The drivers of multidecadal climate variability in the extratropics over the Late Holocene

This thesis investigates the magnitude and drivers of multidecadal variability in Southern Hemisphere extratropical climate over the past 1200‐years with a specific focus on the implications for Tasman Sea storm activity. Contemporary approaches to multiproxy climate reconstructions are unsuitable for resolving coupled ocean-atmosphere variability; therefore I developed a novel multivariate data assimilation (MDA) approach. MDA combines the temporal realism of paleoclimate data with the dynamical consistency of an atmosphere‐ocean global climate model (AOGCM) to produce a multivariate spatial field reconstruction for the Southern Hemisphere at decadal to multidecadal resolution. The reconstruction is analogous to modern reanalysis datasets and is referred to as the Paleoclimate Reanalysis (PaleoR). PaleoR derived spatial fields and climate mode reconstructions show the 13th century transition from Medieval Climate Anomaly (MCA 900-1250) into Little Ice Age (LIA 1400-1900) was characterized by a large-scale, persistent reorganisation of ocean-atmosphere circulation patterns. During the MCA the tropics expanded poleward and the Southern Annular Mode (SAM) was strongly positive. During the early LIA (1400-1600) the tropics contracted equatorward and SAM was strongly negative. During the late LIA (1600-1900) SAM became more neutral to negative and the tropics expanded in conjunction with a persistent La Nina-like mean state, producing ideal conditions for frequent and intense subtropical maritime storms in the Tasman Sea. Commonly referred to as Australian East Coast Cyclones (ECC), this family of storms is most frequent and intense during the late autumn and winter season, they can develop via tropical transition, extratropical transition or in situ cyclogenesis. Seasonal ECC frequency over the 1955‐2012 period is highly correlated to regional ocean-atmosphere circulation patterns. Correlation patterns are used to develop a statistical downscaling approach and estimate autumn‐winter ECC frequency from seasonal reanalysis data over the 1871 to 2012 period, showing the 1950s and 1970s were likely the stormiest decades of the 20th century. Applying the downscaling approach to PaleoR reveals the late LIA (1600‐1900) was likely the stormiest period of the past 1200-years, and most decades of the 19th century were at least as stormy as the 1970s. These ‘ultimate storm centuries’ in the Tasman Sea occurred in conjunction with persistently warm west Pacific sea surface temperatures and negative to neutral SAM. Multidecadal persistence and abrupt shifts in mean climate state are characteristic features of Southern Hemisphere extratropical climate; this variability strongly influences the type and frequency of individual weather events.