Ocean-climate variability and sea level in the North Atlantic region since AD 0

Abstract

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We used reconstructed and model-simulated sea-level variations during the past 1000-3000 years to investigate their cause(s) and assess their suitability as proxy for ocean-related climate variations in the North Atlantic region. We studied salt-marsh deposits in North Carolina, Connecticut, Newfoundland, and Wales (UK). Sea-level reconstructions obtained so far (for Connecticut) show that mean high water varied on centennial and sub-centennial timescales on the order of cms-dms during the past 3000 years. The mean high water fluctuations correlate more or less positively with proxy records of temperature change, but this broad correlation does not hold for the entire record. The most significant correlation found is a consistent phase shift of ca. 125 years between changes in solar activity (as expressed in variations in residual delta 14C) and sea level, suggesting a linkage by one or more mechanisms with a corresponding response time. The fact that sea level continued to fluctuate during periods of low solar variability indicates, however, that other factors are also involved.Sea-level change due to changes in the thermohaline structure of the North Atlantic ocean has been calculated using a coupled ocean-atmosphere model of intermediate complexity (ECBilt). Two 1000-yr simulations are made, one using a constant solar forcing and one using an estimate of historic solar activity. In the solar forced simulation, sea-level variations are a proxy for climate variations. Anomalies in sea-surface temperature (SST) of the northern North Atlantic are generated by the solar radiation changes. These SST anomalies modulate the ocean thermohaline circulation, affecting surface salinities in the northern North Atlantic which are subsequently advected to the deep ocean. The associated deep ocean geopotential thickness anomalies dominate sea level in the northern North Atlantic. Comparison of the simulated steric sea-level variations in the Gulf Stream area for the past 1000 yr with the corresponding part of the mean high water reconstruction for Connecticut shows a satisfactory agreement, indicating that deep ocean salinity anomalies in the northern North Atlantic, induced by changes in irradiance, largely explains the timing of reconstructed mean high water variations. This result has relevance for predictions of sea-level change.