Be-10 in late deglacial climate simulated by ECHAM5-HAM - Part 1: Climatological influences on Be-10 deposition

Abstract

Reconstruction of solar irradiance has only been possible for the Holocene so far. During the last deglaciation, two solar proxies (Be-10 and C-14) deviate strongly, both of them being influenced by climatic changes in a different way. This work addresses the climate influence on Be-10 deposition by means of ECHAM5-HAM atmospheric aerosol-climate model simulations, forced by sea surface temperatures and sea ice extent created by the CSIRO Mk3L coupled climate system model. Three time slice simulations were performed during the last deglaciation: 10 000 BP ("10k"), 11 000 BP ("11k") and 12 000 BP ("12k"), each 30 yr long. The same, theoretical, Be-10 production rate was used in each simulation to isolate the impact of climate on Be-10 deposition. The changes are found to follow roughly the reduction in the greenhouse gas concentrations within the simulations. The 10k and 11k simulations produce a surface cooling which is symmetrically amplified in the 12k simulation. The precipitation rate is only slightly reduced at high latitudes, but there is a northward shift in the polar jet in the Northern Hemisphere, and the stratospheric westerly winds are significantly weakened. These changes occur where the sea ice change is largest in the deglaciation simulations. This leads to a longer residence time of Be-10 in the stratosphere by 30 (10k and 11k) to 80 (12k) days, increasing the atmospheric concentrations (25-30% in 10k and 11k and 100% in 12k). Furthermore the shift of westerlies in the troposphere leads to an increase of tropospheric Be-10 concentrations, especially at high latitudes. The contribution of dry deposition generally increases, but decreases where sea ice changes are largest. In total, the Be-10 deposition rate changes by no more than 20% at mid-to high latitudes, but by up to 50% in the tropics. We conclude that on "long" time scales (a year to a few years), climatic influences on Be-10 deposition remain small (less than 50 %) even though atmospheric concentrations can vary significantly. Averaged over a longer period, all Be-10 produced has to be deposited by mass conservation. This dominates over any climatic influences on Be-10 deposition. Snow concentrations, however, do not follow mass conservation and can potentially be impacted more by climate due to precipitation changes. Quantifying the impact of deglacial climate modulation on Be-10 in terms of preserving the solar signal locally is analysed in an accompanying paper (Heikkila et al., Be-10 in late deglacial climate simulated by ECHAM5-HAM - Part 2: Isolating the solar signal from Be-10 deposition).