DASH: a MATLAB toolbox for paleoclimate data assimilation

Paleoclimate data assimilation (DA) is a tool for reconstructing past climates that directly integrates proxy records with climate model output. Despite the potential for DA to expand the scope of quantitative paleoclimatology, these methods remain difficult to implement in practice due to the multi-faceted requirements and data handling necessary for DA reconstructions, the diversity of DA methods, and the need for computationally efficient algorithms. Here, we present DASH, a MATLAB toolbox designed to facilitate paleoclimate DA analyses. DASH provides command line and scripting tools that implement common tasks in DA workflows. The toolbox is highly modular and is not built around any specific analysis, and thus DASH supports paleoclimate DA for a wide variety of time periods, spatial regions, proxy networks, and algorithms. DASH includes tools for integrating and cataloguing data stored in disparate formats, building state vector ensembles, and running proxy (system) forward models. The toolbox also provides optimized algorithms for implementing ensemble Kalman filters, particle filters, and optimal sensor analyses with variable and modular parameters. This paper reviews the key components of the DASH toolbox and presents examples illustrating DASH's use for paleoclimate DA applications.</p

Snow Cover and Precipitation Impacts on Dry Season Streamflow in the Lower Mekong Basin

Climate change impacts on dry season streamflow in the Mekong River are relatively understudied, despite the fact that water availability during this time is critically important for agricultural and ecological systems. Analyses of two gauging stations (Vientiane and Kratie) in the Lower Mekong Basin (LMB) show significant positive correlations between dry season (March through May, MAM) discharge and upper basin snow cover and local precipitation. Using snow cover, precipitation, and upstream discharge as predictors, we develop skillful regression models for MAM streamflow at Vientiane and Kratie, and force these models with output from a suite of general circulation model (GCM) experiments for the twentieth and twenty-first centuries. The GCM simulations predict divergent trends in snow cover (decreasing) and precipitation (increasing) over the twenty-first century, driving overall negligible long-term trends in dry season streamflow. Our study demonstrates how future changes in dry season streamflow in the LMB will depend on changes in snow cover and precipitation, factors that will need to be considered when assessing the full basin response to other climatic and non-climatic drivers

Paleohydrological Context for Recent Floods and Droughts in the Fraser River Basin, British Columbia, Canada

The recent intensification of floods and droughts in the Fraser River Basin (FRB) of British Columbia has had profound cultural, ecological, and economic impacts that are expected to be exacerbated further by anthropogenic climate change. In part due to short instrumental runoff records, the long-Term stationarity of hydroclimatic extremes in this major North American watershed remains poorly understood, highlighting the need to use high-resolution paleoenvironmental proxies to inform on past streamflow. Here we use a network of tree-ring proxy records to develop 11 subbasin-scale, complementary flood-and drought-season reconstructions, the first of their kind. The reconstructions explicitly target management-relevant flood and drought seasons within each basin, and are examined in tandem to provide an expanded assessment of extreme events across the FRB with immediate implications for water management. We find that past high flood-season flows have been of greater magnitude and occurred in more consecutive years than during the observational record alone. Early 20th century low flows in the drought season were especially severe in both duration and magnitude in some subbasins relative to recent dry periods. Our Fraser subbasin-scale reconstructions provide long-Term benchmarks for the natural flood and drought variability prior to anthropogenic forcing. These reconstructions demonstrate that the instrumental streamflow records upon which current management is based likely underestimate the full natural magnitude, duration, and frequency of extreme seasonal flows in the FRB, as well as the potential severity of future anthropogenically forced events

Tropical sea surface temperatures for the past four centuries reconstructed from coral archives

Most annually resolved climate reconstructions of the Common Era are based on terrestrial data, making it a challenge to independently assess how recent climate changes have affected the oceans. Here as part of the Past Global Changes Ocean2K project, we present four regionally calibrated and validated reconstructions of sea surface temperatures in the tropics, based on 57 published and publicly archived marine paleoclimate data sets derived exclusively from tropical coral archives. Validation exercises suggest that our reconstructions are interpretable for much of the past 400 years, depending on the availability of paleoclimate data within, and the reconstruction validation statistics for, each target region. Analysis of the trends in the data suggests that the Indian, western Pacific, and western Atlantic Ocean regions were cooling until modern warming began around the 1830s. The early 1800s were an exceptionally cool period in the Indo-Pacific region, likely due to multiple large tropical volcanic eruptions occurring in the early nineteenth century. Decadal-scale variability is a quasi-persistent feature of all basins. Twentieth century warming associated with greenhouse gas emissions is apparent in the Indian, West Pacific, and western Atlantic Oceans, but we find no evidence that either natural or anthropogenic forcings have altered El Ni ˜no–Southern Oscillation-related variance in tropical sea surface temperatures. Our marine-based regional paleoclimate reconstructions serve as benchmarks against which terrestrial reconstructions as well as climate model simulations can be compared and as a basis for studying the processes by which the tropical oceans mediate climate variability and change

Coupled modes of North Atlantic Ocean-atmosphere variability and the onset of the Little Ice Age

This research was funded by grants from the US National Science Foundation P2C2 program (AGS-1501856, and AGS-1502224). BIC was supported by the NASA Modeling, Analysis, and Prediction program (NASA #80NSSC17K0265).Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atmospheric circulation anomalies in the Atlantic, but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial timescales. Here we analyze the coupled patterns of drought variability on either side of the North Atlantic Ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin-wide moisture anomalies to ocean and atmosphere processes. A leading mode of broad-scale moisture variability is characterized by distinct patterns of North Atlantic atmosphere circulation and sea surface temperatures. We infer a negative phase of the North Atlantic Oscillation and colder Atlantic sea surface temperatures in the middle of the 15th century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age.Publisher PDFPeer reviewe

A long-term context (931–2005 C.E.) for rapid warming over Central Asia

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 Quaternary Science Reviews 121 (2015): 89-97, doi:10.1016/j.quascirev.2015.05.020.Warming over Mongolia and adjacent Central Asia has been unusually rapid over the past few decades, particularly in the summer, with surface temperature anomalies higher than for much of the globe. With few temperature station records available in this remote region prior to the 1950s, paleoclimatic data must be used to understand annual-to-centennial scale climate variability, to local response to large-scale forcing mechanisms, and the significance of major features of the past millennium such as the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) both of which can vary globally. Here we use an extensive collection of living and subfossil wood samples from temperature-sensitive trees to produce a millennial-length, validated reconstruction of summer temperatures for Mongolia and Central Asia from 931 to 2005 CE. This tree-ring reconstruction shows general agreement with the MCA (warming) and LIA (cooling) trends, a significant volcanic signature, and warming in the 20th and 21st Century. Recent warming (2000-2005) exceeds that from any other time and is concurrent with, and likely exacerbated, the impact of extreme drought (1999-2002) that resulted in massive livestock loss across Mongolia.This research was supported by the National Science Foundation under grants AGS-PRF #1137729, ATM0117442, and AGS0402474

European and Mediterranean hydroclimate responses to tropical volcanic forcing over the last millennium

Volcanic eruptions have global climate impacts, but their effect on the hydrologic cycle is poorly understood. We use a modified version of superposed epoch analysis, an eruption year list collated from multiple data sets, and seasonal paleoclimate reconstructions (soil moisture, precipitation, geopotential heights, and temperature) to investigate volcanic forcing of spring and summer hydroclimate over Europe and the Mediterranean over the last millennium. In the western Mediterranean, wet conditions occur in the eruption year and the following 3 years. Conversely, northwestern Europe and the British Isles experience dry conditions in response to volcanic eruptions, with the largest moisture deficits in posteruption years 2 and 3. The precipitation response occurs primarily in late spring and early summer (April–July), a pattern that strongly resembles the negative phase of the East Atlantic Pattern. Modulated by this mode of climate variability, eruptions force significant, widespread, and heterogeneous hydroclimate responses across Europe and the Mediterranean

A Spatiotemporal Assessment of Extreme Cold in Northwestern North America Following the Unidentified 1809 CE Volcanic Eruption

Two large volcanic eruptions contributed to extreme cold temperatures during the early 1800s, one of the coldest phases of the Little Ice Age. While impacts from the massive 1815 Tambora eruption in Indonesia are relatively well-documented, much less is known regarding an unidentified volcanic event around 1809. Here, we describe the spatial extent, duration, and magnitude of cold conditions following this eruption in northwestern North America using a high-resolution network of tree-ring records that capture past warm-season temperature variability. Extreme and persistent cold temperatures were centered around the Gulf of Alaska, the adjacent Wrangell-St Elias Mountains, and the southern Yukon, while cold anomalies diminished with distance from this core region. This distinct spatial pattern of temperature anomalies suggests that a weak Aleutian Low and conditions similar to a negative phase of the Pacific Decadal Oscillation could have contributed to regional cold extremes after the 1809 eruption

Reconstructing 800 years of summer temperatures in Scotland from tree rings

We thank The Carnegie Trust for the Universities of Scotland for providing funding for Miloš Rydval’s PhD. The Scottish pine network expansion has been an ongoing task since 2007 and funding must be acknowledged to the following projects: EU project ‘Millennium’ (017008-2), Leverhulme Trust project ‘RELiC: Reconstructing 8000 years of Environmental and Landscape change in the Cairngorms (F/00 268/BG)’ and the NERC project ‘SCOT2K: Reconstructing 2000 years of Scottish climate from tree rings (NE/K003097/1)’.This study presents a summer temperature reconstruction using Scots pine tree-ring chronologies for Scotland allowing the placement of current regional temperature changes in a longer-term context. ‘Living-tree’ chronologies were extended using ’subfossil’ samples extracted from nearshore lake sediments resulting in a composite chronology > 800 years in length. The North Cairngorms (NCAIRN) reconstruction was developed from a set of composite blue intensity high-pass and ring-width low-pass chronologies with a range of detrending and disturbance correction procedures. Calibration against July-August mean temperature explains 56.4% of the instrumental data variance over 1866-2009 and is well verified. Spatial correlations reveal strong coherence with temperatures over the British Isles, parts of western Europe, southern Scandinavia and northern parts of the Iberian Peninsula. NCAIRN suggests that the recent summer-time warming in Scotland is likely not unique when compared to multi-decadal warm periods observed in the 1300s, 1500s, and 1730s, although trends before the mid-16th century should be interpreted with some caution due to greater uncertainty. Prominent cold periods were identified from the 16th century until the early 1800s – agreeing with the so-called Little Ice Age observed in other tree-ring reconstructions from Europe - with the 1690s identified as the coldest decade in the record. The reconstruction shows a significant cooling response one year following volcanic eruptions although this result is sensitive to the datasets used to identify such events. In fact, the extreme cold (and warm) years observed in NCAIRN appear more related to internal forcing of the summer North Atlantic Oscillation.Publisher PDFPeer reviewe

The influence of decision-making in tree ring-based climate reconstructions.

Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794-2016 CE at 0.79 (p < 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability