The North Atlantic jet stream: a look at preferred positions, paths and transitions

Hannachi, Abdel; Woollings, Tim; Fraedrich, Klaus

doi:10.1002/qj.959

Cited by 40 publications

(47 citation statements)

References 52 publications

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“…The key result of this letter is that the predictability of the jet decreases systematically, when its trajectory passes through this region. Franzke et al [2012] and Hannachi et al [2012] suggest that when the jet is shifted to the north, it tends to transition south via wave breaking. This transition implies temporary disruption of the zonal flow and passage of trajectories through the less predictable region of PCspace associated with weak or split jets.…”

Section: Flow-dependent Predictability Of the Jetmentioning

confidence: 99%

“…[4] Recent results [Woollings et al, 2010;Franzke et al, 2012;Hannachi et al, 2012] suggest that statistics of North Atlantic eddy-driven jet indices possess significant inhomogeneities, indicating the presence of three regimes: a regime with the maximum wind-speed of the jet shifted south of its climatological mean latitude, one with it close to the mean latitude and one with it shifted north of the mean latitude. Further evidence has been found that the skill in forecasting the jet appears to vary with these regimes, with the skill being lowest when the forecast starts with the jet in the north regime [Frame et al, 2011].…”

Section: Introductionmentioning

confidence: 99%

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Flow‐dependent predictability of the North Atlantic jet

Frame

Methven

Gray

et al. 2013

Geophysical Research Letters

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The North Atlantic eddy‐driven jet is a major component of the large‐scale flow in the northern hemisphere. Here we present evidence from reanalysis and ensemble forecast data for systematic flow‐dependent predictability of the jet during northern hemisphere winter (DJF). It is found that when the jet is weakened or split, it is both less persistent and less predictable. The lack of predictability manifests itself as the onset of an anomalously large instantaneous rate of spread of ensemble forecast members as the jet becomes weakened. This suggests that, as the jet weakens or splits, it enters into a state more sensitive to small differences between ensemble forecast members, rather like the sensitive region between the wings of the Lorenz attractor.

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Section: Flow-dependent Predictability Of the Jetmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Flow‐dependent predictability of the North Atlantic jet

Frame

Methven

Gray

et al. 2013

Geophysical Research Letters

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“…The third EOF mode resembles the ridge regime in Cassou et al [2004]. It is also connected to the Greenland blocking as defined by Scherrer et al [2006] and a northern jet stream location [Woollings et al, 2008, 2010, Hannachi et al, 2012, which is in turn associated with a negative phase of the East Atlantic pattern [Barnston and Livezey, 1987]. This EOF mode shows similarity with the stationary topographic Rossby wave pattern over the southeast Greenland coast, SLP variance over Greenland.…”

Section: Topographic Forcingmentioning

confidence: 73%

“…Next, I represent the geostrophic wind and the vorticity in terms of the perturbation streamfunction Ψ and approximate the smoothed topography with a Fourier series ℎ ( , ) = ∑︀ ∑︀ĥ , exp ( + ), whereĥ , are the Fourier coefficients. This has as steady-state solution [Charney andEliassen, 1949, Holton andHakim, 2013]: [Woollings et al, 2008, 2010, Hannachi et al, 2012.…”

Section: Topographic Forcingmentioning

confidence: 99%

Strong wind events across Greenland�s coast and their influence on the ice sheet, sea ice and ocean

Oltmanns¹

2015

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In winter, Greenland's coastline adjacent to the subpolar North Atlantic and Nordic Seas is characterized by a large land-sea temperature contrast. Therefore, winds across the coast advect air across a horizontal temperature gradient and can result in significant surface heat fluxes both over the ice sheet (during onshore winds) and over the ocean (during offshore winds). Despite their importance, these winds have not been investigated in detail, and this thesis includes the first comprehensive study of their characteristics, dynamics and impacts. Using an atmospheric reanalysis, observations from local weather stations, and remote sensing data, it is suggested that high-speed wind events across the coast are triggered by the superposition of an upper level potential vorticity anomaly on a stationary topographic Rossby wave over Greenland, and that they intensify through baroclinic instability. Onshore winds across Greenland's coast can result in increased melting, and offshore winds drive large heat losses over major ocean convection sites.Strong offshore winds across the southeast coast are unique over Greenland, because the flow is funneled from the vast ice sheet inland into the narrow valley of Ammassalik at the coast, where it can reach hurricane intensity. In this region, the cold air, which formed over the northern ice sheet, is suddenly released during intense downslope wind events and spills over the Irminger Sea where the cold and strong winds can drive heat fluxes of up to 1000 W m −2 , with potential implications for deep water formation. Moreover, the winds advect sea ice away from the coast and out of a major glacial fjord.Simulations of these wind events in Ammassalik with the atmospheric Weather Research and Forecast Model show that mountain wave dynamics contribute to the acceleration of the downslope flow. In order to capture these dynamics, a high model 3 resolution with a detailed topography is needed. The effects of using a different resolution locally in the valley extend far downstream over the Irminger Sea, which has implications for the evolution and distribution of the heat fluxes.

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“…Woollings et al (2010b). A more elegant and robust way is to use a clustering test by applying concepts from homogeneous random point processes combined with copula (Stephenson et al, 2004;Hannachi, 2010;Hannachi et al, 2012). Clustering is a property that does not depend in general on the marginal distributions of the data, and the method of copula attempts to factor out precisely the (undesired) effect of the marginal distributions.…”

Section: Clustering Of the Leading Modesmentioning

confidence: 99%

20th century intraseasonal Asian monsoon dynamics viewed from Isomap

Hannachi

Turner

2013

Nonlin. Processes Geophys.

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Abstract. The Asian summer monsoon is a highdimensional and highly nonlinear phenomenon involving considerable moisture transport towards land from the ocean, and is critical for the whole region. We have used daily ECMWF reanalysis (ERA-40) sea-level pressure (SLP) anomalies on the seasonal cycle, over the region 50-145 • E, 20 • S-35 • N, to study the nonlinearity of the Asian monsoon using Isomap. We have focused on the two-dimensional embedding of the SLP anomalies for ease of interpretation. Unlike the unimodality obtained from tests performed in empirical orthogonal function space, the probability density function, within the two-dimensional Isomap space, turns out to be bimodal. But a clustering procedure applied to the SLP data reveals support for three clusters, which are identified using a three-component bivariate Gaussian mixture model. The modes are found to appear similar to active and break phases of the monsoon over South Asia in addition to a third phase, which shows active conditions over the western North Pacific. Using the low-level wind field anomalies, the active phase over South Asia is found to be characterised by a strengthening and an eastward extension of the Somali jet. However during the break phase, the Somali jet is weakened near southern India, while the monsoon trough in northern India also weakens. Interpretation is aided using the APHRODITE gridded land precipitation product for monsoon Asia. The effect of large-scale seasonal mean monsoon and lower boundary forcing, in the form of ENSO, is also investigated and discussed. The outcome here is that ENSO is shown to perturb the intraseasonal regimes, in agreement with conceptual ideas.

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The North Atlantic jet stream: a look at preferred positions, paths and transitions

Abstract: The transition paths are found to be consistent with transition probabilities. The analysis also shows that wave breaking seems to be the dominant mechanism behind Greenland blocking.

Cited by 40 publications

References 52 publications

Flow‐dependent predictability of the North Atlantic jet

Flow‐dependent predictability of the North Atlantic jet

Strong wind events across Greenland�s coast and their influence on the ice sheet, sea ice and ocean

20th century intraseasonal Asian monsoon dynamics viewed from Isomap

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