1,721,013 research outputs found
Bifurcations of the thermohaline circulation in a simplified three-dimensional model of the world ocean and the effects of inter-basin connectivity
No full textA coarse-resolution, global, three-dimensional, idealised model of the world ocean, based on the thermocline equations, is described. It is demonstrated that, at suitable resolution, such a model can be integrated for hundreds or thousands of millenia, and could therefore form a component of an earth system model designed to investigate climate change on these time scales. As in other ocean models, the thermohaline circulation is found to exhibit multiple stable states. The bifurcation structure of the model has been explored, as a function of surface saline forcing, and is found to depend on the inter-basin connectivity, the topography, and the form of the imposed wind forcing
Linear and double-diffusive-inertial instability at the equator
No full textMotivated by observations of interleaving in the equatorial Pacific, we consider the linear stability of a basic state on an equatorial [beta]-plane which is susceptible to both double-diffusive interleaving, driven by a meridional salinity gradient, and inertial instability driven by meridional shear. In a parameter regime compatible with the observations strong interaction can occur between the two processes, indicating that the stability of the system is dependent on the meridional gradients of both salinity and zonal velocity. Meridional shear is found to enhance the interleaving motion even for values of shear well below the cutoff for inertial instability. In the presence of diffusion inertial instability can also be excited by vertical shear, but only if the shear is comparable to the buoyancy frequency. When double-diffusive driving is weak relative to inertial driving the growth can be oscillatory, in which case the mechanism for instability is viscous–diffusive. In this case interleaving layers can slope downwards towards the fresh side of the front in the fingering regime, inhibiting their own growth
Parameter estimation in an intermediate complexity earth system model using an ensemble Kalman filter
No full textWe describe the development of an efficient method for parameter estimation and ensemble forecasting in climate modelling. The technique is based on the ensemble Kalman filter and is several orders of magnitude more efficient than many others which have been previously used to address this problem. As well as being theoretically (near-)optimal, the method does not suffer from the 'curse of dimensionality' and can comfortably handle multivariate parameter estimation. We demonstrate the potential of this method in identical twin testing with an intermediate complexity coupled AOGCM. The model's climatology is successfully tuned via the simultaneous estimation of 12 parameters. Several minor modifications arc described
by which the method was adapted to a steady state (temporally averaged) case. The method is relatively simple to implement, and with only O(50) model runs required, we believe that optimal parameter estimation is now accessible even to computationally demanding models
Location of potential energy sources and the export of dense water from the Atlantic Ocean
No full textThe export of dense water from the Atlantic and into the Indo-Pacific oceans, and heat transport in the opposite direction, is traditionally attributed to the fact that dense water forms in the Atlantic but not the Pacific. Evidence from two models, presented here, suggests this is an incomplete explanation. It is found that dense water export from the Atlantic depends on a potential energy (PE) sink, associated with dense water downwelling in the Atlantic, that is not balanced by a PE source within the Atlantic. Therefore, increasing the Atlantic PE source reduces dense Atlantic water export. Tide models suggest that the Atlantic PE source may have been much higher at the last glacial maximum. This could have significantly reduced dense water export from the Atlantic Ocean, even if Atlantic overturning was stronger than it is today
An efficient climate forecasting method using an intermediate complexity Earth system model and the ensemble Kalman Filter
Full text linkWe present the implementation and results of a model tuning and ensemble forecasting experiment using an ensemble Kalman filter for the simultaneous estimation of 12 parameters in a low resolution coupled atmosphere-ocean Earth System Model by tuning it to realistic data sets consisting of Levitus ocean temperature/salinity climatology, and NCEP/NCAR atmospheric temperature/humidity reanalysis data. The resulting ensemble of tuned model states is validated by comparing various diagnostics, such as mass and heat transports, to observational estimates and other model results. We show that this ensemble has a very reasonable climatology, with the 3-D ocean in particular having comparable realism to much more expensive coupled numerical models, at least in respect of these averaged indicators. A simple global warming experiment is performed to investigate the response and predictability of the climate to a change in radiative forcing, due to 100 years of 1% per annum atmospheric CO2 increase. The equilibrium surface air temperature rise for this CO2 increase is 4.2±0.1°C, which is approached on a time scale of 1,000 years. The simple atmosphere in this version of the model is missing several factors which, if included, would substantially increase the uncertainty of this estimate. However, even within this ensemble, there is substantial regional variability due to the possibility of collapse of the North Atlantic thermohaline circulation (THC), which switches off in more than one third of the ensemble members. For these cases, the regional temperature is not only 3–5°C colder than in the warmed worlds where the THC remains switched on, but is also 1–2°C colder than the current climate. Our results, which illustrate how objective probabilistic projections of future climate change can be efficiently generated, indicate a substantial uncertainty in the long-term future of the THC, and therefore the regional climate of western Europe. However, this uncertainty is only apparent in long-term integrations, with the initial transient response being similar across the entire ensemble. Application of this ensemble Kalman filtering technique to more complete climate models would improve the objectivity of probabilistic forecasts and hence should lead to significantly increased understanding of the uncertainty of our future climate
Incorporation of the C-GOLDSTEIN efficient climate model into the GENIE framework: "eb_go_gs" configurations of GENIE
Full text linkA computationally efficient, intermediate complexity ocean-atmosphere-sea ice model (C-GOLDSTEIN) has been incorporated into the Grid ENabled Integrated Earth system modelling (GENIE) framework. This involved decoupling of the three component modules that were re-coupled in a modular way, to allow replacement with alternatives and coupling of further components within the framework. The climate model described here (referred to as "eb_go_gs" for short) is the most basic version of GENIE in which atmosphere, ocean and sea ice all play an active role. Among improvements on the original C-GOLDSTEIN model, latitudinal grid resolution is generalized to allow a wider range of surface grids to be used. The ocean, atmosphere and sea-ice components of the "eb_go_gs" configuration of GENIE are individually described, along with details of their coupling. The setup and results from simulations using four different meshes are presented. The four alternative meshes comprise the widely-used 36 × 36 equal-area-partitioning of the Earth surface with 16 depth layers in the ocean, a version in which horizontal and vertical resolution are doubled, a setup matching the horizontal resolution of the dynamic atmospheric component available in the GENIE framework, and a setup with enhanced resolution in high-latitude areas. Results are presented for a spin-up experiment with a baseline parameter set and wind forcing typically used for current studies in which "eb_go_gs" is coupled with the ocean biogeochemistry module of GENIE, as well as for an experiment with a modified parameter set, revised wind forcing, and additional cross-basin transport pathways (Indonesian and Bering Strait throughflows). The latter experiment is repeated with the four mesh variants, with common parameter settings throughout, except for time-step length. Selected state variables and diagnostics are compared in two regards: (i) between simulations at lowest resolution that are obtained with the baseline and modified configurations, predominantly in order to evaluate the revision of the wind forcing, the modification of some key parameters, and the effect of additional transport pathways across the Arctic Ocean and the Indonesian Archipelago; (ii) between simulations with the four meshes, in order to explore various effects of mesh choice.<br/
Stability of the thermohaline circulation in different complexities and resolutions of earth system model (abstract of paper presented at 3rd EGU General Assembly)
No full textWe use the GENIE Earth system modelling framework to examine how the stability / hysteresis diagram of the thermohaline circulation (THC) depends on the use of complex (GCM) or simple (energy-moisture balance) atmosphere models and how it varies with ocean resolution.The model versions all use the GOLDSTEIN frictional geostrophic ocean, but with 3 different horizontal resolutions (and 8 depth layers in each case): (i) 36x36 longitudesine (latitude), (ii) 72x72 longitude-sine(latitude), (iii) 64x32 longitude-latitude. To these we have coupled the Reading Intermediate General Circulation Model (IGCM) at T21 resolution with 7 vertical levels. We contrast this with earlier work using an energy-moisture balance model (EMBM) and ocean resolution (i).For each model version, we construct an ensemble of runs in which we vary atmospheric freshwater transport from the Atlantic to Pacific. In some cases we also vary a parameter controlling equator to pole freshwater transport. The resulting ensembles are run toward equilibrium and then restarts are used to search parameter space for regions of THC bi-stability.The resulting hundreds of thousands of years of 3D ocean-atmosphere model integration were achieved by using UK Grid computing resources, including 6 nodes of the National Grid Service, and additional clusters in Norwich, Southampton and Bristol. A specially developed database system was used to execute and manage the runs.The results are expected to shed light on whether a dynamical atmosphere alters or removes the bi-stability of the THC, and whether THC stability is sensitive to ocean resolution
Multiobjective tuning of GENIE Earth system models
No full textIn order to simulate at the multi-decadal time scale and beyond, climate models rely heavily on parameterisations of physical processes that occur on comparatively small time and spatial scales. A key concern in climate modelling is therefore to find appropriate values for these parameters so that a reasonable climatology is simulated. This is of particularly importance within the GENIE modelling framework where component codes, that are often developed independently, are coupled together to form new Earth system models. In order to produce stable and sensible model output it is almost always necessary to re-tune the parameters of the coupled system. However, as with many design problems, the nonlinear response of a model to its parameters and the often conflicting tuning objectives make this a difficult problem to solve.The general problem of optimising a set of model parameters in order to improve a number of possibly conflicting design objectives is typically approached in one of two ways. One can create a single objective measure of design quality by computing a weighted sum of the individual objectives and seek to find the set of variables that minimise or maximise this measure. Many sophisticated algorithms can be applied to a single objective problem but the weighting factors can be critical in the performance of the optimisation. Alternatively, multiobjective methods can be employed to seek a Pareto set of non-dominated solutions; designs that are superior when all objective measures are considered but that may be inferior when a subset of those objectives are considered. Such a solution set can inform the user of competition in the design goals and allows domain expertise to be applied to select the most appropriate parameter sets for further study.We present the results of applying a multiobjective Non-dominated Sorting Genetic Algorithm (NSGA-II) to tune two models from the GENIE framework. The genie_eb-go-gs (3D frictional geostrophic ocean model, 2D energy moisture balance model and 2D sea-ice) and genie-ig-fi-fi-ml (3D atmosphere, 2D fixed ocean and sea-ice and land surface) models are tuned to appropriate target data sets by minimising multiple measures of model-data mismatch across different physical fields. Grid computing is exploited to perform the large number of concurrent simulations that comprise the generations of the algorithm. Recent advances in the method use Response Surface Modelling (RSM) to provide surrogate models of the underlying objective functions. These RSMs can be searched much more cheaply and extensively to provide considerable performance improvements in the optimisation
A comprehensive, multi-process box-model approach to glacial-interglacial carbon cycling
Full text linkThe canonical question of which physical, chemical or biological mechanisms were responsible for oceanic uptake of atmospheric CO2 during the last glacial is yet unanswered. Insight from paleo proxies has led to a multitude of hypotheses but none so far have been convincingly supported in three dimensional numerical modelling experiments. The processes that influence the CO2 uptake and export production are inter-related and too complex to solve conceptually while complex numerical models are time consuming and expensive to run which severely limits the combinations of mechanisms that can be explored. Instead, an intermediate inverse box model approach is used here in which the whole parameter space is explored. The glacial circulation and biological production states are derived from these using proxies of glacial export production and the need to draw down CO2 into the ocean. We find that circulation patterns which explain glacial observations include reduced Antarctic Bottom Water formation and high latitude mixing and to a lesser extent reduced equatorial upwelling. The proposed mechanism of CO2 uptake by an increase of eddies in the Southern Ocean, leading to a reduced residual circulation, is not supported. Regarding biological mechanisms, an increase in the nutrient utilization in either the equatorial regions or the northern polar latitudes can reduce atmospheric CO2 and satisfy proxies of glacial export production. Consistent with previous studies, CO2 is drawn down more easily through increased productivity in the Antarctic region than the sub-Antarctic, but that violates observations of lower export production there
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