Synoptic-scale Waves Research Articles

Spatial Pattern and Zonal Shift of the North Atlantic Oscillation. Part I: A Dynamical Interpretation

Abstract This paper presents a possible dynamical explanation for why the North Atlantic Oscillation (NAO) pattern exhibits an eastward shift from the period 1958–77 (P1) to the period 1978–97 (P2) or 1998–2007 (P3). First, the empirical orthogonal function analysis of winter mean geopotential heights during P1, P2, and P3 reveals that the NAO dipole anomaly exhibits a northwest–southeast (NW–SE) tilting during P1 but a northeast–southwest (NE–SW) tilting during P2 and P3. The NAO pattern, especially its northern center, undergoes a more pronounced eastward shift from P1 to P2. The composite calculation of NAO events during P1 and P2 also indicates that the negative (positive) NAO phase dipole anomaly can indeed exhibit such a NW–SE (NE–SW) tilting. Second, a linear Rossby wave formula derived in a slowly varying basic flow with a meridional shear is used to qualitatively show that the zonal phase speed of the NAO dipole anomaly is larger (smaller) in higher latitudes and smaller (larger) in lower latitudes during the life cycle of the positive (negative) NAO phases because the core of the Atlantic jet is shifted to the north (south). Such a phase speed distribution tends to cause the different movement speeds of the NAO dipole anomaly at different latitudes, thus resulting in the different spatial tilting of the NAO dipole anomaly depending on the phase of the NAO. The zonal displacement of the northern center of the NAO pattern appears to be more pronounced because the change of the mean flow between two phases of the NAO is more distinct in higher latitudes than in lower latitudes. In addition, a weakly nonlinear analytical solution, based on the assumption of the scale separation between the NAO anomaly and transient synoptic-scale waves, is used to demonstrate that an eastward shift of the Atlantic storm-track eddy activity that is associated with the eastward extension of the Atlantic jet stream is a possible cause of the whole eastward shift of the center of action of the NAO pattern during P2/P3.

Interaction between planetary-scale diffluent flow and synoptic-scale waves during the life cycle of blocking

In this paper, a new transient forced quasi-resonant triad interaction theory in a beta channel is proposed to investigate the interaction between planetary-scale diffluent flow composed of zonal wavenumbers 1–3 and synoptic-scale waves produced continuously by a synoptic-scale vorticity source fixed upstream of an incipient blocking region during the life cycle of blocking. It is shown that the superposition of initial three Rossby waves for zonal wavenumbers 1 (monopole), 2 (dipole), and 3 (monopole), which permit triad quasi-resonance, can represent an incipient blocking event. The synoptic-scale eddies may act to amplify the incipient blocking and to excite a blocking circulation with a strong meander, whose flow pattern depends on the initial amplitudes of the planetary waves and both the intensity and location of preexisting synoptic-scale waves. The onset (decay) of the planetary-scale split-flow blocking is mainly represented by a strong increase (decrease) in the amplitude of the zonal wavenumber 2 component, having a dipole meridional structure related to the preexisting synoptic-scale eddies. The typical persistence time of the model blocking was of about 20 days, consistent with observations of blocking patterns.

Global diagnostic energetics of five state-of-the-art climate models

In this study, the global energy cycle of five state-of-the-art climate models is evaluated in the wave number domain for all seasons. The energy cycle estimates are based on 30 years of 6-hourly data obtained at pressure levels of all models. The models energetics are compared to those obtained from three reanalysis datasets (ERA-40, JRA-25 and NCEP-R2). The results show that the distributions of the energetics integrands and the shape of the various wave number spectra are reasonably well simulated. Many important features can be found in most models, namely both the upscale and downscale energy cascade for the wave–wave interactions of kinetic energy, the downscale energy cascade for the wave–wave interactions of available potential energy and the downscale energy transfer for the zonal–wave interactions of kinetic energy. However, the magnitude in the integrands distributions is generally excessive, yielding too much energy and an overactive energy cycle in the models. Accordingly, this energy excess is also reflected in the various spectra, specially but not exclusively, at the synoptic scale wave numbers for the energy conversion/transfer rates. The well known cold pole bias and the too strong tropospheric jets, along with their dislocation in some cases, still persist in the climate models. These are some of the deficiencies in the models directly implicated in the energy cycle. Apparently, simply increasing the horizontal and vertical resolutions is not enough to eliminate these deficiencies due to somewhat opposite effects achieved by refining both spatial resolutions. Therefore, more accurate physics parameterisations as well as improved numerical schemes and resolution dependence of parameterisations seem to be essential for a significant improvement in the models energetics. Moreover, efforts should be made to improve the physical processes controlling the generation of zonal available potential energy and dissipation of eddy kinetic energy, in which the synoptic scale should be fundamental, as inferred from the excessive energy conversion/transfer rates in the models spectra.

The Impact of Satellite-Derived Polar Winds on Lower-Latitude Forecasts

Abstract The use of atmospheric motion vectors (AMVs) in NWP models continues to be an important source of information in data-sparse regions. These AMVs are derived from a time sequence of images from geostationary and polar-orbiting satellites. NWP centers have documented positive impact on model forecasts not only in regions where the AMVs are measured, but elsewhere as well. One example is the effect of the Moderate Resolution Imaging Spectroradiometer (MODIS) polar winds on forecasts in the middle and lower latitudes. Using a preoperational version of NCEP’s Global Forecast System (GFS), an experiment was run during August and September 2004, with and without the MODIS polar winds. Several cases within this period were analyzed to determine how winds poleward of 70° latitude affect the height and wind fields into lower latitudes. From the five cases examined, it was determined that the addition of the polar winds modifies the mass balance in synoptic-scale waves near the polar jet streams. This change in mass balance is evident in differences in the ageostrophic wind, which has an effect on the speed and amplitude of baroclinic waves that extends from the jet stream into lower latitudes in later forecast times. These results reveal the substantial impact that polar-only observations may have on the predictability of global weather systems.

Synoptic-Scale Flow and Valley Cold Pool Evolution in the Western United States

Abstract Valley cold pools (VCPs), which are trapped, cold layers of air at the bottoms of basins or valleys, pose a significant problem for forecasters because they can lead to several forms of difficult-to-forecast and hazardous weather such as fog, freezing rain, or poor air quality. Numerical models have historically failed to routinely provide accurate guidance on the formation and demise of VCPs, making the forecast problem more challenging. In some case studies of persistent wintertime VCPs, there is a connection between the movement of upper-level waves and the timing of VCP formation and decay. Herein, a 3-yr climatology of persistent wintertime VCPs for five valleys and basins in the western United States is performed to see how often VCP formation and decay coincides with synoptic-scale (∼200–2000 km) wave motions. Valley cold pools are found to form most frequently as an upper-level ridge approaches the western United States and in response to strong midlevel warming. The VCPs usually last as long as the ridge is over the area and usually only end when a trough, and its associated midlevel cooling, move over the western United States. In fact, VCP strength appears to be almost entirely dictated by midlevel temperature changes, which suggests large-scale forcing is dominant for this type of VCP most of the time.

Hovmöller diagrams of climate anomalies in NCEP/NCAR reanalysis from 1948 to 2009

Atmospheric and oceanic reanalysis data of the past 60 years are visualized by time–latitude cross sections which inform about temporal and latitudinal variability, changes, and meridional circulation of the climate system on time scales from years to decades. The diagrams ease the understanding of climate dynamics in a similar way as the Hovmoller diagram has clarified the zonal propagation of synoptic-scale waves in a latitude band. Thus the time–latitude cross sections are referred to as Hovmoller diagrams of climate anomalies. Various parameters are selected at the Earth’s surface level: air temperature, pressure, sea surface temperature, column-integrated atmospheric water vapour (IWV), surface relative humidity, and surface wind. Contour lines of one parameter are overlayed on the color image of the other parameter so that relations and coincidences of trends, oscillations, and sudden changes of the climate are revealed. Temporal variations of IWV and surface relative humidity appear to be related to the strength of the Southern polar front jet. In case of ENSO 1997/1998, the equatorial IWV enhancement is accompanied by acceleration of the meridional circulation cells of the Southern Hemisphere. Meridional surface wind data suggest the existence of double polar cells in both hemispheres. The Southern polar cells switched to a higher circulation speed in 1997, and surface relative humidity increased over Antarctica. A sudden and persistent decrease of surface pressure occurred in Antarctica around 1980. Since 2000, a rapid increase of surface air temperature, sea surface temperature, and IWV occurs in the Northern Hemisphere. These results of NCEP/NCAR reanalysis have to be cross-validated with other reanalyses, climate models, and pure observations which will be a major endeavour.

Impact of Synoptic-Scale Wave Breaking on the NAO and Its Connection with the Stratosphere in ERA-40

Abstract This observational study investigates the impact of North Atlantic synoptic-scale wave breaking on the North Atlantic Oscillation (NAO) and its connection with the stratosphere in winter, as derived from the 40-yr ECMWF Re-Analysis (ERA-40). Anticyclonic (AB) and cyclonic wave breaking (CB) composites are compiled of the temporal and spatial components of the large-scale circulation using a method for the detection of AB and CB events from daily maps of potential vorticity on an isentropic surface. From this analysis a close link between wave breaking, the NAO, and the stratosphere is found: 1) a positive feedback between the occurrence of AB (CB) events and the positive (negative) phase of the NAO is suggested, whereas wave breaking in general without any reference to AB- or CB-like behavior does not affect the NAO, though it preferably emerges from its positive phase. 2) AB strengthens the North Atlantic eddy-driven jet and acts to separate it from the subtropical jet, while CB weakens the eddy-driven jet and tends to merge both jets. 3) AB (CB) events are associated with a stronger (weaker) lower-stratospheric polar vortex, characterized by the 50-hPa northern annular mode. During persistent weak vortex episodes, significantly more frequent CB than AB events are observed concurrently with a significant negative NAO response up to 55 days after the onset of the stratospheric perturbation. Finally, tropospheric wave breaking is related to nonannular stratospheric variability, suggesting an additional sensitivity of wave breaking and, thus, the NAO to specific distortions of the stratospheric polar vortex, rather than solely its strength.

The skeleton of tropical intraseasonal oscillations.

The Madden-Julian oscillation (MJO) is the dominant mode of variability in the tropical atmosphere on intraseasonal timescales and planetary spatial scales. Despite the primary importance of the MJO and the decades of research progress since its original discovery, a generally accepted theory for its essential mechanisms has remained elusive. Here, we present a minimal dynamical model for the MJO that recovers robustly its fundamental features (i.e., its "skeleton") on intraseasonal/planetary scales: (i) the peculiar dispersion relation of d omega/dk approximately = 0, (ii) the slow phase speed of approximately = 5 m/s, and (iii) the horizontal quadrupole vortex structure. This is accomplished here in a model that is neutrally stable on planetary scales; i.e., it is tacitly assumed that the primary instabilities occur on synoptic scales. The key premise of the model is that modulations of synoptic scale wave activity are induced by low-level moisture preconditioning on planetary scales, and they drive the "skeleton" of the MJO through modulated heating. The "muscle" of the MJO--including tilts, vertical structure, etc.--is contributed by other potential upscale transport effects from the synoptic scales.

Nonlinear interaction of internal waves in the coastal zone of the Sea of Japan

Nonlinear interactions of the internal waves of a tidal period with low-frequency synoptic-scale internal waves are studied using instrumental measurements of the current in the coastal zone of the Sea of Japan. In the course of spectral analysis of the data of instrumental measurements, it is found that a maximum in the spectrum of the kinetic energy of coastal waters in the vicinity of the semidiurnal frequency ω0 is surrounded by satellite maxima whose frequencies obey the relation ωs = ω0 ± Ω, where Ω is the characteristic frequency of synoptic-scale internal waves. The spectrum of the anticyclonic current component has a similar structure in the vicinities of the frequency ω0 and its first and second harmonics. The general theory of nonlinear interactions of weakly dispersive waves is used to solve the problem of modulation and the parametric amplification of tidal internal waves in the coastal zone using low-frequency narrow-band internal waviness. As can be judged from the literature, the effect of parametric modulation of tidal internal waves by low-frequency synoptic-scale internal waves has been recorded in the coastal zone of a tidal sea for the first time.

Synoptic scale wave breaking and its potential to drive NAO‐like circulation dipoles: A simplified GCM approach

AbstractRecent studies suggest a synoptic view of the North Atlantic oscillation (NAO) with its positive (negative) phase being the remnant of anticyclonic (cyclonic) synoptic scale wave breaking. This study examines the potential of anticyclonic (AB) and cyclonic wave breaking (CB) to drive NAO‐like meridional circulation dipoles by investigating the synoptic evolution of AB and CB events in a mid‐latitude eddy‐driven jet in a simplified GCM with zonally uniform basic state. First, a method for the detection of such events from daily isentropic maps of potential vorticity and horizontal deformation is constructed. Then, from the obtained sample of events AB‐ and CB‐composites of the upper and lower tropospheric flow are computed, and a distinct spatial and temporal asymmetry in the response to AB and CB events is found. While from the interaction of two AB events (with a mean lifetime of 2.6 days) a strong and short‐lived positive phase NAO‐like dipole is produced at the surface but not at upper levels, single CB events (4.3 days) are found to drive a strong and more persistent negative phase NAO‐like dipole at upper levels but not at the surface. It is concluded that AB (CB) is not capable of driving a positive (negative) phase NAO‐like dipole individually. However, the results suggest that equivalent barotropic NAO‐like variability may arise from the successive occurrence of AB and CB events. Further, a sensitivity to the strength of the stratospheric polar vortex is found with more (less) frequent AB (CB) events under strong vortex conditions. Copyright © 2009 Royal Meteorological Society

Linkage of atmospheric blocks and synoptic-scale Rossby waves: a climatological analysis

Linkage of atmospheric blocks and synoptic-scale Rossby waves: a climatological analysis

Tropical Cyclone Energy Dispersion in a Three-Dimensional Primitive Equation Model: Upper-Tropospheric Influence*

Abstract The three-dimensional (3D) Rossby wave energy dispersion of a tropical cyclone (TC) is studied using a baroclinic primitive equation model. The model is initialized with a symmetric vortex on a beta plane in an environment at rest. The vortex intensifies while becoming asymmetric and moving northwestward because of the beta effect. A synoptic-scale wave train forms in its wake a few days later. The energy-dispersion-induced Rossby wave train has a noticeable baroclinic structure with alternating cyclonic–anticyclonic–cyclonic (anticyclonic–cyclonic–anticyclonic) circulations in the lower (upper) troposphere. A key feature associated with the 3D wave train development is a downward propagation of the relative vorticity and kinetic energy. Because of the vertical differential inertial stability, the upper-level wave train develops faster than the lower-level counterpart. The upper anticyclonic circulation rapidly induces an intense asymmetric outflow jet in the southeast quadrant, and then further influences the lower-level Rossby wave train. On one hand, the outflow jet exerts an indirect effect on the lower-level wave train strength through changing TC intensity and structure. On the other hand, it triggers downward energy propagation that further enhances the lower-level Rossby wave train. A sudden removal of the diabatic heating may initially accelerate the energy dispersion through the increase of the radius of maximum wind and the reduction of the lower-level inflow. The latter may modulate the group velocity of the Rossby wave train through the Doppler shift effect. The 3D numerical results illustrate more complicated Rossby wave energy dispersion characteristics than 2D barotropic dynamics.

An Analysis of West African Dynamics Using a Linearized GCM*

Abstract This study utilizes a linear, primitive equation spherical model to study the development and propagation of easterly wave disturbances over West Africa. Perturbations are started from an initial disturbance consisting of a barotropic vortex and the governing equations are integrated forward in time. The perturbations are introduced into basic states corresponding to the observed dynamical and thermodynamical characteristics of two wet years in the Sahel and two dry years. The model simulations show consistent contrasts in wave activity between the wet and dry years. The waves are markedly stronger in the wet years and show a barotropic structure throughout the troposphere. The waves tend to extend throughout the troposphere to the level of the tropical easterly jet (TEJ) in the wet years, but not in the dry years. The upper-tropospheric shear, which is stronger in wet years, appears to be a key factor in wave development. This shear is dependent on the intensity of the TEJ, suggesting that the TEJ is an important factor in interannual variability in the Sahel. When the overall shear is weak, vertical development is suppressed. Another contrast is that in the dry years the growth rates show a single maximum around 3000–4000 km, but in the wet years there is a second, around 6000–7000 km. This suggests that both synoptic-scale and planetary-scale waves are active in the rainy season of some wet years. Imposing considerations of potential vorticity, the generation of planetary-scale waves implies a strong link between the surface and the TEJ in wet years. Such a link is absent in the dry years. This is likely a major factor in the interannual variability of rainfall in the Sahel.

Tropospheric circulation sensitivity to an interactive stratospheric ozone

[1] For this conceptual study we focus on the impact of interactive stratospheric ozone chemistry on the tropospheric circulation, using the atmosphere-ocean-sea ice general circulation model (AOGCM) ECHO-GiSP with simplified stratospheric chemistry. The model covers the troposphere and middle atmosphere up to 80 km height. Our results show a clear sensitivity of the tropospheric circulation dynamics to the stratospheric chemistry. With enabled interactive stratospheric chemistry the model tends to the negative phase of the Arctic Oscillation (AO) mode. This also includes an enhanced midlatitudinal planetary and synoptic scale wave activity. The strengthening of the synoptic scale waves leads to stronger stormtracks, while the planetary scale waves show larger changes outside this particular latitudes. Another tropospheric region, which is influenced by interactive stratospheric chemistry effects, is the tropical troposphere. Due to changes in lower stratospheric ozone concentrations a significant cooling appears in the positive AO-phase compared to the negative phase.

A comparison study of summer-time synoptic-scale waves in South China and the Yangtze River basin using the TRMM Multi-Satellite Precipitation Analysis daily product

Summer-time synoptic-scale waves in South China and the Yangtze River basin are quantified and compared by means of analyzing the 6-year (1998–2004) TRMM Multi-Satellite Precipitation Analysis (TMPA) daily product. An innovative 3-dimensional spectrum analysis method is applied. The results indicate that synoptic-scale waves appearing in South China prominently propagate westward within a zonal wavenumber range of 9–21 and a frequency range of −0.12–−0.22 cycles day−1, while those in the Yangtze River basin primarily move eastward with the same characteristic wavenumbers of 9–21, but within a frequency range of 0.2–0.29 cycles day−1. Zonal and meridional distributions, and seasonal variations of these waves are further explored and compared. It shows that summer-time synoptic-scale waves in the South China result from the northward migration and oscillation of the Intertropical Convergence Zone (ITCZ) in the western Pacific, whereas the ones in the Yangtze River basin are generally related to the synoptic troughs within the westerly flow, originating from the Qinghai-Tibet Plateau.

Breaking Waves at the Tropopause in the Wintertime Northern Hemisphere: Climatological Analyses of the Orientation and the Theoretical LC1/2 Classification

Abstract Breaking waves on the tropopause are viewed as potential vorticity (PV) streamers on middle-world isentropic levels. A Northern Hemisphere winter climatology of the streamers’ spatial distribution and meridional orientation is derived from the 40-yr ECMWF Re-Analysis (ERA-40) dataset, and used to assess the nature and frequency of occurrence of breaking synoptic-scale waves. The streamers are grouped into two classes related to the so-called cyclonic (LC2) and anticyclonic (LC1) patterns, and the ambient wind strength and wind shear is also noted. It is shown that the occurrence of cyclonic and anticyclonic PV streamers exhibits a distinct spatial variability in the horizontal and the vertical. The majority of cyclonic PV streamers are found on lower isentropic levels that intersect the tropopause at more poleward latitudes, whereas anticyclonic streamers predominate at higher elevations in the subtropics. An analysis of the streamer patterns for the two phases of the North Atlantic Oscillation (NAO) reveals significant differences in the location and frequency of both cyclonic and anticyclonic streamers in the Euro–Atlantic region on the 310-K isentropic level. Likewise, for the two phases of the ENSO and the Pacific–North American (PNA) pattern, there are marked differences in the frequency pattern of cyclonic streamers. An examination of the tropopause-level hemispheric flow pattern at the time of and prior to a streamer’s formation indicates a linkage to the presence or absence of double jet structures.

Interpreting the Opposition between Two Block-Onset Forcing Mechanisms

The opposition between two block-onset forcing mechanisms, previously identified in midtropospheric analyses over the Southern Hemisphere midlatitudes, is analytically interpreted with an idealized model. These mechanisms are the interaction (Finter) between deformation and potential vorticity and the advection (Fadv) of meridionally varying potential vorticity. Weather systems of concern, primarily consisting of planetary- and synoptic-scale waves, mostly fall into two regimes of zonal and meridional wavenumber space in which the opposition between the two block-onset forcing mechanisms is analytically derived. A synoptic interpretation of this opposition is schematically presented within the framework of barotropic dynamics. It is found that whether blocking occurs in diffluent or confluent flow depends upon the critical wavelength associated with the geostrophic flow. Blocking tends to take place in the diffluent flow of long waves in which Finter dominates over Fadv. In addition, blocking also tends to occur in the confluent flow of relative short waves in which Fadv prevails over Finter. An investigation of Rossby wave phase speeds in one diagnosed case reveals a lengthening with time of the dominant wave until it reaches the stationary wavelength on the block-onset day. In this context blocking may be understood as a stationarity and thus persistence of one of the two block-onset forcing mechanisms.

Multiscale Organization of Convection Simulated with Explicit Cloud Processes on an Aquaplanet

This study investigated the multiscale organization of tropical convection on an aquaplanet in a model experiment with a horizontal mesh size of 3.5 km (for a 10-day simulation) and 7 km (for a 40-day simulation). The numerical experiment used the nonhydrostatic icosahedral atmospheric model (NICAM) with explicit cloud physics. The simulation realistically reproduced multiscale cloud systems: eastward-propagating super cloud clusters (SCCs) contained westward-propagating cloud clusters (CCs). SCCs (CCs) had zonal sizes of several thousand (hundred) kilometers; typical propagation speed was 17 (10) m s−1. Smaller convective structures such as mesoscale cloud systems (MCs) of O(10 km) and cloud-scale elements (<10 km) were reproduced. A squall-type cluster with high cloud top (z > 16 km) of O(100 km) area was also reproduced. Planetary-scale equatorial waves (with wavelengths of 10 000 and 40 000 km) had a major influence on the eastward propagation of the simulated SCC; destabilization east of the SCC facilitated generation of new CCs at the eastern end of the SCC. Large-scale divergence fields associated with the waves enhanced the growth of deep clouds in the CCs. A case study of a typical SCC showed that the primary mechanism forcing westward propagation varies with the life stages of the CCs or with vertical profiles of zonal wind. Cold pools and synoptic-scale waves both affected CC organization. Cloud-scale elements systematically formed along the edges of cold pools to sustain simulated MCs. The location, movement, and duration of the MCs varied with the large-scale conditions.

The Nature of a Heat Wave in Eastern Argentina Occurring during SALLJEX

Abstract This note describes the physical processes associated with the occurrence of a heat wave over central Argentina during the austral summer of 2002/03, during which the South American Low-Level Jet Experiment (SALLJEX) was carried out. The SALLJEX heat wave that lasted between 25 January and 2 February 2003 was punctuated by extreme conditions during its last 3 days, with the highest temperature recorded over the last 35 yr at several stations of the region. It was found that not only the activity of synoptic-scale waves, but also the intraseasonal oscillation variability, had a strong impact on the temperature evolution during this summer. During the weeks previous to the heat wave development, an intensified South Atlantic convergence zone (SACZ) dominated the atmospheric conditions over tropical South America. Temperatures started to increase in the subtropics due to the subsidence and diabatic warming associated with the SACZ, as depicted by SALLJEX upper-air observations. An extratropical anticyclone that evolved along southern South America further intensified subsidence conditions. By the end of January the warming processes associated with SACZ activity weakened, while horizontal temperature advection began to dominate over central Argentina due to the intensification of the South American low-level jet. This mechanism led to temperature extremes by 2 February with temperature anomalies at least two standard deviations larger than the climatological mean values. Intense solar heating favored by strong subsidence was responsible for the heat wave until 31 January, after which horizontal temperature advection was the primary process associated with the temperature peak.

Characteristics of the Atlantic Storm-Track Eddy Activity and Its Relation with the North Atlantic Oscillation

Abstract This study focuses on feedbacks of the high-frequency eddy activity onto the quasi-stationary circulation, particularly with regard to the North Atlantic Oscillation (NAO). The methodology consists of analyzing NCEP–NCAR reanalysis data and sensitivity runs from a high-resolution nonhydrostatic regional model. Consistent with recent studies, results show that the jet displacement characteristic of the NAO phenomenon depends strongly on the dynamics of the synoptic-scale waves and the way they break. Positive and negative phases of the NAO are closely related to anticyclonic and cyclonic wave breaking, respectively. Indeed, the high-frequency momentum flux whose sign is directly related to the type of wave breaking is correlated with the NAO index over the Atlantic. The peak of the momentum flux signal precedes that of the NAO by a few days suggesting that wave breaking is triggering NAO events. Two examples illustrate the significant impact of single storms, in particular those occurring in the east coast of the United States. The wave breaking at the end of their life cycle can suddenly change the NAO index in few days, and as the return to equilibrium takes generally a longer time, it can even affect the sign of the NAO during an entire month. An important issue determining the NAO phase is related to upstream effects. By considering a domain extending from the eastern Pacific to western Europe and by forcing the regional model with real data at the western boundary, sensitivity runs show that the right sign of the NAO index can be recovered. It indicates that waves coming from the eastern Pacific are crucial for determining the NAO phase. According to their spatial scales and frequencies when they reach the Atlantic domain, they can break one way or another and push the Atlantic jet equatorward or poleward. Synoptic waves with periods between 5 and 12 days break anticyclonically whereas those with periods between 2 and 5 days break both anticyclonically and cyclonically with a predominance for cyclonic wave breaking. Another crucial factor concerns surface effects. Cyclonic wave breaking in the upper levels is strongly connected with an explosive cyclonic development at the surface accompanied by strong surface moisture fluxes whereas such an explosive growth is not present in the anticyclonic wave breaking case. Finally, it is proposed that these results are not only useful for explaining the intraseasonal variations of the NAO but would serve also as a basis for understanding its interannual and interdecadal variations.