Self-Oscillations, External Forcings, and Climate Predictability

Abstract The Qinghai–Tibetan Plateau and Arctic both have an important influence on global climate, but the correlation between climate variations in these two regions remains unclear. Here we reconstructed and compared the summer temperature anomalies over the past 1,120 yr (900–2019 CE) in the Qinghai–Tibetan Plateau and Arctic. The temperature correlation during the past millennium in these two regions has a distinct centennial variation caused by volcanic eruptions. Furthermore, the abrupt weak-to-strong transition in the temperature correlation during the sixteenth century could be analogous to this type of transition during the Modern Warm Period. The former was forced by volcanic eruptions, while the latter was controlled by changes in greenhouse gases. This implies that anthropogenic, as opposed to natural, forcing has acted to amplify the teleconnection between the Qinghai–Tibetan Plateau and Arctic during the Modern Warm Period.

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Estimating the limit of decadal-scale climate predictability using observational data

Climate Dynamics ◽

10.1007/s00382-015-2662-6 ◽

2015 ◽

Vol 46 (5-6) ◽

pp. 1563-1580 ◽

Cited By ~ 26

Author(s):

Ruiqiang Ding ◽

Jianping Li ◽

Fei Zheng ◽

Jie Feng ◽

Deqiang Liu

Keyword(s):

Observational Data ◽

Climate Predictability ◽

Decadal Scale

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Climate predictability experiments with a general circulation model

Climate Dynamics ◽

10.1007/bf00219500 ◽

1996 ◽

Vol 12 (4) ◽

pp. 261-278 ◽

Cited By ~ 37

Author(s):

L. Bengtsson ◽

K. Arpe ◽

E. Roeckner ◽

U. Schulzweida

Keyword(s):

General Circulation Model ◽

General Circulation ◽

Circulation Model ◽

Climate Predictability

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SEASONAL VARIABILITY OF THE CURRENTS IN THE OFFSHORE REGION OF SURF ZONE AND EFFECTS OF EXTERNAL FORCINGS ON THE CURRENTS

Journal of Japan Society of Civil Engineers Ser B2 (Coastal Engineering) ◽

10.2208/kaigan.77.2_i_355 ◽

2021 ◽

Vol 77 (2) ◽

pp. I_355-I_360

Author(s):

Shinya UMEDA ◽

Junichi NINOMIYA ◽

Mamoru ARITA ◽

Masatoshi YUHI

Keyword(s):

Seasonal Variability ◽

Surf Zone ◽

External Forcings ◽

Offshore Region

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A Numerical Study of Plane Ice-Sheet Flow

Journal of Glaciology ◽

10.1017/s002214300001546x ◽

1986 ◽

Vol 32 (111) ◽

pp. 139-160 ◽

Cited By ~ 39

Author(s):

K. Hutter ◽

S. Yakowitz ◽

F. Szidarovszky

Keyword(s):

Surface Temperature ◽

Thermal State ◽

Numerical Study ◽

Ice Sheets ◽

Ice Sheet ◽

Tangential Velocity ◽

Rigid Base ◽

Geothermal Heat ◽

Functional Relations ◽

External Forcings

AbstractThe plane steady flow of a grounded ice sheet is numerically analysed using the approximate model of Morland or Hutter. In this, the ice behaves as a non-linear viscous fluid with a strongly temperature-dependent rate factor, and ice sheets are assumed to be long and shallow. The climate is assumed to be prescribed via the accumulation/ablation distribution and the surface temperature, both of which are functions of position and unknown height. The rigid base exerts external forcings via the normal heat flow, the geothermal heat, and a given basal sliding condition connecting the tangential velocity, tangential traction, and normal traction. The functional relations are those of Morland (1984) or motivated by his work. We use equations in his notation.The governing equations and boundary conditions in dimensionless form are briefly stated and dimensionless variables are related to their physical counterparts. The thermo-mechanical parabolic boundary-value problem, found to depend on physical scales, constitutive properties, and external forcing functions, has been numerically solved. For reasons of stability, the numerical integration must proceed from the ice divide towards the margin, which requires a special analysis of the ice divide. We present this analysis and then describe the versatility and limitations of the constructed computer code.Results of extensive computations are shown. In particular, we prove that the Morland–Hutter model for ice sheets is only applicable when sliding is sufficiently large (satisfying inequality (30)). In the range of the validity of this inequality, it is then demonstrated that of all physical scaling parameters only a single π-product influences the geometry and the flow within the ice sheet. We analyse the role played by advection, diffusion, and dissipation in the temperature distribution, and discuss the significance of the rheological non-linearities. Variations of the external forcings, such as accumulation/ablation conditions, free surface temperature, and geothermal heat, demonstrate the sensitivity of the ice-sheet geometry to accumulation conditions and the robustness of the flow to variations in the thermal state. We end with a summary of results and a critical review of the model.

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The Meridional Shift of the Midlatitude Westerlies over Arid Central Asia during the Past 21 000 Years Based on the TraCE-21ka Simulations

Journal of Climate ◽

10.1175/jcli-d-19-0798.1 ◽

2020 ◽

Vol 33 (17) ◽

pp. 7455-7478

Author(s):

Nanxuan Jiang ◽

Qing Yan ◽

Zhiqing Xu ◽

Jian Shi ◽

Ran Zhang

Keyword(s):

Indian Ocean ◽

North Atlantic ◽

North Atlantic Ocean ◽

Last Deglaciation ◽

The Past ◽

The North ◽

External Forcings ◽

The Indian Ocean ◽

The Last Deglaciation ◽

The North Atlantic

AbstractTo advance our knowledge of the response of midlatitude westerlies to various external forcings, we investigate the meridional shift of midlatitude westerlies over arid central Asia (ACA) during the past 21 000 years, which experienced more varied forcings than the present day based on a set of transient simulations. Our results suggest that the evolution of midlatitude westerlies over ACA and driving factors vary with time and across seasons. In spring, the location of midlatitude westerlies over ACA oscillates largely during the last deglaciation, driven by meltwater fluxes and continental ice sheets, and then shows a long-term equatorward shift during the Holocene controlled by orbital insolation. In summer, orbital insolation dominates the meridional shift of midlatitude westerlies, with poleward and equatorward migration during the last deglaciation and the Holocene, respectively. From a thermodynamic perspective, variations in zonal winds are linked with the meridional temperature gradient based on the thermal wind relationship. From a dynamic perspective, variations in midlatitude westerlies are mainly induced by anomalous sea surface temperatures over the Indian Ocean through the Matsuno–Gill response and over the North Atlantic Ocean by the propagation of Rossby waves, or both, but their relative importance varies across forcings. Additionally, the modeled meridional shift of midlatitude westerlies is broadly consistent with geological evidence, although model–data discrepancies still exist. Overall, our study provides a possible scenario for a meridional shift of midlatitude westerlies over ACA in response to various external forcings during the past 21 000 years and highlights important roles of both the Indian Ocean and the North Atlantic Ocean in regulating Asian westerlies, which may shed light on the behavior of westerlies in the future.

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Deglacial climate and ice sheet evolution of the Ross Embayment, Antarctica

10.26686/wgtn.17142116 ◽

2021 ◽

Author(s):

◽

Daniel P. Lowry

Keyword(s):

Spatial Pattern ◽

Climate Models ◽

Ice Sheet ◽

Model Parameters ◽

Atmospheric Conditions ◽

Last Deglaciation ◽

Proxy Records ◽

External Forcings ◽

Wide Range ◽

Grounding Line

<p>Reconstructing past grounding-line evolution can help inform future sea level projections by constraining marine ice sheet sensitivities to changes in climate. The Ross Embayment, the largest sector of Antarctica, experienced substantial grounding-line retreat since the Last Glacial Maximum. However, different interpretations for the timing and spatial pattern of deglacial grounding-line retreat in this region persist, suggesting either very high or low sensitivity to external forcings. Complicating matters is the sparse paleoclimate record, which is limited spatially and temporally. In this thesis, I address these issues by analysing the output of two transient climate simulations in relation to Antarctic ice core and marine sediment records, and performing and analysing the largest ensemble to date of regional ice sheet model simulations of the last deglaciation in the Ross Sea. The climate models and paleoclimate proxy records exhibit key differences in the timing, magnitude and duration of millennial-scale climate change events through the deglacial period. Using this diverse set of deglacial climate trajectories as ocean and atmosphere forcings, the ice sheet model ensemble produces a wide range of ice sheet responses, supporting the view that external forcings are the main drivers of past grounding-line retreat in the region. The simulations demonstrate that atmospheric conditions early in the deglacial period can enhance or diminish ice sheet sensitivity to rising ocean temperatures, thereby controlling the initial timing and spatial pattern of grounding-line retreat. Through the Holocene, grounding-line position is more sensitive to sub-shelf melt rates as the ocean cavity below the ice shelf expands. Model parameters that control the physical properties of the bed, deformation of the continental shelf, and rheological properties of the ice strongly influence the sensitivity of ice sheets to external forcing. Basin-wide differences in these forcings, driven by oceanic and atmospheric circulation, and spatial heterogeneity of bed properties likely contribute to the asynchronous pattern of retreat in the eastern and western parts of the embayment, as indicated by marine and terrestrial proxy records.</p>

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A unifying framework for iceberg-calving models

Journal of Glaciology ◽

10.3189/002214310794457173 ◽

2010 ◽

Vol 56 (199) ◽

pp. 822-830 ◽

Cited By ~ 52

Author(s):

Jason M. Amundson ◽

Martin Truffer

Keyword(s):

Empirical Relationship ◽

Ice Thickness ◽

Ice Shelf ◽

Physical Explanation ◽

Positive Feedbacks ◽

First Order ◽

External Forcings ◽

Iceberg Calving ◽

Calving Rate ◽

Glacier Flow

AbstractWe propose a general framework for iceberg-calving models that can be applied to any calving margin. The framework is based on mass continuity, the assumption that calving rate and terminus velocity are not independent and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset. The theoretical, near steady-state analysis used to support and analyze the framework indicates that calving rate is governed, to first order, by ice thickness, thickness gradient, strain rate, mass-balance rate and backwards melting of the terminus; the analysis furthermore provides a physical explanation for a previously derived empirical relationship for ice-shelf calving (Alley and others, 2008). In the calving framework the pre- and post-calving terminus thicknesses are given by two unknown but related functions. The functions can vary independently of changes in glacier flow and geometry, and can therefore account for variations in calving behavior due to external forcings and/or self-sustaining calving processes (positive feedbacks). Although the calving framework does not constitute a complete calving model, any thickness-based calving criterion can easily be incorporated into the framework. The framework should be viewed as a guide for future attempts to parameterize calving.

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