THE SENSITIVITY OF NUMERICAL SIMULATION TO OROGRAPHY SPECIFICATION IN THE LOW RESOLUTION SPECTRAL MODEL－PART I: THE EFFECTS OF OROGRAPHY ON THE ATMOSPHERIC GENERAL CIRCULATION

In order to identify the sensitivity of the numerical simulation to the orography specification in a low resolution spectral model, two sets of numerical experiments for full-mountain and no-mountain cases are performed. By comparing the results, it is possible to determine the eflects of mountains on the atmospheric general circulation.This is a global, spectral model incorporating the primitive equations sugmented by physical parameterization and mountains, with five equally-spaced sigma levels in the vertical ang a triangular truncation at wavenum-ber 10 in the horizontal.Analysis of results supports earlier work by demonstrating that the low resolution global spectral model is capable of simulating the major features of global general circulation and indicates that it is necessary to consider the effects of mountains on stationary disturbances in the numerical simulation. The simulations show that topography plays an important role in intensifying heat sources for maintenance of disturbances.All the     

THE EFFECTS OF TOPOGRAPHY ON THE SUMMER ATMOS-PHERIC ENERGETICS OF THE NORTHERN HEMISPHERE IN A LOW-RESOLUTION GLOBAL SPECTRAL MODEL

An analysis is made of the effects of topography on the summer atmospheric energetics of the Northern Hemisphere in a low-resolution global spectral model. The numerical model is a global, spectral, primitive equation model with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. The model includes comparatively full physical processes.Each term of the energy budget equations is calculated in four specific latitudinal belts (81.11°S-11.53°S; 11.53°S-11.53°N; 11.53°N-46.24°N; 46.24°N-81.11°N) from a five-year simulation with mountains and a one-year simulation without mountains, respectively. Differences between them are compared and statistically tested. The results show that synoptical scale waves transport available potential energy and kinetic energy to long waves and increase conversion from available potential energy of the zonal flow to eddy’s and from the eddy kinetic energy to the zonal kinetic energy in region 3 (11.53°N-46.24°N) due to mountains; topography intensifies the atmospheric baroclinity in region 3, consequently the baroclinic conversion of atmosphere energy is increased. The seasonal characteristics associated with the summer atmospheric energy source in region 3 are caused by seasonal variation of the solar radiation and the land-ocean contrasts and independent of topographic effects. The mechanism of topographic effects on the increase of long wave kinetic energy is also discussed.     

Robustness and the Paradox of Bridging Organizations: The Exit Problem in Regional Water Governance Networks in Central America

Bridging organizations facilitate a range of governance processes, including cooperation and social learning, and are theorized to be a key component of robust governance systems. In this article, we use node removal simulations to test structural hypotheses of robustness in a regional water governance network in Central America. We investigate the response of network measures supporting core governance processes to the targeted removal of bridging organizations and other actors, which we compare to random and centrality-based simulations. The results indicate removing bridging organizations has a greater impact on the network than any other type of actor, suggesting bridging organizations are critical to the robustness of the governance system. Furthermore, network structures supporting cooperation may be less robust than structures facilitating social learning. We conclude with policy implications of the research findings as they relate to the exit problem in governance systems with a large presence of international development actors.     

The role of meltwater-induced subsurface ocean warming in regulating the Atlantic meridional overturning in glacial climate simulations

The Community Climate System Model version 3, (CCSM3) is used to investigate the effect of the high latitude North Atlantic subsurface ocean temperature response in idealized freshwater hosing experiments on the strength of the Atlantic meridional overturning circulation (AMOC). The hosing experiments covered a range of input magnitudes at two locations in a glacial background state. Subsurface subpolar ocean warms when freshwater is added to the high latitude North Atlantic (NATL cases) and weakly cools when freshwater is added to the Gulf of Mexico (GOM cases). All cases show subsurface ocean warming in the Southern Hemisphere (SH). The sensitivity of the AMOC response to the location and magnitude of hosing is related to the induced subsurface temperature response, which affects the magnitude of the large-scale meridional pressure gradient at depth through the effect on upper ocean density. The high latitude subsurface warming induced in the NATL cases lowers the upper ocean density in the deepwater formation region enhancing a density reduction by local freshening. In the GOM cases the effect of SH warming partially offsets the effect of the high latitude freshening on the meridional density gradient. Following the end of hosing, a brief convective event occurs in the largest NATL cases which flushes some of the heat stored in the subsurface layers. This fuels a rapid rise in AMOC that lasts less than a couple of decades before subsequent freshening from increases in precipitation and sea ice melt reverses the initial increase in the meridional density gradient. Thereafter AMOC recovery slows to the rate found in comparable GOM cases. The result for these glacial transient hosing experiments is that the pace of the longer recovery is not sensitive to location of the imposed freshwater forcing.     

近10年来"国际古气候模拟比较计划(PMIP)"回顾和未来古气候模拟研究热点

国际古气候模拟比较计划(PMIP)是古气候领域里的一项长期开展的大型国际研究计划.该计划的主旨是利用气候数值模式,通过开展一系列针对古气候变化的模拟试验理解全球气候变化的机理,确定气候系统内部的各种影响气候变化的关键反馈因子;通过比较不同气候模式对不同于现代气候(古气候)的模拟能力评价模式的优劣,进而促进气候模式的发展;同时也为全球变化背景下利用气候模式预估未来气候变化提供借鉴.PMIP白1991年酝酿开始至今经历了PMIP1(1991～2001年)、PMlP2(2002～2007年)两个阶段,并正在开展第三阶段(PMfP3)(2008～)的研究.本文简要回顾了近10年来PMIP研究进展及概述了未来PMIP将要开展研究的重点领域和方向,主要包括4个主题:1)基准(benchmak)时段(6kaB.P.和21kaB.P.)模拟;2)间冰期和温暖期气候模拟;3)快速气候变化模拟;4)不确定性研究.     

PAH concentrations in Coquina (Donax spp.) on a sandy beach shoreline impacted by a marine oil spill

The BP MC252 well failure in the Gulf of Mexico, April 2010 caused concern for crude oil and polycyclic aromatic hydrocarbon (PAHs) exposure along the sandy beaches of the Florida Panhandle. We began collections of Coquina clams (Donax spp.) from the surf zone of Florida Panhandle beaches to monitor PAH contamination to compliment analysis of surf zone sand samples. These clams had higher levels of PAHs relative to ambient sand, and this allowed us to continue to monitor PAH levels after sand concentrations fell below limits of detection. PAH levels in the Coquina tissues were highly variable, perhaps indicative of the heterogeneous distribution of oil and tar on the beaches and exposure to tar particles. Overall, PAH levels decreased continuously in both sand and Coquina tissues, reaching limits of detection within one and two years respectively after oil landed on Florida Panhandle beaches. Our work suggests these surf zone molluscs may be used to monitor pollutant exposure along high energy sandy beach shorelines.     

A comparison of PMIP2 model simulations and the MARGO proxy reconstruction for tropical sea surface temperatures at last glacial maximum

Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm⁻², which translates to equilibrium climate sensitivity for doubling of atmospheric CO₂ of 2.6–3.1°C.     

A Numerical Study of the Impact of Greenhouse Gases on the South Atlantic Ocean Climatology

The National Center for Atmospheric Research Community Climate SystemModel (NCAR CCSM, version 3) numerical coupled model is used tounderstand the climatic impacts on the South Atlantic Ocean due toindustrialization and consequent increase of greenhouse gasemission. Two experiments are analyzed: the first one withtrace/greenhouse gases at pre-industrial levels and a second one wherepresent day levels were adopted. The results show that the annualaveraged sea surface temperature, sea level pressure and barotropictransport intensify and precipitation weakens from one period to thenext. With respect to the seasonal cycle, the sea surface temperaturewarms relative to the pre-industrial period mainly during the winterand spring; while sea level pressure presents higher values in summerand autumn. Barotropic transport has revealed significant differencesbetween the two experiments at middle and high latitudes. Increasedtransport is associated with the intensification of the SubtropicalGyre and the Antarctic Circumpolar Current. Changes in barotropictransport and sea surface temperature leads to an intensification ofthe Polar Front and associated gradients. Examination of theprecipitation field differences showed an increase over the Amazonregion and along the South Atlantic Convergence Zone, during summer.The changes in sea surface temperature, sea-level pressure andbarotropic transport from the pre-industrial period to the present daywere more pronounced at high latitudes. These reach almost 1 °Cand 11Sv between 45-60° S, respectively. Majordifferences in precipitation are confined to the tropics.     

THE SENSITIVITY OF THE NUMERICAL SIMULATION TO OROGRAPHY SPECIFICATION IN THE LOWRESOLUTION SPECTRAL MODEL－PART II: IMPACT OF THE SMOOTHED OROGRAPHY AND RIPPLES ON SIMULATIONS

In order to investigate the impact of the smoothed orography and the spurious orographic ripples on simu-lations in the low-resolution spectral model, three different numerical tests, that is, the unsmoothed orography scheme, the smoothed orography scheme and non-ripples scheme are performed. In this paper, the model used by us is the same as Part I except for orographic specification.The results from simulations indicate that, as far as the climatic simulation is concerned, some aspects of the simulated stationary disturbances, zonal and meridional wind, temperature and precipitation in the low-resolu-tion spectral model with properly smoothed mountains are significantly improved, especially in winter hemis-phere.The deep ripples in the model with the unsmoothed orography produce spurious high pressure regions at the surface with subsidence, and suppress rainfall, causing an unrealistic splitting of the precipitation area in northern winter and summer. Removal of tbe deep ripples by using the special procedure for smoothing topog-raphy allows a strong upward motion in the ripple area with heavy rainfall, eliminating the unrelistic split in the precipitation area.