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Steven B Feldstein - One of the best experts on this subject based on the ideXlab platform.

  • what drives the North Atlantic Oscillation s temperature anomaly pattern part i the growth and decay of the surface air temperature anomalies
    Journal of the Atmospheric Sciences, 2020
    Co-Authors: Joseph P Clark, Steven B Feldstein
    Abstract:

    AbstractComposite analysis is used to examine the physical processes that drive the growth and decay of the surface air temperature anomaly pattern associated with the North Atlantic Oscillation (N...

  • the positive North Atlantic Oscillation with downstream blocking and middle east snowstorms the large scale environment
    Journal of Climate, 2015
    Co-Authors: Dehai Luo, Yao Yao, Aiguo Dai, Steven B Feldstein
    Abstract:

    AbstractIn this study, the atmospheric conditions for the December 2013 Middle East snowstorm are examined from a case study perspective and by performing a composite analysis of extreme winter events from 1950 to 2013 using reanalysis data. It is revealed that this snowstorm arises from the occurrence of an omega (Ω)-type European blocking (EB) with a strong downstream trough that is associated with a southward-displaced positive-phase North Atlantic Oscillation (NAO+) event. In the anomaly field, the EB exhibits a Northeast–southwest (NE–SW)-tilted dipole structure. The Ω-type EB transports cold air into the Middle East and produces snowfall within the trough over the Middle East.The composite analysis shows that the location of cold temperatures depends strongly on the tilting direction and strength of the EB dipole anomaly. The NE–SW [Northwest–southeast (NW–SE)]-tilted EB dipole occurs with a southward (Northward)-displaced NAO+ event. The NE–SW-tilted EB dipole anomaly is associated with an arching-...

  • weather regime transitions and the interannual variability of the North Atlantic Oscillation part ii dynamical processes
    Journal of the Atmospheric Sciences, 2012
    Co-Authors: Dehai Luo, Jing Cha, Steven B Feldstein
    Abstract:

    AbstractIn this study, attention is focused on identifying the dynamical processes that contribute to the negative North Atlantic Oscillation (NAO)− to positive NAO (NAO+) and NAO+ to NAO− transitions that occur during 1978–90 (P1) and 1991–2008 (P2). By constructing Atlantic ridge (AR) and Scandinavian blocking (SBL) indices, the composite analysis demonstrates that in a stronger AR (SBL) winter NAO− (NAO+) event can more easily transition into an NAO+ (NAO−) event. Composites of 300-hPa geopotential height anomalies for the NAO− to NAO+ and NAO+ to NAO− transition events during P1 and P2 are calculated. It is shown for P2 (P1) that the NAO+ to SBL to NAO− (NAO− to AR to NAO+) transition results from the retrograde drift of an enhanced high-latitude, large-scale, positive (negative) anomaly over Northern Europe during the decay of the previous NAO+ (NAO−) event. This finding cannot be detected for NAO events without transition.Moreover, it is found that the amplification of retrograding wavenumber 1 is m...

  • is the North Atlantic Oscillation a breaking wave
    Journal of the Atmospheric Sciences, 2004
    Co-Authors: Christian Franzke, Sukyoung Lee, Steven B Feldstein
    Abstract:

    Abstract Given the recent observational evidence that the positive (negative) phase of the North Atlantic Oscillation (NAO) is the remnant of anticyclonic (cyclonic) wave breaking, this study uses a multilevel primitive equation model to investigate important dynamical attributes of the above wave breaking behavior. For this purpose, a hierarchy of different basic states (two- and three-dimensional) and initial perturbations are used. With the three-dimensional climatological flow as the basic state, it is found that initial perturbations located equatorward (poleward) and upstream of the climatological Atlantic jet lead to wave breaking similar to that of the positive (negative) NAO phase. Consistently, analysis of observational data indeed shows that the Pacific storm track is displaced equatorward (poleward) prior the onset of the positive (negative) NAO phase. This result suggests that the latitudinal position of the Pacific storm track plays an important role for determining the phase of the NAO. Sen...

  • synoptic view of the North Atlantic Oscillation
    Journal of the Atmospheric Sciences, 2004
    Co-Authors: James J Benedict, Sukyoung Lee, Steven B Feldstein
    Abstract:

    Abstract This article investigates the synoptic characteristics of individual North Atlantic Oscillation (NAO) events by examining the daily evolution of the potential temperature field on the nominal tropopause (the 2-PVU surface). This quantity is obtained from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset for the winter season. For both phases, the NAO is found to originate from synoptic-scale waves. As these waves evolve into the low-frequency NAO pattern, they break anticyclonically for the positive phase and cyclonically for the negative phase. The results of this analysis suggest that it is the remnants of these breaking waves that form the physical entity of the NAO. Throughout the NAO events, for both phases, the NAO is maintained by the successive breaking of upstream synoptic-scale waves. When synoptic-scale disturbances are no longer present, mixing processes play an important role in the NAO decay. As in other recent ...

James W Hurrell - One of the best experts on this subject based on the ideXlab platform.

  • the role of the North Atlantic Oscillation in european climate projections
    Climate Dynamics, 2017
    Co-Authors: Clara Deser, James W Hurrell, Adam S Phillips
    Abstract:

    This study highlights the expected range of projected winter air temperature and precipitation trends over the next 30–50 years due to unpredictable fluctuations of the North Atlantic Oscillation (NAO) superimposed upon forced anthropogenic climate change. The findings are based on a 40-member initial-condition ensemble of simulations covering the period 1920–2100 conducted with the Community Earth System Model version 1 (CESM1) at 1° spatial resolution. The magnitude (and in some regions, even the sign) of the projected temperature and precipitation trends over Europe, Russia and parts of the Middle East vary considerably across the ensemble depending on the evolution of the NAO in each individual member. Thus, internal variability of the NAO imparts substantial uncertainty to future changes in regional climate over the coming decades. To validate the model results, we apply a simple scaling approach that relates the margin-of-error on a trend to the statistics of the interannual variability. In this way, we can obtain the expected range of projected climate trends using the interannual statistics of the observed NAO record in combination with the model’s radiatively-forced response (given by the ensemble-mean of the 40 simulations). The results of this observationally-based estimate are similar to those obtained directly from the CESM ensemble, attesting to the fidelity of the model’s representation of the NAO and the utility of this approach. Finally, we note that the interannual statistics of the NAO and associated surface climate impacts are subject to uncertainty due to sampling fluctuations, even when based on a century of data.

  • an overview of the North Atlantic Oscillation
    Geophysical monograph, 2013
    Co-Authors: James W Hurrell, Yochanan Kushnir, Geir Ottersen, Martin Visbeck
    Abstract:

    The North Atlantic Oscillation (NAO) is one of the most prominent and recurrent patterns of atmospheric circulation variability. It dictates climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during boreal winter, so variations in the NAO are important to society and for the environment. Understanding the processes that govern this variability is, therefore, of high priority, especially in the context of global climate change. This review, aimed at a scientifically diverse audience, provides general background material for the other chapters in the monograph, and it synthesizes some of their central points. It begins with a description of the spatial structure of climate and climate variability, including how the NAO relates to other prominent patterns of atmospheric circulation variability. There is no unique way to define the spatial structure of the NAO, or thus its temporal evolution, but several common approaches are illustrated. The relationship between the NAO and variations in surface temperature, storms and precipitation, and thus the economy, as well as the ocean and ecosystem responses to NAO variability, are described. Although the NAO is a mode of variability internal to the atmosphere, indices of it exhibit decadal variability and trends. That not all of its variability can be attributed to intraseasonal stochastic atmospheric processes points to a role for external forcings and, perhaps, a small but useful amount of predictability. The surface, stratospheric and anthropogenic processes that may influence the phase and amplitude of the NAO are reviewed.

  • the summer North Atlantic Oscillation past present and future
    Journal of Climate, 2009
    Co-Authors: C K Folland, Jeff Knight, Hans W Linderholm, D Fereday, Sarah Ineson, James W Hurrell
    Abstract:

    Summer climate in the North Atlantic‐European sector possesses a principal pattern of year-to-year variability that is the parallel to the well-known North Atlantic Oscillation in winter. This summer North Atlantic Oscillation (SNAO) is defined here as the first empirical orthogonal function (EOF) of observed summertime extratropical North Atlantic pressure at mean sea level. It is shown to be characterized by a more Northerly location and smaller spatial scale than its winter counterpart. The SNAO is also detected by cluster analysis and has a near-equivalent barotropic structure on daily and monthly time scales. Although of lesser amplitude than its wintertime counterpart, the SNAO exerts a strong influence on Northern European rainfall, temperature, and cloudiness through changes in the position of the North Atlantic storm track. It is, therefore, of key importance in generating summer climate extremes, including flooding, drought, and heat

  • the North Atlantic Oscillation past present and future
    Proceedings of the National Academy of Sciences of the United States of America, 2001
    Co-Authors: Martin Visbeck, James W Hurrell, Lorenzo M Polvani, Heidi Cullen
    Abstract:

    The climate of the Atlantic sector exhibits considerable variability on a wide range of time scales. A substantial portion is associated with the North Atlantic Oscillation (NAO), a hemispheric meridional Oscillation in atmospheric mass with centers of action near Iceland and over the subtropical Atlantic. NAO-related impacts on winter climate extend from Florida to Greenland and from Northwestern Africa over Europe far into Northern Asia. Over the last 3 decades, the phase of the NAO has been shifting from mostly negative to mostly positive index values. Much remains to be learned about the mechanisms that produce such low frequency changes in the North Atlantic climate, but it seems increasingly likely that human activities are playing a significant role.

  • the North Atlantic Oscillation
    Science, 2001
    Co-Authors: James W Hurrell, Yochanan Kushnir, Martin Visbeck
    Abstract:

    The North Atlantic Oscillation (NAO) dictates climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during winter. It strongly affects agricultural yields, water management, fish inventories, and terrestrial ecology. In their Perspective, Hurrell, Kushnir, and Visbeck report recent research into the NAO discussed at an American Geophysical Union Chapman Conference at the end of 2000. Much remains to be learned about the NAO, but it seems increasingly less likely that natural variability is the cause for the recent upward NAO trend.

Jurgen Bader - One of the best experts on this subject based on the ideXlab platform.

Thomas L Delworth - One of the best experts on this subject based on the ideXlab platform.

  • the North Atlantic Oscillation as a driver of rapid climate change in the Northern hemisphere
    Nature Geoscience, 2016
    Co-Authors: Thomas L Delworth, Fanrong Zeng, Gabriel A Vecchi, Xiaosong Yang, Liping Zhang, Rong Zhang
    Abstract:

    The North Atlantic Oscillation has varied markedly on multidecadal timescales. Analyses of climate simulations show that these variations have contributed to Arctic sea ice loss, Northern Hemisphere warming and tropical storm activity.

  • the ocean s response to North Atlantic Oscillation variability
    Geophysical monograph, 2013
    Co-Authors: Martin Visbeck, Thomas L Delworth, Ruth G Curry, Robert R Dickson, Eric P Chassignet, Gerd Krahmann
    Abstract:

    The North Atlantic Oscillation (NAO) is the dominant mode of atmospheric variability in the North Atlantic Sector. Basin scale changes in the atmospheric forcing significantly affect properties and circulation of the ocean. Part of the response is local and rapid (surface temperature, mixed-layer depth, upper ocean heat content, surface Ekman transport, sea ice cover). However, the geostrophically balanced large-scale horizontal and overturning circulation can take several years to adjust to changes in the forcing. The delayed response is non-local in the sense that waves and the mean circulation communicate perturbations at the air-sea interface to other parts of the Atlantic basin. A delayed and non-local response can potentially give rise to oscillatory behavior if there is significant feedback from the ocean to the atmosphere. We conjecture that, on decadal and longer time scales, changes in the ocean's heat storage and transport should have an increasingly important impact on the climate. Finally, changes in the ocean circulation and distribution of heat and freshwater will also alter ventilation rates and pathways. Thus we expect a change in the net uptake of gases (e.g., O 2 , CO 2 ), altered nutrient balance, and changes in the dispersion of marine life. We review what is known about the oceanic response to changes in NAO-induced forcing from combined theoretical, numerical experimentation and observational perspectives.

  • implications of the recent trend in the arctic North Atlantic Oscillation for the North Atlantic thermohaline circulation
    Journal of Climate, 2000
    Co-Authors: Thomas L Delworth, Keith W Dixon
    Abstract:

    Most projections of greenhouse gas‐induced climate change indicate a weakening of the thermohaline circulation (THC) in the North Atlantic in response to increased freshening and warming in the subpolar region. These changes reduce high-latitude upper-ocean density and therefore weaken the THC. Using ensembles of numerical experiments with a coupled ocean‐atmosphere model, it is found that this weakening could be delayed by several decades in response to a sustained upward trend in the Arctic/North Atlantic Oscillation during winter, such as has been observed over the last 30 years. The stronger winds over the North Atlantic associated with this trend extract more heat from the ocean, thereby cooling and increasing the density of the upper ocean and thus opposing the previously described weakening of the THC. This result is of particular importance if the positive trend in the Arctic/North Atlantic Oscillation is a response to increasing greenhouse gases, as has been recently suggested.

  • oceanic influence on the North Atlantic Oscillation and associated Northern hemisphere climate variations 1959 1993
    Geophysical Research Letters, 2000
    Co-Authors: Vikram M Mehta, Max J Suarez, Julia V Manganello, Thomas L Delworth
    Abstract:

    The North Atlantic Oscillation (NAO) ex- hibits variations at interannual to multidecadal time scales and is associated with climate variations over eastern North America, the North Atlantic, Europe, and North Africa. Therefore, it is very important to understand causes of these NAO variations and assess their predictability. It has been hypothesized, based on observations, that sea surface temperature (SST) and sea-ice variations in the North Atlantic Ocean influ- ence the NAO. We describe results of an ensemble of sixteen experiments with an atmospheric general cir- culation model in which we used observed SST and sea-ice boundary conditions globally during 1949-1993. We show that multiyear NAO and associated climate variations can be simulated reasonably accurately if re- sults from a large number of experiments are averaged. We also show that the ambiguous results of previous NAO modeling studies were strongly influenced by the ensemble size, which was much smaller than that in the present study. The implications of these results for understanding and predictability of the NAO are dis- cussed.

Carley Iles - One of the best experts on this subject based on the ideXlab platform.

  • role of the North Atlantic Oscillation in decadal temperature trends
    Environmental Research Letters, 2017
    Co-Authors: Carley Iles, Gabriele C Hegerl
    Abstract:

    Global temperatures have undergone periods of enhanced warming and pauses over the last century, with greater variations at local scales due to internal variability of the climate system. Here we investigate the role of the North Atlantic Oscillation (NAO) in decadal temperature trends in the Northern Hemisphere for periods with large decadal NAO trends. Using a regression based technique we find a best estimate that trends in the NAO more than halved (reduced by 57%, 5%–95%: 47%–63%) the winter warming over the Northern Hemisphere extratropics (NH; 30N–90N) from 1920–1971 and account for 45% (±14%) of the warming there from 1963–1995, with larger impacts on regional scales. Over the period leading into the so-called warming hiatus, 1989–2013, the NAO reduced NH winter warming to around one quarter (24%; 19%–31%) of what it would have been, and caused large negative regional trends, for example, in Northern Eurasia. Warming is more spatially uniform across the Northern Hemisphere after removing the NAO influence in winter, and agreement with multi-model mean simulated trends improves. The impact of the summer NAO is much weaker, but still discernible over Europe, North America and Greenland, with the downward trend in the summer NAO from 1988–2012 reducing warming by about a third in Northern Europe and a half in North America. A composite analysis using CMIP5 control runs suggests that the ocean response to prolonged NAO trends may increase the influence of decadal NAO trends compared to estimates based on interannual regressions, particularly in the Arctic. Results imply that the long-term NAO trends over the 20th century alternately masked or enhanced anthropogenic warming, and will continue to temporarily offset or enhance its effects in the future.