Comparison of a 1500 Year-Long ENSO-Sensitive Ice Core Proxy Record to Modeled Solar Forcing Mechanisms

A calibrated 1500 year-long glaciochemical proxy record for the strength of the wintertime (November-March) Aleutian Low (ALOW) from the Mt. Logan summit (PR Col; 5300 m a.s.l.) ice core [Na+] time series reveals a strong ALOW from ca. 650-900 A.D., ca. 1300-1550 A.D., and ca. 1700-1998 A.D., and a weaker ALOW from ca. 900-1300 A.D. and ca. 1550-1700 A.D. The proxy record was calibrated to instrumental sea-level pressure data using standard regression techniques, and verified using statistical, spectral, and spatial correlation analyses. This record is consistent with ENSO-sensitive paleoclimate proxies from the Pacific basin that indicate El Niño-like conditions during the Little Ice Age (LIA) and La Niña- like conditions during the so-called Medieval Climate Anomaly (MCA). The Mt. Logan ALOW proxy record is significantly correlated with atmospheric Δ14C records (r = 0.39, p<0.05), supporting an influence of solar forcing on late Holocene North Pacific climate. The increased frequency of El Niño conditions during the LIA relative to the MCA is consistent with the modeled "ocean thermostat" mechanism whereby weak solar irradiance during the LIA promotes El Niño-like conditions, and strong MCA solar irradiance promotes La Niña-like conditions by modifying the east-west SST gradient in the tropical Pacific. We will also discuss the consistency of these paleoproxy records with models invoking solar-induced changes in the north-south temperature gradient at the tropopause. In these model results, weaker solar irradiance causes cooling and weaker westerly winds in the stratosphere, inducing a tropospheric response that includes expansion of the polar cell, contraction of the Hadley cell, and enhanced meridional circulation.