scholarly journals Dynamical Attribution of Recent Variability in Atlantic Overturning

2016 ◽  
Vol 29 (9) ◽  
pp. 3339-3352 ◽  
Author(s):  
Helen R. Pillar ◽  
Patrick Heimbach ◽  
Helen L. Johnson ◽  
David P. Marshall
Keyword(s):  
Time Series ◽  
Time Scales ◽  
Surface Wind ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Buoyancy Forcing ◽  
Freshwater Forcing ◽  
Decadal Trend ◽  
Improved Model ◽  
Short Time

Abstract Attributing observed variability of the Atlantic meridional overturning circulation (AMOC) to past changes in surface forcing is challenging but essential for detecting any influence of anthropogenic forcing and reducing uncertainty in future climate predictions. Here, quantitative estimates of separate contributions from wind and buoyancy forcing to AMOC variations at 25°N are obtained. These estimates are achieved by projecting observed atmospheric anomalies onto model-based dynamical patterns of AMOC sensitivity to surface wind, thermal, and freshwater forcing over the preceding 15 years. Local wind forcing is shown to dominate AMOC variability on short time scales, whereas subpolar heat fluxes dominate on decadal time scales. The reconstructed transport time series successfully reproduces most of the interannual variability observed by RAPID–MOCHA. However, the apparent decadal trend in the RAPID–MOCHA time series is not captured, requiring improved model representation of ocean adjustment to subpolar heat fluxes over at least the past two decades and highlighting the importance of sustained monitoring of the high-latitude North Atlantic.

scholarly journals Locally and Remotely Forced Subtropical AMOC Variability: A Matter of Time Scales

2020 ◽  
Vol 33 (12) ◽  
pp. 5155-5172
Author(s):  
Quentin Jamet ◽  
William K. Dewar ◽  
Nicolas Wienders ◽  
Bruno Deremble ◽  
Sally Close ◽  
...  
Keyword(s):  
Time Series ◽  
Time Scales ◽  
North Atlantic ◽  
Time Scale ◽  
Low Frequency ◽  
Meridional Overturning Circulation ◽  
Open Boundary ◽  
Overturning Circulation ◽  
Decadal Scale ◽  
Meridional Overturning

AbstractMechanisms driving the North Atlantic meridional overturning circulation (AMOC) variability at low frequency are of central interest for accurate climate predictions. Although the subpolar gyre region has been identified as a preferred place for generating climate time-scale signals, their southward propagation remains under consideration, complicating the interpretation of the observed time series provided by the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array–Western Boundary Time Series (RAPID–MOCHA–WBTS) program. In this study, we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low-frequency AMOC variability. We analyze for this a set of four ensembles of a regional (20°S–55°N), eddy-resolving (1/12°) North Atlantic oceanic configuration, where surface forcing and open boundary conditions are alternatively permuted from fully varying (realistic) to yearly repeating signals. Their analysis reveals the predominance of local, atmospherically forced signal at interannual time scales (2–10 years), whereas signals imposed by the boundaries are responsible for the decadal (10–30 years) part of the spectrum. Due to this marked time-scale separation, we show that, although the intergyre region exhibits peculiarities, most of the subtropical AMOC variability can be understood as a linear superposition of these two signals. Finally, we find that the decadal-scale, boundary-forced AMOC variability has both northern and southern origins, although the former dominates over the latter, including at the site of the RAPID array (26.5°N).

scholarly journals The Response of an Idealized Ocean Basin to Variable Buoyancy Forcing

2005 ◽  
Vol 35 (5) ◽  
pp. 601-615 ◽  
Author(s):  
M. A. Lucas ◽  
J. J. Hirschi ◽  
J. D. Stark ◽  
J. Marotzke
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean Basin ◽  
Meridional Overturning Circulation ◽  
Quasi Equilibrium ◽  
Vertical Diffusion ◽  
Strong Response ◽  
Buoyancy Forcing

Abstract The response of an idealized ocean basin to variable buoyancy forcing is examined. A general circulation model that employs a Gent–McWilliams mixing parameterization is forced by a zonally constant restoring surface temperature profile, which varies with latitude and time over a period P. In each experiment, 17 different values of P are studied, ranging from 6 months to 32 000 yr. The model's meridional overturning circulation (MOC) exhibits a very strong response on all time scales greater than 15 yr, up to and including the longest forcing time scales examined. The peak-to-peak values of the MOC oscillations reach up to 125% of the steady-state maximum MOC and exhibit resonance-like behavior, with a maximum at centennial to millennial forcing periods (depending on the vertical diffusivity). This resonance-like behavior stems from the existence of two adjustment time scales, one of which is set by the vertical diffusion and the other of which is set by the basin width. Furthermore, the linearity of the response as well as its lag with the forcing varies with the forcing period. The considerable deviation from the quasi-equilibrium response at all time scales above 15 yr is surprising and suggests a potentially important role of the ocean circulation for climate, even at Milankovich time scales.

scholarly journals Warm-air entrainment and advection during alpine blowing snow events

2020 ◽  
Vol 14 (9) ◽  
pp. 2795-2807
Author(s):  
Nikolas O. Aksamit ◽  
John W. Pomeroy
Keyword(s):  
Time Series ◽  
Hydrological Cycle ◽  
Surface Wind ◽  
Variable Interval ◽  
Air Entrainment ◽  
Heat Fluxes ◽  
Time Averaging ◽  
Blowing Snow ◽  
Near Surface ◽  
Scaling Relationship

Abstract. Blowing snow transport has considerable impact on the hydrological cycle in alpine regions both through the redistribution of the seasonal snowpack and through sublimation back into the atmosphere. Alpine energy and mass balances are typically modeled with time-averaged approximations of sensible and latent heat fluxes. This oversimplifies nonstationary turbulent mixing in complex terrain and may overlook important exchange processes for hydrometeorological prediction. To determine if specific turbulent motions are responsible for warm- and dry-air advection during blowing snow events, quadrant analysis and variable interval time averaging was used to investigate turbulent time series from the Fortress Mountain Snow Laboratory alpine study site in the Canadian Rockies, Alberta, Canada, during the winter of 2015–2016. By analyzing wind velocity and sonic temperature time series with concurrent blowing snow, such turbulent motions were found to supply substantial sensible heat to near-surface wind flows. These motions were responsible for temperature fluctuations of up to 1 ∘C, a considerable change for energy balance estimation. A simple scaling relationship was derived that related the frequency of dominant downdraft and updraft events to their duration and local variance. This allows for the first parameterization of entrained or advected energy for time-averaged representations of blowing snow sublimation and suggests that advection can strongly reduce thermodynamic feedbacks between blowing snow sublimation and the near-surface atmosphere. The downdraft and updraft scaling relationship described herein provides a significant step towards a more physically based blowing snow sublimation model with more realistic mixing of atmospheric heat. Additionally, calculations of return frequencies and event durations provide a field-measurement context for recent findings of nonstationarity impacts on sublimation rates.

scholarly journals The Importance of Wind and Buoyancy Forcing for the Boundary Density Variations and the Geostrophic Component of the AMOC at 26°N

2014 ◽  
Vol 44 (9) ◽  
pp. 2387-2408 ◽  
Author(s):  
Irene Polo ◽  
Jon Robson ◽  
Rowan Sutton ◽  
Magdalena Alonso Balmaseda
Keyword(s):  
Density Gradient ◽  
Time Scales ◽  
Wind Stress ◽  
Coastal Upwelling ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Wind Forcing ◽  
Western Boundary ◽  
Decadal Time Scale ◽  
Buoyancy Forcing

Abstract It is widely thought that changes in both the surface buoyancy fluxes and wind stress drive variability in the Atlantic meridional overturning circulation (AMOC), but that they drive variability on different time scales. For example, wind forcing dominates short-term variability through its effects on Ekman currents and coastal upwelling, whereas buoyancy forcing is important for longer time scales (multiannual and decadal). However, the role of the wind forcing on multiannual to decadal time scales is less clear. Here the authors present an analysis of simulations with the Nucleus for European Modelling of the Ocean (NEMO) ocean model with the aim of explaining the important drivers of the zonal density gradient at 26°N, which is directly related to the AMOC. In the experiments, only one of either the wind stress or the buoyancy forcing is allowed to vary in time, whereas the other remains at its seasonally varying climatology. On subannual time scales, variations in the density gradient, and in the AMOC minus Ekman, are driven largely by local wind-forced coastal upwelling at both the western and eastern boundaries. On decadal time scales, buoyancy forcing related to the North Atlantic Oscillation dominates variability in the AMOC. Interestingly, however, it is found that wind forcing also plays a role at longer time scales, primarily impacting the interannual variability through the excitation of Rossby waves in the central Atlantic, which propagate westward to interact with the western boundary, but also by modulating the decadal time-scale response to buoyancy forcing.

scholarly journals Short-term instabilities in γ Velorum: a search for strange modes in variable stars

2004 ◽  
Vol 193 ◽  
pp. 247-250
Author(s):  
John Bentley ◽  
P.L. Cottrell
Keyword(s):  
Time Series ◽  
Time Scales ◽  
Short Duration ◽  
Line Profile ◽  
Stellar Wind ◽  
High Frequency ◽  
Variable Stars ◽  
Short Term ◽  
Short Time ◽  
Rayet Star

AbstractStrange mode instabilities in Wolf-Rayet stars have been proposed as the cause of clumping in the stellar wind. This, in turn, is suggested as a cause of line profile variations found on short time scales. High-frequency, short-duration, time-series spectroscopy was performed on the Wolf-Rayet star γ Velorum and analysis confirmed the results of a previous study of γ Velorum’s stellar wind which showed that the wind is indeed clumped, most likely from the predicted strange mode pulsations.

scholarly journals Prevailing Surface Wind Direction during Air–Sea Heat Exchange

2019 ◽  
Vol 32 (17) ◽  
pp. 5601-5617 ◽  
Author(s):  
Fumiaki Ogawa ◽  
Thomas Spengler
Keyword(s):  
Heat Exchange ◽  
Time Scales ◽  
Wind Direction ◽  
Surface Wind ◽  
Reanalysis Data ◽  
Heat Fluxes ◽  
Prevailing Wind ◽  
Prevailing Wind Direction ◽  
Mean Wind Speed ◽  
Time Averages

AbstractWhile the climatological-mean sensible and latent heat fluxes are remarkably well described using climatological-mean fields in the bulk flux formulas, this study shows that a significant fraction of the climatological-mean wind speed in the midlatitudes is associated with wind variations on synoptic time scales. Hence, the prevailing wind direction associated with the most intense air–sea heat exchange can differ from the mean wind direction. To pinpoint these striking differences between the climatological and synoptic viewpoint, this study presents a global climatology of the prevailing surface wind direction during air–sea heat exchanges calculated for instantaneous and time-averaged reanalysis data. The interpretation of the fluxes in the lower latitudes is basically unaffected by the different time averages, highlighting the time-mean nature of the circulation in the lower latitudes. In the midlatitudes, however, the prevailing wind direction features a significant equatorward component for subweekly time averages and reverts to pure westerlies for longer time averages. These findings pinpoint the necessity to consider subweekly time scales, in particular along the midlatitude SST fronts, to describe the air–sea heat exchange in a physically consistent way.

scholarly journals Atlantic Subsurface Temperatures: Response to a Shutdown of the Overturning Circulation and Consequences for Its Recovery

2007 ◽  
Vol 20 (19) ◽  
pp. 4884-4898 ◽  
Author(s):  
J. Mignot ◽  
A. Ganopolski ◽  
A. Levermann
Keyword(s):  
Time Scales ◽  
Temperature Inversion ◽  
Meridional Overturning Circulation ◽  
Subsurface Water ◽  
Subsurface Temperature ◽  
Vertical Temperature ◽  
Overturning Circulation ◽  
Intermediate Depth ◽  
Freshwater Forcing ◽  
Cold Case

Abstract Using the coupled climate model of intermediate complexity, CLIMBER-3α, changes in the vertical thermal structure associated with a shutdown of the Atlantic meridional overturning circulation (AMOC) are investigated. When North Atlantic Deep Water formation is inhibited by anomalous freshwater forcing, intermediate depth ventilation can remain active and cool the subsurface water masses (i.e., the “cold case”). However, if intermediate ventilation is completely suppressed, relatively warm water coming from the south penetrates to a high northern latitude beneath the halocline and induces a strong vertical temperature inversion between the surface and intermediate depth (i.e., the “warm case”). Both types of temperature anomalies emerge within the first decade after the beginning of the freshwater perturbation. The sign of subsurface temperature anomaly has a strong implication for the recovery of the AMOC once the anomalous freshwater forcing is removed. While the AMOC recovers from the cold case on centennial time scales, the recovery is much more rapid (decadal time scales) when ventilation is completely suppressed and intermediate depths are anomalously warm. This is explained by a more rapid destabilization of the water column after cessation of the anomalous flux due to a strong vertical temperature inversion. A suite of sensitivity experiments with varying strength and duration of the freshwater perturbation and a larger value of background vertical diffusivity demonstrate robustness of the phenomenon. Implications of the simulated subsurface temperature response to the shutdown of the AMOC for future climate and abrupt climate changes of the past are discussed.

Energetics of a Rotating Wind-forced Horizontal Convection Model of a Reentrant Channel

2020 ◽  
Author(s):  
Varvara E. Zemskova ◽  
Brian L. White ◽  
Alberto Scotti
Keyword(s):  
Potential Energy ◽  
Southern Ocean ◽  
Wind Stress ◽  
Energy Budget ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
Stress Profile ◽  
Overturning Circulation ◽  
Available Potential Energy ◽  
Buoyancy Forcing

AbstractWe present numerical results for an idealized rotating, buoyancy- and windforced channel as a simple model for the Southern Ocean branch of the Meridional Overturning Circulation (MOC). Differential buoyancy forcing is applied along the top horizontal surface, with surface cooling at one end (to represent the pole) and surface warming at the other (to represent the equatorial region) and a zonally re-entrant channel to represent the Antarctic Circumpolar Current (ACC). Zonally-uniform surface wind forcing is applied with a similar pattern to the westerlies and easterlies with varying magnitude relative to the buoyancy forcing. The problem is solved numerically using a 3D DNS model based on a finite-volume solver for the Boussinesq Navier-Stokes equations with rotation. The overall dynamics, including large-scale overturning, baroclinic eddying, turbulent mixing, and resulting energy cascades are studied by calculating terms in the energy budget using the local Available Potential Energy framework. The basic physics of the overturning in the Southern Ocean are investigated at multiple scales and the output from the fully-resolved DNS simulations is compared with the results from previous studies of the global (ECCO2) and Southern Ocean eddy-permitting state estimates. We find that both the magnitude and shape of the zonal wind stress profile are important to the spatial pattern of the overturning circulation. However, the available potential energy budget and the diapycnal mixing are not significantly affected by the surface wind stress and are primarily set by the buoyancy forcing at the surface.

SCALE INVARIANCE IN SELECTIN-MEDIATED LEUKOCYTE ROLLING

Fractals ◽  
2004 ◽  
Vol 12 (02) ◽  
pp. 235-241 ◽  
Author(s):  
MICHAEL R. KING
Keyword(s):  
Time Series ◽  
Time Scales ◽  
Hurst Exponent ◽  
White Blood Cells ◽  
Spatial Arrangement ◽  
Range Analysis ◽  
Rescaled Range Analysis ◽  
Long Time ◽  
Short Time ◽  
Over Time

White blood cells slowly roll along the walls of blood vessels, due to the coordinated formation and breakage of chemical selectin-carbohydrate bonds. Using detailed computer simulations of cells rolling on a selectin surface under flow, we show the time series of the cell translational velocity to be fractal in nature over time scales ranging from 22–211 ms. A rescaled range analysis was performed to determine the Hurst exponent of the velocity time series, for simulations of cells rolling on either a uniform or punctate distribution of P-selectin molecules. The rolling behavior was found to exhibit two very distinct regimes, with a negative Hurst exponent ranging from -(1.2-0.6) over time scales of 23-27 ms, and a positive Hurst exponent of +0.47±0.03 over time scales of 27-211 ms. The short-time Hurst exponent was found to be a strong function of the molecular distribution and also a function of average molecular density, while the long-time Hurst exponent was unchanged over all conditions studied. The implication is that the short-time adhesive behavior of cells interacting with a reactive surface is sensitive to the spatial arrangement of molecules, and the total number of molecules on the surface.

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