scholarly journals Spatial Pattern and Zonal Shift of the North Atlantic Oscillation. Part II: Numerical Experiments

2010 ◽  
Vol 67 (9) ◽  
pp. 2827-2853 ◽  
Author(s):  
Dehai Luo ◽  
Linhao Zhong ◽  
Rongcai Ren ◽  
Chunzai Wang
Keyword(s):  
Spatial Pattern ◽  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Large Scale ◽  
Storm Track ◽  
Lower Latitude ◽  
Eddy Activity ◽  
The Core ◽  
The North ◽  
Nao Pattern

Abstract In this part, the spatial evolution of an initial dipole anomaly in a prescribed jet is at first investigated by numerically solving linear and nonlinear models without forcing in order to examine how the spatial pattern of a dipole anomaly depends on the meridional distribution of a specified jet. It is shown that in a linear experiment an initial symmetric dipole anomaly in the meridional direction can evolve into a northeast–southwest (NE–SW) or northwest–southeast (NW–SE) tilted dipole structure if the core of this jet is in higher latitudes (the north) or in lower latitudes (the south). This is in agreement with the result predicted by the linear Rossby wave theory in slowly varying media. The conclusion also holds for the nonlinear and unforced experiment. North Atlantic Oscillation (NAO) events are then reproduced in a fully nonlinear barotropic model with a wavemaker that mimics the Atlantic storm-track eddy activity. In the absence of topography the spatial tilting of the eddy-driven NAO pattern is found to be independent of the NAO phase. The eddy-driven NAO pattern for the positive (negative) phase can exhibit a NE–SW (NW–SE) tilting only when the core of a prescribed jet prior to the NAO is confined in the higher latitude (lower latitude) region. However, in the presence of the wavenumber-2 topography (two oceans and continents) in the Northern Hemisphere the spatial tilting of the eddy-driven NAO dipole anomaly can be dependent on the NAO phase. Even when the specified basic flow prior to the NAO is uniform, the eddy-driven positive (negative) NAO phase dipole anomaly can also show a NE–SW (NW–SE) tilting because the northward (southward) shift of the excited westerly jet can occur in the presence of topography. In addition, it is found that when the wavemaker is closer to the position of the initial NAO, the eddy-driven positive (negative) NAO phase pattern can display a whole eastward shift and a more distinct NE–SW (NW–SE) tilting. This thus explains why the first empirical orthogonal function of the NAO pattern observed during 1998–2007 exhibits a more pronounced NE–SW tilting than during 1978–97. It appears that the latitudinal shift of the jet, the large-scale topography, and the zonal position of the Atlantic storm-track eddy activity are three important factors for controlling the spatial tilting and zonal shift of eddy-driven NAO dipole anomalies.

Weather Regime Transitions and the Interannual Variability of the North Atlantic Oscillation. Part II: Dynamical Processes

2012 ◽  
Vol 69 (8) ◽  
pp. 2347-2363 ◽  
Author(s):  
Dehai Luo ◽  
Jing Cha ◽  
Steven B. Feldstein
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Large Scale ◽  
Storm Track ◽  
Constructive Interference ◽  
Dynamical Processes ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation ◽  
Regime Transitions

Abstract In 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 more important for the NAO− to NAO+ transition during P1, but the marked reintensification and retrograde movement of both wavenumbers 1 and 2 after the NAO+ event decays is crucial for the NAO+ to NAO− transition during P2. It is further shown that destructive (constructive) interference between wavenumbers 1 and 2 over the North Atlantic during P1 (P2) is responsible for the subsequent weak NAO+ (strong NAO−) anomaly associated with the NAO− to NAO+ (NAO+ to NAO−) transition. Also, the weakening (strengthening) of the vertically integrated zonal wind (upstream Atlantic storm track) is found to play an important role in the NAO regime transition.

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

2010 ◽  
Vol 67 (9) ◽  
pp. 2805-2826 ◽  
Author(s):  
Dehai Luo ◽  
Zhihui Zhu ◽  
Rongcai Ren ◽  
Linhao Zhong ◽  
Chunzai Wang
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Function Analysis ◽  
Storm Track ◽  
Phase Speed ◽  
Mean Flow ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation ◽  
Nao Pattern

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.

scholarly journals Influence of the North Atlantic oscillation on the atmospheric levels of benzo[a]pyrene over Europe

2021 ◽  
Author(s):  
Pedro Jiménez-Guerrero ◽  
Nuno Ratola
Keyword(s):  
Spatial Pattern ◽  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Transport Model ◽  
Regional Scale ◽  
The South ◽  
Climatic Fluctuations ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

AbstractThe atmospheric concentration of persistent organic pollutants (and of polycyclic aromatic hydrocarbons, PAHs, in particular) is closely related to climate change and climatic fluctuations, which are likely to influence contaminant’s transport pathways and transfer processes. Predicting how climate variability alters PAHs concentrations in the atmosphere still poses an exceptional challenge. In this sense, the main objective of this contribution is to assess the relationship between the North Atlantic Oscillation (NAO) index and the mean concentration of benzo[a]pyrene (BaP, the most studied PAH congener) in a domain covering Europe, with an emphasis on the effect of regional-scale processes. A numerical simulation for a present climate period of 30 years was performed using a regional chemistry transport model with a 25 km spatial resolution (horizontal), higher than those commonly applied. The results show an important seasonal behaviour, with a remarkable spatial pattern of difference between the north and the south of the domain. In winter, higher BaP ground levels are found during the NAO+ phase for the Mediterranean basin, while the spatial pattern of this feature (higher BaP levels during NAO+ phases) moves northwards in summer. These results show deviations up to and sometimes over 100% in the BaP mean concentrations, but statistically significant signals (p<0.1) of lower changes (20–40% variations in the signal) are found for the north of the domain in winter and for the south in summer.

scholarly journals Trends in compound extremes of air temperature and precipitation in Eastern Europe

2021 ◽  
Author(s):  
Elena Vyshkvarkova ◽  
Olga Sukhonos
Keyword(s):  
Eastern Europe ◽  
North Atlantic Oscillation ◽  
Air Temperature ◽  
North Atlantic ◽  
Large Scale ◽  
The North ◽  
Temperature And Precipitation ◽  
Atmosphere System ◽  
Daily Data ◽  
The North Atlantic Oscillation

Abstract The spatial distribution of compound extremes of air temperature and precipitation was studied over the territory of Eastern Europe for the period 1950–2018 during winter and spring. Using daily data on air temperature and precipitation, we calculated the frequency and trends of the four indices – cold/dry, cold/wet, warm/dry and warm/wet. Also, we studying the connection between these indices and large-scale processes in the ocean-atmosphere system such as North Atlantic Oscillation, East Atlantic Oscillation and Scandinavian Oscillation. The results have shown that positive trends in the region are typical of the combinations with the temperatures above the 75th percentile, i.e., the warm extremes in winter and spring. Negative trends were obtained for the cold extremes. Statistically significant increase in the number of days with warm extremes was observed in the northern parts of the region in winter and spring. The analysis of the impacts of the large-scale processes in oceans-atmosphere system showed that the North Atlantic Oscillation index has a strong positive and statistically significant correlation with the warm indices of compound extremes in the northern part of Eastern Europe in winter, while the Scandinavian Oscillation shows the opposite picture.

scholarly journals The North Atlantic Oscillation and the North Atlantic Jet Variability: Precursors to NAO Regimes and Transitions

2012 ◽  
Vol 69 (12) ◽  
pp. 3763-3787 ◽  
Author(s):  
Dehai Luo ◽  
Jing Cha
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Zonal Wind ◽  
Storm Track ◽  
Dominant Negative ◽  
Wind Condition ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation ◽  
North Atlantic Jet

Abstract In this paper, precursors to the North Atlantic Oscillation (NAO) and its transitions are investigated to understand the dynamical cause of the interdecadal NAO variability from dominant negative (NAO−) events during 1950–77 (P1) to dominant positive (NAO+) events during 1978–2010 (P2). It is found that the phase of the NAO event depends strongly on the latitudinal position of the North Atlantic jet (NAJ) prior to the NAO onset. The NAO− (NAO+) events occur frequently when the NAJ core prior to the NAO onset is displaced southward (northward), as the situation within P1 (P2). Thus, the northward (southward) shift of the NAJ from its mean position is a precursor to the NAO+ (NAO−) event. This finding is further supported by results obtained from a weakly nonlinear model. Furthermore, the model results show that, when the Atlantic mean zonal wind exceeds a critical strength under which the dipole anomaly prior to the NAO onset is stationary, in situ NAO− (NAO+) events, which are events not preceded by opposite events, can occur frequently during P1 (P2) when the Atlantic storm track is not too strong. This mean zonal wind condition is easily satisfied during P1 and P2. However, when the Atlantic storm track (mean zonal wind) prior to the NAO onset is markedly intensified (weakened), the NAO event can undergo a transition from one phase to another, especially in a relatively strong background westerly wind, the Atlantic storm track has to be strong enough to produce a phase transition.

The Variability of the Atlantic Storm Track and the North Atlantic Oscillation: A Link between Intraseasonal and Interannual Variability

2011 ◽  
Vol 68 (3) ◽  
pp. 577-601 ◽  
Author(s):  
Dehai Luo ◽  
Yina Diao ◽  
Steven B. Feldstein
Keyword(s):  
North Atlantic Oscillation ◽  
Interannual Variability ◽  
North Atlantic ◽  
Storm Track ◽  
Positive Phase ◽  
Negative Phase ◽  
Time Interval ◽  
European Continent ◽  
Eddy Activity ◽  
Nao Index

Abstract The winter-mean North Atlantic Oscillation (NAO) index has been mostly positive since the 1980s, with a linear upward trend during the period from 1978 to 1990 (P1) and a linear downward trend during the period from 1991 to 2009 (P2). Further calculations show that the Atlantic storm-track eddy activity is more intense during P2 than during P1, which is statistically significant at the 90% confidence level for a t test. This study proposes a hypothesis that the change in the trend of the positive NAO index from P1 to P2 may be associated with the marked intensification of the Atlantic storm track during P2. A generalized nonlinear NAO model is used to explain the observed trend of the positive NAO index within P2. It is found that even when the Atlantic storm-track eddies are less intense, a positive-phase NAO event can form under the eddy forcing if the planetary-scale wave has an initial value with a low-over-high dipole structure during P1 and P2. A blocking flow can occur in the downstream side (over Europe) of the Atlantic basin as a result of the energy dispersion of Rossby waves during the decay of the positive-phase NAO event. This blocking flow does not strictly correspond to a negative-phase NAO event because the blocking stays mainly over the European continent. However, when the Atlantic storm-track eddies are rather strong, the blocking flow occurring over the European continent is enhanced and can retrograde into the Atlantic region and finally become a long-lived negative-phase NAO event. In this case, the NAO event can transit from the positive phase to the negative phase. Thus, the winter-mean NAO index during P2 will inevitably decline because of the increase in days of negative-phase NAO events in winter because the Atlantic storm track exhibits a marked intensification in the time interval. The transition of the NAO event from the positive phase to the negative phase can also be observed only when the downstream development of the Atlantic storm-track eddy activity is rather prominent. Thus, it appears that there is a physical link between intraseasonal and interannual time scales of the NAO when the Atlantic storm track exhibits an interannual variability.

scholarly journals Impact of the North Atlantic Oscillation on Transatlantic Flight Routes and Clear-Air Turbulence

2016 ◽  
Vol 55 (3) ◽  
pp. 763-771 ◽  
Author(s):  
Jung-Hoon Kim ◽  
William N. Chan ◽  
Banavar Sridhar ◽  
Robert D. Sharman ◽  
Paul D. Williams ◽  
...  
Keyword(s):  
New York ◽  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Large Scale ◽  
Jet Streams ◽  
Weather Patterns ◽  
Teleconnection Patterns ◽  
The North ◽  
Air Turbulence ◽  
The North Atlantic Oscillation

AbstractThe variation of wind-optimal transatlantic flight routes and their turbulence potential is investigated to understand how upper-level winds and large-scale flow patterns can affect the efficiency and safety of long-haul flights. In this study, the wind-optimal routes (WORs) that minimize the total flight time by considering wind variations are modeled for flights between John F. Kennedy International Airport (JFK) in New York, New York, and Heathrow Airport (LHR) in London, United Kingdom, during two distinct winter periods of abnormally high and low phases of North Atlantic Oscillation (NAO) teleconnection patterns. Eastbound WORs approximate the JFK–LHR great circle (GC) route following northerly shifted jets in the +NAO period. Those WORs deviate southward following southerly shifted jets during the −NAO period, because eastbound WORs fly closely to the prevailing westerly jets to maximize tailwinds. Westbound WORs, however, spread meridionally to avoid the jets near the GC in the +NAO period to minimize headwinds. In the −NAO period, westbound WORs are north of the GC because of the southerly shifted jets. Consequently, eastbound WORs are faster but have higher probabilities of encountering clear-air turbulence than westbound ones, because eastbound WORs are close to the jet streams, especially near the cyclonic shear side of the jets in the northern (southern) part of the GC in the +NAO (−NAO) period. This study suggests how predicted teleconnection weather patterns can be used for long-haul strategic flight planning, ultimately contributing to minimizing aviation’s impact on the environment.

scholarly journals The Basic Ingredients of the North Atlantic Storm Track. Part I: Land–Sea Contrast and Orography

2009 ◽  
Vol 66 (9) ◽  
pp. 2539-2558 ◽  
Author(s):  
David James Brayshaw ◽  
Brian Hoskins ◽  
Michael Blackburn
Keyword(s):  
North Atlantic ◽  
Rocky Mountains ◽  
Large Scale ◽  
Storm Track ◽  
Storm Tracks ◽  
Cold Pool ◽  
North American Continent ◽  
The North ◽  
The North Atlantic ◽  
The Impact

Abstract Understanding and predicting changes in storm tracks over longer time scales is a challenging problem, particularly in the North Atlantic. This is due in part to the complex range of forcings (land–sea contrast, orography, sea surface temperatures, etc.) that combine to produce the structure of the storm track. The impact of land–sea contrast and midlatitude orography on the North Atlantic storm track is investigated through a hierarchy of GCM simulations using idealized and “semirealistic” boundary conditions in a high-resolution version of the Hadley Centre atmosphere model (HadAM3). This framework captures the large-scale essence of features such as the North and South American continents, Eurasia, and the Rocky Mountains, enabling the results to be applied more directly to realistic modeling situations than was possible with previous idealized studies. The physical processes by which the forcing mechanisms impact the large-scale flow and the midlatitude storm tracks are discussed. The characteristics of the North American continent are found to be very important in generating the structure of the North Atlantic storm track. In particular, the southwest–northeast tilt in the upper tropospheric jet produced by southward deflection of the westerly flow incident on the Rocky Mountains leads to enhanced storm development along an axis close to that of the continent’s eastern coastline. The approximately triangular shape of North America also enables a cold pool of air to develop in the northeast, intensifying the surface temperature contrast across the eastern coastline, consistent with further enhancements of baroclinicity and storm growth along the same axis.

scholarly journals Impacts of the North Atlantic Oscillation on Winter Precipitations and Storm Track Variability in Southeast Canada and Northeast US

2020 ◽  
Author(s):  
Julien Chartrand ◽  
Francesco Salvatore Rocco Pausata
Keyword(s):  
United States ◽  
North America ◽  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Storm Track ◽  
The United States ◽  
Eastern North America ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract. The North Atlantic Oscillation (NAO) affects atmospheric variability from eastern North America to Europe. Although the link between the NAO and winter precipitations in the eastern North America have been the focus of previous work, only few studies have hitherto provided clear physical explanations on these relationships. In this study we revisit and extend the analysis of the effect of the NAO on winter precipitations over a large domain covering southeast Canada and the northeastern United States. Furthermore, here we use the recent ERA5 reanalysis dataset (1979–2018), which currently has the highest available horizontal resolution for a global reanalysis (0.25°), to track extratropical cyclones to delve into the physical processes behind the relationship between NAO and precipitation, snowfall, snowfall-to-precipitation ratio (S/P), and snow cover depth anomalies in the region. In particular, our results show that positive NAO phases are associated with less snowfall over a wide region covering Nova Scotia, New England and the Mid-Atlantic of the United States relative to negative NAO phases. Henceforth, a significant negative correlation is also seen between S/P and the NAO over this region. This is due to a decrease (increase) in cyclogenesis of coastal storms near the United States east coast during positive (negative) NAO phases, as well as a northward (southward) displacement of the mean storm track over North America.

On the Zonal Structure of the North Atlantic Oscillation and Annular Modes

2009 ◽  
Vol 66 (2) ◽  
pp. 332-352 ◽  
Author(s):  
Edwin P. Gerber ◽  
Geoffrey K. Vallis
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Intraseasonal Variability ◽  
Storm Track ◽  
Zonal Structure ◽  
Annular Modes ◽  
Synoptic Variability ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract The zonal structure and dynamics of the dipolar patterns of intraseasonal variability in the extratropical atmosphere—namely, the North Atlantic Oscillation (NAO) and the so-called annular modes of variability—are investigated in an idealized general circulation model. Particular attention is focused on the relationships linking the zonal structure of the stationary waves, synoptic variability (i.e., the storm tracks), and the zonal structure of the patterns of intraseasonal variability. Large-scale topography and diabatic anomalies are introduced to modify and concentrate the synoptic variability, establishing a recipe for a localized storm track. Comparison of the large-scale forcing, synoptic variability, and patterns of intraseasonal variability suggests a nonlinear relationship between the large-scale forcing and the variability. It is found that localized NAO-like patterns arise from the confluence of topographic and diabatic forcing and that the patterns are more localized than one would expect based on superposition of the responses to topography and thermal forcing alone. The connection between the eddy life cycle of growth and decay and the localization of the intraseasonal variability is investigated. Both the termination of the storm track and the localization of the intraseasonal variability in the GCM depend on a difluent region of weak upper-level flow, where eddies break and dissipate rather than propagate energy forward through downstream development. The authors' interpretation suggests that the North Atlantic storm track and the NAO are two manifestations of the same phenomenon. Conclusions from the GCM study are critiqued by comparison with observations.

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