253 research outputs found

    The Role of Climate and Bed Topography on the Evolution of the Tasman Glacier Since the Last Glacial Maximum

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    Mountain glaciers respond to climatic changes by advancing or retreating, leaving behind a potentially powerful record of climate through moraine deposition. Estimates of past climate have been made based on the moraine record alone, using geometrical arguments; however, these methods necessarily ignore the effects of glacier dynamics and bed modification. Here, a one-dimensional coupled mass balance-flowline model is used to place constraints on the climate of the Late-glacial (13.5ā€“11.6 kyr ago) and Last Glacial Maximum (LGM, 28 ā€“ 17.5 kyr ago) based on the well-mapped and -dated moraines at Tasman Glacier/Lake Pukaki, South Island, New Zealand. Due to the highly-dynamic nature of the system, distinct longitudinal bed profiles are considered for each of the glaciations modelled; the reconstructions show that terminal overdeepenings are likely present in all bed profiles, and hundreds of metres of sediment has been deposited in the glacier valley since the LGM. Using the coupled model and calculated bed topography, a 2.2Ā°C temperature depression from the present is necessary to reproduce the Lateglacial ice extent, and 7.0Ā°C is required for the early LGM, assuming presentday precipitation. The modelled Late-glacial ice extent is more sensitive to precipitation variability than that during the LGM, but the Tasman Glacier during both periods is primarily driven by temperature changes. While the Tasman Glacier shrank between the early and late LGM, modelling demonstrates that changes in bed topography due to erosion, transport and deposition of sediment are a major driver in reduction of glacier extent; a temperature increase of only 0.1Ā°C is required to cause the transition between the two periods, which may be attributable to interannual, zero-trend climate variability. Thus, the consideration of the coupled glacier-sediment system is critical in accurately reconstructing past climate. Future work focusing on modelling this coupled system, such that the bed profile can evolve interactively with glacier flow, will be critical in better resolving transient events such as the early to late LGM transition

    The Spatial Structure of the Annual Cycle in Surface Temperature: Amplitude, Phase, and Lagrangian History

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    The climatological annual cycle in surface air temperature, defined by its amplitude and phase lag with respect to solar insolation, is one of the most familiar aspects of the climate system. Here, the authors identify three first-order features of the spatial structure of amplitude and phase lag and explain them using simple physical models. Amplitude and phase lag 1) are broadly consistent with a land and ocean end-member mixing model but 2) exhibit overlap between land and ocean and, despite this overlap, 3) show a systematically greater lag over ocean than land for a given amplitude. Based on previous work diagnosing relative ocean or land influence as an important control on the extratropical annual cycle, the authors use a Lagrangian trajectory model to quantify this influence as the weighted amount of time that an ensemble of air parcels has spent over ocean or land. This quantity explains 84% of the spaceā€“time variance in the extratropical annual cycle, as well as features 1 and 2. All three features can be explained using a simple energy balance model with land and ocean surfaces and an advecting atmosphere. This model explains 94% of the spaceā€“time variance of the annual cycle in an illustrative midlatitude zonal band when incorporating the results of the trajectory model. The aforementioned features of annual variability in surface air temperature thus appear to be explained by the coupling of land and ocean through mean atmospheric circulation.Earth and Planetary Science

    The value of initial condition large ensembles to robust adaptation decision-making

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    Ā© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Mankin, J. S., Lehner, F., Coats, S., & McKinnon, K. A. The value of initial condition large ensembles to robust adaptation decision-making. Earth's Future, 8(10), (2020): e2012EF001610, doi:10.1029/2020EF001610.The origins of uncertainty in climate projections have major consequences for the scientific and policy decisions made in response to climate change. Internal climate variability, for example, is an inherent uncertainty in the climate system that is undersampled by the multimodel ensembles used in most climate impacts research. Because of this, decision makers are left with the question of whether the range of climate projections across models is due to structural model choices, thus requiring more scientific investment to constrain, or instead is a set of equally plausible outcomes consistent with the same warming world. Similarly, many questions faced by scientists require a clear separation of model uncertainty and that arising from internal variability. With this as motivation and the renewed attention to large ensembles given planning for Phase 7 of the Coupled Model Intercomparison Project (CMIP7), we illustrate the scientific and policy value of the attribution and quantification of uncertainty from initial condition large ensembles, particularly when analyzed in conjunction with multimodel ensembles. We focus on how large ensembles can support regionalā€scale robust adaptation decisionā€making in ways multimodel ensembles alone cannot. We also acknowledge several recently identified problems associated with large ensembles, namely, that they are (1) resource intensive, (2) redundant, and (3) biased. Despite these challenges, we show, using examples from hydroclimate, how large ensembles provide unique information for the scientific and policy communities and can be analyzed appropriately for regionalā€scale climate impacts research to help inform risk management in a warming world.F. L. has been supported by the Swiss NSF (grant no. PZ00P2_174128), the NSF Division of Atmospheric and Geospace Sciences (grant no. AGSā€0856145, Amendment 87), and the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the U.S.Department of Energyā€™s Office of Biological & Environmental Research (BER) via NSF IA 1844590. This is SOEST publication no. 11115
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