On short (15-year) to mid-term (30-year) time-scales how the Earth's surface temperature evolves can be dominated by internal variability as demonstrated by the global-warming pause or 'hiatus'. In ...this study, we use six single model initial-condition large ensembles (SMILEs) and the Coupled Model Intercomparison Project 5 (CMIP5) to visualise the role of internal variability in controlling possible observable surface temperature trends in the short-term and mid-term projections from 2019 onwards. We confirm that in the short-term, surface temperature trend projections are dominated by internal variability, with little influence of structural model differences or warming pathway. Additionally we demonstrate that this result is independent of the model-dependent estimate of the magnitude of internal variability. Indeed, and perhaps counter intuitively, in all models a lack of warming, or even a cooling trend could be observed at all individual points on the globe, even under the largest greenhouse gas emissions. The near-equivalence of all six SMILEs and CMIP5 demonstrates the robustness of this result to the choice of models used. On the mid-term time-scale, we confirm that structural model differences and scenario uncertainties play a larger role in controlling surface temperature trend projections than they did on the shorter time-scale. In addition we show that whether internal variability still dominates, or whether model uncertainties and internal variability are a similar magnitude, depends on the estimate of internal variability, which differs between the SMILEs. Finally we show that even out to thirty years large parts of the globe (or most of the globe in MPI-GE and CMIP5) could still experience no-warming due to internal variability.
Separating how model-to-model differences in the forced response (U
) and internal variability (U
) contribute to the uncertainty in climate projections is important, but challenging. Reducing U
...increases confidence in projections, while U
characterises the range of possible futures that might occur purely by chance. Separating these uncertainties is limited in traditional multi-model ensembles because most models have only a small number of realisations; furthermore, some models are not independent. Here, we use six largely independent single model initial-condition large ensembles to separate the contributions of U
and U
in projecting 21st-century changes of temperature, precipitation, and their temporal variability under strong forcing (RCP8.5). We provide a method that produces similar results using traditional multi-model archives. While U
is larger than U
for both temperature and precipitation changes, U
is larger than U
for the changes in temporal variability of both temperature and precipitation, between 20° and 80° latitude in both hemispheres. Over large regions and for all variables considered here except temporal temperature variability, models agree on the sign of the forced response whereas they disagree widely on the magnitude. Our separation method can readily be extended to other climate variables.
Effects of volcanism on tropical variability Maher, Nicola; McGregor, Shayne; England, Matthew H. ...
Geophysical research letters,
28 July 2015, Letnik:
42, Številka:
14
Journal Article
Recenzirano
Odprti dostop
The effects of large tropical volcanic eruptions on Indo‐Pacific tropical variability are investigated using 122 historical ensemble members from the Coupled Model Intercomparison Project 5. ...Radiative forcing due to volcanic aerosols in the stratosphere is found to increase the likelihood of a model climatic response that projects onto both the El Niño–Southern Oscillation and the Indian Ocean Dipole (IOD). Large eruptions are associated with co‐occurring El Niño and positive IOD events in the ensemble means that peak 6–12 months after the volcanic forcing peaks, marking a significant increase in the likelihood of each event occurring in the Southern Hemisphere (SH) spring/summer posteruption. There is also an ensemble mean La Niña‐like response in the third SH summer posteruption, which coincides with a significant increase in the likelihood of a La Niña occurring. Taken together with the initial cooling, this La Niña‐like response may increase the persistence of the cool global average surface temperature anomaly after an eruption.
Key Points
The likelihood of a positive IOD and El Niño increases after an eruption
The likelihood of a La Niña increases in the third summer posteruption
The La Niña response may increase the persistence of the initial cool anomaly
The Max Planck Institute Grand Ensemble (MPI‐GE) is the largest ensemble of a single comprehensive climate model currently available, with 100 members for the historical simulations (1850–2005) and ...four forcing scenarios. It is currently the only large ensemble available that includes scenario representative concentration pathway (RCP) 2.6 and a 1% CO2 scenario. These advantages make MPI‐GE a powerful tool. We present an overview of MPI‐GE, its components, and detail the experiments completed. We demonstrate how to separate the forced response from internal variability in a large ensemble. This separation allows the quantification of both the forced signal under climate change and the internal variability to unprecedented precision. We then demonstrate multiple ways to evaluate MPI‐GE and put observations in the context of a large ensemble, including a novel approach for comparing model internal variability with estimated observed variability. Finally, we present four novel analyses, which can only be completed using a large ensemble. First, we address whether temperature and precipitation have a pathway dependence using the forcing scenarios. Second, the forced signal of the highly noisy atmospheric circulation is computed, and different drivers are identified to be important for the North Pacific and North Atlantic regions. Third, we use the ensemble dimension to investigate the time dependency of Atlantic Meridional Overturning Circulation variability changes under global warming. Last, sea level pressure is used as an example to demonstrate how MPI‐GE can be utilized to estimate the ensemble size needed for a given scientific problem and provide insights for future ensemble projects.
Key Points
The 100‐member MPI‐GE is currently the largest publicly available ensemble of a comprehensive climate model
MPI‐GE currently has the most forcing scenarios of all large ensemble projects: RCP2.6, RCP4.5, RCP8.5, and 1% CO2
The power of MPI‐GE is to estimate the forced response and internal variability, including changing variability, to unprecedented precision
Abstract
The representation of tropical precipitation is evaluated across three generations of models participating in phases 3, 5, and 6 of the Coupled Model Intercomparison Project (CMIP). Compared ...to state-of-the-art observations, improvements in tropical precipitation in the CMIP6 models are identified for some metrics, but we find no general improvement in tropical precipitation on different temporal and spatial scales. Our results indicate overall little changes across the CMIP phases for the summer monsoons, the double-ITCZ bias, and the diurnal cycle of tropical precipitation. We find a reduced amount of drizzle events in CMIP6, but tropical precipitation occurs still too frequently. Continuous improvements across the CMIP phases are identified for the number of consecutive dry days, for the representation of modes of variability, namely, the Madden–Julian oscillation and El Niño–Southern Oscillation, and for the trends in dry months in the twentieth century. The observed positive trend in extreme wet months is, however, not captured by any of the CMIP phases, which simulate negative trends for extremely wet months in the twentieth century. The regional biases are larger than a climate change signal one hopes to use the models to identify. Given the pace of climate change as compared to the pace of model improvements to simulate tropical precipitation, we question the past strategy of the development of the present class of global climate models as the mainstay of the scientific response to climate change. We suggest the exploration of alternative approaches such as high-resolution storm-resolving models that can offer better prospects to inform us about how tropical precipitation might change with anthropogenic warming.
Abstract
In this study, we investigate whether the Pacific decadal oscillation (PDO) can enhance or diminish El Niño Southern Oscillation (ENSO) temperature and precipitation teleconnections over ...North America using five single model initial-condition large ensembles (SMILEs). The use of SMILEs facilitates a statistically robust comparison of ENSO events that occur during different phases of the PDO. We find that a positive PDO enhances winter and spring El Niño temperature and precipitation teleconnections and diminishes La Niña teleconnections. A negative PDO has the opposite effect. The modulation of ENSO by the PDO is mediated by differences in the location and strength of the Aleutian Low and Pacific Jet during ENSO events under different phases of the PDO. This modulation is a simple combination of the individual effects of the PDO and ENSO over North America. Finally, we show that ENSO and the PDO can be used to evaluate the likelihood of the occurrence of temperature and precipitation anomalies in different regions, but cannot be used as a deterministic predictor of these anomalies due to the large variability between individual events.
The latest generation of climate model simulations are used to investigate the occurrence of hiatus periods in global surface air temperature in the past and under two future warming scenarios. ...Hiatus periods are identified in three categories: (i) those due to volcanic eruptions, (ii) those associated with negative phases of the Interdecadal Pacific Oscillation (IPO), and (iii) those affected by anthropogenically released aerosols in the mid‐twentieth century. The likelihood of future hiatus periods is found to be sensitive to the rate of change of anthropogenic forcing. Under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption. We further demonstrate that most nonvolcanic hiatuses across Coupled Model Intercomparison Project 5 (CMIP5) models are associated with enhanced cooling in the equatorial Pacific linked to the transition to a negative IPO phase.
Key PointsHiatuses are modulated by volcanic eruptions and anthropogenic aerosolsClimate models consistently link nonvolcanic hiatuses to IPO variabilityHiatuses are extremely rare after the mid‐21st century under high‐emissions
Interannual to decadal variability in the Pacific Ocean is a prominent feature of Earth’s climate system, with global teleconnections. Recent studies have identified Pacific decadal variability as a ...major driver of periods of rapid and slower global mean surface air temperature change. Here, we use an eddy-permitting global ocean model to investigate the role of the observed 1992–2011 trade wind intensification and concurrent trends in surface atmospheric variables over the Pacific associated with the negative phase of the Interdecadal Pacific Oscillation (IPO) in driving ocean circulation and heat content changes. We find a strengthening of the Equatorial Undercurrent (EUC) in response to strengthened winds, which brings cooler water to the surface of the eastern Pacific and an increase in the Pacific shallow overturning cells (PSOC), which in turn drives additional heat into the subsurface western Pacific. The wind acceleration also results in an increase in the strength and subsequent heat transport of the Indonesian throughflow (ITF), which transports some of the additional heat from the western Pacific into the Indian Ocean. The circulation changes result in warming of the subsurface western Pacific, cooling of the upper eastern Pacific Ocean and warming of the subsurface Indian Ocean, with an overall increase in Indo-Pacific heat content. Further experiments impose a symmetric reversal of the atmospheric state to examine how the ocean would behave if the winds (and other atmospheric variables) were to revert to their initial state. This mimics a return to the neutral phase of the IPO, characterised by a weakening of the Pacific trade winds. In response we find a slowdown of the EUC and the PSOC, which results in a return to climatological SST conditions in the western and eastern Pacific. The ITF also slows towards its original strength. However, the subsurface temperature, heat content and ITF responses are not symmetric due to an overall increase in the surface heat flux into the ocean associated with the cooler surface of the Pacific. There may also be irreversible heat transport across the thermocline via diapycnal mixing, further contributing to this asymmetry. The net result of the experiment is that the Indo-Pacific subsurface ocean is warmer than it was in its initial state.
There is an error in figures 1, 2, and 3 (corresponding to Figures 1, 4 and 5 of the original publication) and figures S2-S9 where the units were originally printed as oC but should be oC yr−1. The ...corrected figures and Supplementary material (stacks.iop.org/ERL/15/109502/mmedia) are reproduced below.
Multi‐model simulations show a post‐Pinatubo eruption sequence of Pacific sea surface temperatures (SSTs) that includes a La Niña‐like pattern the third northern winter after an eruption, opposite in ...sign to what was observed after Pinatubo. This leads to the loss of hindcast skill for years in the 1990s affected by the Pinatubo eruption because the post‐eruption internal variability of the climate system did not match the multi‐model forced response. Agung (1963) and El Chichón (1982) happened to have post‐eruption Pacific SST sequences more similar to the multi‐model response and thus do not degrade prediction skill as measured by anomaly pattern correlation in the hindcasts. Thus, decadal hindcast skill is reduced if the post‐eruption randomly occurring internal El Niño variability in the observations deviates from the multi‐model forced response that, by definition, averages out internal variability in favor of the forced response.
Key Points
Climate effects from Pinatubo eruption negatively affect decadal prediction skill
Different volcanic eruptions have different effects on hindcast skill
Climate models produce a La Nina‐like response after a major eruption
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