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.

/pdf/an-overview-of-the-north-atlantic-oscillation-3uug0y0py4.pdf

An overview of the North Atlantic Oscillation

The primary focus of this review is tropical-extratropical interactions and especially the issues involved in determining the response of the extratropical atmosphere to tropical forcing associated with sea surface temperature (SST) anomalies. The review encompasses observations, empirical studies, theory and modeling of the extratropical teleconnections with a focus on developments over the Tropical Oceans-Global Atmosphere (TOGA) decade and the current state of understanding. In the tropical atmosphere, anomalous SSTs force anomalies in convection and large-scale overturning with subsidence in the descending branch of the local Hadley circulation. The resulting strong upper tropospheric divergence in the tropics and convergence in the subtropics act as a Rossby wave source. The climatological stationary planetary waves and associated jet streams, especially in the northern hemisphere, can make the total Rossby wave sources somewhat insensitive to the position of the tropical heating that induces them and thus can create preferred teleconnection response patterns, such as the Pacific-North American (PNA) pattern. However, a number of factors influence the dispersion and propagation of Rossby waves through the atmosphere, including zonal asymmetries in the climatological state, transients, and baroclinic and nonlinear effects. Internal midlatitude sources can amplify perturbations. Observations, modeling, and theory have clearly shown how storm tracks change in response to changes in quasi-stationary waves and how these changes generally feedback to maintain or strengthen the dominant perturbations through vorticity and momentum transports. The response of the extratropical atmosphere naturally induces changes in the underlying surface, so that there are changes in extratropical SSTs and changes in land surface hydrology and moisture availability that can feedback and influence the total response. Land surface processes are believed to be especially important in spring and summer. Anomalous SSTs and tropical forcing have tended to be strongest in the northern winter, and teleconnections in the southern hemisphere are weaker and more variable and thus more inclined to be masked by natural variability. Occasional strong forcing in seasons other than winter can produce strong and identifiable signals in the northern hemisphere and, because the noise of natural variability is less, the signal-to-noise ratio can be large. The relative importance of tropical versus extratropical SST forcings has been established through numerical experiments with atmospheric general circulation models (AGCMs). Predictability of anomalous circulation and associated surface temperature and precipitation in the extratropics is somewhat limited by the difficulty of finding a modest signal embedded in the high level of noise from natural variability in the extratropics, and the complexity and variety of the possible feedbacks. Accordingly, ensembles of AGCM runs and time averaging are needed to identify signals and make predictions. Strong anomalous tropical forcing provides opportunities for skillful forecasts, and the accuracy and usefulness of forecasts is expected to improve as the ability to forecast the anomalous SSTs improves, as models improve, and as the information available from the mean and the spread of ensemble forecasts is better utilized.

/pdf/progress-during-toga-in-understanding-and-modeling-global-bqacmww353.pdf

Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface temperatures

The advances in our understanding of extratropical atmosphere‐ocean interaction over the past decade and a half are examined, focusing on the atmospheric response to sea surface temperature anomalies. The main goal of the paper is to assess what was learned from general circulation model (GCM) experiments over the recent two decades or so. Observational evidence regarding the nature of the interaction and dynamical theory of atmospheric anomalies forced by surface thermal anomalies is reviewed. Three types of GCM experiments used to address this problem are then examined: models with fixed climatological conditions and idealized, stationary SST anomalies; models with seasonally evolving climatology forced with realistic, time-varying SST anomalies; and models coupled to an interactive ocean. From representative recent studies, it is argued that the extratropical atmosphere does respond to changes in underlying SST although the response is small compared to internal (unforced) variability. Two types of interactions govern the response. One is an eddy-mediated process, in which a baroclinic response to thermal forcing induces and combines with changes in the position or strength of the storm tracks. This process can lead to an equivalent barotropic response that feeds back positively on the ocean mixed layer temperature. The other is a linear, thermodynamic interaction in which an equivalent-barotropic low-frequency atmospheric anomaly forces a change in SST and then experiences reduced surface thermal damping due to the SST adjustment. Both processes contribute to an increase in variance and persistence of low-frequency atmospheric anomalies and, in fact, may act together in the natural system.

/pdf/atmospheric-gcm-response-to-extratropical-sst-anomalies-1isb295h3v.pdf

Atmospheric GCM Response to Extratropical SST Anomalies: Synthesis and Evaluation*

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. This NAO 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 may be influenced, however, by ocean and sea-ice processes.

/pdf/north-atlantic-oscillation-concepts-and-studies-sh5cxw245m.pdf

North atlantic oscillation - concepts and studies

Starting from the assumption that the atmosphere is the primary source of variability internal to the midlatitude atmosphere–ocean system on intraseasonal to interannual timescales, the authors construct a simple stochastically forced, one-dimensional, linear, coupled energy balance model. The coupled system is then dissected into partially coupled and uncoupled systems in order to quantify the effects of coupling. The simplicity of the model allows for analytic evaluation of many quantities of interest, including power spectra, total variance, lag covariance between atmosphere and ocean, and surface flux spectra. The model predicts that coupling between the atmosphere and ocean in the midlatitudes will enhance the variance in both media and will decrease the energy flux between the atmosphere and the ocean. The model also demonstrates that specification of historical midlatitude sea surface temperature anomalies as a boundary condition for an atmospheric model will not generally lead to a correc...

/pdf/the-basic-effects-of-atmosphere-ocean-thermal-coupling-on-53tjuy8t2p.pdf

The Basic Effects of Atmosphere–Ocean Thermal Coupling on Midlatitude Variability*

Using an atmospheric global spectral model, it is shown that the winter atmosphere in the midlatitudes is capable of reacting to prescribed sea surface temperature (SST) anomalies in the northwest Atlantic with two very different responses. The nature of the response is determined by the climatological conditions of the winter regime. Experiments are performed using either the perpetual November or January conditions with or without the prescribed SST anomalies. Warm SST anomalies in November result in a highly significant anomalous ridge downstream over the Atlantic with a nearly equivalent barotropic structure; in January, the response is a statistically less significant trough. The presence of the SST anomalies also causes a northward (southward) shift of the Atlantic storm track in the November (January) cases. A diagnostic analysis of the anomalous heat advection in the simulations reveals that in the January cases, the surface heating is offset primarily by the strong horizontal cold advect...

The Differences between Early and Midwinter Atmospheric Responses to Sea Surface Temperature Anomalies in the Northwest Atlantic

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The Differences between Early and Midwinter Atmospheric Responses to Sea Surface Temperature Anomalies in the Northwest Atlantic