Interhemispheric antiphasing of neotropical precipitation during the past millennium

Abstract

Uncertainty about the influence of anthropogenic radiative forcing on the position and strength of convective rainfall in the Intertropical Convergence Zone (ITCZ) inhibits our ability to project future tropical hydroclimate change in a warmer world. Paleoclimatic and modeling data inform on the timescales and mechanisms of ITCZ variability; yet a comprehensive, long-term perspective remains elusive. Here, we quantify the evolution of neotropical hydroclimate over the preindustrial past millennium (850 to 1850 CE) using a synthesis of 48 paleo-records, accounting for uncertainties in paleo-archive age models. We show that an interhemispheric pattern of precipitation antiphasing occurred on multicentury timescales in response to changes in natural radiative forcing. The conventionally defined “Little Ice Age” (1450 to 1850 CE) was marked by a clear shift toward wetter conditions in the southern neotropics and a less distinct and spatiotemporally complex transition toward drier conditions in the northern neotropics. This pattern of hydroclimatic change is consistent with results from climate model simulations indicating that a relative cooling of the Northern Hemisphere caused a southward shift in the thermal equator across the Atlantic basin and a southerly displacement of the ITCZ in the tropical Americas, with volcanic forcing as the principal driver. These findings are at odds with proxy-based reconstructions of ITCZ behavior in the western Pacific basin, where changes in ITCZ width and intensity, rather than mean position, appear to have driven hydroclimate transitions over the last millennium. This reinforces the idea that ITCZ responses to external forcing are region specific, complicating projections of the tropical precipitation response to global warming.

Keywords: Intertropical Convergence Zone; hydroclimate; last millennium; neotropics; paleoclimate.

Fig. 1.

Observed precipitation trends and vertical atmospheric motion. (A) Trend (mm/y) in annual precipitation from 1970 to 2016 CE. Regions of significance (P ≤ 0.1) are marked by black dots. Precipitation data are the mean of three products (see Materials and Methods). (B) Vertical atmospheric motion (Omega) monthly mean (June minus January) (Pascal/s) from National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis 1 (1981 to 2010 CE). The seasonal shift in ITCZ position and development of summer monsoons produce opposite responses in each hemisphere. Negative values represent upward vertical atmospheric movement. Paleoclimate record locations: circles, lake sediment; diamonds, speleothems; asterisks, ice; and squares, ocean sediment. Numbers correspond to SI Appendix, Table S1. Yellow locations are part of the Core region group.

Fig. 2.

Results from neotropical paleoclimate record analysis. (A) Paleoclimate proxy MCEOF1 loadings (circles) onto MCPC1 of the ALL ensemble. The outer ring, inner ring, and central colors depict the 2σ high, median, and 2σ low loading values, respectively. Circles outlined in yellow are speleothem δ¹³C, ice accumulation for the Quelccaya Ice Cap, and lipid δD for El Junco Lake. (B and C) Median (solid) and 2σ range (dashed) of MCPC1 for (B) All, (C) Northern (red), Southern (blue; multiplied by −1 for visual comparison), and Core (green) records from the ALL proxy ensemble. (D) Median (solid black) and 2σ range (dashed black) of proxy MCPC1 and median (solid blue) and 2σ range (dashed blue) of 1,000 realizations of the Atlantic meridional SST gradient (north – south). (E) Median (solid black) and 2σ range (dashed black) of MCPC2 and median (solid blue) and 2σ range (dashed blue) of 1,000 realizations of the Pacific zonal SST gradient (west – east).

Fig. 3.

Results from neotropical paleoclimate data and CESM1-LME analyses. (A) Paleoclimate record MCEOF1 loadings onto MCPC1 of the ALL ensemble (circles as described in Fig. 2). Background colors: EOF1 loadings for each grid cell onto PC1 of the annual precipitation field in the CESM1-LME all-forcing simulations. (B) Model PC1 of precipitation in the tropical Americas (black), meridional (north – south) Atlantic SST gradient (dashed blue), approximation (Approx.) of the meridional Atlantic SST gradient (solid blue) from a weighted combination of the global, interhemispheric mean temperature difference (north – south) and maximum North Atlantic streamfunction (series shown in SI Appendix, Fig. S6C), and PC2 of Atlantic basin SST (red). (C) Model PC2 of precipitation in the tropical Americas (black) and PC2 or PC3 of Pacific–Indian tropical SST (red) (linear trend removed from both series). For Β, the correlations are between the regression model and the meridional Atlantic SST gradient (blue text) and PC1 of precipitation and the meridional Atlantic SST gradient (black text). (D–F) PRECIP δ¹⁸O proxy and volcanic-only model results. (G–I) NON-PRECIP δ¹⁸O proxy and solar-only model results. The 200-y low-pass filter was applied to all series prior to analysis. EOFs corresponding to the Atlantic and Pacific–Indian SST PC and precipitation PC2 for each ensemble are shown in Fig. 4 and SI Appendix, Fig. S5, respectively. Ann. Precip., annual precipitation.

Fig. 4.

Results from the CESM1-LME mean and proxy-based climate field reconstruction of Mann et al. (70). (A–D) EOF2 of Atlantic Basin SST for (A) all-forcing, (B) volcanic-only, and (C) solar-only simulations and (D) proxy data. (E–H) Tropical Pacific–Indian SST (E) EOF3 for all-forcing, (F) EOF2 for volcanic-only, (G) EOF3 for solar-only, and (H) EOF2 for proxy data. Green diamonds, locations of SST records used to produce ocean-basin SST gradient reconstructions shown in Fig. 2.