Inter-decadal climate oscillations in the Tasmanian sector of the Southern Hemisphere: Evidence from tree rings over the past three millennia

Abstract

The characterization of inter-decadal climate variability in the Southern Hemisphere is severely constrained by the shortness of the instrumental climate records. To help relieve this constraint, we have developed and analyzed a reconstruction of warm-season (November–April) temperatures from Tasmanian tree rings that now extends back to 800 B.C. A detailed analysis of this reconstruction in the time and frequency domains indicates that much of the inter-decadal variability is principally confined to four frequency bands with mean periods of 31, 57, 77, and 200 years. These oscillations are stable and robust with respect to the progressive development and extension of the tree-ring reconstruction back in time. They account for approximately 12% of the overall variance in the annual temperature estimates and 41% of the inter-decadal variance (i.e. periods >10 years). Using singular spectrum analysis, we estimate the overall inter-decadal temperature signal contributed by these four oscillations and compare its recent behavior to an anomalous warming that has been in progress over Tasmania since about 1965. We find that 51% of the mean anomalous warming can be accounted for by these oscillations. Prediction error filtering is also used to forecast the inter-decadal temperature signal 30 years into the future. In so doing, we show how a future greenhouse warming signal over Tasmania could be masked by these natural oscillations unless they are taken into account.