Late Holocene changes in precipitation in northwest Tasmania and their potential links to shifts in the Southern Hemisphere westerly winds

Abstract

Accurate projections of future climate changes in regions susceptible to drought depend on a good understanding of past climate changes and the processes driving them. In the absence of longer term instrumental data, paleoclimate data are needed. In this study we develop a precipitation reconstruction for Rebecca Lagoon (41°11′S, 144°41′E), northwest Tasmania. First, the relationship between scanning reflectance spectroscopy measurements of sediment cores in the visible spectrum (380–730nm) and instrumental precipitation record (1912–2009) was used to develop a model to reconstruct precipitation back in time. Results showed that the ratio of reflectance between 660 and 670nm (i.e., reflectance at 660nm/reflectance at 670nm; a measure of pigment diagenesis) was significantly related to annual precipitation. A calibration model was developed (R=−0.56, pauto<0.001, RMSEP=43.0mm yr−1, 5year triangular filtered data, calibration period 1912–2009). Second, this calibration-in-time model was used to reconstruct late Holocene precipitation changes over the last ~3000years. This showed relatively dry conditions from ca. 3100–2800cal yr BP, wet conditions from ca. 2800–2400cal yr BP, dry conditions from ca. 2400–2000calyr BP, and variable conditions after this. Relatively wet conditions occurred from ca. 500cal yr BP to the late AD 1800s (ca. 50cal. yr BP). The precipitation reconstruction indicates that conditions were relatively dry for the 20th century compared to the last ~3000years. In particular, the dry period measured in recent decades is one of the most intense in at least the last 500years. As precipitation in this region is primarily driven by the Southern Hemisphere westerly winds, these changes are discussed in terms of shifts in westerly wind strength and/or position.