Ice-age thermal response and climatic role of the surface Atlantic Ocean, 40°N to 63°N

Abstract

The three orbital periods—eccentricity, obliquity, and precession—dominate the sea surface temperature (SST) response of the North Atlantic from 40°N to 55°N. The 100,000-yr eccentricity rhythm is strong everywhere and dominant in most records. The 23,000-yr precession rhythm is very strong near 40°N, with power diminishing northward to negligible levels at 54°N. The 41,000-yr obliquity rhythm is strongest at 50°N to 55°N and diminishes southward to negligible values at 40°N. Within their regions of optimal development, each periodicity is at least as powerful as any observed in long-term oceanic records. The phasing of SST against the same frequency component in δ18O (∼ice volume) records and against the relevant orbital insolation rhythm constrains the possible origin of each oceanic response. Given that the 41,000-yr and 100,000-yr SST rhythms at latitudes 50°N to 55°N are synchronous with ice-volume responses to insolation forcing at the same periodicity, we infer that the size of high-latitude ice sheets regulates the SST response through the agency of regional atmospheric temperatures. At the 23,000-yr rhythm, however, the oceanic response lags so far behind the ice signal that two other explanations are suggested: (1) variable influxes of icebergs and meltwater from rapidly fluctuating mid-latitude ice sheets; and (2) variable advection of heat from low latitudes and from the southern hemisphere. Viewed in terms of its impact on ice-sheet mass balance, the high-latitude North Atlantic at the 41,000-yr and 100,000-yr periods provides positive feedback through the heat fluxes and negative feedback through the moisture flux. The heat flux is probably a small component of the total hemispheric heat transfer, but even small changes of sensible heat could have a significant impact on the mass balance of marine ice sheets at high latitudes. The energetic circulation in the subtropical gyre of the North Atlantic provides negative feedback of heat, but a potentially very powerful moisture feedback to the mid-latitude ice sheets at the 23,000-yr rhythm.