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

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

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

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

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

Warming over Mongolia and 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, 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

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

Strip‐Bark Morphology and Radial Growth Trends in Ancient Pinus sibirica Trees From Central Mongolia

Some of the oldest and most important trees used for dendroclimatic reconstructions develop strip‐bark morphology, in which only a portion of the stem contains living tissue. Yet the ecophysiological factors initiating strip bark and the potential effect of cambial dieback on annual ring widths and tree‐ring estimates of past climate remain poorly understood. Using a combination of field observations and tree‐ring data, we investigate the causes and timing of cambial dieback events in Pinus sibirica strip‐bark trees from central Mongolia and compare the radial growth rates and trends of strip‐bark and whole‐bark trees over the past 515 years. Results indicate that strip bark is more common on the southern aspect of trees, and dieback events were most prevalent in the 19th century, a cold and dry period. Further, strip‐bark and whole‐bark trees have differing centennial trends, with strip‐bark trees exhibiting notably large increases in ring widths at the beginning of the 20th century. We find a steeper positive trend in the strip‐bark chronology relative to the whole‐bark chronology when standardizing with age‐dependent splines. We hypothesize that localized warming on the southern side of stems due to solar irradiance results in physiological damage and dieback and leads to increasing tree‐ring increment along the living portion of strip‐bark trees. Because the impact of cambial dieback on ring widths likely varies depending on species and site, we suggest conducting a comparison of strip‐bark and whole‐bark ring widths before statistically treating ring‐width data for climate reconstructions

A specific case in the classification of woods by FTIR and chemometric: discrimination of Fagales from Malpighiales

Fourier transform infrared (FTIR) spectroscopic data was used to classify wood samples from nine species within the Fagales and Malpighiales using a range of multivariate statistical methods. Taxonomic classification of the family Fagaceae and Betulaceae from Angiosperm Phylogenetic System Classification (APG II System) was successfully performed using supervised pattern recognition techniques. A methodology for wood sample discrimination was developed using both sapwood and heartwood samples. Ten and eight biomarkers emerged from the dataset to discriminate order and family, respectively. In the species studied FTIR in combination with multivariate analysis highlighted significant chemical differences in hemicelluloses, cellulose and guaiacyl (lignin) and shows promise as a suitable approach for wood sample classification