scholarly journals Interdecadal Variability of the Warm Arctic and Cold Eurasia Pattern and Its North Atlantic Origin

2018 ◽  
Vol 31 (15) ◽  
pp. 5793-5810 ◽  
Author(s):  
Mi-Kyung Sung ◽  
Seon-Hwa Kim ◽  
Baek-Min Kim ◽  
Yong-Sang Choi
Keyword(s):  
Twentieth Century ◽  
Rossby Wave ◽  
North Atlantic ◽  
Storm Track ◽  
Stationary Wave ◽  
Interdecadal Variability ◽  
Temperature Perturbation ◽  
Mean Flow ◽  
The North ◽  
The North Atlantic

This study investigates the origin of the interdecadal variability in the warm Arctic and cold Eurasia (WACE) pattern, which is defined as the second empirical orthogonal function of surface air temperature (SAT) variability over the Eurasian continent in Northern Hemisphere winter, by analyzing the Twentieth Century Reanalysis dataset. While previous studies highlight recent enhancement of the WACE pattern, ascribing it to anthropogenic warming, the authors found that the WACE pattern has experienced a seemingly periodic interdecadal variation over the twentieth century. This long-term variation in the Eurasian SAT is attributable to the altered coupling between the Siberian high (SH) and intraseasonal Rossby wave emanating from the North Atlantic, as the local wave branch interacts with the SH and consequentially enhances the continental temperature perturbation. It is further identified that these atmospheric circulation changes in Eurasia are largely controlled by the decadal amplitude modulation of the climatological stationary waves over the North Atlantic region. The altered decadal mean condition of stationary wave components brings changes in local baroclinicity and storm track activity over the North Atlantic, which jointly change the intraseasonal Rossby wave generation and propagation characteristics as well. With simple stationary wave model experiments, the authors confirm how the altered mean flow condition in the North Atlantic acts as a source for the growth of the Rossby wave that leads to the change in the downstream WACE pattern.

scholarly journals Tropical and Extratropical Responses of the North Atlantic Atmospheric Circulation to a Sustained Weakening of the MOC

2009 ◽  
Vol 22 (11) ◽  
pp. 3146-3155 ◽  
Author(s):  
David J. Brayshaw ◽  
Tim Woollings ◽  
Michael Vellinga
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Western Europe ◽  
Storm Track ◽  
Meridional Overturning Circulation ◽  
Boreal Winter ◽  
Mean Flow ◽  
Near Surface ◽  
The North ◽  
The North Atlantic

Abstract The tropospheric response to a forced shutdown of the North Atlantic Ocean’s meridional overturning circulation (MOC) is investigated in a coupled ocean–atmosphere GCM [the third climate configuration of the Met Office Unified Model (HadCM3)]. The strength of the boreal winter North Atlantic storm track is significantly increased and penetrates much farther into western Europe. The changes in the storm track are shown to be consistent with the changes in near-surface baroclinicity, which can be linked to changes in surface temperature gradients near regions of sea ice formation and in the open ocean. Changes in the SST of the tropical Atlantic are linked to a strengthening of the subtropical jet to the north, which, combined with the enhanced storm track, leads to a pronounced split in the jet structure over Europe. EOF analysis and stationary box indices methods are used to analyze changes to the North Atlantic Oscillation (NAO). There is no consistent signal of a change in the variability of the NAO, and while the changes in the mean flow project onto the positive NAO phase, they are significantly different from it. However, there is a clear eastward shift of the NAO pattern in the shutdown run, and this potentially has implications for ocean circulation and for the interpretation of proxy paleoclimate records.

scholarly journals The Life Cycle of Upper-Level Troughs and Ridges: A Novel Detection Method, Climatologies and Lagrangian Characteristics

2020 ◽  
Author(s):  
Sebastian Schemm ◽  
Michael Sprenger
Keyword(s):  
Life Cycle ◽  
Rossby Wave ◽  
North Atlantic ◽  
Potential Temperature ◽  
Three Dimensional ◽  
Storm Track ◽  
Pressure Potential ◽  
The North ◽  
The North Atlantic ◽  
Upper Level

Abstract. A novel method is introduced to identify and track the life cycle of upper-level troughs and ridges. The aim is to close the existing gap between methods that detect the initiation phase of upper-level Rossby wave development and methods that detect Rossby wave breaking and decaying waves. The presented method quantifies the horizontal trough and ridge orientation and identifies the corresponding trough and ridge axes. The trough and ridge axes allow us to study the dynamics of pre- and post-trough or ridge regions separately. The tracking allows us to study the temporal evolution of the trough or ridge orientation. The method is based on the curvature of the geopotential height at a given isobaric surface and is computationally efficient. First, the algorithm is introduced in detail, and several illustrative applications, such as a downstream development from the North Atlantic into the Mediterranean, and seasonal climatologies are discussed. For example, the climatological trough and ridge orientations reveal strong zonal and meridional asymmetry. Over land, most troughs and ridges are anticyclonically oriented, while they are cyclonically oriented over the main oceanic storm tracks. The cyclonic orientation increases towards the poles, while the anticyclonic orientation increases towards the equator. Trough detection frequencies are climatologically high downstream of the Rocky Mountains and over East Asia and Eastern Europe, but are remarkably low downstream of Greenland. Furthermore, the detection frequencies of troughs are high at the end of the Pacific storm track, but no comparable signal is seen over the North Atlantic. During El Niño-affected winters, troughs and ridges tilt anomalously strong cyclonically over North America and the North Atlantic, in agreement with previous findings based on traditional variance-based diagnostics such as E vectors. During La Niña the situation is essentially reversed. Finally, the identified troughs and ridges are used as starting points for 24-hour backward parcel trajectories, and a discussion of the distribution of pressure, potential temperature and potential vorticity changes along the flow path is provided to give insight into the three-dimensional nature of troughs and ridges.

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 The life cycle of upper-level troughs and ridges: a novel detection method, climatologies and Lagrangian characteristics

2020 ◽  
Vol 1 (2) ◽  
pp. 459-479
Author(s):  
Sebastian Schemm ◽  
Stefan Rüdisühli ◽  
Michael Sprenger
Keyword(s):  
Life Cycle ◽  
Rossby Wave ◽  
North Atlantic ◽  
Potential Temperature ◽  
Three Dimensional ◽  
Storm Track ◽  
Pressure Potential ◽  
The North ◽  
The North Atlantic ◽  
Upper Level

Abstract. A novel method is introduced to identify and track the life cycle of upper-level troughs and ridges. The aim is to close the existing gap between methods that detect the initiation phase of upper-level Rossby wave development and methods that detect Rossby wave breaking and decaying waves. The presented method quantifies the horizontal trough and ridge orientation and identifies the corresponding trough and ridge axes. These allow us to study the dynamics of pre- and post-trough–ridge regions separately. The method is based on the curvature of the geopotential height at a given isobaric surface and is computationally efficient. Spatiotemporal tracking allows us to quantify the maturity of troughs and ridges and could also be used to study the temporal evolution of the trough or ridge orientation. First, the algorithm is introduced in detail, and several illustrative applications – such as a downstream development from the North Atlantic into the Mediterranean – and seasonal climatologies are discussed. For example, the climatological trough and ridge orientations reveal strong zonal and meridional asymmetry: over land, most troughs and ridges are anticyclonically oriented, while they are cyclonically oriented over the main oceanic storm tracks; the cyclonic orientation increases toward the poles, while the anticyclonic orientation increases toward the Equator. Trough detection frequencies are climatologically high downstream of the Rocky Mountains and over East Asia and eastern Europe but are remarkably low downstream of Greenland. Furthermore, the detection frequencies of troughs are high at the end of the North Pacific storm track and at the end of the North Atlantic storm track over the British Isles. During El Niño-affected winters, troughs and ridges exhibit an anomalously strong cyclonic tilt over North America and the North Atlantic, in agreement with previous findings based on traditional variance-based diagnostics such as E vectors. During La Niña, the situation is essentially reversed. The orientation of troughs and ridges also depends on the jet position. For example, during midwinter over the Pacific, when the subtropical jet is strongest and located farthest equatorward, cyclonically oriented troughs and ridges dominate the climatology. Finally, the identified troughs and ridges are used as starting points for 24 h backward parcel trajectories, and a discussion of the distribution of pressure, potential temperature and potential vorticity changes along the trajectories is provided to give insight into the three-dimensional nature of troughs and ridges.

Evidence for a Rapid Global Climate Shift across the Late 1960s

2007 ◽  
Vol 20 (12) ◽  
pp. 2721-2744 ◽  
Author(s):  
Peter G. Baines ◽  
Chris K. Folland
Keyword(s):  
Southern Hemisphere ◽  
North Atlantic ◽  
Amazon Basin ◽  
Storm Track ◽  
Central Pacific ◽  
Atlantic Sector ◽  
The North ◽  
The North Atlantic ◽  
The 1950S ◽  
African Sahel

Abstract It is shown that a number of important characteristics of the global atmospheric circulation and climate changed in a near-monotonic fashion over the decade, or less, centered on the late 1960s. These changes were largest or commonest in tropical regions, the Southern Hemisphere, and the Atlantic sector of the Northern Hemisphere. Some, such as the decrease in rainfall in the African Sahel, are well known. Others appear to be new, but their combined extent is global and dynamical linkages between them are evident. The list of affected variables includes patterns of SST; tropical rainfall in the African Sahel and Sudan, the Amazon basin, and northeast Brazil; pressure and SST in the tropical North Atlantic and the west and central Pacific; various branches of the southern Hadley circulation and the southern subtropical jet stream; the summer North Atlantic Oscillation; south Greenland temperature; the Southern Hemisphere storm track; and, quite likely, the Antarctic sea ice boundary. These changes are often strongest in the June–August season; changes are also seen in December–February but are generally smaller. In Greenland, annual mean temperature seems to be affected strongly, reflecting similar changes in SST throughout the year in the higher latitudes of the North Atlantic. Possible causes for these coordinated changes are briefly evaluated. The most likely candidates appear to be a likely reduction in the northward oceanic heat flux associated with the North Atlantic thermohaline circulation in the 1950s to 1970s, which was nearly in phase with a rapid increase in anthropogenic aerosol emissions during the 1950s and 1960s, particularly over Europe and North America.

scholarly journals The North Atlantic Eddy Heat Transport and Its Relation with the Vertical Tilting of the Gulf Stream Axis

2017 ◽  
Vol 47 (6) ◽  
pp. 1281-1289 ◽  
Author(s):  
A. M. Treguier ◽  
C. Lique ◽  
J. Deshayes ◽  
J. M. Molines
Keyword(s):  
Numerical Model ◽  
Heat Transport ◽  
North Atlantic ◽  
Gulf Stream ◽  
Mean Flow ◽  
The North ◽  
Spatial Shift ◽  
Eddy Heat Flux ◽  
Eddy Heat Transport ◽  
The North Atlantic

AbstractCorrelations between temperature and velocity fluctuations are a significant contribution to the North Atlantic meridional heat transport, especially at the northern boundary of the subtropical gyre. In satellite observations and in a numerical model at ⅞° resolution, a localized pattern of positive eddy heat flux is found northwest of the Gulf Stream, downstream of its separation at Cape Hatteras. It is confined to the upper 500 m. A simple kinematic model of a meandering jet can explain the surface eddy flux, taking into account a spatial shift between the maximum velocity of the jet and the maximum cross-jet temperature gradient. In the Gulf Stream such a spatial shift results from the nonlinear temperature profile and the vertical tilting of the velocity profile with depth. The numerical model suggests that the meandering of the Gulf Stream could account, at least in part, for the large eddy heat transport (of order 0.3 PW) near 36°N in the North Atlantic and for its compensation by the mean flow.

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.

scholarly journals The Life Cycle of the North Atlantic Storm Track*

2015 ◽  
Vol 72 (2) ◽  
pp. 821-833 ◽  
Author(s):  
Lenka Novak ◽  
Maarten H. P. Ambaum ◽  
Rémi Tailleux
Keyword(s):  
Heat Flux ◽  
Life Cycle ◽  
North Atlantic ◽  
Storm Track ◽  
High Heat ◽  
High Heat Flux ◽  
Dominant Type ◽  
The North ◽  
Eddy Heat Flux ◽  
The North Atlantic

Abstract The North Atlantic eddy-driven jet exhibits latitudinal variability with evidence of three preferred latitudinal locations: south, middle, and north. Here the authors examine the drivers of this variability and the variability of the associated storm track. The authors investigate the changes in the storm-track characteristics for the three jet locations and propose a mechanism by which enhanced storm-track activity, as measured by upstream heat flux, is responsible for cyclical downstream latitudinal shifts in the jet. This mechanism is based on a nonlinear oscillator relationship between the enhanced meridional temperature gradient (and thus baroclinicity) and the meridional high-frequency (periods of shorter than 10 days) eddy heat flux. Such oscillations in baroclinicity and heat flux induce variability in eddy anisotropy, which is associated with the changes in the dominant type of wave breaking and a different latitudinal deflection of the jet. The authors’ results suggest that high heat flux is conducive to a northward deflection of the jet, whereas low heat flux is conducive to a more zonal jet. This jet-deflecting effect was found to operate most prominently downstream of the storm-track maximum, while the storm track and the jet remain anchored at a fixed latitudinal location at the beginning of the storm track. These cyclical changes in storm-track characteristics can be viewed as different stages of the storm track’s spatiotemporal life cycle.

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