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Found 10 results

  1. Relationship between Tropical Pacific SST and global atmospheric angular momentum in coupled models Authors: Huei−Ping Huang, Matthew Newman, Richard Seager, Yochanan Kushnir and Participating CMIP2+ Modeling Groups First Published: January 2004 Abstract: The sensitivity parameter S1 = ∆AAM/∆SST, where ∆AAM and ∆SST represent the anomalies of global atmospheric angular momentum (AAM) and tropical Pacific sea surface temperature (SST) in the NINO3.4 region, is compared for the CMIP2+ coupled models. The parameter quantifies the strength of atmospheric zonal mean zonal wind response to SST anomaly in the equatorial Pacific, an important process for the climate system. Although the simulated ∆AAM and ∆SST are found to exhibit great disparity, their ratios agree better among the coupled models (and with observation) with no significant outliers. This indicates that the processes that connect the AAM anomaly to tropical SST anomaly are not sensitive to the base SST and the detail of convective heating and are relatively easy to reproduce by the coupled models. Through this robust ∆SST−∆AAM relationship, the model bias in tropical Pacific SST manifests itself in the bias in atmospheric angular momentum. The value of S1 for an atmospheric model forced by observed SST is close to that for a coupled model with a similar atmospheric component, suggesting that the ∆SST− ∆AAM relationship is dominated by a one−way influence of the former forcing the latter. The physical basis for the ∆SST−∆AAM relationship is explored using a statistical equilibrium argument that links ∆SST to the anomaly of tropical tropospheric temperature. The resulting meridional gradient of tropospheric temperature is then linked to the change in zonal wind in the subtropical jets, the main contributor to ∆AAM, by thermal wind balance. Link to Paper: Credit goes to Tom @Isotherm for finding this paper - thank you.
  2. Centennial Trend and Decadal-to-Interdecadal Variability of Atmospheric Angular Momentum in CMIP3 and CMIP5 Simulations Authors: Houk Paek and Huei-Ping Huang First Published: 26th November, 2012 Published on line: 31st May, 2013 Abstract: The climatology and trend of atmospheric angular momentum from the phase 3 and the phase 5 Climate Model Intercomparison Project (CMIP3 and CMIP5, respectively) simulations are diagnosed and validated with the Twentieth Century Reanalysis (20CR). It is found that CMIP5 models produced a significantly smaller bias in the twentieth-century climatology of the relative MR and omega MΩ angular momentum compared to CMIP3. The CMIP5 models also produced a narrower ensemble spread of the climatology and trend of MR and MΩ. Both CMIP3 and CMIP5 simulations consistently produced a positive trend in MR and MΩ for the twentieth and twenty-first centuries. The trend for the twenty-first century is much greater, reflecting the role of greenhouse gas (GHG) forcing in inducing the trend. The simulated increase in MR for the twentieth century is consistent with reanalysis. Both CMIP3 and CMIP5 models produced a wide range of magnitudes of decadal and interdecadal variability of MRcompared to 20CR. The ratio of the simulated standard deviation of decadal or interdecadal variability to its observed counterpart ranges from 0.5 to over 2.0 for individual models. Nevertheless, the bias is largely random and ensemble averaging brings the ratio to within 18% of the reanalysis for decadal and interdecadal variability for both CMIP3 and CMIP5. The twenty-first-century simulations from both CMIP3 and CMIP5 produced only a small trend in the amplitude of decadal or interdecadal variability, which is not statistically significant. Thus, while GHG forcing induces a significant increase in the climatological mean of angular momentum, it does not significantly affect its decadal-to-interdecadal variability in the twenty-first century. Link to Paper:
  3. Near-term Climate Predictions of the North Atlantic Region - YouTube Presentation Presenters/Authors: Dr. Nick Dunstone, Doug Smith, Adam Scaife, Leon Hermanson, Rosie Eade, Niall Robinson, Martin Andrews and Jeff Knight Presentation Team from: The UK Met Office, Hadley Centre Presentation Date: 6th April, 2017 (at St Andrews, Scotland) Abstract: None (but see below) Link to full YouTube presentation (57 minutes): This brilliant "balanced" presentation was based on a paper published by the same authors (on the 17th October, 2016) with the title "Skilful predictions of the winter North Atlantic Oscillation one year ahead". The full paper is still behind a "Nature GeoScience" paywall on this site: Abstract to paper: The winter North Atlantic Oscillation is the primary mode of atmospheric variability in the North Atlantic region and has a profound influence on European and North American winter climate. Until recently, seasonal variability of the North Atlantic Oscillation was thought to be largely driven by chaotic and inherently unpredictable processes. However, latest generation seasonal forecasting systems have demonstrated significant skill in predicting the North Atlantic Oscillation when initialized a month before the onset of winter. Here we extend skilful dynamical model predictions to more than a year ahead. The skill increases greatly with ensemble size due to a spuriously small signal-to-noise ratio in the model, and consequently larger ensembles are projected to further increase the skill in predicting the North Atlantic Oscillation. We identify two sources of skill for second-winter forecasts of the North Atlantic Oscillation: climate variability in the tropical Pacific region and predictable effects of solar forcing on the stratospheric polar vortex strength. We also identify model biases in Arctic sea ice that, if reduced, may further increase skill. Our results open possibilities for a range of new climate services, including for the transport, energy, water management and insurance sectors.
  4. How Predictable Are the Arctic and North Atlantic Oscillations? Exploring the Variability and Predictability of the Northern Hemisphere Authors: Daniela I. V. Domeisena, Gualtiero Badin and Inga M. Koszalka Published: 18th January, 2018 Abstract: The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) describe the dominant part of the variability in the Northern Hemisphere extratropical troposphere. Because of the strong connection of these patterns with surface climate, recent years have shown an increased interest and an increasing skill in forecasting them. However, it is unclear what the intrinsic limits of short-term predictability for the NAO and AO patterns are. This study compares the variability and predictability of both patterns, using a range of data and index computation methods for the daily NAO and AO indices. Small deviations from Gaussianity are found along with characteristic decorrelation time scales of around one week. In the analysis of the Lyapunov spectrum it is found that predictability is not significantly different between the AO and NAO or between reanalysis products. Differences exist, however, between the indices based on EOF analysis, which exhibit predictability time scales around 12–16 days, and the station-based indices, exhibiting a longer predictability of 18–20 days. Both of these time scales indicate predictability beyond that currently obtained in ensemble prediction models for short-term predictability. Additional longer-term predictability for these patterns may be gained through local feedbacks and remote forcing mechanisms for particular atmospheric conditions. Link to full paper (on the Researchgate website): Please note that there is also a fully downloadable pdf version on the Researchgate site but only directly to your own pc or device.
  5. A real-time Global Warming Index Authors: Dr. K. Haustein, M. R. Allen, P. M. Forster, F. E. L. Otto, D. M. Mitchell, H. D. Matthews and D. J. Frame Published: 13th November, 2017 Abstract: We propose a simple real-time index of global human-induced warming and assess its robustness to uncertainties in climate forcing and short-term climate fluctuations. This index provides improved scientific context for temperature stabilisation targets and has the potential to decrease the volatility of climate policy. We quantify uncertainties arising from temperature observations, climate radiative forcings, internal variability and the model response. Our index and the associated rate of human-induced warming is compatible with a range of other more sophisticated methods to estimate the human contribution to observed global temperature change. Link to full paper:
  6. The South Pacific Meridional Mode: A Mechanism for ENSO-like Variability Authors: Honghai Zhang, Amy Clement and Pedro Di Nezio Published: 9th July, 2013 Abstract: In this study, the authors investigate the connection between the South Pacific atmospheric variability and the tropical Pacific climate in models of different degrees of coupling between the atmosphere and ocean. A robust mode of variability, defined as the South Pacific meridional mode (SPMM), is identified in a multimodel ensemble of climate model experiments where the atmosphere is only thermodynamically coupled to a slab ocean mixed layer. The physical interpretation of the SPMM is nearly identical to the North Pacific meridional mode (NPMM) with the off-equatorial southeast trade wind variability altering the latent heat flux and sea surface temperature (SST) and initiating a wind–evaporation–SST feedback that propagates signals into the tropics. The authors also show that a positive cloud feedback plays a role in the development of this mode, but this effect is model dependent. While physically analogous to the NPMM, the SPMM has a stronger expression in the equatorial Pacific and directly perturbs the zonal gradients of SST and sea level pressure (SLP) on the equator, thus leading to ENSO-like variability despite the lack of ocean–atmosphere dynamical coupling. Further analysis suggests that the SPMM is also active in fully coupled climate models and observations. This study highlights the important role of the Southern Hemisphere in tropical climate variability and suggests that including observations from the data-poor South Pacific could improve the ENSO predictability. Link to full paper:
  7. The Northern Hemisphere Extratropical Atmospheric Circulation Response to ENSO: How Well Do We Know It and How Do We Evaluate Models Accordingly? Authors: Clara Deser, Isla R. Simpson, Karen A. McKinnon and Adam S. Phillips Published: 14th March, 2017 Abstract: Application of random sampling techniques to composite differences between 18 El Niño and 14 La Niña events observed since 1920 reveals considerable uncertainty in both the pattern and amplitude of the Northern Hemisphere extratropical winter sea level pressure (SLP) response to ENSO. While the SLP responses over the North Pacific and North America are robust to sampling variability, their magnitudes can vary by a factor of 2; other regions, such as the Arctic, North Atlantic, and Europe are less robust in their SLP patterns, amplitudes, and statistical significance. The uncertainties on the observed ENSO composite are shown to arise mainly from atmospheric internal variability as opposed to ENSO diversity. These observational findings pose considerable challenges for the evaluation of ENSO teleconnections in models. An approach is proposed that incorporates both pattern and amplitude uncertainty in the observational target, allowing for discrimination between true model biases in the forced ENSO response and apparent model biases that arise from limited sampling of non-ENSO-related internal variability. Large initial-condition coupled model ensembles with realistic tropical Pacific sea surface temperature anomaly evolution during 1920–2013 show similar levels of uncertainty in their ENSO teleconnections as found in observations. Because the set of ENSO events in each of the model composites is the same (and identical to that in observations), these uncertainties are entirely attributable to sampling fluctuations arising from internal variability, which is shown to originate from atmospheric processes. The initial-condition model ensembles thus inform the interpretation of the single observed ENSO composite and vice versa. Link to full paper:
  8. Recent Progress in Understanding and Predicting Atlantic Decadal Climate Variability Authors: S. G. Yeager and J. I. Robson Published: 18th April, 2017 Abstract: Purpose of Review Recent Atlantic climate prediction studies are an exciting new contribution to an extensive body of research on Atlantic decadal variability and predictability that has long emphasized the unique role of the Atlantic Ocean in modulating the surface climate. We present a survey of the foundations and frontiers in our understanding of Atlantic variability mechanisms, the role of the Atlantic Meridional Overturning Circulation (AMOC), and our present capacity for putting that understanding into practice in actual climate prediction systems. Recent Findings The AMOC—or more precisely, the buoyancy-forced thermohaline circulation (THC) that encompasses both overturning and gyre circulations—appears to underpin decadal timescale prediction skill in the subpolar North Atlantic in retrospective forecasts. Skill in predicting more wide-ranging climate variations, including those over land, is more limited, but there are indications this could improve with more advanced models. Summary Preliminary successes in the field of initialized Atlantic climate prediction confirm the climate relevance of low-frequency Atlantic Ocean dynamics and suggest that useful decadal climate prediction is a realizable goal. Link to full paper:
  9. Intrinsic and atmospherically forced variability of the AMOC : insights from a large-ensemble ocean hindcast Authors: Leroux S., Penduff T., Bessieres L., Molines J. M., Brankart J. M., Sérazin Guillaume, Barnier B. and Terray L. Published: 1st February, 2018 Abstract: This study investigates the origin and features of interannual-decadal Atlantic meridional overturning circulation (AMOC) variability from several ocean simulations, including a large (50 member) ensemble of global, eddy-permitting (1/4 degrees) ocean-sea ice hindcasts. After an initial stochastic perturbation, each member is driven by the same realistic atmospheric forcing over 1960-2015. The magnitude, spatiotemporal scales, and patterns of both the atmospherically forced and intrinsic-chaotic interannual AMOC variability are then characterized from the ensemble mean and ensemble spread, respectively. The analysis of the ensemble-mean variability shows that the AMOC fluctuations north of 40 degrees N are largely driven by the atmospheric variability, which forces meridionally coherent fluctuations reaching decadal time scales. The amplitude of the intrinsic interannual AMOC variability never exceeds the atmospherically forced contribution in the Atlantic basin, but it reaches up to 100% of the latter around 35 degrees S and 60% in the Northern Hemisphere midlatitudes. The intrinsic AMOC variability exhibits a large-scale meridional coherence, especially south of 25 degrees N. An EOF analysis over the basin shows two large-scale leading modes that together explain 60% of the interannual intrinsic variability. The first mode is likely excited by intrinsic oceanic processes at the southern end of the basin and affects latitudes up to 40 degrees N; the second mode is mostly restricted to, and excited within, the Northern Hemisphere midlatitudes. These features of the intrinsic, chaotic variability (intensity, patterns, and random phase) are barely sensitive to the atmospheric evolution, and they strongly resemble the "pure intrinsic' interannual AMOC variability that emerges in climatological simulations under repeated seasonal-cycle forcing. These results raise questions about the attribution of observed and simulated AMOC signals and about the possible impact of intrinsic signals on the atmosphere. Link to full paper:
  10. Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review Authors: Martha W. Buckley and John Marshall Published: 22nd December, 2015 Abstract: This is a review about the Atlantic Meridional Overturning Circulation (AMOC), its mean structure, temporal variability, controlling mechanisms, and role in the coupled climate system. The AMOC plays a central role in climate through its heat and freshwater transports. Northward ocean heat transport achieved by the AMOC is responsible for the relative warmth of the Northern Hemisphere compared to the Southern Hemisphere and is thought to play a role in setting the mean position of the Intertropical Convergence Zone north of the equator. The AMOC is a key means by which heat anomalies are sequestered into the ocean's interior and thus modulates the trajectory of climate change. Fluctuations in the AMOC have been linked to low‐frequency variability of Atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses. On intra‐annual timescales, variability in AMOC is large and primarily reflects the response to local wind forcing; meridional coherence of anomalies is limited to that of the wind field. On interannual to decadal timescales, AMOC changes are primarily geostrophic and related to buoyancy anomalies on the western boundary. A pacemaker region for decadal AMOC changes is located in a western “transition zone” along the boundary between the subtropical and subpolar gyres. Decadal AMOC anomalies are communicated meridionally from this region. AMOC observations, as well as the expanded ocean observational network provided by the Argo array and satellite altimetry, are inspiring efforts to develop decadal predictability systems using coupled atmosphere‐ocean models initialized by ocean data. Link to full paper: