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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. 

Abstract Continued global warming is expected to result in reduced precipitation and a drier climate in Central America. Projections of future changes are highly uncertain, however, due to the spatial resolution limitations of models and insufficient observational data coverage across space and time. Paleoclimate proxy data are therefore critical for understanding regional climate responses during times of global climate reorganization. Here we present two lake‐sediment based records of precipitation variability in Guatemala along with a synthesis of Central American hydroclimate records spanning the last millennium (800–2000 CE). The synthesis reveals that regional climate changes have been strikingly heterogeneous, even over relatively short distances. Our analysis further suggests that shifts in the mean position of the Intertropical Convergence Zone, which have been invoked by numerous studies to explain variability in Central American and circum‐Caribbean proxy records, cannot alone explain the observed pattern of hydroclimate variability. Instead, interactions between several ocean‐atmosphere processes and their disparate influences across variable topography appear to have resulted in complex precipitation responses. These complexities highlight the difficulty of reconstructing past precipitation changes across Central America and point to the need for additional paleo‐record development and analysis before the relationships between external forcing and hydroclimate change can be robustly determined. Such efforts should help anchor model‐based predictions of future responses to continued global warming. 

Abstract We use an ensemble of simulations of a coupled model (NCAR Community Earth System Model) driven by natural radiative forcing estimates over the pre‐industrial past millennium to test the efficacy of methods designed to remove forced variability from proxy‐based climate reconstructions and estimate residual internal variability (e.g., a putative “Atlantic Multidecadal Oscillation”). Within the framework of these experiments, the forced component of surface temperature change can be estimated accurately from the ensemble mean, and the internal variability of each of the independent realizations can be accurately assessed by subtracting off that estimate. We show in this case, where the true internal variability is known, that regression‐based methods of removing the forced component from proxy reconstructions will, in the presence of uncertainties in the underlying natural radiative forcing, fail to yield accurate estimates thereof, incorrectly attributing unresolved forced features (and multidecadal spectral peaks associated with them) to internal variability. 

Past research argues for an internal multidecadal (40- to 60-year) oscillation distinct from climate noise. Recent studies have claimed that this so-termed Atlantic Multidecadal Oscillation is instead a manifestation of competing time-varying effects of anthropogenic greenhouse gases and sulfate aerosols. That conclusion is bolstered by the absence of robust multidecadal climate oscillations in control simulations of current-generation models. Paleoclimate data, however, do demonstrate multidecadal oscillatory behavior during the preindustrial era. By comparing control and forced “Last Millennium” simulations, we show that these apparent multidecadal oscillations are an artifact of pulses of volcanic activity during the preindustrial era that project markedly onto the multidecadal (50- to 70-year) frequency band. We conclude that there is no compelling evidence for internal multidecadal oscillations in the climate system. 

Abstract. The primary scientific objective of MexiDrill, the Basin of MexicoDrilling Program, is development of a continuous, high-resolution∼400 kyr lacustrine record of tropical North Americanenvironmental change. The field location, in the densely populated,water-stressed Mexico City region gives this record particular societalrelevance. A detailed paleoclimate reconstruction from central Mexico willenhance our understanding of long-term natural climate variability in theNorth American tropics and its relationship with changes at higher latitudes.The site lies at the northern margin of the Intertropical Convergence Zone(ITCZ), where modern precipitation amounts are influenced by sea surfacetemperatures in the Pacific and Atlantic basins. During the Last GlacialMaximum (LGM), more winter precipitation at the site is hypothesized to have beena consequence of a southward displacement of the mid-latitude westerlies. Itthus represents a key spatial node for understanding large-scalehydrological variability of tropical and subtropical North America and isat an altitude (2240 m a.s.l.), typical of much of western North America. In addition, its sediments contain a rich record of pre-Holocene volcanichistory; knowledge of the magnitude and frequency relationships of thearea's explosive volcanic eruptions will improve capacity for riskassessment of future activity. Explosive eruption deposits will also be usedto provide the backbone of a robust chronology necessary for fullexploitation of the paleoclimate record. Here we report initial resultsfrom, and outreach activities of, the 2016 coring campaign.