Passive tracers in a general circulation model of the Southern Ocean     

I. G. Stevens  D. P. Stevens 《Annales Geophysicae》1999,17(7):971-982

Passive tracers are used in an off-line version of the United Kingdom Fine Resolution Antarctic Model (FRAM) to highlight features of the circulation and provide information on the inter-ocean exchange of water masses. The use of passive tracers allows a picture to be built up of the deep circulation which is not readily apparent from examination of the veloCity or density fields. Comparison of observations with FRAM results gives good agreement for many features of the Southern Ocean circulation. Tracer distributions are consistent with the concept of a global “conveyor belt” with a return path via the Agulhas retroflection region for the replenishment of North Atlantic Deep Water.  相似文献   

SCUBA: a common-user submillimetre camera operating on the James Clerk Maxwell Telescope     

W. S. Holland  E. I. Robson  W. K. Gear  C. R. Cunningham  J. F. Lightfoot  T. Jenness  R. J. Ivison  J. A. Stevens  P. A. R. Ade  M. J. Griffin  W. D. Duncan  J. A. Murphy  D. A. Naylor 《Monthly notices of the Royal Astronomical Society》1999,303(4):659-672

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A possible active galactic nucleus in M82?     

K. A. Wills  A. Pedlar  T. W. B. Muxlow  I. R. Stevens 《Monthly notices of the Royal Astronomical Society》1999,305(3):680-684

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Beach ridges of Dali Lake in Inner Mongolia reveal precipitation variation during the Holocene     

Mengyao Jiang  Zhiyong Han  Xusheng Li  Yong Wang  Thomas Stevens  Jun Cheng  Cunjuan Lv  Yuwen Zhou  Qianqian Yang  Zhiwei Xu  Shuangwen Yi  Huayu Lu 《第四纪科学杂志》2020,35(5):716-725

Variability in East Asian summer monsoon precipitation during the Holocene remains of debate. In this study, we use a closed lake with well-dated lake beach ridges located on the margin of the East Asian summer monsoon, a region highly sensitive to monsoon precipitation changes, to obtain a temporal sequence of water volume in North China. The elevation of each beach ridge calibrated to the modern lake level was surveyed. Optically stimulated luminescence dating of undisturbed sediments of beach ridges was performed. The lake area and water volume corresponding to each beach ridge were calculated using a digital elevation model. This study reveals relatively reduced monsoon precipitation from ~12 to 7 ka interrupted by strengthening of the monsoon circulation to a maximum from ~7 to ~5 ka and followed by greatly reduced monsoon intensity until the present day. These results demonstrate that changes in the East Asian summer monsoon precipitation may not be directly driven by global temperature or atmospheric CO₂ content. Rather, we suggest that variation in the the monsoon margin precipitation is probably mainly driven by ice volume and subordinately by the summer solar insolation difference between mid-latitude land and low-latitude ocean.  相似文献   

The Influence of Wave Energy and Sediment Transport on Seagrass Distribution     

Andrew W. Stevens  Jessica R. Lacy 《Estuaries and Coasts》2012,35(1):92-108

A coupled hydrodynamic and sediment transport model (Delft3D) was used to simulate the water levels, waves, and currents associated with a seagrass (Zostera marina) landscape along a 4-km stretch of coast in Puget Sound, WA, USA. A hydroacoustic survey of seagrass percent cover and nearshore bathymetry was conducted, and sediment grain size was sampled at 53 locations. Wave energy is a primary factor controlling seagrass distribution at the site, accounting for 73% of the variability in seagrass minimum depth and 86% of the variability in percent cover along the shallow, sandy portions of the coast. A combination of numerical simulations and a conceptual model of the effect of sea-level rise on the cross-shore distribution of seagrass indicates that the area of seagrass habitat may initially increase and that wave dynamics are an important factor to consider in predicting the effect of sea-level rise on seagrass distributions in wave-exposed areas.  相似文献   

Modeling the Hydrodynamic and Morphologic Response of an Estuary Restoration   总被引：1，自引：0，他引：1  

Douglas A. George  Guy Gelfenbaum  Andrew W. Stevens 《Estuaries and Coasts》2012,35(6):1510-1529

Estuary evolution is investigated using the hydrodynamic and sediment transport model, Delft3D, to study the response of a dammed tidal basin to restored tidal processes. The development of decadal (10-year) morphological simulations of the restored estuary required simplifying several data inputs and implementing a time-scale acceleration technique. An innovative river sediment discharge schematization was developed that connected sediment discharge to morphological change in the estuary. Mud erodibility parameters were determined from laboratory analysis of sediment cores from the modern lakebed and statistical refinement with a Bayes network of the probability of occurrence. The changing estuary morphology appears to have a dominant impact on the physical habitat (substrate, inundation frequency, mean salinity, and salinity range). The numerical model provides a tool to compare the functions of the historical estuary and possible future alternatives for a restored estuary. Sensitivity of the morphological model to sediment types and erodibility parameters was also examined. A conceptual model covering morphology and indicators of physical habitat for three phases of estuary evolution during restoration is presented that could be applied to estuarine systems that are severely out of equilibrium.  相似文献   

The end of continental growth by TTG magmatism   总被引：1，自引：0，他引：1  

Angelique Laurie  Gary Stevens  Jeroen van Hunen 《地学学报》2013,25(2):130-136

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A modeling approach for assessing the effect of multiple alpine lakes in sequence on nutrient transport   总被引：1，自引：1，他引：0  

Dave M. Epstein  Bethany T. Neilson  Keli J. Goodman  David K. Stevens  Wayne A. Wurtsbaugh 《Aquatic Sciences - Research Across Boundaries》2013,75(2):199-212

The effects of a single lake on downstream water chemistry may be compounded by the presence of additional lakes within the watershed, augmenting or negating the effects of the first lake. Multiple, linked lakes are a common feature of many watersheds and these resemble reactors in series often studied in engineering. The effects of multiple lakes in series on nutrient transport are largely unexplored. We populated and calibrated a simple lake model to investigate the role of a sub-alpine lake (Bull Trout Lake (BTL), Rocky Mountains, USA) on the transport of the macronutrients during the summer of 2008. Further, we developed a sequential model in which four identical lakes (copies of the BTL model) were connected in series. All lakes in the sequence retarded the flux of nutrients, thus slowing their transport downstream. The first lake in the sequence dramatically altered stream water chemistry and served as a sink for C and P and a source of N, while additional lakes downstream became sources of C, N and P. Although additional downstream lakes resulted in important changes to water chemistry and nutrient transport, the nature of the changes were similar from Lakes 2 to 4 and the magnitude of the changes diminished with distance downstream. Our lake model served as an effective tool for assessing the nutrient budget of the lake and the hypothetical effect of multiple lakes in sequence in a landscape limnology framework.  相似文献   

Rossby wave dynamics of the North Pacific extra-tropical response to El Niño: importance of the basic state in coupled GCMs     

Andrew Dawson  Adrian J. Matthews  David P. Stevens 《Climate Dynamics》2011,37(1-2):391-405

The extra-tropical response to El Niño in a “low” horizontal resolution coupled climate model, typical of the Intergovernmental Panel on Climate Change fourth assessment report simulations, is shown to have serious systematic errors. A high resolution configuration of the same model has a much improved response that is similar to observations. The errors in the low resolution model are traced to an incorrect representation of the atmospheric teleconnection mechanism that controls the extra-tropical sea surface temperatures (SSTs) during El Niño. This is due to an unrealistic atmospheric mean state, which changes the propagation characteristics of Rossby waves. These erroneous upper tropospheric circulation anomalies then induce erroneous surface circulation features over the North Pacific. The associated surface wind speed and direction errors create erroneous surface flux and upwelling anomalies which finally lead to the incorrect extra-tropical SST response to El Niño in the low resolution model. This highlights the sensitivity of the climate response to a single link in a chain of complex climatic processes. The correct representation of these processes in the high resolution model indicates the importance of horizontal resolution in resolving such processes.  相似文献   

Climate feedback efficiency and synergy     

Thorsten Mauritsen  Rune G. Graversen  Daniel Klocke  Peter L. Langen  Bjorn Stevens  Lorenzo Tomassini 《Climate Dynamics》2013,41(9-10):2539-2554

Earth’s climate sensitivity to radiative forcing induced by a doubling of the atmospheric CO₂ is determined by feedback mechanisms, including changes in atmospheric water vapor, clouds and surface albedo, that act to either amplify or dampen the response. The climate system is frequently interpreted in terms of a simple energy balance model, in which it is assumed that individual feedback mechanisms are additive and act independently. Here we test these assumptions by systematically controlling, or locking, the radiative feedbacks in a state-of-the-art climate model. The method is shown to yield a near-perfect decomposition of change into partial temperature contributions pertaining to forcing and each of the feedbacks. In the studied model water vapor feedback stands for about half the temperature change, CO₂-forcing about one third, while cloud and surface albedo feedback contributions are relatively small. We find a close correspondence between forcing, feedback and partial surface temperature response for the water vapor and surface albedo feedbacks, while the cloud feedback is inefficient in inducing surface temperature change. Analysis suggests that cloud-induced warming in the upper tropical troposphere, consistent with rising convective cloud anvils in a warming climate enhances the negative lapse-rate feedback, thereby offsetting some of the warming that would otherwise be attributable to this positive cloud feedback. By subsequently combining feedback mechanisms we find a positive synergy acting between the water vapor feedback and the cloud feedback; that is, the combined cloud and water vapor feedback is greater than the sum of its parts. Negative synergies surround the surface albedo feedback, as associated cloud and water vapor changes dampen the anticipated climate change induced by retreating snow and ice. Our results highlight the importance of treating the coupling between clouds, water vapor and temperature in a deepening troposphere.  相似文献