NADW is compensated by an expansion of the North Atlantic subpolar gyre and an acceleration of the North Atlantic Current, indicating greater deep water formation. Contractions of the North Atlantic subpolar gyre enable warm water anomalies, which evolved in response to deep convection events in the Southern Ocean, to penetrate farther to the north, eventually weakening the AMOC and closing a quasi-centennial cycle. Gyre contractions are accompanied by increases in sea level of up to 20 cm/century in some areas of the North Atlantic. In the Southern Ocean itself, the heat loss during the convective regime results in a sea surface height decrease on the order of 10 cm/century, with a maximum of 30 cm/century in the Weddell Sea. Hence, the impact of the Southern Ocean Centennial Variability (SOCV) on regional as well as North Atlantic sea level is of the same order of magnitude as the rise of global average sea level during the 20th century, which amounts to about 15–20 cm. This suggests that internal variability on a centennial time scale cannot be neglected a priori in assessments of 20th and 21st century AMOC and regional sea level change.