This work quantitatively evaluates the fidelity with which the northern annular mode (NAM), southern annular mode (SAM), Pacific–North American pattern (PNA), El Niño–Southern Oscillation (ENSO), ...Pacific decadal oscillation (PDO), Atlantic multidecadal oscillation (AMO), and the first-order mode interactions are represented in Earth system model (ESM) output from the CMIP6 archive. Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families, and specific ESM realizations relative to ERA5. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climatemodes. Substantial variability in skill scoresmanifests across realizations fromindividual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM–PNA first-order interaction and underestimate the NAM–AMO connection. These results suggest that the choice of ESMand ESM realizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near term.
SIGNIFICANCE STATEMENT: Internal climate variability occurs over multiple spatial and temporal scales and is encapsulated in a series of internal climate modes. The representation of such modes in climate models is a critically important aspect of model fidelity. Analyses presented herein uses several skill scores to evaluate both the spatial and temporal manifestations of these climate modes in the CMIP6 generation of Earth system models (ESMs). There is marked variability in model fidelity for these modes and this variability in credibility within the current climate has important implications for the choice of specific ESMs and ESM realizations in making climate projections.
We compare the performance of several modes of variability across six U.S. climate modeling groups, with a focus on identifying robust improvements in recent models including those participating in ...phase 6 of the Coupled Model Intercomparison Project (CMIP) compared to previous versions. In particular, we examine the representation of the Madden–Julian oscillation (MJO), El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), the quasi-biennial oscillation (QBO) in the tropical stratosphere, and the dominant modes of extratropical variability, including the southern annular mode (SAM), the northern annular mode (NAM) and the closely related North Atlantic Oscillation (NAO), and the Pacific–North American pattern (PNA). Where feasible, we explore the processes driving these improvements through the use of “intermediary” experiments that utilize model versions between CMIP3/5 and CMIP6 as well as targeted sensitivity experiments in which individual modeling parameters are altered. We find clear and systematic improvements in the MJO and QBO and in the teleconnection patterns associated with the PDO and ENSO. Some gains arise from better process representation, while others (e.g., the QBO) from higher resolution that allows for a greater range of interactions. Our results demonstrate that the incremental development processes in multiple climate model groups lead to more realistic simulations over time.
This work investigates the interdecadal variations of the relationship between the El Niño–Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM), further explores possible mechanisms, ...and finally considers a recent switch in the ENSO–EAWM relationship. The 23-yr sliding correlation between the Niño-3.4 index and the EAWM index reveals an obvious low-frequency oscillation with a period of about 50 yr in the ENSO–EAWM relationship. Warm ENSO events during high-correlation periods are associated with an unusually weak East Asian trough, a positive phase of the North Pacific Oscillation (NPO), significant southerly wind anomalies along coastal East Asia, and warmer East Asian continent and adjacent oceans. However, there are no robust and significant anomalies in the EAWM-related circulation during low-correlation periods. Because of the southeastward shift of the Walker circulation, the area of anomalously high pressure in the western Pacific retreats south of 25°N, confining it to the region of the Philippine Sea. In this sense, the Pacific–East Asian teleconnection is not well established. Consequently, ENSO’s impact on the EAWM is suppressed. Additionally, the low-frequency oscillation of the ENSO–EAWM relationship might be attributable to the combined effect of the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation owing to their modulation on the establishment of the NPO teleconnection. The observation of two full cycles of the ENSO–EAWM relationship, a transition to negative PDO in the early 2000s and an enhancement of the Walker circulation in the late 1990s, suggests a recovery of the ENSO–EAWM relationship.
El Niño-Southern Oscillation complexity Timmermann, Axel; An, Soon-Il; Kug, Jong-Seong ...
Nature (London),
07/2018, Volume:
559, Issue:
7715
Journal Article
Peer reviewed
Open access
El Niño events are characterized by surface warming of the tropical Pacific Ocean and weakening of equatorial trade winds that occur every few years. Such conditions are accompanied by changes in ...atmospheric and oceanic circulation, affecting global climate, marine and terrestrial ecosystems, fisheries and human activities. The alternation of warm El Niño and cold La Niña conditions, referred to as the El Niño-Southern Oscillation (ENSO), represents the strongest year-to-year fluctuation of the global climate system. Here we provide a synopsis of our current understanding of the spatio-temporal complexity of this important climate mode and its influence on the Earth system.
The study of the interannual and intraseasonal modulation of tropical cyclogenesis (TCG) in the northwest Pacific (NWP) basin has received significant attention in the past. Most previous studies ...have focused on the individual impact of El Niño‐southern oscillation (ENSO) or Madden–Julian Oscillation on TCG, whereas the combined effect of ENSO and another leading intraseasonal oscillation mode, quasi‐biweekly oscillation (QBWO), on NWP TCG has rarely been studied. Therefore, this study systemically investigates the modulation of NWP TCG by the QBWO under different ENSO phases. A significant modulation of TCG by QBWO in the ENSO neutral years is observed, with significantly increased (decreased) TCG over the NWP in QBWO active (inactive) phases. Moreover, during El Niño years and La Niña years, the modulation of TCG by QBWO is not significant and no distinctive difference in the TCG frequency can be detected. Further analyses show that the modulation of TCG by the QBWO is mainly related to the changes in large‐scale environment and energy that affect TCG associated with active or inactive phases of the QBWO under different ENSO phases. Large‐scale environmental factors and energy are more favourable for TCG during ENSO neutral years compared with El Niño and La Niña years. These results are associated with QBWO cycle under the impact of background (e.g., boundary layer moisture and zonal wind shear) which is determined by ENSO conditions.
A 10–20‐day filtered outgoing longwave radiation anomalies in quasi‐biweekly oscillation active and inactive phases in climatology (a, b), neutral years (c, d), El Niño years (e, f) and La Niña years (g, h) (unit: W/m2). The black dots indicate the locations of tropical cyclogenesis which is calculated from Japan Meteorological Agency dataset.
The Madden–Julian Oscillation (MJO) is the leading mode of sub‐seasonal variability in the tropical atmosphere and is a source of predictability for extratropical weather through its teleconnections. ...MJO teleconnection patterns can be modulated by the El Niño–Southern Oscillation (ENSO) on seasonal to interannual time scales. However, changes over decadal time scales are less well understood. ERA5 reanalysis data are used to show that the boreal winter MJO teleconnection pattern in the Northern Hemisphere has changed in recent decades in line with changes in the Pacific Decadal Oscillation and Atlantic Multidecadal Variability. Changes are seen in the circulation, temperature and precipitation responses. In particular, from 1997, intraseasonal cold anomalies appear over Europe and the eastern United States due to MJO convection over the western Pacific; these were not present 20 years previously. The decadal variability observed is not the product of aliasing of ENSO modulation of the teleconnection.
Plain Language Summary
Weather in different regions of the globe can be linked by planetary‐scale atmospheric waves, and these links can help forecasters to predict the weather. One such link, or teleconnection pattern, connects changes in rainfall over Indonesia and the tropical Pacific (from a weather system called the Madden–Julian Oscillation or MJO) to changes in the weather in North America and Europe. This study assesses this teleconnection pattern in two separate time periods (roughly the mid‐1970s to mid‐1990s and mid‐1990s to late 2010s) to analyze if and how it has changed. We find that the pattern has changed, and that this is due to large‐scale changes in the background state of the atmosphere. These changes in the link between the tropics and extratropics will have implications for weather forecasts on weekly to monthly time scales.
Key Points
The extratropical response to the Madden‐Julian Oscillation has changed on decadal time scales
This decadal variability coincides with changes in low‐frequency oceanic modes in both the Pacific and Atlantic basins
Changes on decadal time scales are different to those modulated by the El Niño‐Southern Oscillation on interannual scales
We present a general Bayesian hierarchical framework for conducting nonstationary frequency analysis of multiple hydrologic variables. In this, annual maxima from each variable are assumed to follow ...a generalized extreme value (GEV) distribution in which the location parameter is allowed to vary in time. A Gaussian elliptical copula is used to model the joint distribution of all variables. We demonstrate the utility of this framework with a joint frequency analysis model of annual peak snow water equivalent (SWE), annual peak flow, and annual peak reservoir elevation at Taylor Park dam in Colorado, USA. Indices of large‐scale climate drivers—El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) are used as covariates to model temporal nonstationarity. The Bayesian framework provides the posterior distribution of the model parameters and consequently the return levels. Results show that performing a multivariate joint frequency analysis reduces the uncertainty in return level estimates and better captures multivariate dependence compared to an independent model.
Plain Language Summary
In this study, we develop a method for determining the probability of occurrence of rare hydrologic events (e.g., floods). Utilizing modern statistical methods, we are able to estimate occurrence probabilities for multiple hydrologic variables simultaneously while incorporating climate information that changes in time. We apply this technique to estimate occurrence probabilities for streamflow, reservoir elevation, and snow levels for the Taylor Park reservoir in Colorado, USA. This method provides several benefits over traditional methods including reduction of uncertainty and a flexible model structure which allows for the incorporation of climate information.
Key Points
A model for nonstationary multivariate hydrologic frequency analysis is developed
The model allows for the incorporation of climate covariates and the specification of a nonlinear dependence between variables
Multivariate frequency analysis capture the dependence between multiple hydrologic variables
Interannual variation of seasonal-mean tropical convection over the Indo-Pacific region is primarily controlled by El Niño–Southern Oscillation (ENSO). For example, during El Niño winters, ...seasonal-mean convection around the Maritime Continent becomes weaker than normal, while that over the central to eastern Pacific is strengthened. Similarly, subseasonal convective activity, which is associated with the Madden–Julian oscillation (MJO), is influenced by ENSO. The MJO activity tends to extend farther eastward to the date line during El Niño winters and contract toward the western Pacific during La Niña winters. However, the overall level of MJO activity across the Maritime Continent does not change much in response to the ENSO. It is shown that the boreal winter MJO amplitude is closely linked with the stratospheric quasi-biennial oscillation (QBO) rather than with ENSO. The MJO activity around the Maritime Continent becomes stronger and more organized during the easterly QBO winters. The QBO-related MJO change explains up to 40% of interannual variation of the boreal winter MJO amplitude. This result suggests that variability of the MJO and the related tropical–extratropical teleconnections can be better understood and predicted by taking not only the tropospheric circulation but also the stratospheric mean state into account. The seasonality of the QBO–MJO link and the possible mechanism are also discussed.
The prediction skill of the North Atlantic Oscillation (NAO) in boreal winter is assessed in the operational models of the WCRP/WWRP Subseasonal-to-Seasonal (S2S) prediction project. Model ...performance in representing the contribution of different processes to the NAO forecast skill is evaluated. The S2S models with relatively higher stratospheric vertical resolutions (high-top models) are in general more skillful in predicting the NAO than those models with relatively lower stratospheric resolutions (low-top models). Comparison of skill is made between different groups of forecasts based on initial condition characteristics: phase and amplitude of the NAO, easterly and westerly phases of the quasi-biennial oscillation (QBO), warm and cold phases of ENSO, and phase and amplitude of the Madden–Julian oscillation (MJO). The forecasts with a strong NAO in the initial condition are more skillful than with a weak NAO. Those with negative NAO tend to have more skillful predictions than positive NAO. Comparisons of NAO skill between forecasts during easterly and westerly QBO and between warm and cold ENSO show no consistent difference for the S2S models. Forecasts with strong initial MJO tend to bemore skillful in the NAO prediction than weak MJO. Among the eight phases of MJO in the initial condition, phases 3–4 and phase 7 have better NAO forecast skills compared with the other phases. The results of this study have implications for improving our understanding of sources of predictability of the NAO. The situation dependence of the NAO prediction skill is likely useful in identifying “windows of opportunity” for subseasonal to seasonal predictions.
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