Climate response to large, high-latitude and low-latitude volcanic eruptions in the Community Climate System Model

Explosive volcanism is known to be a leading natural cause of climate change. The second half of the 13th century was likely the most volcanically perturbed half-century of the last 2000 years, although none of the major 13th century eruptions have been clearly attributed to specific volcanoes. This period was in general a time of transition from the relatively warm Medieval period to the colder Little Ice Age, but available proxy records are insufficient on their own to clearly assess whether this transition is associated with volcanism. This context motivates our investigation of the climate system sensitivity to high- and low-latitude volcanism using the fully coupled NCAR Community Climate System Model (CCSM3). We evaluate two sets of ensemble simulations, each containing four volcanic pulses, with the first set representing them as a sequence of tropical eruptions and the second representing eruptions occurring in the mid-high latitudes of both the Northern and Southern hemispheres. The short-term, direct radiative impacts of tropical and high- latitude eruptions include significant cooling over the continents in summer and cooling over regions of increased sea-ice concentration in Northern Hemisphere (NH) winter. A main dynamical impact of moderate tropical eruptions is a winter warming pattern across northern Eurasia. Furthermore, both ensembles show significant reductions in global precipitation, especially in the summer monsoon regions. The most important long-term impact is the cooling of the high-latitude NH produced by multiple tropical eruptions, suggesting that positive feedbacks associated with ice and snow cover could lead to long-term climate cooling in the Arctic

Impact of Abrupt Deglacial Climate Change on Tropical Atlantic Subsurface Temperatures

Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and sub-thermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition

Paleoclimate constraints on the spatiotemporal character of past and future droughts

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(22), (2020): 9883-9903, https://doi.org/10.1175/JCLI-D-20-0004.1.Machine-learning-based methods that identify drought in three-dimensional space–time are applied to climate model simulations and tree-ring-based reconstructions of hydroclimate over the Northern Hemisphere extratropics for the past 1000 years, as well as twenty-first-century projections. Analyzing reconstructed and simulated drought in this context provides a paleoclimate constraint on the spatiotemporal characteristics of simulated droughts. Climate models project that there will be large increases in the persistence and severity of droughts over the coming century, but with little change in their spatial extent. Nevertheless, climate models exhibit biases in the spatiotemporal characteristics of persistent and severe droughts over parts of the Northern Hemisphere. We use the paleoclimate record and results from a linear inverse modeling-based framework to conclude that climate models underestimate the range of potential future hydroclimate states. Complicating this picture, however, are divergent changes in the characteristics of persistent and severe droughts when quantified using different hydroclimate metrics. Collectively our results imply that these divergent responses and the aforementioned biases must be better understood if we are to increase confidence in future hydroclimate projections. Importantly, the novel framework presented herein can be applied to other climate features to robustly describe their spatiotemporal characteristics and provide constraints on future changes to those characteristics.This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement 1852977. JAF was also supported by the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the U.S. Department of Energy's Office of Biological & Environmental Research (BER) via National Science Foundation IA 1844590. JS was supported in part by the U.S. National Science Foundation through Grants AGS-1602920 and AGS-1805490, and by the National Oceanic and Atmospheric Administration by Grant NA20OAR4310425. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1) for producing and making available their model output. For CMIP, the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portal. We thank the editor and two reviewers for comments that greatly improved the quality of this manuscript. This is SOEST Publication No. 11116 and LDEO Publication No. 8450.2021-04-1

Impact of abrupt deglacial climate change on tropical Atlantic subsurface temperatures

Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and subthermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition

Regional and global forcing of glacier retreat during the last deglaciation

The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global warming associated with rising greenhouse gas concentrations. By comparison, the importance of greenhouse gases in driving glacier retreat during the most recent deglaciation, the last major interval of global warming, is unclear due to uncertainties in the timing of retreat around the world. Here we use recently improved cosmogenic-nuclide production-rate calibrations to recalculate the ages of 1,116 glacial boulders from 195 moraines that provide broad coverage of retreat in mid-to-low-latitude regions. This revised history, in conjunction with transient climate model simulations, suggests that while several regional-scale forcings, including insolation, ice sheets and ocean circulation, modulated glacier responses regionally, they are unable to account for global-scale retreat, which is most likely related to increasing greenhouse gas concentrations.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Nature Publishing Group. The published article can be found at: http://www.nature.com/ncomms/2015/150821/ncomms9059/full/ncomms9059.htm

The role of North Brazil Current transport in the paleoclimate of the Brazilian Nordeste margin and paleoceanography of the western tropical Atlantic during the late Quaternary

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014): 3-13, doi:10.1016/j.palaeo.2014.05.030.Reconstructions of surface paleoceanographic conditions of the western equatorial Atlantic and past climates of the adjacent Northeast Brazilian (the "Nordeste") continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0° 20’ N on the equatorial Brazilian continental slope. The record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near- modern to approximately 110 Ka. High-resolution XRF analysis provides insight into the paleoclimate history of the Nordeste during the last glacial interval. Several large-amplitude and abrupt peaks are observed in the time series of Ti/Ca and are usually accompanied by peaks of Fe/K. Together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from Nordeste rivers into the Atlantic. Within the limits of dating accuracy, most Ti/Ca peaks correlate with Heinrich events in the North Atlantic. This record thus corroborates, and extends back in time, the previous record of Arz et al (1998) determined on sediment cores from farther southeast along the Nordeste margin. Stable oxygen isotopic analysis and Mg/Ca paleothermometry on the near- surface-dwelling planktonic foraminiferal species Globierinoides ruber find that mean sea-surface temperature (SST) during glacial time (20 to 55 Ka, n = 97) was 23.89 ± 0.79 °C and the mean SST during the late Holocene (0 to 5 Ka, n = 14) was 26.89 ± 0.33 °C. SSTs were 0.5 to 2 °C higher and inferred sea-surface salinities were lower during most of the periods of elevated Ti/Ca, thus, as observed in previous studies, the western equatorial Atlantic was warm (at least locally) and the adjacent southern tropical continent was wet at the same time that the high-latitude North Atlantic was cold. Using the SYNTRACE-CCSM3 fully coupled climate model with transient forcing for the period 22 Ka to present, we find that decreased transport of the North Brazil Current co-occurs with reduced Atlantic meridional overturning circulation, and colder-than-normal SSTs in the North Atlantic region. These simulated conditions are invariably associated with significantly increased precipitation in the Nordeste region.Funding for the cruise and post-cruise science was provided to PAB by NSF-OCE-0823650

Temporal and spatial structure of multi‐millennial temperature changes at high latitudes during the Last Interglacial

The Last Interglacial (LIG, 129–116 thousand of years BP, ka) represents a test bed for climate model feedbacks in warmer-than-present high latitude regions. However, mainly because aligning different palaeoclimatic archives and from different parts of the world is not trivial, a spatio-temporal picture of LIG temperature changes is difficult to obtain. Here, we have selected 47 polar ice core and sub-polar marine sediment records and developed a strategy to align them onto the recent AICC2012 ice core chronology. We provide the first compilation of high-latitude temperature changes across the LIG associated with a coherent temporal framework built between ice core and marine sediment records. Our new data synthesis highlights non-synchronous maximum temperature changes between the two hemispheres with the Southern Ocean and Antarctica records showing an early warming compared to North Atlantic records. We also observe warmer than present-day conditions that occur for a longer time period in southern high latitudes than in northern high latitudes. Finally, the amplitude of temperature changes at high northern latitudes is larger compared to high southern latitude temperature changes recorded at the onset and the demise of the LIG. We have also compiled four data-based time slices with temperature anomalies (compared to present-day conditions) at 115 ka, 120 ka, 125 ka and 130 ka and quantitatively estimated temperature uncertainties that include relative dating errors. This provides an improved benchmark for performing more robust model-data comparison. The surface temperature simulated by two General Circulation Models (CCSM3 and HadCM3) for 130 ka and 125 ka is compared to the corresponding time slice data synthesis. This comparison shows that the models predict warmer than present conditions earlier than documented in the North Atlantic, while neither model is able to produce the reconstructed early Southern Ocean and Antarctic warming. Our results highlight the importance of producing a sequence of time slices rather than one single time slice averaging the LIG climate conditions

Investigating the Direct Meltwater Effect in Terrestrial Oxygenâ Isotope Paleoclimate Records Using an Isotopeâ Enabled Earth System Model

Variations in terrestrial oxygenâ isotope reconstructions from ice cores and speleothems have been primarily attributed to climatic changes of surface air temperature, precipitation amount, or atmospheric circulation. Here we demonstrate with the fully coupled isotopeâ enabled Community Earth System Model an additional process contributing to the oxygenâ isotope variations during glacial meltwater events. This process, termed â the direct meltwater effect,â involves propagating large amounts of isotopically depleted meltwater throughout the hydrological cycle and is independent of climatic changes. We find that the direct meltwater effect can make up 15â 35% of the δ18O signals in precipitation over Greenland and eastern Brazil for large freshwater forcings (0.25â 0.50 sverdrup (106 m3/s)). Model simulations further demonstrate that the direct meltwater effect increases with the magnitude and duration of the freshwater forcing and is sensitive to both the location and shape of the meltwater. These new modeling results have important implications for past climate interpretations of δ18O.Key PointsA portion of the δ18O signal in landâ based paleoclimate proxies can be attributed to the direct meltwater effect instead of climatic changesThe direct meltwater effect can make up 15â 35% of the δ18O signals in precipitation in Greenland and eastern Brazil for large meltwater eventsThe direct meltwater effect increases with the magnitude and duration of the freshwater forcing and is sensitive to location and shape dependentPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141374/1/grl56782_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141374/2/grl56782-sup-0001-Supporting_Information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141374/3/grl56782.pd

Agreement between reconstructed and modeled boreal precipitation of the Last Interglacial

The last extended time period when climate may have been warmer than today was during the Last Interglacial (LIG; ca. 129 to 120 thousand years ago). However, a global view of LIG precipitation is lacking. Here, seven new LIG climate models are compared to the first global database of proxies for LIG precipitation. In this way, models are assessed in their ability to capture important hydroclimatic processes during a different climate. The models can reproduce the proxy-based positive precipitation anomalies from the preindustrial period over much of the boreal continents. Over the Southern Hemisphere, proxy-model agreement is partial. In models, LIG boreal monsoons have 42% wider area than in the preindustrial and produce 55% more precipitation and 50% more extreme precipitation. Austral monsoons are weaker. The mechanisms behind these changes are consistent with stronger summer radiative forcing over boreal high latitudes and with the associated higher temperatures during the LIG