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  • North Atlantic Oscillation (NAO)

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    Here are the current Papers & Articles under the research topic North Atlantic Oscillation (NAO). Click on the title of a paper you are interested in to go straight to the full paper.

    The North Atlantic Oscillation - Learning Guide
    The common pressure features seen in the North Atlantic Ocean are for large regions of relatively high pressure centred over the Azores islands (west of Portugal, known as the sub-tropical or Azores high) and low pressure centred over Iceland (the sub-polar or Icelandic low). The NAO describes the relative changes in pressure between these two regions (Azores minus Iceland), and was discovered in the 1920s by Sir Gilbert Walker. This has a strong influence on winter weather and climate patterns in Europe and North America, and can extend further into northern Asia if phases are prolonged. Acting like a giant seesaw, the NAO leads to changes in the intensity and location of the North Atlantic jet stream - ribbons of very fast winds high in the atmosphere that influence the movement of regions of low pressure (depressions) and their associated storms.

    Drivers of North Atlantic Oscillation Events
    Published June 2013.
    This work is set out to quantify the contribution of tropical and extratropical atmospheric forcing mechanisms to the formation of the North Atlantic Oscillation (NAO) pattern. Although the NAO varies on a wide range of time scales, we focus on 10–60 d. At these time scales, mechanisms are at play in the atmosphere that can generate the characteristic dipole pattern. We focus on the tropical Rossby Wave Source (RWS) and extratropical eddy activity. Anomalous tropical and extratropical vorticity forcing associated with the NAO is derived from atmospheric reanalysis data and applied in an idealised barotropic model. Also, using winds from composites of the NAO, the vorticity forcing is derived inversely from the barotropic vorticity equation. Both types of forcing are imposed in the barotropic model in the tropics and extratropics, respectively. An important result is that the tropics dampen the NAO as a result of a negative feedback generated in the extratropics. The damping is strongest, about 30%, for the negative phase of the NAO. For the positive phase, the damping is about 50% smaller. The results show that the barotropic vorticity equation can represent the dynamics of both tropical and extratropical forcing related to the formation of the NAO patterns.

    A robust empirical seasonal prediction of winter NAO and surface climate
    A key determinant of winter weather and climate in Europe and North America is the North Atlantic Oscillation (NAO), the dominant mode of atmospheric variability in the Atlantic domain. Skilful seasonal forecasting of the surface climate in both Europe and North America is reflected largely in how accurately models can predict the NAO. Most dynamical models, however, have limited skill in seasonal forecasts of the winter NAO. A new empirical model is proposed for the seasonal forecast of the winter NAO that exhibits higher skill than current dynamical models. The empirical model provides robust and skilful prediction of the December-January-February (DJF) mean NAO index using a multiple linear regression (MLR) technique with autumn conditions of sea-ice concentration, stratospheric circulation, and sea-surface temperature. The predictability is, for the most part, derived from the relatively long persistence of sea ice in the autumn. The lower stratospheric circulation and sea-surface temperature appear to play more indirect roles through a series of feedbacks among systems driving NAO evolution. This MLR model also provides skilful seasonal outlooks of winter surface temperature and precipitation over many regions of Eurasia and eastern North Americ

    Drivers of North Atlantic Polar Front jet stream variability
    Polar front jet stream variability is responsible for instances of extreme weather and is crucial for regional climate change. The North Atlantic Polar Front jet stream is of particular significance to the heavily populated areas of western Europe and eastern North America as storm track variability, atmospheric modes of variability such as the North Atlantic Oscillation (NAO), temperature and rainfall are all intimately linked with jet stream changes. Although seasonal and interannual variability are often attributed to internal variability, there are several possible drivers of polar front jet stream changes that are reviewed in this study. Cryospheric effects from sea-ice extent and snow cover, oceanic effects from North Atlantic sea-surface temperatures and tropical influences such as the El-Niño Southern Oscillation, and stratospheric effects due to stratospheric circulation variability, solar variability, volcanic eruptions and the Quasi-Biennial Oscillation are all identified in the literature as factors that impact on the Atlantic Polar Front jet stream. These drivers of jet stream variability can oppose or reinforce one another, and there are some indications of possible nonlinear interactions between them. We also review the modelling of jet stream variability. While a consensus has now been reached that some observed drivers can be reproduced in climate models, we conclude that improved understanding of more recently identified drivers of the Atlantic extratropical jet stream is crucial for making progress in regional climate predictions on all timescales from months to decades ahead.

    Dynamical Link between the Barents–Kara Sea Ice and the Arctic Oscillation
    2016 paper. Abstract:
    The recent accelerated Arctic sea ice decline has been proposed as a possible forcing factor for midlatitude circulation changes, which can be projected onto the Arctic Oscillation (AO) and/or North Atlantic Oscillation (NAO) mode. However, the timing and physical mechanisms linking AO responses to the Arctic sea ice forcing are not entirely understood. In this study, the authors suggest a connection between November sea ice extent in the Barents and Kara Seas and the following winter’s atmospheric circulation in terms of the fast sea ice retreat and the subsequent modification of local air–sea heat fluxes. In particular, the dynamical processes that link November sea ice in the Barents and Kara Seas with the development of AO anomalies in February is explored. In response to the lower-tropospheric warming associated with the initial thermal effect of the sea ice loss, the large-scale atmospheric circulation goes through a series of dynamical adjustment processes: The decelerated zonal-mean zonal wind anomalies propagate gradually from the subarctic to midlatitudes in about one month. The equivalent barotropic AO dipole pattern develops in January because of wave–mean flow interaction and firmly establishes itself in February following the weakening and warming of the stratospheric polar vortex. This connection between sea ice loss and the AO mode is robust on time scales ranging from interannual to decadal. Therefore, the recent winter AO weakening and the corresponding midlatitude climate change may be partly associated with the early winter sea ice loss in the Barents and Kara Seas.

    Dynamics of the ENSO teleconnection and NAO variability in the North Atlantic-European late winter
    The winter extratropical teleconnection of El Niño-Southern Oscillation (ENSO) in the North Atlantic-European (NAE) sector remains controversial, concerning both the amplitude of its impacts and the underlying dynamics. However, a well-established response is a late-winter (January-March) signal in sea-level pressure (SLP) consisting of a dipolar pattern that resembles the North Atlantic Oscillation (NAO). Clarifying the relationship between this “NAO like” ENSO signal and the actual NAO is the focus of this study. The ENSO-NAE teleconnection and NAO signature are diagnosed by means of linear regression onto the sea-surface temperature (SST) Niño3.4 index and an EOF-based NAO index, respectively, using long-term reanalysis data (NOAA-20CR, ERA-20CR). While the similarity in SLP is evident, the analysis of anomalous upper-tropospheric geopotential height, zonal wind, transient-eddy momentum flux, as well as precipitation and meridional eddy heat flux, suggests that there is no dynamical link between the phenomena. The observational results are further confirmed by analyzing two 10-member ensembles of atmosphere-only simulations (using an intermediate-complexity and a state-of-the-art model) with prescribed SSTs over the 20th century. The SST forced variability in the Northern Hemisphere is dominated by the extratropical ENSO teleconnection, which provides modest but significant SLP skill in the NAE midlatitudes. The regional internally-generated variability, estimated from residuals around the ensemble mean, corresponds to the NAO pattern. It is concluded that distinct dynamics are at play in the ENSO-NAE teleconnection and NAO variability, and caution is advised when interpreting the former in terms of the latter.

    How Predictable Are the Arctic and North Atlantic Oscillations? Exploring the Variability and Predictability of the Northern Hemisphere
    The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) describe the dominant part of the variability in the Northern Hemisphere extratropical troposphere. Due to 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/AO indices. Small deviations from Gaussianity are found and 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.

    Interdecadal variability of the ENSO–North Atlantic Oscillation connection in boreal summer
    Understanding the combined effect of El Ni ̃no – Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) is of great importance for climate seasonal prediction (extreme climate events in particular). Results in this study show that during the last hundred years (1900 to present), the ENSO – NAO connection experiences a notable interdecadal change in summer (June – August) according toa 21-year sliding correlation between them, namely, from no significant correlation(uncoupling) before the mid-1970s to a significant correlation (coupling) after themid-1970s. Comparison analysis between the coupling epoch (1977 – 1997) and the uncoupling epoch (1958 – 1976) shows that the most pronounced circulation anomalies take place over the extratropical North Pacific. Further analysis and numerical experiments suggest that a poleward-propagating Rossby wave train,possibly enhanced by sea-surface-temperature anomalies in the extratropical North Pacific associated with the developing phases of ENSO during the later epoch, is responsible for connecting the ENSO signal with the NAO.

    Negative NAO and cold Eurasian winters: How exceptional was the winter of 1962/1963?
    2007 paper. Abstract:
    Since the mid 1980s frequent positive phases of the North Atlantic Oscillation (NAO) linked to a higher-than-normal pressure difference between the Azores high and Icelandic low pressure centres and stronger westerlies (Hurrell, 1995; Thompson and Wallace, 1998; Osborn, 2006) have led to higher winter temperatures over most of western Europe. Harsher winters were more frequent between the 1950s and the 1980s (see Figure 9 in Graham et al., 2006). Even though mild winters similar to the recent ones did occur they alternated with colder winter seasons. The best remembered is probably the winter of 1962/1963 when temperatures were well below zero across most of Europe. Figure 1 illustrates the unusual amounts of snow that occurred at lower elevations in Switzerland. With the exception of western Ireland, most of the British Isles was covered by a blanket of snow for most of the winter (Met Office).

    North Atlantic Oscillation – Concepts And Studies
    This paper aims to provide a comprehensive review of previous studies and concepts concerning the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) and its recent homologue, the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM), are the most prominent modes of variability in the Northern Hemisphere winter climate. The NAO teleconnection is characterised by a meridional displacement of atmospheric mass over the North Atlantic area. Its state is usually expressed by the standardised air pressure difference between the Azores High and the Iceland Low. ThisNAO index is a measure of the strength of the westerly flow (positive with strong westerlies, and vice versa). Together with the El Nino/Southern Oscillation (ENSO) phenomenon, the NAO is a major source of seasonal to interdecadal variability in the global atmosphere. On interannual and shorter time scales, the NAO dynamics can be explained as a purely internal mode of variability of the atmospheric circulation. Interdecadal variability maybe influenced, however, by ocean and sea-ice processes.

    Snow–(N)AO Teleconnection and Its Modulation by the Quasi-Biennial Oscillation
    This study explores the wintertime extratropical atmospheric response to Siberian snow anomalies in fall,using observations and two distinct atmospheric general circulation models. The role of the quasi-biennial oscillation (QBO) in modulating this response is discussed by differentiating easterly and westerly QBO years. The remote influence of Siberian snow anomalies is found to be weak in the models, especially in the stratosphere where the ‘‘Holton–Tan’’ effect of the QBO dominates the simulated snow influence on the polar vortex. At the surface, discrepancies between composite analyses from observations and model results question the causal relationship between snow and the atmospheric circulation, suggesting that the atmosphere might have driven snow anomalies rather than the other way around. When both forcings are combined, the simulations suggest destructive interference between the response to positive snow anomalies and easterly QBO (and vice versa), at odds with the hypothesis that the snow–North Atlantic Oscillation/Arctic Oscillation [(N)AO] teleconnection in recent decades has been promoted by the QBO. Although model limitations in capturing the relationship exist, altogether these results suggest that the snow–(N)AO teleconnection may be a stochastic artifact rather than a genuine atmospheric response to snow-cover variability.This study adds to a growing body of evidence suggesting that climate models do not capture a robust and stationary snow–(N)AO relationship. It also highlights the need for extending observations and/or improvingmodels to progress on this matter.

    The Central Role of Ocean Dynamics in Connecting the NAO to the Extratropical Component of the AMO
    The relationship between the North Atlantic Oscillation (NAO) and Atlantic sea surface temperature(SST) variability is investigated using models and observations. Coupled climate models are used in which the ocean component is either a fully dynamic ocean or a slab ocean with no resolved ocean heat transport. Ontime scales less than 10 yr, NAO variations drive a tripole pattern of SST anomalies in both observations and models. This SST pattern is a direct response of the ocean mixed layer to turbulent surface heat flux anomalies associated with the NAO. On time scales longer than 10 yr, a similar relationship exists between the NAO and the tripole pattern of SST anomalies in models with a slab ocean. A different relationship exists both for the observations and for models with a dynamic ocean. In these models, a positive (negative) NAO anomaly leads, after a decadal-scale lag, to a monopole pattern of warming (cooling) that resembles the Atlantic multidecadal oscillation (AMO), although with smaller-than-observed amplitudes of tropical SST anomalies.Ocean dynamics are critical to this decadal-scale response in the models. The simulated Atlantic meridional overturning circulation (AMOC) strengthens (weakens) in response to a prolonged positive (negative) phase of the NAO, thereby enhancing (decreasing) poleward heat transport, leading to broad-scale warming(cooling). Additional simulations are used in which heat flux anomalies derived from observed NAO variations from 1901 to 2014 are applied to the ocean component of coupled models. It is shown that ocean dynamics allow models to reproduce important aspects of the observed AMO, mainly in the Subpolar Gyre.

    The Linear Sensitivity of the North Atlantic Oscillation and Eddy-Driven Jet to SSTs
    The North Atlantic Oscillation (NAO) and eddy-driven jet contain a forced component arising from sea surface temperature (SST) variations. Due to large amounts of internal variability, it is not trivial to determine where and to what extent SSTs force the NAO and jet. A linear statistical–dynamic method is employed with a large climate ensemble to compute the sensitivities of the winter and summer NAO and jet speed and latitude to the SSTs. Key regions of sensitivity are identified in the Indian and Pacific basins, and the North Atlantic tripole. Using the sensitivity maps and a long observational SST dataset, skillful reconstructions of the NAO and jet time series are made. The ability to skillfully forecast both the winter and summer NAO using only SST anomalies is also demonstrated. The linear approach used here allows precise attribution of model forecast signals to SSTs in particular regions. Skill comes from the Atlantic and Pacific basins on short lead times, while the Indian Ocean SSTs may contribute to the longer-term NAO trend. However, despite the region of high sensitivity in the Indian Ocean, SSTs here do not provide significant skill on interannual time scales, which highlights the limitations of the imposed SST approach. Given the impact of the NAO and jet on Northern Hemisphere weather and climate, these results provide useful information that could be used for improved attribution and forecasting.

    The Role of Linear Interference in the Annular Mode Response to Tropical SST Forcing
    Recent observational and modeling studies have demonstrated a link between eastern tropical Pacific Ocean (TPO) warming associated with the El Niño–Southern Oscillation (ENSO) and the negative phase of the wintertime northern annular mode (NAM). The TPO–NAM link involves a Rossby wave teleconnection from the tropics to the extratropics, and an increase in polar stratospheric wave driving that in turn induces a negative NAM anomaly in the stratosphere and troposphere. Previous work further suggests that tropical Indian Ocean (TIO) warming is associated with a positive NAM anomaly, which is of opposite sign to the TPO case. The TIO case is, however, difficult to interpret because the TPO and TIO warmings are not independent. To better understand the dynamics of tropical influences on the NAM, the current study investigates the NAM response to imposed TPO and TIO warmings in a general circulation model. The NAM responses to the two warmings have opposite sign and can be of surprisingly similar amplitude even though the TIO forcing is relatively weak. It is shown that the sign and strength of the NAM response is often simply related to the phasing, and hence the linear interference, between the Rossby wave response and the climatological stationary wave. The TPO (TIO) wave response reinforces (attenuates) the climatological wave and therefore weakens (strengthens) the stratospheric jet and leads to a negative (positive) NAM response. In additional simulations, it is shown that decreasing the strength of the climatological stationary wave reduces the importance of linear interference and increases the importance of nonlinearity. This work demonstrates that the simulated extratropical annular mode response to climate forcings can depend sensitively on the amplitude and phase of the climatological stationary wave and the wave response.

    The dynamics of NAO teleconnection pattern growth and decay
    This investigation performs both diagnostic analyses with NCEP/NCAR re-analysis data and forced, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the North Atlantic Oscillation (NAO) teleconnection pattern.The results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks. The positive NAO phase is found to develop after anomalous wave train propagation across the North Pacic to the east coast of North America.This contrasts with the negative NAO phase which appeared to develop in situ. Both high-frequency (period<10 days) and low-frequency (period>10 days) transient eddy fluxes drive the NAO growth. After the NAO anomaly attains its maximum amplitude,the high-frequency transient eddy fluxes continue to drive the NAO anomaly in a manner that is consistent with a positive feedback process.discussed.


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