Centennial-scale solar forcing of the South American Monsoon System recorded in stalagmites.

Valdir F Novello,Ivo Karmann,Plínio F Jaqueto,Ricardo I F Trindade,Mathias Vuille,Jean S Moquet,Nicolás M Stríkis,R Lawrence Edwards,Hai Cheng,Marcos Saito De Paula,Gelvam A Hartmann,Francisco W Cruz

doi:10.1038/srep24762

Abstract

The South American Monsoon System (SAMS) is generally considered to be highly sensitive to Northern Hemisphere (NH) temperature variations on multi-centennial timescales. The direct influence of solar forcing on moisture convergence in global monsoon systems on the other hand, while well explored in modeling studies, has hitherto not been documented in proxy data from the SAMS region. Hence little is known about the sensitivity of the SAMS to solar forcing over the past millennium and how it might compete or constructively interfere with NH temperature variations that occurred primarily in response to volcanic forcing. Here we present a new annually-resolved oxygen isotope record from a 1500-year long stalagmite recording past changes in precipitation in the hitherto unsampled core region of the SAMS. This record details how solar variability consistently modulated the strength of the SAMS on centennial time scales during the past 1500 years. Solar forcing, besides the previously recognized influence from NH temperature changes and associated Intertropical Convergence Zone (ITCZ) shifts, appears as a major driver affecting SAMS intensity at centennial time scales.

Highlights

Several studies have shown that monsoon systems and other large-scale convergence zones around the world are influenced by solar variability[1,2,3,4,5,6,7,8,9,10,11,12]
The record is developed from two stalagmites (ALHO6 and CUR4) collected from Pau d’Alho and Curupira caves respectively, and is representative of South American Monsoon System (SAMS) variability in its active core, covering the last 1500 years with accurate geochronology based on U-Th ages and annual layer counting
The solar irradiance is in clear anti-phase with the δ 18O variability of the stalagmite (Supplementary Fig. S9), a slight phase discrepancy is apparent before 1100 AD, with the solar forcing leading the δ 18O response

Summary

Introduction

Several studies have shown that monsoon systems and other large-scale convergence zones around the world are influenced by solar variability[1,2,3,4,5,6,7,8,9,10,11,12]. Meehl et al.[12], for example, reported that peaks in solar forcing increase the energy input to the surface ocean at subtropical latitudes, thereby enhancing evaporation and near-surface moisture, which is carried by the trade winds to the convergence zones Through this mechanism convective activity in the regions influenced by the upward branches of Hadley and Walker cells can be intensified, resulting in strengthened regional tropical precipitation regimes due to enhanced solar forcing[8]. Most of the NH temperature excursions over the past millennium, including the LIA cooling, occurred first and foremost in response to volcanic forcing and were only to a very small extent influenced by solar variability[20] In this sense, the NH warming/cooling and the direct solar irradiance influence on tropical convergence zones can be considered two separate forcings, which at times may oppose or constructively interfere with one another in modulating the SAMS intensity. The mean growth rate of ALHO6 and CUR4 is 0.17 mm/yr and 0.20 mm/yr, respectively, yielding an average δ 18O resolution of 1.1 and 0.7 years per sample, respectively

Methods

Results

Conclusion