Mid-holocene hydroclimate change over tropical South America

Abstract

The South American Summer Monsoon (SASM) system is a vital system that supports the existence of the unique biomes and ecology of tropical to subtropical South America, as well as the economies of the South American population. In order to improve our understanding of the South American climate system, this thesis uses a model-proxy data comparison approach to characterize and understand the mechanisms driving precipitation response during the mid-Holocene.

First, a new speleothem oxygen isotopic record (δ18O) from Angelica Cave in central Brazil shows that precipitation under the South Atlantic Convergence Zone (SACZ) remained stable over the mid-to-late Holocene. The central Brazilian record shows that rainfall under the core SACZ is insensitive to changes in the monsoon strength within the core SASM region and that the latitudinal position of the convergence zone did not shift significantly over time. The steady precipitation trend is in contrast to hydroclimate datasets from the western Amazon and northeastern Brazil which respectively show a strengthening and weakening of rainfall over the same time interval in an east-west dipole pattern.

Next, the spatial variation of hydroclimate records over the mid-to-late Holocene is investigated using an Atmospheric General Circulation Model (AGCM), the ECHAM4.6. Mid-Holocene and pre-industrial scenario experiments ran with the ECHAM4.6 coupled to a slab ocean model show that while reduced precipitation in the western Amazon and Andes is a result of a weakened SASM during the mid-Holocene, the wetter condition in northeastern Brazil is due to a weakened Nordeste Low, resulting in less atmospheric subsidence over the northeast, in addition to an intensified Intertropical Convergence Zone (ITCZ) towards the south. The model results also show an increase in precipitation rates along the northern region of the SACZ and a decrease along the southern regions, while precipitation along the core of the SACZ remained similar between the two scenarios with no latitudinal shift of the core axis. Therefore, the SACZ responds in a north–south dipole pattern to the increasing SASM strength over the mid-to-late Holocene, rather than a latitudinal shift, thus agreeing with the invariant speleothem δ18O trend from central Brazil.

Further experiments with the ECHAM4.6 forced with prescribed sea surface temperatures (SST) are used to investigate the impact of the El Niño–Southern Oscillation (ENSO) on the climatological precipitation and precipitation δ18O over South America during the mid-Holocene. Results show that the removal of ENSO variability on an otherwise unchanged annual cycle in SST results in a relatively minor change in the climatological precipitation. This means that the reductions in ENSO variability (amplitude and frequency) during the mid-Holocene is likely a negligible factor in explaining mid-Holocene precipitation trends in tropical to subtropical South America.

In contrast, mid-Holocene insolation and a ‘La Niña-like’ mean state change in SSTs result in a prominent east-west dipole pattern in precipitation response between western Amazon and the northeast of South America. However, only the insolation experiment manages to reproduce the dipole pattern in precipitation δ18O values. This is in part due to insolation changes having the greatest impact on western Amazon precipitation during the peak monsoon months, whereas, the change in the mean state SST has the greatest impact during the dry winter season, thus resulting in the varying δ18O responses. We further show that the location of anomalous subsidence in the insolation experiment is concentrated over the eastern tropical Atlantic (~30°E), compared to subsidence over the northeast of the continent (~60°E) in the mean state experiment, further resulting in different extents of drying in the region.