Ummenhofer Caroline C.

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Ummenhofer
First Name
Caroline C.
ORCID
0000-0002-9163-3967

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Early assessment of seasonal forage availability for mitigating the impact of drought on East African pastoralists

2015-11 , Vrieling, Anton , Meroni, Michele , Mude, Andrew G. , Chantarat, Sommarat , Ummenhofer, Caroline C. , de Bie, Kees (C.A.J.M.)

Pastoralist households across East Africa face major livestock losses during drought periods that can cause persistent poverty. For Kenya and southern Ethiopia, an existing index insurance scheme aims to reduce the adverse effects of such losses. The scheme insures individual households through an area-aggregated seasonal forage scarcity index derived from remotely-sensed normalized difference vegetation index (NDVI) time series. Until recently, insurance contracts covered animal losses and indemnity payouts were consequently made late in the season, based on a forage scarcity index incorporating both wet and dry season NDVI data. Season timing and duration were fixed for the whole area (March-September for long rains, October-February for short rains). Due to demand for asset protection insurance (pre-loss intervention) our aim was to identify earlier payout options by shortening the temporal integration period of the index. We used 250m-resolution 10-day NDVI composites for 2001-2014 from the Moderate Resolution Imaging Spectroradiometer (MODIS). To better describe the period during which forage develops, we first retrieved per-pixel average season start- and end-dates using a phenological model. These dates were averaged per insurance unit to obtain unit-specific growing period definitions. With these definitions a new forage scarcity index was calculated. We then examined if shortening the temporal period further could effectively predict most (>90%) of the interannual variability of the new index, and assessed the effects of shortening the period on indemnity payouts. Our analysis shows that insurance payouts could be made one to three months earlier as compared to the current index definition, depending on the insurance unit. This would allow pastoralists to use indemnity payments to protect their livestock through purchase of forage, water, or medicines.

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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.

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Extreme weather and climate events with ecological relevance : a review

2017-05 , Ummenhofer, Caroline C. , Meehl, Gerald A.

Robust evidence exists that certain extreme weather and climate events, especially daily temperature and precipitation extremes, have changed in regard to intensity and frequency over recent decades. These changes have been linked to human-induced climate change, while the degree to which climate change impacts an individual extreme climate event (ECE) is more difficult to quantify. Rapid progress in event attribution has recently been made through improved understanding of observed and simulated climate variability, methods for event attribution and advances in numerical modelling. Attribution for extreme temperature events is stronger compared with other event types, notably those related to the hydrological cycle. Recent advances in the understanding of ECEs, both in observations and their representation in state-of-the-art climate models, open new opportunities for assessing their effect on human and natural systems. Improved spatial resolution in global climate models and advances in statistical and dynamical downscaling now provide climatic information at appropriate spatial and temporal scales. Together with the continued development of Earth System Models that simulate biogeochemical cycles and interactions with the biosphere at increasing complexity, these make it possible to develop a mechanistic understanding of how ECEs affect biological processes, ecosystem functioning and adaptation capabilities. Limitations in the observational network, both for physical climate system parameters and even more so for long-term ecological monitoring, have hampered progress in understanding bio-physical interactions across a range of scales. New opportunities for assessing how ECEs modulate ecosystem structure and functioning arise from better scientific understanding of ECEs coupled with technological advances in observing systems and instrumentation.

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Increased typhoon activity in the Pacific deep tropics driven by Little Ice Age circulation changes

2020-11-16 , Bramante, James F. , Ford, Murray R. , Kench, Paul S. , Ashton, Andrew D. , Toomey, Michael R. , Sullivan, Richard M. , Karnauskas, Kristopher B. , Ummenhofer, Caroline C. , Donnelly, Jeffrey P.

The instrumental record reveals that tropical cyclone activity is sensitive to oceanic and atmospheric variability on inter-annual and decadal scales. However, our understanding of the influence of climate on tropical cyclone behaviour is restricted by the short historical record and the sparseness of prehistorical reconstructions, particularly in the western North Pacific, where coastal communities suffer loss of life and livelihood from typhoons annually. Here, to explore past regional typhoon dynamics, we reconstruct three millennia of deep tropical North Pacific cyclogenesis. Combined with existing records, our reconstruction demonstrates that low-baseline typhoon activity prior to 1350 ce was followed by an interval of frequent storms during the Little Ice Age. This pattern, concurrent with hydroclimate proxy variability, suggests a centennial-scale link between Pacific hydroclimate and tropical cyclone climatology. An ensemble of global climate models demonstrates a migration of the Pacific Walker circulation and variability in two Pacific climate modes during the Little Ice Age, which probably contributed to enhanced tropical cyclone activity in the tropical western North Pacific. In the next century, projected changes to the Pacific Walker circulation and expansion of the tropics will invert these Little Ice Age hydroclimate trends, potentially reducing typhoon activity in the deep tropical Pacific.

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400 years of summer hydroclimate from stable isotopes in Iberian trees

2016-11 , Andreu-Hayles, Laia , Ummenhofer, Caroline C. , Barriendos, Mariano , Schleser, Gerhard H. , Helle, Gerhard , Leuenberger, Markus , Gutierrez, Emilia , Cook, Edward R.

Tree rings are natural archives that annually record distinct types of past climate variability depending on the parameters measured. Here, we use ring-width and stable isotopes in cellulose of trees from the northwestern Iberian Peninsula (IP) to understand regional summer hydroclimate over the last 400 years and the associated atmospheric patterns. Spatial correlations between tree rings and gridded climate products demonstrate that isotope signatures in the targeted Iberian pine forests are very sensitive to water availability during the summer period, and are mainly controlled by stomatal conductance. Non-linear methods based on extreme events analysis allow for capturing distinct seasonal climatic variability recorded by tree-ring parameters and asymmetric signals of the associated atmospheric features. Moreover, years with extreme high (low) values in the tree-ring records were characterised by coherent large-scale atmospheric circulation patterns with reduced (enhanced) moisture transport onto the northwestern IP. These analyses of extremes revealed that high/low proxy values do not necessarily correspond to mirror images in the atmospheric anomaly patterns, suggesting different drivers of these patterns and the corresponding signature recorded in the proxies. Regional hydroclimate features across the broader IP and western Europe during extreme wet/dry summers detected by the northwestern IP trees compare favourably to an independent multicentury sea level pressure and drought reconstruction for Europe. These independent sources of past climate validate our findings that attribute non-linear moisture signals recorded by extreme tree-ring values to distinct large-scale atmospheric patterns and allow for 400-yr reconstructions of the frequency of occurrence of extreme conditions in summer hydroclimate.

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Extremes in East African hydroclimate and links to Indo-Pacific variability on interannual to decadal timescales

2017-07 , Ummenhofer, Caroline C. , Kulüke, Marco , Tierney, Jessica E.

East African hydroclimate exhibits considerable variability across a range of timescales, with implications for its population that depends on the region’s two rainy seasons. Recent work demonstrated that current state-of-the-art climate models consistently underestimate the long rains in boreal spring over the Horn of Africa while overestimating the short rains in autumn. This inability to represent the seasonal cycle makes it problematic for climate models to project changes in East African precipitation. Here we consider whether this bias also has implications for understanding interannual and decadal variability in the East African long and short rains. Using a consistent framework with an unforced multi-century global coupled climate model simulation, the role of Indo-Pacific variability for East African rainfall is compared across timescales and related to observations. The dominant driver of East African rainfall anomalies critically depends on the timescale under consideration: Interannual variations in East African hydroclimate coincide with significant sea surface temperature (SST) anomalies across the Indo-Pacific, including those associated with the El Niño-Southern Oscillation (ENSO) in the eastern Pacific, and are linked to changes in the Walker circulation, regional winds and vertical velocities over East Africa. Prolonged drought/pluvial periods in contrast exhibit anomalous SST predominantly in the Indian Ocean and Indo-Pacific warm pool (IPWP) region, while eastern Pacific anomalies are insignificant. We assessed dominant frequencies in Indo-Pacific SST and found the eastern equatorial Pacific dominated by higher-frequency variability in the ENSO band, while the tropical Indian Ocean and IPWP exhibit lower-frequency variability beyond 10 years. This is consistent with the different contribution to regional precipitation anomalies for the eastern Pacific versus Indian Ocean and IPWP on interannual and decadal timescales, respectively. In the model, the dominant low-frequency signal seen in the observations in the Indo-Pacific is not well-represented as it instead exhibits overly strong variability on subdecadal timescales. The overly strong ENSO-teleconnection likely contributes to the overestimated role of the short rains in the seasonal cycle in the model compared to observations.

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The El Niño – La Niña cycle and recent trends in supply and demand of net primary productivity in African drylands

2016-07 , Abdi, Abdulhakim , Vrieling, Anton , Yengoh, Genesis T. , Anyamba, Assaf , Seaquist, Jonathan , Ummenhofer, Caroline C. , Ardö, Jonas

Inter-annual climatic variability over a large portion of sub-Saharan Africa is under the influence of the El Niño-Southern Oscillation (ENSO). Extreme variability in climate is a threat to rural livelihoods in sub-Saharan Africa, yet the role of ENSO in the balance between supply and demand of net primary productivity (NPP) over this region is unclear. Here, we analyze the impact of ENSO on this balance in a spatially explicit framework using gridded population data from the WorldPop project, satellite-derived data on NPP supply, and statistical data from the United Nations. Our analyses demonstrate that between 2000 and 2013 fluctuations in the supply of NPP associated with moderate ENSO events average ±2.8 g C m-2 yr-1 across sub-Saharan drylands. The greatest sensitivity is in arid Southern Africa where a +1oC change in the Niño-3.4 sea surface temperature index is associated with a mean change in NPP supply of -6.6 g C m-2 yr-1. Concurrently, the population-driven trend in NPP demand averages 3.5 g C m-2 yr-1 over the entire region with densely populated urban areas exhibiting the highest mean demand for NPP. Our findings highlight the importance of accounting for the role ENSO plays in modulating the balance between supply and demand of NPP in sub-Saharan drylands. An important implication of these findings is that increase in NPP demand for socio-economic metabolism must be taken into account within the context of climate-modulated supply

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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.

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