Denniston Rhawn F.

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Rhawn F.

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  • Preprint
    Decoupling of monsoon activity across the northern and southern Indo-Pacific during the Late Glacial
    ( 2017-09) Denniston, Rhawn F. ; Asmerom, Yemane ; Polyak, Victor J. ; Wanamaker, Alan D. ; Ummenhofer, Caroline C. ; Humphreys, William F. ; Cugley, John ; Woods, David ; Lucker, Stephanie
    Recent studies of stalagmites from the Southern Hemisphere tropics of Indonesia revealed two shifts in monsoon activity not apparent in records from the Northern Hemisphere sectors of the Austral-Asian monsoon system: an interval of enhanced rainfall at ~19 ka, immediately prior to Heinrich Stadial 1, and a sharp increase in precipitation at ~9 ka. Determining whether these events are site-specific or regional is important for understanding the full range of sensitivities of the Austral-Asian monsoon. We present a discontinuous 40 kyr carbon isotope record of stalagmites from two caves in the Kimberley region of the north-central Australian tropics. Heinrich stadials are represented by pronounced negative carbon isotopic anomalies, indicative of enhanced rainfall associated with a southward shift of the intertropical convergence zone and consistent with hydroclimatic changes observed across Asia and the Indo- Pacific. Between 20-8 ka, however, the Kimberley stalagmites, like the Indonesian record, reveal decoupling of monsoon behavior from Southeast Asia, including the early deglacial wet period (which we term the Late Glacial Pluvial) and the abrupt strengthening of early Holocene monsoon rainfall.
  • Preprint
    Extreme rainfall activity in the Australian tropics reflects changes in the El Niño/Southern Oscillation over the last two millennia
    ( 2015-03) Denniston, Rhawn F. ; Villarini, Gabriele ; Gonzales, Angelique N. ; Wyrwoll, Karl-Heinz ; Polyak, Victor J. ; Ummenhofer, Caroline C. ; Lachniet, Matthew S. ; Wanamaker, Alan D. ; Humphreys, William F. ; Woods, David ; Cugley, John
    Assessing temporal variability in extreme rainfall events prior to the historical era is complicated by the sparsity of long-term ‘direct’ storm proxies. Here we present a 2200-yr-long, accurate and precisely dated record of cave flooding events from the northwest Australian tropics that we interpret, based on an integrated analysis of meteorological data and sediment layers within stalagmites, as representing a proxy for extreme rainfall events derived primarily from tropical cyclones (TCs) and secondarily from the regional summer monsoon. This time series reveals substantial multi-centennial variability in extreme rainfall, with elevated occurrence rates characterizing the twentieth century, the period 850-1450 CE, and 50-400 CE; reduced activity marks 1450-1650 CE and 500-850 CE. These trends are similar to reconstructed numbers of TCs in the North Atlantic and Caribbean basins, and form temporal and spatial patterns best explained by secular changes in the dominant mode of the El Niño-Southern Oscillation (ENSO), the primary driver of modern TC variability. We thus attribute long-term shifts in cyclogenesis in both the central Australian and North Atlantic sectors over the past two millennia to entrenched El Niño or La Niña states of the tropical Pacific. The influence of ENSO on monsoon precipitation in this region of northwest Australia is muted, but ENSO-driven changes to the monsoon may have complemented changes to TC activity.
  • Article
    A stalagmite test of North Atlantic SST and Iberian hydroclimate linkages over the last two glacial cycles
    (Copernicus Publications on behalf of the European Geosciences Union, 2018-12-11) Denniston, Rhawn F. ; Houts, Amanda N. ; Asmerom, Yemane ; Wanamaker, Alan D. ; Haws, Jonathan A. ; Polyak, Victor J. ; Thatcher, Diana L. ; Altan-Ochir, Setsen ; Borowske, Alyssa C. ; Breitenbach, Sebastian F. M. ; Ummenhofer, Caroline C. ; Regala, Frederico T. ; Benedett, Michael M. ; Bicho, Nuno F.
    Close coupling of Iberian hydroclimate and North Atlantic sea surface temperature (SST) during recent glacial periods has been identified through the analysis of marine sediment and pollen grains co-deposited on the Portuguese continental margin. While offering precisely correlatable records, these time series have lacked a directly dated, site-specific record of continental Iberian climate spanning multiple glacial cycles as a point of comparison. Here we present a high-resolution, multi-proxy (growth dynamics and δ13C, δ18O, and δ234U values) composite stalagmite record of hydroclimate from two caves in western Portugal across the majority of the last two glacial cycles (∼220 ka). At orbital and millennial scales, stalagmite-based proxies for hydroclimate proxies covaried with SST, with elevated δ13C, δ18O, and δ234U values and/or growth hiatuses indicating reduced effective moisture coincident with periods of lowered SST during major ice-rafted debris events, in agreement with changes in palynological reconstructions of continental climate. While in many cases the Portuguese stalagmite record can be scaled to SST, in some intervals the magnitudes of stalagmite isotopic shifts, and possibly hydroclimate, appear to have been somewhat decoupled from SST.
  • Article
    Hydroclimate variability from western Iberia (Portugal) during the Holocene: insights from a composite stalagmite isotope record
    (SAGE Publications, 2020-03-07) Thatcher, Diana L. ; Wanamaker, Alan D. ; Denniston, Rhawn F. ; Asmerom, Yemane ; Polyak, Victor J. ; Fullick, Daniel ; Ummenhofer, Caroline C. ; Gillikin, David P. ; Haws, Jonathan A.
    Iberia is predicted under future warming scenarios to be increasingly impacted by drought. While it is known that this region has experienced multiple intervals of enhanced aridity over the Holocene, additional hydroclimate-sensitive records from Iberia are necessary to place current and future drying into a broader perspective. Toward that end, we present a multi-proxy composite record from six well-dated and overlapping speleothems from Buraca Gloriosa (BG) cave, located in western Portugal. The coherence between the six stalagmites in this composite stalagmite record illustrates that climate (not in-cave processes) impacts speleothem isotopic values. This record provides the first high-resolution, precisely dated, terrestrial record of Holocene hydroclimate from west-central Iberia. The BG record reveals that aridity in western Portugal increased secularly from 9.0 ka BP to present, as evidenced by rising values of both carbon (δ13C) and oxygen (δ18O) stable isotope values. This trend tracks the decrease in Northern Hemisphere summer insolation and parallels Iberian margin sea surface temperatures (SST). The increased aridity over the Holocene is consistent with changes in Hadley Circulation and a southward migration of the Intertropical Convergence Zone (ITCZ). Centennial-scale shifts in hydroclimate are coincident with changes in total solar irradiance (TSI) after 4 ka BP. Several major drying events are evident, the most prominent of which was centered around 4.2 ka BP, a feature also noted in other Iberian climate records and coinciding with well-documented regional cultural shifts. Substantially, wetter conditions occurred from 0.8 ka BP to 0.15 ka BP, including much of the ‘Little Ice Age’. This was followed by increasing aridity toward present day. This composite stalagmite proxy record complements oceanic records from coastal Iberia, lacustrine records from inland Iberia, and speleothem records from both northern and southern Spain and depicts the spatial and temporal variability in hydroclimate in Iberia.
  • Article
    Sensitivity of northwest Australian tropical cyclone activity to ITCZ migration since 500 CE
    (American Association for the Advancement of Science, 2023-01) Denniston, Rhawn F. ; Ummenhofer, Caroline C. ; Emanuel, Kerry ; Ingrosso, Roberto ; Pausata, Francesco S. R. ; Wanamaker, Alan D. ; Lachniet, Matthew S. ; Carr, Kenneth T. ; Asmerom, Yemane ; Polyak, Victor J. ; Nott, Jonathan ; Zhang, Wei ; Villarini, Gabriele ; Cugley, John ; Brooks, Darren ; Woods, David ; Humphreys, William F.
    Tropical cyclones (TCs) regularly form in association with the intertropical convergence zone (ITCZ), and thus, its positioning has implications for global TC activity. While the poleward extent of the ITCZ has varied markedly over past centuries, the sensitivity with which TCs responded remains poorly understood from the proxy record, particularly in the Southern Hemisphere. Here, we present a high-resolution, composite stalagmite record of ITCZ migrations over tropical Australia for the past 1500 years. When integrated with a TC reconstruction from the Australian subtropics, this time series, along with downscaled climate model simulations, provides an unprecedented examination of the dependence of subtropical TC activity on meridional shifts in the ITCZ. TCs tracked the ITCZ at multidecadal to centennial scales, with a more southward position enhancing TC-derived rainfall in the subtropics. TCs may play an increasingly important role in Western Australia's moisture budgets as subtropical aridity increases due to anthropogenic warming.