Comment on cp-2022-6

doi:10.5194/cp-2022-6-rc2

Abstract

The South American Monsoon System (SAMS) is the main driver of regional hydroclimate variability across tropical and subtropical South America. It is best recorded on paleoclimatic timescales by stable oxygen isotope proxies, which are more spatially representative of regional hydroclimate than proxies for local precipitation alone. Network studies of proxies that can isolate regional influences lend particular insight into various environmental characteristics that modulate hydroclimate, such as atmospheric circulation variability and changes in the regional energy budget as well as understanding the climate system sensitivity to external forcings. To extract the coherent modes of variability of the SAMS over the Last Millennium (LM), we use a Monte Carlo Empirical Orthogonal Function (MCEOF) decomposition of 14 δ18O proxy records and compare them with modes extracted from a similar decomposition using isotope-enabled climate models. The two leading modes reflect the isotopic expression of the upper-tropospheric monsoon circulation (Bolivian High – Nordeste Low waveguide) and the latitudinal displacement of the South Atlantic Convergence Zone (SACZ), respectively. The spatial characteristics of these modes appear to be robust features of the LM hydroclimate over South America and are reproduced both in the proxy data and in isotope-enabled climate models, regardless of the nature of the model-imposed external forcing. Model analyses suggests that the local isotopic composition is primarily a reflection of an upstream rainout processes. The proxy data document that the SAMS was characterized by considerable temporal variability throughout the LM, with significant departures from the mean state during both the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The monsoon was intensified during the LIA over the central and western parts of tropical South America and the South Atlantic Convergence Zone (SACZ) was displaced to the southwest. These centennial-scale changes in monsoon intensity over the LM are underestimated in climate models, complicating the attribution of changes on these timescales to specific forcings and pointing toward areas of important model development.

Highlights

The annual hydrologic cycle in northern and central South America is dominated by distinct wet and dry seasons 35 following the development and demise of the South American Monsoon System (SAMS) (Vera et al, 2006)
We present an analysis of the SAMS using a proxy network derived solely from oxygen isotope records covering the Last Millennium (LM), all located within the SAMS domain
EOF1 represents the isotopic response to the Bolivian High-Nordeste Low (BH-NL) system: an upper tropospheric Rossby wave train induced by diabatic heating from convection over the core monsoon region in the western 350 Amazon (Lenters and Cook, 1997; Chen et al, 1999) (Fig. 4)

Summary

Introduction

The annual hydrologic cycle in northern and central South America is dominated by distinct wet and dry seasons 35 following the development and demise of the South American Monsoon System (SAMS) (Vera et al, 2006). The mature phase of the monsoon occurs during the austral summer in December, January, and February (DJF) when the region receives the majority of its annual precipitation (Zhou and Lau, 1998; Garreaud et al, 2009). Historical hydroclimate 40 observations do exist from this region over the 20th century, such records are not long enough to characterize multidecadal to centennial changes, which are critical for interpreting and constraining projections of future change. The onset of the SAMS is initiated by the increase in austral summer insolation over the southern hemisphere driving the seasonal southward progression of the Intertropical Convergence Zone (ITCZ) and strengthening the onshore 45 flow of moisture from the tropical Atlantic toward the South American continent (Zhou and Lau, 1998). The SALLJ transports moisture into the subtropics and feeds the South Atlantic Convergence Zone (SACZ), an axis of NW-SE convective activity in southeastern Brazil (Kodama, 1992, 1993, Carvalho et al, 2004)

Results

Discussion

Conclusion