Abstract
Abstract. Past attempts to reconstruct the Southern Annular Mode (SAM) using paleo-archives have resulted in records which can differ significantly from one another prior to the window over which the proxies are calibrated. This study attempts to quantify not only the skill with which we may expect to reconstruct the SAM but also to assess the contribution of regional bias in proxy selection and the impact of non-stationary proxy–SAM teleconnections on a resulting reconstruction. This is achieved using a pseudoproxy framework with output from the GFDL CM2.1 global climate model. Reconstructions derived from precipitation fields perform better, with 89 % of the reconstructions calibrated over a 61 year window able to reproduce at least 50 % of the inter-annual variance in the SAM, as opposed to just 25 % for surface air temperature (SAT)-derived reconstructions. Non-stationarity of proxy–SAM teleconnections, as defined here, plays a small role in reconstructions, but the range in reconstruction skill is not negligible. Reconstructions are most likely to be skilful when proxies are sourced from a geographically broad region with a network size of at least 70 proxies.
Highlights
The Southern Annular Mode (SAM), which describes the intensity and latitudinal location of the subtropical westerly jet, is the leading mode of atmospheric variability in the Southern Hemisphere
This study aims to quantify the uncertainties raised by the aforementioned assumptions within a modelling framework, similar to Batehup et al (2015), and seeks to address the following questions: (1) What impacts do the proxy network size and calibration window size have on the skill of a resulting reconstruction? (2) How does the geographical distribution of the proxies affect reconstruction skill? (3)
A true correlation of −0.3 between precipitation and the SAM may become anything ranging from −0.65 to 0.1 when evaluated over a shorter 31 year window (Fig. 2d)
Summary
The Southern Annular Mode (SAM), which describes the intensity and latitudinal location of the subtropical westerly jet, is the leading mode of atmospheric variability in the Southern Hemisphere. Over the last five decades, the SAM has shown a trend towards more positive values, consistent with a poleward intensification of the surface westerly winds, which has been largely attributed to anthropogenic forcing, such as stratospheric ozone depletion and the increase in atmospheric CO2 (Son et al, 2008; Lee and Feldstein, 2013; Previdi and Polvani, 2014). Both high-frequency (3–4 months) and low-frequency (16 years) variability, as derived from reanalysis experiments, has been observed in the SAM (Raphael and Holland, 2006).
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