scholarly journals Intraseasonal Dynamical Evolution of the Northern Annular Mode

2005 ◽  
Vol 18 (18) ◽  
pp. 3820-3839 ◽  
Author(s):  
Brent A. McDaniel ◽  
Robert X. Black
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Dynamical Evolution ◽  
Stationary Wave ◽  
Index Of Refraction ◽  
Annular Mode ◽  
Wave Forcing ◽  
Planetary Scale ◽  
The North ◽  
The North Atlantic

Abstract The Northern Hemisphere annular mode (NAM) accounts for a significant fraction of the extratropical wintertime atmospheric variability. The dynamics of NAM events have been studied on monthly time scales, but little is known about the physical mechanisms that give rise to NAM variability on shorter time scales. Composite diagnostic analyses based on daily NAM indices are performed with a goal of identifying the dominant processes responsible for the growth and decay of large-amplitude positive and negative NAM events on short intraseasonal time scales. Transformed Eulerian mean, piecewise potential vorticity inversions, and regional Plumb flux diagnoses are employed to deduce the proximate forcings of the zonal-mean wind tendency during maturing and declining NAM stages. A remarkable degree of reverse symmetry is observed between the zonal-mean dynamical evolution of positive and negative NAM events. Anomalous equatorward and downward (poleward and upward) Eliassen–Palm fluxes are observed during the maturation of positive (negative) NAM events, consistent with index of refraction considerations and an indirect downward stratospheric influence. The associated patterns of anomalous wave driving provide the primary forcing of the zonal wind tendency field. Spectral analyses reveal that both the stratospheric and tropospheric patterns of wave driving are primarily due to low-frequency planetary-scale eddies. Regional wave activity flux diagnoses further illustrate that this wave-driving pattern represents the zonal-mean manifestation of planetary-scale anomalies over the North Atlantic that are linked to local anomalies in stationary wave forcing. The decay of NAM events coincides with the collapse in the pattern of anomalous stationary wave forcing over the North Atlantic region. Our diagnostic results indicate that both (i) synoptic eddies and (ii) direct downward stratospheric forcing provide second-order reinforcing contributions to the intraseasonal dynamical evolution of NAM events.

Variability of the Meridional Overturning in the North Atlantic from the 50-Year GECCO State Estimation

2008 ◽  
Vol 38 (9) ◽  
pp. 1913-1930 ◽  
Author(s):  
Armin Köhl ◽  
Detlef Stammer
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Special Focus ◽  
Western Boundary ◽  
Consistent Estimate ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract The German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-yr period 1952–2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO–Massachusetts Institute of Technology (MIT) ocean circulation model using its adjoint. This GECCO estimate is analyzed here for the period 1962–2001 with respect to decadal and longer-term changes of the meridional overturning circulation (MOC) of the North Atlantic. A special focus is on the maximum MOC values at 25°N. Over this period, the dynamically self-consistent synthesis stays within the error bars of H. L. Bryden et al., but reveals a general increase of the MOC strength. The variability on decadal and longer time scales is decomposed into contributions from different processes. Changes in the model’s MOC strength are strongly influenced by the southward communication of density anomalies along the western boundary originating from the subpolar North Atlantic, which are related to changes in the Denmark Strait overflow but are only marginally influenced by water mass formation in the Labrador Sea. The influence of density anomalies propagating along the southern edge of the subtropical gyre associated with baroclinically unstable Rossby waves is found to be equally important. Wind-driven processes such as local Ekman transport explain a smaller fraction of the variability on those long time scales.

scholarly journals Dynamical Response of the Oceanic Eddy Field to the North Atlantic Oscillation: A Model–Data Comparison

2004 ◽  
Vol 34 (12) ◽  
pp. 2615-2629 ◽  
Author(s):  
Thierry Penduff ◽  
Bernard Barnier ◽  
W. K. Dewar ◽  
James J. O'Brien
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic Oscillation ◽  
Time Scales ◽  
North Atlantic ◽  
Distribution Model ◽  
Altimeter Data ◽  
Dynamical Response ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract Observational studies have shown that in many regions of the World Ocean the eddy kinetic energy (EKE) significantly varies on interannual time scales. Comparing altimeter-derived EKE maps for 1993 and 1996, Stammer and Wunsch have mentioned a significant meridional redistribution of EKE in the North Atlantic Ocean and suggested the possible influence of the North Atlantic Oscillation (NAO) cycle. This hypothesis is examined using 7 yr of Ocean Topography Experiment (TOPEX)/Poseidon altimeter data and three ⅙°-resolution Atlantic Ocean model simulations performed over the period 1979–2000 during the French “CLIPPER” experiment. The subpolar–subtropical meridional contrast of EKE in the real ocean appears to vary on interannual time scales, and the model reproduces it realistically. The NAO cycle forces the meridional contrast of energy input by the wind. The analysis in this paper suggests that after 1993 the large amplitude of the NAO cycle induces changes in the transport of the baroclinically unstable large-scale circulation (Gulf Stream/North Atlantic Current) and, thus, changes in the EKE distribution. Model results suggest that NAO-like fluctuations were not followed by EKE redistributions before 1994, probably because NAO oscillations were weaker. Strong NAO events after 1994 were followed by gyre-scale EKE fluctuations with a 4–12-month lag, suggesting that complex, nonlinear adjustment processes are involved in this oceanic adjustment.

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 Impact of Anomalous Ocean Heat Transport on the North Atlantic Oscillation

2005 ◽  
Vol 18 (23) ◽  
pp. 4955-4969 ◽  
Author(s):  
Fabio D’Andrea ◽  
Arnaud Czaja ◽  
John Marshall
Keyword(s):  
North Atlantic Oscillation ◽  
Heat Transport ◽  
Time Scales ◽  
North Atlantic ◽  
Ocean Dynamics ◽  
Ocean Heat Transport ◽  
The North ◽  
The North Atlantic ◽  
Wind Driven Circulation ◽  
The North Atlantic Oscillation

Abstract Coupled atmosphere–ocean dynamics in the North Atlantic is studied by means of a simple model, featuring a baroclinic three-dimensional atmosphere coupled to a slab ocean. Anomalous oceanic heat transport due to wind-driven circulation is parameterized in terms of a delayed response to the change in wind stress curl due to the North Atlantic Oscillation (NAO). Climate variability for different strengths of ocean heat transport efficiency is analyzed. Two types of behavior are found depending on time scale. At interdecadal and longer time scales, a negative feedback is found that leads to a reduction in the spectral power of the NAO. By greatly increasing the efficiency of ocean heat transport, the NAO in the model can be made to completely vanish from the principal modes of variability at low frequency. This suggests that the observed NAO variability at these time scales must be due to mechanisms other than the interaction with wind-driven circulation. At decadal time scales, a coupled oscillation is found in which SST and geopotential height fields covary.

scholarly journals Ventilation time scales of the North Atlantic subtropical cell revealed by coral radiocarbon from the Cape Verde Islands

2015 ◽  
Vol 30 (7) ◽  
pp. 938-948 ◽  
Author(s):  
Alvaro Fernandez ◽  
Thomas J. Lapen ◽  
Rasmus Andreasen ◽  
Peter K. Swart ◽  
Christopher D. White ◽  
...  
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Cape Verde ◽  
Cape Verde Islands ◽  
Ventilation Time ◽  
The North ◽  
The North Atlantic ◽  
Subtropical Cell

An inter-basin teleconnection from the North Atlantic to the subarctic North Pacific at multidecadal time scales

2019 ◽  
Vol 54 (1-2) ◽  
pp. 807-822 ◽  
Author(s):  
Zhanqiu Gong ◽  
Cheng Sun ◽  
Jianping Li ◽  
Juan Feng ◽  
Fei Xie ◽  
...  
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
North Pacific ◽  
The North ◽  
Subarctic North Pacific ◽  
The North Atlantic
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