Summary
Knowledge of environmental variability and change during the Medieval Climate Anomaly (MCA ; ~800 to 1300 - 1400 AD) in the Southern Hemisphere has been characterized by a La Nina-like mean climate state in the Pacific Basin, largely deduced from proxy records located in the equatorial and tropical Pacific Ocean. In addition, the MCA climate has been previously discussed in terms of air/sea surface temperature, and precipitation variability. Little attention has been placed on the atmosphere/ocean circulation anomalies that forced the interpreted temperature and precipitation patterns seen across the Southwest Pacific region. In this paper, we reconstruct the atmospheric circulation anomalies in the Australasian and Pacific sectors of the Southern Hemisphere. These past circulation patterns are derived from a synoptic climate typology related to regional MCA climate anomalies, including: Eastern Australian coastal behaviour and hindcast wave climate; subtropical sea-surface temperature anomalies from corals; New Zealand precipitation from tree ring, speleothem and glaciosedimentary records; South Polynesian voyaging and migration routes; Southern Australian mega-lake hydrology; extratropical sea-level pressure anomalies derived from East and West Antarctic ice core glaciochemistry; and Antarctic lake hydrological balance and katabatic wind records. The proxies define three intervals of different atmospheric circulation anomalies: (i)~600 to 1050 AD, (ii) ~1050 to 1300 AD, and (iii) ~1300 to 1550 AD. The broadscale anomalies define a low-frequency progression in the phases of the dominant extratropical climate modes like the Pacific-South American (PSA) and the Southern Annular Mode, which are significant influences on ENSO teleconnected areas, and generate regionally distinct atmospheric pressure anomalies across the midlatitudes. Our hypothesis is that the circulation regime progression seen in the past is similar to that recorded in the instrumental record over the past 50 years, with the change in mean climate state at 1976/1977 AD being an analogue to the type of mean state change at ~1300 AD.