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  • American Meteorological Society  (16)
  • 1
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    American Meteorological Society ; 2021
    In:  Journal of Climate Vol. 34, No. 3 ( 2021-02), p. 1023-1045
    In: Journal of Climate, American Meteorological Society, Vol. 34, No. 3 ( 2021-02), p. 1023-1045
    Kurzfassung: Observational analyses suggest that a significant fraction of the tropical Pacific decadal variability (TPDV) (~60%–70%) is energized by the combined action of extratropical precursors of El Niño–Southern Oscillation (ENSO) originating from the North and South Pacific. Specifically, the growth and decay of the basin-scale TPDV pattern (time scale = ~1.5–2 years) is linked to the following sequence: ENSO precursors (extratropics, growth phase) → ENSO (tropics, peak phase) → ENSO successors (extratropics, decay phase) resulting from ENSO teleconnections. This sequence of teleconnections is an important physical basis for Pacific climate predictability. Here we examine the TPDV and its connection to extratropical dynamics in 20 models from phase 5 of the Coupled Model Intercomparison Project (CMIP). We find that most models (~80%) can simulate the observed spatial pattern ( R 〉 0.6) and frequency characteristics of the TPDV. In 12 models, more than 65% of the basinwide Pacific decadal variability (PDV) originates from TPDV, which is comparable with observations (~70%). However, despite reproducing the basic spatial and temporal statistics, models underestimate the influence of the North and South Pacific ENSO precursors to the TPDV, and most of the models’ TPDV originates in the tropics. Only 35%–40% of the models reproduce the observed extratropical ENSO precursor patterns ( R 〉 0.5). Models with a better representation of the ENSO precursors show 1) better basin-scale signatures of TPDV and 2) stronger ENSO teleconnections from/to the tropics that are consistent with observations. These results suggest that better representation of ENSO precursor dynamics in CMIP may lead to improved Pacific decadal variability dynamics and predictability.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Unbekannt
    Verlag: American Meteorological Society
    Publikationsdatum: 2021
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
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  • 2
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    American Meteorological Society ; 2012
    In:  Journal of Climate Vol. 25, No. 22 ( 2012-11-15), p. 7781-7801
    In: Journal of Climate, American Meteorological Society, Vol. 25, No. 22 ( 2012-11-15), p. 7781-7801
    Kurzfassung: Air–sea fluxes from the Community Climate System Model version 4 (CCSM4) are compared with the Coordinated Ocean-Ice Reference Experiment (CORE) dataset to assess present-day mean biases, variability errors, and late twentieth-century trend differences. CCSM4 is improved over the previous version, CCSM3, in both air–sea heat and freshwater fluxes in some regions; however, a large increase in net shortwave radiation into the ocean may contribute to an enhanced hydrological cycle. The authors provide a new baseline for assessment of flux variance at annual and interannual frequency bands in future model versions and contribute a new metric for assessing the coupling between the atmospheric and oceanic planetary boundary layer (PBL) schemes of any climate model. Maps of the ratio of CCSM4 variance to CORE reveal that variance on annual time scales has larger error than on interannual time scales and that different processes cause errors in mean, annual, and interannual frequency bands. Air temperature and specific humidity in the CCSM4 atmospheric boundary layer (ABL) follow the sea surface conditions much more closely than is found in CORE. Sensible and latent heat fluxes are less of a negative feedback to sea surface temperature warming in the CCSM4 than in the CORE data with the model’s PBL allowing for more heating of the ocean’s surface.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Englisch
    Verlag: American Meteorological Society
    Publikationsdatum: 2012
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
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  • 3
    In: Journal of Climate, American Meteorological Society, Vol. 34, No. 7 ( 2021-04), p. 2549-2565
    Kurzfassung: In the past 40 years, the global annual mean surface temperature has experienced a nonuniform warming, differing from the spatially uniform warming simulated by the forced responses of large multimodel ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, with intermittent and spatially varying warming trends along the Northern Hemisphere (NH) midlatitudes and a slight cooling in the tropical eastern Pacific. In particular, this “wavy” pattern of temperature changes over the NH midlatitudes features strong cooling over Eurasia in boreal winter. Here, we show that these nonuniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperatures (SSTs), via a global atmospheric teleconnection. Using six reanalyses, we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century. This tropically driven teleconnection is associated with a Pacific SST pattern resembling the interdecadal Pacific oscillation (IPO), and hereafter referred to as the IPO-related bipolar teleconnection (IPO-BT). Further, two paleo-reanalysis reconstruction datasets show that the IPO-BT is a robust recurrent mode over the past 400 and 2000 years. The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales, favoring a warming (cooling) episode in the Arctic accompanied by cooling (warming) over Eurasia and the Southern Ocean (SO). Thus, the spatial nonuniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Unbekannt
    Verlag: American Meteorological Society
    Publikationsdatum: 2021
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
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  • 4
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    American Meteorological Society ; 2018
    In:  Journal of Climate Vol. 31, No. 11 ( 2018-06-01), p. 4309-4327
    In: Journal of Climate, American Meteorological Society, Vol. 31, No. 11 ( 2018-06-01), p. 4309-4327
    Kurzfassung: Multidecadal hydroclimate variability has been expressed as “megadroughts” (dry periods more severe and prolonged than observed over the twentieth century) and corresponding “megapluvial” wet periods in many regions around the world. The risk of such events is strongly affected by modes of coupled atmosphere–ocean variability and by external impacts on climate. Accurately assessing the mechanisms for these interactions is difficult, since it requires large ensembles of millennial simulations as well as long proxy time series. Here, the Community Earth System Model (CESM) Last Millennium Ensemble is used to examine statistical associations among megaevents, coupled climate modes, and forcing from major volcanic eruptions. El Niño–Southern Oscillation (ENSO) strongly affects hydroclimate extremes: larger ENSO amplitude reduces megadrought risk and persistence in the southwestern United States, the Sahel, monsoon Asia, and Australia, with corresponding increases in Mexico and the Amazon. The Atlantic multidecadal oscillation (AMO) also alters megadrought risk, primarily in the Caribbean and the Amazon. Volcanic influences are felt primarily through enhancing AMO amplitude, as well as alterations in the structure of both ENSO and AMO teleconnections, which lead to differing manifestations of megadrought. These results indicate that characterizing hydroclimate variability requires an improved understanding of both volcanic climate impacts and variations in ENSO/AMO teleconnections.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Englisch
    Verlag: American Meteorological Society
    Publikationsdatum: 2018
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
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  • 5
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    American Meteorological Society ; 2021
    In:  Journal of Climate Vol. 34, No. 21 ( 2021-11), p. 8579-8597
    In: Journal of Climate, American Meteorological Society, Vol. 34, No. 21 ( 2021-11), p. 8579-8597
    Kurzfassung: Characterizing variability in the global water cycle is fundamental to predicting impacts of future climate change; understanding the role of the Pacific Walker circulation (PWC) in the regional expression of global water cycle changes is critical to understanding this variability. Water isotopes are ideal tracers of the role of the PWC in global water cycling because they retain information about circulation-dependent processes including moisture source, transport, and delivery. We collated publicly available measurements of precipitation δ 18 O ( δ 18 O P ) and used novel data processing techniques to synthesize long (34 yr), globally distributed composite records from temporally discontinuous δ 18 O P measurements. We investigated relationships between global-scale δ 18 O P variability and PWC strength, as well as other possible drivers of global δ 18 O P variability—including El Niño–Southern Oscillation (ENSO) and global mean temperature—and used isotope-enabled climate model simulations to assess potential biases arising from uneven geographical distribution of the observations or our data processing methodology. Covariability underlying the δ 18 O P composites is more strongly correlated with the PWC ( r = 0.74) than any other index of climate variability tested. We propose that the PWC imprint in global δ 18 O P arises from multiple complementary processes, including PWC-related changes in moisture source and transport length, and a PWC- or ENSO-driven “amount effect” in tropical regions. The clear PWC imprint in global δ 18 O P implies a strong PWC influence on the regional expression of global water cycle variability on interannual to decadal time scales, and hence that uncertainty in the future state of the PWC translates to uncertainties in future changes in the global water cycle.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Unbekannt
    Verlag: American Meteorological Society
    Publikationsdatum: 2021
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
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  • 6
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    American Meteorological Society ; 2021
    In:  Journal of Climate Vol. 34, No. 2 ( 2021-01), p. 449-464
    In: Journal of Climate, American Meteorological Society, Vol. 34, No. 2 ( 2021-01), p. 449-464
    Kurzfassung: The majority of future projections in the Coupled Model Intercomparison Project (CMIP5) show more frequent exceedances of the 5 mm day −1 rainfall threshold in the eastern equatorial Pacific rainfall during El Niño, previously described in the literature as an increase in “extreme El Niño events”; however, these exceedance frequencies vary widely across models, and in some projections actually decrease. Here we combine single-model large ensemble simulations with phase 5 of the Coupled Model Intercomparison Project (CMIP5) to diagnose the mechanisms for these differences. The sensitivity of precipitation to local SST anomalies increases consistently across CMIP-class models, tending to amplify extreme El Niño occurrence; however, changes to the magnitude of ENSO-related SST variability can drastically influence the results, indicating that understanding changes to SST variability remains imperative. Future El Niño rainfall intensifies most in models with 1) larger historical cold SST biases in the central equatorial Pacific, which inhibit future increases in local convective cloud shading, enabling more local warming; and 2) smaller historical warm SST biases in the far eastern equatorial Pacific, which enhance future reductions in stratus cloud, enabling more local warming. These competing mechanisms complicate efforts to determine whether CMIP5 models under- or overestimate the future impacts of climate change on El Niño rainfall and its global impacts. However, the relation between future projections and historical biases suggests the possibility of using observable metrics as “emergent constraints” on future extreme El Niño, and a proof of concept using SSTA variance, precipitation sensitivity to SST, and regional SST trends is presented.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Unbekannt
    Verlag: American Meteorological Society
    Publikationsdatum: 2021
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
    Standort Signatur Einschränkungen Verfügbarkeit
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  • 7
    In: Journal of Climate, American Meteorological Society, Vol. 26, No. 18 ( 2013-09-15), p. 6915-6936
    Kurzfassung: The past 1500 years provide a valuable opportunity to study the response of the climate system to external forcings. However, the integration of paleoclimate proxies with climate modeling is critical to improving the understanding of climate dynamics. In this paper, a climate system model and proxy records are therefore used to study the role of natural and anthropogenic forcings in driving the global climate. The inverse and forward approaches to paleoclimate data–model comparison are applied, and sources of uncertainty are identified and discussed. In the first of two case studies, the climate model simulations are compared with multiproxy temperature reconstructions. Robust solar and volcanic signals are detected in Southern Hemisphere temperatures, with a possible volcanic signal detected in the Northern Hemisphere. The anthropogenic signal dominates during the industrial period. It is also found that seasonal and geographical biases may cause multiproxy reconstructions to overestimate the magnitude of the long-term preindustrial cooling trend. In the second case study, the model simulations are compared with a coral δ18O record from the central Pacific Ocean. It is found that greenhouse gases, solar irradiance, and volcanic eruptions all influence the mean state of the central Pacific, but there is no evidence that natural or anthropogenic forcings have any systematic impact on El Niño–Southern Oscillation. The proxy climate relationship is found to change over time, challenging the assumption of stationarity that underlies the interpretation of paleoclimate proxies. These case studies demonstrate the value of paleoclimate data–model comparison but also highlight the limitations of current techniques and demonstrate the need to develop alternative approaches.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Englisch
    Verlag: American Meteorological Society
    Publikationsdatum: 2013
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
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  • 8
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    American Meteorological Society ; 2016
    In:  Bulletin of the American Meteorological Society Vol. 97, No. 5 ( 2016-05-01), p. 735-754
    In: Bulletin of the American Meteorological Society, American Meteorological Society, Vol. 97, No. 5 ( 2016-05-01), p. 735-754
    Kurzfassung: The climate of the past millennium provides a baseline for understanding the background of natural climate variability upon which current anthropogenic changes are superimposed. As this period also contains high data density from proxy sources (e.g., ice cores, stalagmites, corals, tree rings, and sediments), it provides a unique opportunity for understanding both global and regional-scale climate responses to natural forcing. Toward that end, an ensemble of simulations with the Community Earth System Model (CESM) for the period 850–2005 (the CESM Last Millennium Ensemble, or CESM-LME) is now available to the community. This ensemble includes simulations forced with the transient evolution of solar intensity, volcanic emissions, greenhouse gases, aerosols, land-use conditions, and orbital parameters, both together and individually. The CESM-LME thus allows for evaluation of the relative contributions of external forcing and internal variability to changes evident in the paleoclimate data record, as well as providing a longer-term perspective for understanding events in the modern instrumental period. It also constitutes a dynamically consistent framework within which to diagnose mechanisms of regional variability. Results demonstrate an important influence of internal variability on regional responses of the climate system during the past millennium. All the forcings, particularly large volcanic eruptions, are found to be regionally influential during the preindustrial period, while anthropogenic greenhouse gas and aerosol changes dominate the forced variability of the mid- to late twentieth century.
    Materialart: Online-Ressource
    ISSN: 0003-0007 , 1520-0477
    Sprache: Englisch
    Verlag: American Meteorological Society
    Publikationsdatum: 2016
    ZDB Id: 2029396-3
    ZDB Id: 419957-1
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  • 9
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    American Meteorological Society ; 2014
    In:  Journal of Climate Vol. 27, No. 7 ( 2014-04-01), p. 2735-2756
    In: Journal of Climate, American Meteorological Society, Vol. 27, No. 7 ( 2014-04-01), p. 2735-2756
    Kurzfassung: Drier future conditions are projected for the arid southwest of North America, increasing the chances of the region experiencing severe and prolonged drought. To examine the mechanisms of decadal variability, 47 global climate model historical simulations performed for phase 5 of the Coupled Model Intercomparison Project (CMIP5) were assessed. On average, the CMIP5 models have higher climatological precipitation over the past century in southwestern North America than current instrumental or reanalysis products. The timing of the winter peak in climatological precipitation over California and Nevada is accurately represented. Models with resolutions coarser than 2° show a larger spread in the location and strength of the North American monsoon ridge and subsequent summer precipitation, in comparison with the higher-resolution models. Less than 20% of decadal variability in wintertime precipitation over California is associated with North Pacific sea surface temperature anomalies, a larger proportion than is associated with the tropical forcing but not sufficient for making decadal drought predictions. North American monsoon precipitation is strongly associated with local land temperatures on interannual-to-decadal time scales attributable to evaporative cooling and radiation changes driven by varying cloud cover. Soil moisture in Texas and Oklahoma in April is shown to be positively correlated with monsoon precipitation for the following summer, indicating a potential source of nonoceanic interseasonal persistence in southwestern North American hydroclimate. To make meaningful decadal predictions in the future, it is likely that forecasting will move away from sea surface temperature–driven anomaly patterns, and focus on land surface processes, which can allow persistence of precipitation anomalies via feedbacks.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Englisch
    Verlag: American Meteorological Society
    Publikationsdatum: 2014
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
    Standort Signatur Einschränkungen Verfügbarkeit
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  • 10
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    American Meteorological Society ; 2012
    In:  Journal of Climate Vol. 25, No. 6 ( 2012-03-15), p. 2129-2145
    In: Journal of Climate, American Meteorological Society, Vol. 25, No. 6 ( 2012-03-15), p. 2129-2145
    Kurzfassung: The El Niño–Southern Oscillation (ENSO) response to anthropogenic climate change is assessed in the following 1° nominal resolution Community Climate System Model, version 4 (CCSM4) Coupled Model Intercomparison Project phase 5 (CMIP5) simulations: twentieth-century ensemble, preindustrial control, twenty-first-century projections, and stabilized 2100–2300 “extension runs.” ENSO variability weakens slightly with CO2; however, various significance tests reveal that changes are insignificant at all but the highest CO2 levels. Comparison with the 1850 control simulation suggests that ENSO changes may become significant on centennial time scales; the lack of signal in the twentieth- versus twenty-first-century ensembles is due to their limited duration. Changes to the mean state are consistent with previous studies: a weakening of the subtropical wind stress curl, an eastward shift of the tropical convective cells, a reduction in the zonal SST gradient, and an increase in vertical thermal stratification take place as CO2 increases. The extratropical thermocline deepens throughout the twenty-first century, with the tropical thermocline changing slowly in response. The adjustment time scale is set by the relevant ocean dynamics, and the delay in its effect on ENSO variability is not diminished by increasing ensemble size. The CCSM4 results imply that twenty-first-century simulations may simply be too short for identification of significant tropical variability response to climate change. An examination of atmospheric teleconnections, in contrast, shows that the remote influences of ENSO do respond rapidly to climate change in some regions, particularly during boreal winter. This suggests that changes to ENSO impacts may take place well before changes to oceanic tropical variability itself become significant.
    Materialart: Online-Ressource
    ISSN: 0894-8755 , 1520-0442
    RVK:
    Sprache: Englisch
    Verlag: American Meteorological Society
    Publikationsdatum: 2012
    ZDB Id: 246750-1
    ZDB Id: 2021723-7
    Standort Signatur Einschränkungen Verfügbarkeit
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