Spatial Reconstructions of Asian Monsoon Climate Variability Over the Past Millennium from Long Tree-Ring Records

Details Spatial Reconstructions of Asian Monsoon Climate Variability Over the Past Millennium from Long Tree-Ring Records Spatial Reconstructions of Asian Monsoon Climate Variability Over the Past Millennium from Long Tree-Ring Records

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmpp24a..03c&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #PP24A-03

Physics

0473 Paleoclimatology And Paleoceanography (3344, 4900), 1637 Regional Climate Change, 1812 Drought, 1833 Hydroclimatology, 4920 Dendrochronology

Scientific paper

We present the first spatial reconstructions of Asian monsoon climate variability over the past millennium from long tree-ring records. The reconstructions, for both the monsoon (JJA) and pre-monsoon (MAM) seasons, are based on a 534-point grid of instrumental Palmer Drought Severity Indices (PDSI) covering all of monsoon Asia and an irregular network of 312 annual tree-ring chronologies over most of the same domain. The seasonal reconstructions were initially estimated at each grid point using a local "point-by-point regression" (PPR) method that has been used successfully in reconstructing drought over North America. Different levels of predictor variable screening applied in PPR produced a 5-member ensemble of reconstructions for each season. The estimated noise level in these reconstructions was relatively high (average cross-validation R2 over the 534 grid point domain typically <0.30). In addition, the lengths of the grid point reconstructions varied over space due to the variable length tree-ring series available for use in PPR. For these reasons, each ensemble member was iteratively refined using a local variant of PPR to improve its reconstructions, with missing values imputed as necessary, to produce complete fields extending back to AD 1000 over all 534 grid point locations. An ensemble average for each season, with estimated uncertainties, was then calculated and used for analysis. The reconstructions reveal the occurrence of some persistent "megadroughts" in the past that appear to be unprecedented in the instrumental records. These megadroughts are not restricted to any particular part of "Monsoon Asia", but the ones in Southeast Asia stand out particularly strong. Comparisons made between these drought reconstructions and a companion field of SST reconstructions for the tropical Pacific back to AD 1400, based on independent tree-ring data from the American Southwest and Mexico, suggest that unusual ENSO variability is a contributor to the development of past severe droughts in monsoon Asia. An association between monsoon drought variability and explosive volcanism is also indicated. Implications for future work, especially related to expanding and improving the tree-ring network for reconstruction in certain regions, are also discussed.

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