Comparing the spatial patterns of climate change in the 9^th and 5^th millennia B.P. from TRACE-21 model simulations

Abstract

Abstract. The spatial patterns of global temperature and precipitation changes, as well as corresponding large-scale circulation patterns during the latter part of the 9th and 5th millennia B.P. (4800–4500 versus 4500–4000 years B.P. and 9200–8800 versus 8800–8000 years B.P.) are compared through a group of transient simulations using Community Climate System Model version 3 (CCSM3). Both periods are characterized by significant sea surface temperature decreases over the North Atlantic south of Iceland. Temperatures were also colder across the northern hemisphere, but warmer in the southern hemisphere. Significant precipitation decreases are seen over most of the northern hemisphere, especially over Eurasia and the Asian monsoon regions, indicating a weaker summer monsoon. Large precipitation anomalies over northern South America and adjacent ocean regions are related to a southward displacement of the Inter Tropical Convergence Zone (ITCZ). Climate changes in the late 9th millennium B.P. (The 8.2 ka B.P. event) are widely considered to have been caused by a large fresh water discharge into the northern Atlantic, which is confirmed in a meltwater forcing sensitivity experiment, but this was not the cause of changes occurring between the early and latter half of the 5th millennium B.P. We speculate that long-term changes in insolation related to precessional forcing led to cooling, which passed a threshold around 4500 years B.P., leading to a reduction in the Atlantic meridional overturning circulation (AMOC) and associated teleconnections across the globe. The onset of the Neoglacial occurred around this time, and the subsequent changes in glacierization have persisted, modulated by internal centennial-scale ocean-atmosphere variability. We suggest that the 4.2 ka B.P. event was one of several late Holocene multi-century fluctuations that were embedded in a longer-term, lower frequency change in climate, linked to orbital forcing.