years before present

Abstract: A modern summer-climate analogue of western United States 6 ka palaeoclimate, based on broad-scale patterns from proxy data and general-circulation-model simulations, provides details concerning its sum-mer synoptic climatology. August 1955 serves as a close analogue, featuring an amplified subtropical ridge aloft and anticyclonic flow near the surface over the central United States and stronger westerly flow along the Canadian/northwest United States border. The August 1955 summer-temperature pattern, similar to those from general-circulation-model simulations, exhibits positive anomalies throughout most of the continental interior. The August 1955 precipitation pattern corresponds very closely with patterns of effective moisture from proxy data, showing widespread aridity and some stronger monsoonal activity in the Southwest. The analogue suggests that the amplified subtropical ridge is a very important feature of 6 ka palaeoclimate, with the circulation pattern differing from that in the early Holocene, implying interactions between both Milankov-itch and sub-Milankovitch forcing factors. Key words: Western United States, modern analogue, summer palaeoclimate, subtropical ridge, proxy data, 500 mb heights, synoptic climatology, monsoon, August 1955