South American Monsoon Variability Over the Last Millennium Captured by Oxygen Isotope Paleorecords and Isotope-enabled Models
Abstract
Austral summer rainfall associated with the South American Monsoon System (SAMS) plays a key role in South American hydroclimate. Paleoclimate records are a critical source of information for understanding the variability of the SAMS over the last millennium (LM; 850 1850 CE). Stable oxygen isotope records in tropical ice cores and carbonate from lakes and speleothems are particularly valuable proxies for SAMS variability due to their ability to archive physical processes key to the system, including those processes which are distal from the record location, such as the degree of rainout upstream (DRU) and atmospheric circulation variability. These proxy records nevertheless also contain information about local climate, which complicates the interpretation of individual records. We evaluate a network of 14 stable oxygen isotope records from across the SAMS domain to extract a common signal of SAMS variability over the LM, while separating the signal from the background noise of local hydroclimate variability. We apply a Monte Carlo Empirical Orthogonal Function (MCEOF) decomposition to extract the leading modes of variability characterizing the core features of the monsoon system. The physical underpinnings of these statistical modes are established through comparison with dynamic analogs in the present-day SAMS. The first three modes of variability correspond to 1) an upper-tropospheric Rossby wave response to condensational heating over the Amazon basin during the mature phase of the monsoon, exciting or relaxing the Bolivian High-Nordeste Low system, 2) changes in intensity and location of the monsoon convective axis (South Atlantic Convergence Zone, SACZ), and 3) SAMS variability modulated by the El Niño-Southern Oscillation. In parallel, an identical analysis is performed based on two isotope-enabled climate models that replicate the basic structure of the leading modes found in the proxy-based decomposition. Though the models do not accurately capture the low-frequency component of the paleorecord, these results highlight the inherent stability of these spatial modes of SAMS variability across the LM and help to establish a baseline envelope of variability from the paleorecords against which future changes can be compared.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMGC51B..04O