Large Ensemble Experiments Research Articles

Event-to-event intensification of the hydrologic cycle from 1.5\u2009\xb0C to a 2\u2009\xb0C warmer world

The Paris agreement was adopted to hold the global average temperature increase to well below 2 °C and pursue efforts to limit it to 1.5 °C. Here, we investigate the event-to-event hydroclimatic intensity, where an event is a pair of adjacent wet and dry spells, under future warming scenarios. According to a set of targeted multi-model large ensemble experiments, event-wise intensification will significantly increase globally for an additional 0.5 °C warming beyond 1.5 °C. In high latitudinal regions of the North American continent and Eurasia, this intensification is likely to involve overwhelming increases in wet spell intensity. Western and Eastern North America will likely experience more intense wet spells with negligible changes of dry spells. For the Mediterranean region, enhancement of dry spells seems to be dominating compared to the decrease in wet spell strength, and this will lead to an overall event-wise intensification. Furthermore, the extreme intensification could be 10 times stronger than the mean intensification. The high damage potential of such drastic changes between flood and drought conditions poses a major challenge to adaptation, and the findings suggest that risks could be substantially reduced by achieving a 1.5 °C target.

PROJECTION OF FUTURE STORM SURGES AROUND THE KOREAN PENINSULA BASED ON LARGE ENSEMBLE CLIMATE EXPERIMENTS

Projection of future storm surge height (SSH) based on results of ensemble climate experiments performed using a general climate model (hereafter, GCM) under future climate conditions is ready to start at the regional scale for disaster prevention against storm surge. However, there are limitation to estimate future SSHs with particular return periods which required in coastal structure design because of the lack of sample numbers of storm surge events on local scale. To obtain a large number of samples in localized catastrophic events, Mizuta et al. (2016) carried out an unprecedentedly large ensemble of climate simulations using a high-resolution global climate model over 5000 years. In this study, spatial pattern of storm surge values around the Korean Peninsula (hereafter, the KP) with 100-years return period are assessed based on the large ensemble experiments.

Assessment of long-term impact of storm surges around the Korean Peninsula based on a large ensemble of climate projections

This study assesses the long-term impact of storm surges due to typhoons around the Korean Peninsula (KP) resulting from climate change. Long-term projection is conducted based on the large ensemble experiments on so-called d4PDF for the past and +4 K future climate conditions over 5000 years by a single atmospheric global climate model developed by the Meteorological Research Institute of Japan. The use of a large ensemble is to enable one to obtain probabilistic future changes in low-frequency of extreme storm surge events. The properties of typhoons which may directly and indirectly have an effect on the KP in terms of past and future climate conditions is extracted from d4PDF. These are employed as the driving force in the projection of future storm surges around the KP. The storm surge heights (SSH) around the KP are projected to increase in the future climate except for around some areas in the south coast. The magnitude of future change of SSH varies spatially. The maximum variation was estimated to be 0.36 m (9.9%) with a 100-year return period in the west area of the southeastern coast of the KP. The locations of the areas vulnerable to storm surge shift to the north area of the western region and to the west area of the southern regions in the Korean Peninsula under the future climate. The characteristic of future change to areas where high SSHs will occur coincides with that of the typhoon tracks.

Multi-Decadal Trend and Decadal Variability of the Regional Sea Level over the Indian Ocean since the 1960s: Roles of Climate Modes and External Forcing

Previous studies suggest that anthropogenic warming has affected the multi-decadal trend patterns of sea level over the Indian Ocean (IO). This effect, however, has not been quantified. Using observational datasets combined with large ensemble experiments from two climate models, this paper assesses the effects of natural internal variability versus external forcing on the observed, multi-decadal trend pattern and the decadal sea level anomaly (SLA) of the IO since the 1960s. Because the global mean sea level rise (SLR), which results largely from external forcing, has been removed before the examination, the paper focuses on the regionally uneven distribution of trend and SLA. The impacts of climate modes are quantified using a Bayesian Dynamic Linear Model. For the regional trend pattern of 1958–2005, the effects of internal variability dominate external forcing. Over the Seychelles area where sea-level variations obtain the maximum, internal variability (external forcing) contributes 81% (19 ± 2.4%) of the observed trend. For decadal SLA, internal variability is the predominant cause, with a standard deviation (STD) ratio of externally forced/observed SLA being 18 ± 17% over Seychelles and 17 ± 11% near the Indonesian Throughflow (ITF) area. Climate modes account for most observed SLA during boreal winter, with the total effects of decadal ENSO, Indian Ocean Dipole (IOD), and monsoon accounting for 78–86% of the observed STD near the Seychelles region, ITF area, and coasts of Sumatra and the Bay of Bengal. During summer, climate modes explain 95% of observed STD near the ITF but only 58–67% in other regions. Decadal ENSO dominates the SLA in the south tropical IO for both seasons and near the coasts of Sumatra and the Bay during winter. Decadal IOD and monsoon, however, control the coastal SLA during summer. Remote and local winds over the IO are the main drivers for decadal SLA, while the Pacific influence via the ITF is strong mainly in the southeast basin.

The influences of El Nino and Arctic sea-ice on the QBO disruption in February 2016

The westerly phase of the quasi-biennial oscillation (QBO) was unexpectedly disrupted by an anomalous easterly near 40 hPa (~23 km) in February 2016. At the same time, a very strong El Nino and a very low Arctic sea-ice concentration in the Barents and Kara Sea were present. Previous studies have shown that the disruption of the QBO was primarily caused by the momentum transport of the atmospheric waves in the Northern Hemisphere. Our results indicate that the tropical waves evident over the Atlantic, Africa, and the western Pacific were associated with extratropical disturbances. Moreover, we suggest that the El Nino and sea-ice anomalies in 2016 account for approximately half of the disturbances and waves based on multiple regression analysis of the observational/reanalysis data and large-ensemble experiments using an atmospheric global climate model.

Role of the Atlantic Multidecadal Variability in modulating the climate response to a Pinatubo-like volcanic eruption

The modulation by the Atlantic multidecadal variability (AMV) of the dynamical climate response to a Pinatubo-like eruption is investigated for the boreal winter season based on a suite of large ensemble experiments using the CNRM-CM5 Coupled Global Circulation Model. The volcanic eruption induces a strong reduction and retraction of the Hadley cell during 2 years following the eruption and independently of the phase of the AMV. The mean extratropical westerly circulation simultaneously weakens throughout the entire atmospheric column, except at polar Northern latitudes where the zonal circulation is slightly strengthened. Yet, there are no significant changes in the modes of variability of the surface atmospheric circulation, such as the North Atlantic Oscillation (NAO), in the first and the second winters after the eruption. Significant modifications over the North Atlantic sector are only found during the third winter. Using clustering techniques, we decompose the atmospheric circulation into weather regimes and provide evidence for inhibition of the occurrence of negative NAO-type circulation in response to volcanic forcing. This forced signal is amplified in cold AMV conditions and is related to sea ice/atmosphere feedbacks in the Arctic and to tropical-extratropical teleconnections. Finally, we demonstrate that large ensembles of simulations are required to make volcanic fingerprints emerge from climate noise at mid-latitudes. Using small size ensemble could easily lead to misleading conclusions especially those related to the extratropical dynamics, and specifically the NAO.

Investigating the zonal wind response to SST warming using transient ensemble AGCM experiments

The response of the atmospheric circulation to greenhouse gas-induced SST warming is investigated using large ensemble experiments with two AGCMs, with a focus on the robust feature of the poleward shift of the eddy driven jet. In these experiments, large ensembles of simulations are conducted by abruptly switching the SST forcing on from January 1st to focus on the wintertime circulation adjustment. A hybrid, finite amplitude wave activity budget analysis is performed to elucidate the nonlinear and irreversible aspects of the eddy-mean flow interaction during the adjustment of the zonal wind towards a poleward shifted state. The results confirm the results from earlier more idealized studies, particularly the importance of reduced dissipation of wave activity, in which the midlatitude decrease of effective diffusivity appears to be dominant. This reduction in dissipation increases the survival of midlatitude waves. These surviving waves, when reaching the upper propagation level in the upper troposphere, are subject to the influence of the increase of reflection phase speed at the poleward side of the mean jet, and thus more waves are reflected equatorward across the jet, giving rise to a poleward transport of momentum and thus an eddy momentum flux convergence for the poleward shift. The relative importance of wave breaking-induced PV mixing versus diabatic PV source in the evolution of the Lagrangian PV gradient is also investigated. The former plays the dominant role in the PV gradient formation during the initial phase of the jet shift, while the latter actually opposes the evolution of the Lagrangian PV gradient at times.

Mechanistic site-based emulation of a global ocean biogeochemical model (MEDUSA 1.0) for parametric analysis and calibration: an application of the Marine Model Optimization Testbed (MarMOT 1.1)

Abstract. Biogeochemical ocean circulation models used to investigate the role of plankton ecosystems in global change rely on adjustable parameters to capture the dominant biogeochemical dynamics of a complex biological system. In principle, optimal parameter values can be estimated by fitting models to observational data, including satellite ocean colour products such as chlorophyll that achieve good spatial and temporal coverage of the surface ocean. However, comprehensive parametric analyses require large ensemble experiments that are computationally infeasible with global 3-D simulations. Site-based simulations provide an efficient alternative but can only be used to make reliable inferences about global model performance if robust quantitative descriptions of their relationships with the corresponding 3-D simulations can be established. The feasibility of establishing such a relationship is investigated for an intermediate complexity biogeochemistry model (MEDUSA) coupled with a widely used global ocean model (NEMO). A site-based mechanistic emulator is constructed for surface chlorophyll output from this target model as a function of model parameters. The emulator comprises an array of 1-D simulators and a statistical quantification of the uncertainty in their predictions. The unknown parameter-dependent biogeochemical environment, in terms of initial tracer concentrations and lateral flux information required by the simulators, is a significant source of uncertainty. It is approximated by a mean environment derived from a small ensemble of 3-D simulations representing variability of the target model behaviour over the parameter space of interest. The performance of two alternative uncertainty quantification schemes is examined: a direct method based on comparisons between simulator output and a sample of known target model "truths" and an indirect method that is only partially reliant on knowledge of the target model output. In general, chlorophyll records at a representative array of oceanic sites are well reproduced. The use of lateral flux information reduces the 1-D simulator error considerably, consistent with a major influence of advection at some sites. Emulator robustness is assessed by comparing actual error distributions with those predicted. With the direct uncertainty quantification scheme, the emulator is reasonably robust over all sites. The indirect uncertainty quantification scheme is less reliable at some sites but scope for improving its performance is identified. The results demonstrate the strong potential of the emulation approach to improve the effectiveness of site-based methods. This represents important progress towards establishing a robust site-based capability that will allow comprehensive parametric analyses to be achieved for improving global models and quantifying uncertainty in their predictions.

Amplification of the influence of solar flux variations on the winter stratosphere by planetary waves

Recent observational evidence has suggested that variations in solar activity may affect winter stratospheric polar ozone and temperature levels. The paucity of direct sunlight available during this season points strongly to a dynamical mechanism. We have carried out several large ensemble experiments within the middle atmosphere and the coupled middle atmosphere and lower thermosphere to simulate the radiative/dynamical coupling via planetary waves for a range of solar fluxes. In the former case, the model response in the winter stratosphere was linear and of the order of the summer stratopause forcing, whilst in the latter, the level of correlation in the winter stratosphere remained high, but was diluted over a wider volume. The inclusion of the upper atmosphere enhanced the winter polar stratospheric response by a factor of three.