1,721,009 research outputs found
Evolving Bayesian Emulators for Structured Chaotic Time Series, with Application to Large Climate Models
No full textWe develop Bayesian dynamic linear model Gaussian processes for emulation of time series output for computer models that may exhibit chaotic behavior, but where this behavior retains some underlying structure. The statistical technology is particularly suited to emulating the time series output of large climate models that exhibit this feature and where we want samples from the posterior of the emulator to evolve in the same way as dynamic processes in the computer model do. The methodology combines key features of good uncertainty quantification (UQ) methods such as using complex mean functions to capture large-scale signals within parameter space, with dynamic linear models in a way that allows UQ to borrow strength from the Bayesian time series literature. We present an MCMC algorithm for sampling from the posterior of the emulator parameters when the roughness lengths of the Gaussian process are unknown. We discuss an interpretation of the results of this algorithm that allows us to use MCMC to fix the correlation lengths, making future online samples from the emulator tractable when used in practical applications where online MCMC is infeasible. We apply this methodology to emulate the Atlantic Meridional Overturning Circulation (AMOC) as a time series output of the fully coupled non--flux-adjusted atmosphere-ocean general circulation model HadCM3.
Read More: http://epubs.siam.org/doi/abs/10.1137/12090091
Corrigendum to: Sévellec, F., J. J.-M. Hirschi, and A. T. Blaker, 2013: On the near-inertial resonance of the Atlantic meriodional overturning circulation. J. Phys. Oceanogr., 43, 2661–2672, doi:10.1175/JPO-D-13-092.1.
No full textNorth Atlantic SST anomalies and the cold North European weather events of winter 2009/10 and December 2010
Full text linkNorthern Europe experienced consecutive periods of extreme cold weather in the winter of 2009/2010 and in late 2010. These periods were characterised by a tripole pattern in North Atlantic sea surface temperature (SST) anomalies and exceptionally negative phases of the North Atlantic Oscillation (NAO). A global Ocean-Atmosphere General Circulation Model (OAGCM) is used to investigate the ocean’s role in influencing North Atlantic and European climate. Observed SST anomalies are used to force the atmospheric model and the resultant changes in atmospheric conditions over Northern Europe are examined. We observe differences between the atmospheric responses in the winter of 2009/2010 and the early winter of 2010. Our experiments suggest that North Atlantic SST anomalies did not significantly affect the development of the negative NAO phase in the cold winter of 2009/2010. However, in November and December 2010 the large-scale North Atlantic SST anomaly pattern leads to significant shift in the atmospheric circulation over the North Atlantic towards a NAO negative phase. Therefore, our results indicate that SST anomalies in November/December 2010 were particularly conducive to the development of a negative NAO phase which culminated in the extreme cold weather conditions experienced over Northern Europe in December 2010
Identifying and removing structural biases in climate models with history matching
Full text linkWe describe the method of history matching, a method currently used to help quantify parametric uncertainty in climate models, and argue for its use in identifying and removing structural biases in climate models at the model development stage. We illustrate the method using an investigation of the potential to improve upon known ocean circulation biases in a coupled non-flux-adjusted climate model (the third Hadley Centre Climate Model; HadCM3). In particular, we use history matching to investigate whether or not the behaviour of the Antarctic Circumpolar Current (ACC), which is known to be too strong in HadCM3, represents a structural bias that could be corrected using the model parameters. We find that it is possible to improve the ACC strength using the parameters and observe that doing this leads to more realistic representations of the sub-polar and sub-tropical gyres, sea surface salinities (both globally and in the North Atlantic), sea surface temperatures in the sinking regions in the North Atlantic and in the Southern Ocean, North Atlantic Deep Water flows, global precipitation, wind fields and sea level pressure. We then use history matching to locate a region of parameter space predicted not to contain structural biases for ACC and SSTs that is around 1 % of the original parameter space. We explore qualitative features of this space and show that certain key ocean and atmosphere parameters must be tuned carefully together in order to locate climates that satisfy our chosen metrics. Our study shows that attempts to tune climate model parameters that vary only a handful of parameters relevant to a given process at a time will not be as successful or as efficient as history matching
On the Near-Inertial Resonance of the Atlantic Meridional Overturning Circulation
Full text linkThe Atlantic meridional overturning circulation (AMOC) is a crucial component of the global climate system. It is responsible for around a quarter of the global northward heat transport and contributes to the mild European climate. Observations and numerical models suggest a wide range of AMOC variability. Recent results from an ocean general circulation model (OGCM) in a high-resolution configuration (¼°) suggest the existence of superinertial variability of the AMOC. In this study, the validity of this result in a theoretical framework is tested. At a low Rossby number and in the presence of Rayleigh friction, it is demonstrated that, unlike a typical forced damped oscillator (which shows subinertial resonance), the AMOC undergoes both super- and subinertial resonances (except at low latitudes and for high friction). A dimensionless number Sr, measuring the ratio of ageo- to geostrophic forcing (i.e., the zonal versus meridional pressure gradients), indicates which of these resonances dominates. If Sr ≪ 1, the AMOC variability is mainly driven by geostrophic forcing and shows subinertial resonance. Alternatively and consistent with the recently published ¼° OGCM experiments, if Sr ≫ 1, the AMOC variability is mainly driven by the ageostrophic forcing and shows superinertial resonance. In both regimes, a forcing of ±1 K induces an AMOC variability of ±10 Sv (1 Sv ≡ 106 m3 s−1) through these near-inertial resonance phenomena. It is also shown that, as expected from numerical simulations, the spatial structure of the near-inertial AMOC variability corresponds to equatorward-propagating waves equivalent to baroclinic Poincaré waves. The long-time average of this resonance phenomenon, raising and depressing the pycnocline, could contribute to the mixing of the ocean stratification
Dynamical evolution of ENSO in a warming background: A review of recent trends & future projections
No full textThe wide-spread implications of El Niño-Southern Oscillation (ENSO) on global and regional climate necessitates a better understanding of how the underlying interannual dynamics have changed over recent years. Year-to-year changes in ENSO impact terrestrial and marine habitats, water availability, food security and social stability (Santoso et al., 2017). With abundant evidence of a warming climate, it is imperative to understand how a large-scale climatic oscillation such as ENSO is evolving and influencing changes in large-scale atmospheric circulation patterns (Alizadeh et al., 2022; Cai et al., 2021). Furthermore, quantifying the influence of the ocean on changes in this climatic pattern is an interesting and important question to answer. Evaluating the ability of models to appropriately represent the underlying physics and dynamical changes impacting the spatiotemporal extent and the intensity of ENSO is crucial to understanding ocean-climate teleconnections and changes in climatic extremes. In this study, we review and evaluate the representation of ENSO in several high-resolution CMIP6 and HighResMIP models and forced ocean-only simulations focusing on the ability of current state-of-the-art models to represent central equatorial pacific warming and cooling. This evaluation involves looking at the development and propagation of warm temperature anomalies on surface and sub-surface levels in the equatorial Pacific and understanding the differences in simulating surface heat budget and exchange with the overlying atmosphere and the deeper ocean. Surface and sub-surface (up to 200m depth) temperature anomalies in the Niño 3.4 region were calculated from modelled data and were then compared with anomalies from observational and reanalysis temperature datasets (like EN4, ORAS5). We find good agreement in the timing and vertical structure of surface/sub-surface temperature anomalies in the forced model simulations, particularly during strong ENSO years. Moreover, the genesis of sub-surface anomalies and their further propagation to the surface was well simulated in the forced simulations. The vertical coherence of temperature anomalies was relatively more pronounced in forced ocean-only simulations than in coupled high-resolution model runs. Furthermore, we comment on the shortcomings and suggest potential improvements that can be made in the models that could improve the model's ability to capture ENSO strength and variability
Tuning without over-tuning: parametric uncertainty quantification for the NEMO ocean model
No full textIn this paper we discuss climate model tuning and present an iterative automatic tuning method from the statistical science literature. The method, which we refer to here as iterative refocussing (though also known as history matching), avoids many of the common pitfalls of automatic tuning procedures that are based on optimisation of a cost function, principally the over-tuning of a climate model due to using only partial observations. This avoidance comes by seeking to rule out parameter choices that we are confident could not reproduce the observations, rather than seeking the model that is closest to them (a procedure that risks over-tuning). We comment on the state of climate model tuning and illustrate our approach through three waves of iterative refocussing of the NEMO (Nucleus for European Modelling of the Ocean) ORCA2 global ocean model run at 2° resolution. We show how at certain depths the anomalies of global mean temperature and salinity in a standard configuration of the model exceeds 10 standard deviations away from observations and show the extent to which this can be alleviated by iterative refocussing without compromising model performance spatially. We show how model improvements can be achieved by simultaneously perturbing multiple parameters, and illustrate the potential of using low-resolution ensembles to tune NEMO ORCA configurations at higher resolutions
Characterising surface discrepancies and vertical coherence of ocean temperature anomalies in CMIP6 HighResMIP during ENSO events
No full textConsidering the widespread implications of El Niño-Southern Oscillation (ENSO) on global and regional climate, through atmospheric and oceanic teleconnections, understanding its changing inter-annual dynamics in the context of a warming climate is crucial. Therefore, evaluating the ability of current generation high-resolution climate models to accurately simulate the spatio-temporal characteristics and underlying dynamics of ENSO, is essential for improving future projections. In this study, we review and evaluate the representation of ENSO in several high-resolution coupled climate model simulations from the CMIP6 HighResMIP project, along with two ocean-only simulations forced with surface fluxes from atmospheric reanalyses. Our evaluation concentrates on the ability of current state-of-the-art models to represent central equatorial Pacific warming and cooling. We assess the representation of surface and sub-surface spatio-temporal characteristics of the equatorial Pacific Ocean temperature anomalies that define ENSO events against observation-based ocean temperature datasets. We observe a strong alignment in both the timing and vertical structure of temperature anomalies in the ocean-only model simulations with observations, particularly during strong ENSO events. The genesis of sub-surface anomalies and their further vertical propagation to the surface is well simulated in the atmosphere-forced ocean-only model runs. However, several high-resolution coupled model runs underestimated the magnitude of sub-surface temperature anomalies and showed significant diversity in representing typical ENSO characteristics. The vertical coherence of temperature anomalies was more pronounced in forced ocean-only simulations compared to coupled model runs. The underestimation of sub-surface temperature anomalies and the large diversity in characteristics in coupled model runs indicate potential shortcomings in accurately representing the genesis and evolution of temperature anomalies. Furthermore, potential hypotheses have been discussed to explain the observed model diversity and shortcomings of coupled model runs compared to the ocean-only model simulations
Fast linked analyses for scenario-based hierarchies
No full textWhen using computer models to provide policy support it is normal to encounter ensembles that test only a handful of feasible or idealized decision scenarios. We present a new methodology for performing multilevel emulation of a complex model as a function of any decision within a predefined class that makes specific use of a scenario ensemble of opportunity on a fast or early version of a simulator and a small, well-chosen, design on our current simulator of interest. The method exploits a geometrical approach to Bayesian inference and is designed to be fast, to facilitate detailed diagnostic checking of our emulators by allowing us to carry out many analyses very quickly. Our motivating application involved constructing an emulator for the UK Met Office Hadley Centre coupled climate model HadCM3 as a function of carbon dioxide forcing, which was part of a ‘RAPID’ programme deliverable to the UK Met Office funded by the Natural Environment Research Council. Our application involved severe time pressure as well as limited access to runs of HadCM3 and a scenario ensemble of opportunity on a lower resolution version of the model
A numerical model study of the effects of interannual timescale wave propagation on the predictability of the Atlantic meridional overturning circulation
Full text linkWe investigate processes leading to uncertainty in forecasts of the Atlantic meridional overturning circulation (AMOC). A climate model is used to supply initial conditions for ensemble simulations in which members initially have identical ocean states but perturbed atmosphere states. Baroclinic transports diverge on interannual timescales even though the ocean is not eddy-permitting. Interannual fluctuations of the model AMOC in the subtropical gyre are caused by westward propagating Rossby waves. Divergence of the predicted AMOC with time occurs because the waves develop different phases in different ensemble members predominantly due to differences in eastern boundary windstress curl. These windstress fluctuations communicate with interior ocean transports via modifications to the vertical velocity and the vortex stretching term dw/dz. Consequently, errors propagate westwards resulting in longer predictability times in the interior ocean compared with the eastern boundary. Another source of divergence is transport anomalies propagating along the Gulf Stream (and other boundary currents). The propagation mechanism seems to be predominantly advection by mean currents, and we show that the arrival of westward propagating waves can trigger development of these anomalies. The mean state of the AMOC has a small effect on interannual predictability in the subtropical gyre, most likely because eastern boundary windstress curl predictability is not strongly dependent on the state of the AMOC in the subtropics. Eastern boundary windstress curl was more predictable at 45{degree sign}N when the AMOC was in a strongly decreasing state, but, unlike at 30{degree sign}N, no mechanism was found linking windstress curl fluctuations with deep transports
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