Climate dynamics during the Medieval Climate Anomaly

Two millennial-length hydroclimatic proxy records from close to the centers of action of the winter North Atlantic Oscillation (NAO) indicate that the Medieval Climate Anomaly (MCA) was characterized by a pervasive positive phase of the NAO. Supplementary Proxy Surrogate reconstructions based on climate model results and proxy data indicate a clear shift to weaker NAO conditions into the Little Ice Age (LIA). Multidecadal NAO variability results in synoptic-scale shifts in surface pressure, wind fields, and precipitation. Globally distributed terrestrial and marine proxies suggest that this NAO shift is one aspect of a global MCA-LIA climate transition that is hypothesized to be coupled to prevailing La Nina-like conditions amplified by an intensified Atlantic meridional overturning circulation (AMOC) during the MCA. The majority of high-resolution palaeoceanographic data that span the last millennium are consistent with the hypothesis that the MCA (LIA) was characterized by more (less) intense AMOC. There are multiple datasets, however, including the Na ion ice core proxy from the Greenland Ice Sheet, that indicate enhanced storminess and cyclone frequency across the North Atlantic, and thus a positive rather than a negative NAO phase, during the LIA. A possible explanation for this discrepancy in proxy records has been provided by ensemble simulations of the cyclone-resolving Climate Community System Model (CCSM) coupled ocean-atmosphere general circulation model for the Maunder Minimum during the LIA. These simulations indicate major mid-latitude blocking anticyclones and reduced cyclone frequency constructions for the LIA, consistent with NAO negative phase. Simultaneously, however, the simulations suggest an increased intensity of cyclones when anticyclones break down during the LIA. I will here discuss the multi-proxy reconstruction of NAO variability over the last millennium, how it fits in a global climate dynamical model for the MCA, and how it can be reconciled with contradicting proxy records.