Abstract
Abstract. Simulations of climate of the last millennium (LM) show that external forcing had a major contribution to the evolution of temperatures; warmer and colder periods like the Medieval Climate Anomaly (MCA; ca. 950–1250 CE) and the Little Ice Age (LIA; ca. 1450–1850 CE) were critically influenced by changes in solar and volcanic activity. Even if this influence is mainly observed in terms of temperatures, evidence from simulations and reconstructions shows that other variables related to atmospheric dynamics and hydroclimate were also influenced by external forcing over some regions. In this work, simulations from the Coupled Model Intercomparison Project Phase 5 and Paleoclimate Modelling Intercomparison Project Phase 3 (CMIP5/PMIP3) are analyzed to explore the influence of external forcings on the dynamical and hydrological changes during the LM at different spatial and temporal scales. Principal component (PC) analysis is used to obtain the modes of variability governing the global evolution of climate and to assess their correlation with the total external forcing at multidecadal to multicentennial timescales. For shorter timescales, a composite analysis is used to address the response to specific events of external forcing like volcanic eruptions. The results show coordinated long-term changes in global circulation patterns, which suggest expansions and contractions of the Hadley cells and latitudinal displacements of westerlies in response to external forcing. For hydroclimate, spatial patterns of drier and wetter conditions in areas influenced by the North Atlantic Oscillation (NAO), Northern Annular Mode (NAM), and Southern Annular Mode (SAM) and alterations in the intensity and distribution of monsoons and convergence zones are consistently found. Similarly, a clear short-term response is found in the years following volcanic eruptions. Although external forcing has a greater influence on temperatures, the results suggest that dynamical and hydrological variations over the LM exhibit a direct response to external forcing at both long and short timescales that is highly dependent on the particular simulation and model.
Highlights
Reconstructions and model simulations have shown that the evolution of temperatures during the last millennium (LM) was influenced by both external forcing and internal variability (Fernández-Donado et al, 2013; Schurer et al, 2013)
As reported by FernándezDonado et al (2013) in the case of the pre-PMIP3 LM experiments, there is a quasi-linear response of temperatures to changes in external forcing, with the major warmings occurring in periods with high solar activity and rise in greenhouse gases (GHGs) and with coolings occurring in response to lower solar forcing and increased volcanic activity
Since the average of all ensemble members cancels out uncorrelated contributions of internal variability, the resulting ensemble mean constitutes a smoothed estimation of the forced response, and the residuals of subtracting the ensemble mean from each ensemble member are an estimation of internal variability above 31-year timescales (Crowley, 2000; PAGES2k-PMIP3 group, 2015)
Summary
Reconstructions and model simulations have shown that the evolution of temperatures during the last millennium (LM) was influenced by both external forcing and internal variability (Fernández-Donado et al, 2013; Schurer et al, 2013). Like eastern Africa and tropical South America, showed increased precipitation in the north during the MCA and in the south during the LIA (Anchukaitis and Tierney, 2013; Vuille et al, 2012) Such coordinated changes in atmospheric dynamics and hydroclimate, with out-of-phase regional behaviors during the MCA and LIA, are suggestive of large-scale responses to external forcing. The latitudinal distribution of these changes in both tropical and extratropical areas is suggestive of a mechanism based on displacements of the Intertropical Convergence Zone (ITCZ) and expansions and contractions of the Hadley cells (Newton et al, 2006; Graham et al, 2011) These changes may have contributed to the alteration of modes of variability like the NAO and Northern Annular Mode (NAM) in the Northern Hemisphere (NH) and Southern Annular Mode (SAM) in the Southern Hemisphere (SH). Our analyses allow us to evaluate whether global responses in temperature, dynamics, and hydroclimate to external forcing are manifest in climate models, what their expected spatial distribution is, and whether the simulated responses are consistent with those obtained from reconstructed data
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