scholarly journals North Atlantic variability driven by stochastic forcing in a simple model

2012 ◽  
Vol 64 (1) ◽  
pp. 18695 ◽  
Author(s):  
Rebecca Legatt ◽  
Igor V. Polyakov ◽  
UmaS. Bhatt ◽  
Xiangdong Zhang ◽  
Roman V. Bekryaev
Keyword(s):  
Simple Model ◽  
North Atlantic ◽  
Stochastic Forcing ◽  
North Atlantic Variability

scholarly journals A last millennium perspective on North Atlantic variability: exploiting synergies between models and proxy data

2017 ◽  
Vol 25 (1) ◽  
pp. 61-67 ◽  
Author(s):  
Pablo Ortega ◽  
Jon Robson ◽  
Paola Moffa-Sanchez ◽  
David Thornalley ◽  
Didier Swingedouw
Keyword(s):  
North Atlantic ◽  
Last Millennium ◽  
Proxy Data ◽  
North Atlantic Variability

scholarly journals Contributions of Atmospheric Stochastic Forcing and Intrinsic Ocean Modes to North Atlantic Ocean Interdecadal Variability

2020 ◽  
Vol 33 (6) ◽  
pp. 2351-2370 ◽  
Author(s):  
Olivier Arzel ◽  
Thierry Huck
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
General Circulation ◽  
Thermal Structure ◽  
Ocean Dynamics ◽  
Dimensionless Number ◽  
Stochastic Forcing ◽  
The North ◽  
Wide Range ◽  
The North Atlantic

AbstractAtmospheric stochastic forcing associated with the North Atlantic Oscillation (NAO) and intrinsic ocean modes associated with the large-scale baroclinic instability of the North Atlantic Current (NAC) are recognized as two strong paradigms for the existence of the Atlantic multidecadal oscillation (AMO). The degree to which each of these factors contribute to the low-frequency variability of the North Atlantic is the central question in this paper. This issue is addressed here using an ocean general circulation model run under a wide range of background conditions extending from a supercritical regime where the oceanic variability spontaneously develops in the absence of any atmospheric noise forcing to a damped regime where the variability requires some noise to appear. The answer to the question is captured by a single dimensionless number Γ measuring the ratio between the oceanic and atmospheric contributions, as inferred from the buoyancy variance budget of the western subpolar region. Using this diagnostic, about two-thirds of the sea surface temperature (SST) variance in the damped regime is shown to originate from atmospheric stochastic forcing whereas heat content is dominated by internal ocean dynamics. Stochastic wind stress forcing is shown to substantially increase the role played by damped ocean modes in the variability. The thermal structure of the variability is shown to differ fundamentally between the supercritical and damped regimes, with abrupt modifications around the transition between the two regimes. Ocean circulation changes are further shown to be unimportant for setting the pattern of SST variability in the damped regime but are fundamental for a preferred time scale to emerge.

scholarly journals Precipitação na América do Sul: Médias Climáticas e Padrões da Variabilidade no Período entre 1979 e 2008

2016 ◽  
Vol 9 (1) ◽  
pp. 032
Author(s):  
Éder Leandro Maier ◽  
Juliana Costi ◽  
Sandra Barreira ◽  
Jefferson Cardia Simões
Keyword(s):  
South America ◽  
North Atlantic ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Principal Component ◽  
Monthly Precipitation ◽  
Tropical Regions ◽  
The North ◽  
North Atlantic Variability

Este artigo discute os principais padrões médios e anômalos da precipitação sobre a América do Sul no período 1979–2008. Para isso foram manipulados dados mensais da precipitação observada em 890 estações meteorológicas localizadas na Argentina, Bolívia, Brasil, Paraguai e Uruguai ao longo desse período de trinta anos. As médias climáticas foram subtraídas das amostras, originando as anomalias, as quais foram agrupadas por meio da Análise das Componentes Principais em dois modos. No modo T se identificou 6 componentes principais, que explicam 35% da variância e representam 12 padrões espaciais anômalos originados, principalmente, pelo fenômeno El Niño–Oscilação Sul (ENOS) e pela variabilidade do Atlântico Norte. No modo S foram identificadas 8 zonas em que a variabilidade temporal das séries anômalas é semelhante, sendo que o ENOS prevalece no controle das anomalias nas zonas situadas na região equatorial e extratropical, além disso, a variabilidade do Atlântico Norte pode maximizar ou minimizar os impactos do ENOS. A frequência de recorrência desses estresses hídrico variam entre 20 e 60 meses.  This article discusses mean and anomalous rainfall patterns over South America in the period 1979–2008. For that we handled monthly precipitation data observed at 890 meteorological stations located in Argentina, Bolivia, Brazil, Paraguay and Uruguay over this thirty years period. Climatic means were subtracted from the data, resulting in anomalies that were grouped by Principal Component Analysis in two modes. We identified 6 main components in the T mode, which explain 35% of the variance and represent 12 anomalous spatial patterns originated mainly by El Niño–Southern Oscillation (ENSO) phenomenon and the North Atlantic variability. In mode S, we identified eight zones where the series temporal variability is also anomalous, and the ENOS prevails as the anomalies controller in the equatorial and extra tropical regions. Further, North Atlantic variability may maximize or minimize the ENSO impact. The frequency of these recurrent water stresses range from 20 to 60 months. Keywords: Precipitation, South America, PCA  

scholarly journals Observed Decadal North Atlantic Tripole SST Variability. Part II: Diagnosis of Mechanisms

2012 ◽  
Vol 69 (1) ◽  
pp. 51-64 ◽  
Author(s):  
Edwin K. Schneider ◽  
Meizhu Fan
Keyword(s):  
Heat Flux ◽  
Simple Model ◽  
North Atlantic ◽  
Time Scale ◽  
Coupled Model ◽  
Decadal Time Scale ◽  
Weather Noise ◽  
The North ◽  
Sst Variability ◽  
Flux Response

Abstract In Part I of this study, the atmospheric weather noise for 1951–2000 was inferred from an atmospheric analysis in conjunction with SST-forced AGCM simulations and used to force interactive ensemble coupled GCM simulations of the North Atlantic SST variability. Here, results from those calculations are used in conjunction with a simple stochastically forced coupled model of the decadal time scale North Atlantic tripole SST variability to examine the mechanisms associated with the tripole SST variability. The diagnosed tripole variability is found to be characterized by damped, delayed oscillator dynamics, similar to what has been found by other investigators. However, major differences here, affecting the signs of two of the crucial parameters of the simple model, are that the atmospheric heat flux feedback damps the tripole pattern and that a counterclockwise intergyre gyre-like circulation enhances the tripole pattern. Delayed oscillator dynamics are still obtained because the sign of the dynamically important parameter, proportional to the product of these two parameters, is unchanged. Another difference with regard to the dynamical processes included in the simple model is that the major contribution to the ocean’s dynamical heat flux response to the weather noise wind stress is through a delayed modulation of the mean gyres, rather than from the simultaneous intergyre gyre response. The power spectrum of a revised simple model forced by white noise has a less prominent decadal peak using the parameter values and dynamics diagnosed here than in previous investigations. Decadal time scale retrospective predictions made with this version of the simple model are no better than persistence.

scholarly journals Measurements of nitrogen oxides and a simple model of NOyfate in the remote North Atlantic marine atmosphere

1998 ◽  
Vol 103 (D11) ◽  
pp. 13489-13503 ◽  
Author(s):  
Matthew C. Peterson ◽  
Richard E. Honrath ◽  
David D. Parrish ◽  
Samuel J. Oltmans
Keyword(s):  
Simple Model ◽  
Nitrogen Oxides ◽  
North Atlantic ◽  
Marine Atmosphere

scholarly journals Stochastic Forcing of Ocean Variability by the North Atlantic Oscillation

2009 ◽  
Vol 39 (1) ◽  
pp. 162-184 ◽  
Author(s):  
Kettyah C. Chhak ◽  
Andrew M. Moore ◽  
Ralph F. Milliff
Keyword(s):  
North Atlantic Oscillation ◽  
Ocean Circulation ◽  
Rossby Wave ◽  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
Stochastic Forcing ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract At middle and high latitudes, the magnitude of stochastic wind stress forcing of the ocean by atmospheric variability on synoptic time scales (i.e., “weather” related variability) is comparable to that of the seasonal cycle. Stochastic forcing may therefore have a significant influence on the ocean circulation, climate, and ocean predictability. Here, the influence of stochastic forcing associated with the North Atlantic Oscillation on the subtropical gyre circulation of the North Atlantic is explored in an eddy-permitting quasigeostrophic framework. For the North Atlantic winds used in this study, the root-mean-square of the annual average Ekman pumping velocity of the seasonal cycle between 35° and 52°N is 1.3 × 10−7 m s−1, while the wintertime standard deviation of the stochastic component of the North Atlantic Oscillation over the same latitude band is 2.2 × 10−7 m s−1. Significant stochastically induced variability in the ocean circulation occurs near the western boundary region and along the western margins of the abyssal plains associated with vortex stretching, energy release from the mean flow, and the generation of topographic Rossby waves. Variability arises from a combination of two effects, depending on the measure of variance used: growth of unstable modes of the underlying circulation and modal interference resulting from their nonnormal nature, which dominates during the first 10 days or so of perturbation growth. Near the surface, most of the variability is associated with large-scale changes in the barotropic circulation, although more than 20% of the energy and enstrophy variability is associated with small-scale baroclinic waves. In the deep ocean, much of the stochastically induced variability is apparently due to topographic Rossby wave activity along the continental rise and ocean ridges. Previous studies have demonstrated that rectification of topographic Rossby wave–induced circulations in the western North Atlantic may contribute to the western boundary current recirculation zones. The authors suggest that a source of topographic Rossby wave energy, significant enough to rectify the mean ocean circulation, may arise from stochastic forcing by large-scale atmospheric forcing, such as the North Atlantic Oscillation and other atmospheric teleconnection patterns.

scholarly journals Stochastic Forcing of the North Atlantic Wind-Driven Ocean Circulation. Part I: A Diagnostic Analysis of the Ocean Response to Stochastic Forcing

2006 ◽  
Vol 36 (3) ◽  
pp. 300-315 ◽  
Author(s):  
Kettyah C. Chhak ◽  
Andrew M. Moore ◽  
Ralph F. Milliff ◽  
Grant Branstator ◽  
William R. Holland ◽  
...  
Keyword(s):  
Wind Stress ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Induced Response ◽  
Western Boundary ◽  
Transient Growth ◽  
Stochastic Forcing ◽  
Stochastic Component ◽  
The North ◽  
The North Atlantic

Abstract At midlatitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that associated with the seasonal cycle. Stochastic forcing is therefore likely to have a significant influence on the ocean circulation. In this work, the influence of the stochastic component of the wind stress forcing on the large-scale, wind-driven circulation of the North Atlantic Ocean is examined. To this end, a quasigeostrophic model of the North Atlantic was forced with estimates of the stochastic component of wind stress curl obtained from the NCAR Community Climate Model. Analysis reveals that much of the stochastically induced variability in the ocean circulation occurs in the vicinity of the western boundary and some major bathymetric features. Thus, the response is localized even though the stochastic forcing occurs over most of the ocean basin. Using the ideas of generalized stability theory, the stochastically induced response in the ocean circulation can be interpreted as a linear interference of the nonorthogonal eigenmodes of the system. This linear interference process yields transient growth of stochastically induced perturbations. By examining the model pseudospectra, it is seen that the nonnormal nature of the system enhances the transient growth of perturbation enstrophy and therefore elevates and maintains the variance of the stochastically induced circulations in the aforementioned regions. The primary causes of nonnormality in the enstrophy norm are bathymetry and the western boundary current circulation.

scholarly journals Stochastic Forcing of the North Atlantic Wind-Driven Ocean Circulation. Part II: An Analysis of the Dynamical Ocean Response Using Generalized Stability Theory

2006 ◽  
Vol 36 (3) ◽  
pp. 316-334 ◽  
Author(s):  
Kettyah C. Chhak ◽  
Andrew M. Moore ◽  
Ralph F. Milliff ◽  
Grant Branstator ◽  
William R. Holland ◽  
...  
Keyword(s):  
Wind Stress ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Boundary Region ◽  
Western Boundary ◽  
Transient Growth ◽  
Wind Stress Curl ◽  
Stochastic Forcing ◽  
The North ◽  
The North Atlantic

Abstract As discussed in Part I of this study, the magnitude of the stochastic component of wind stress forcing is comparable to that of the seasonal cycle and thus will likely have a significant influence on the ocean circulation. By forcing a quasigeostrophic model of the North Atlantic Ocean circulation with stochastic wind stress curl data from the NCAR CCM3, it was found in Part I that much of the stochastically induced variability in the ocean circulation is confined to the western boundary region and some major topographic features even though the stochastic forcing is basinwide. This can be attributed to effects of bathymetry and vorticity gradients in the basic state on the system eigenmodes. Using generalized stability theory (GST), it was found in Part I that transient growth due to the linear interference of nonnormal eigenmodes enhances the stochastically induced variance. In the present study, the GST analysis of Part I is extended and it is found that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system. These eigenmodes are confined primarily to the western boundary region and are composed of long Rossby wave packets that are Doppler shifted by the Gulf Stream to have eastward group velocity. Linear interference of these eigenmodes yields transient growth of stochastically induced perturbations, and it is this process that maintains the variance of the stochastically induced circulations. Analysis of the large-scale circulation also reveals that the system possesses a large number of degrees of freedom, which has significant implications for ocean prediction. Sensitivity studies show that the results and conclusions of this study are insensitive and robust to variations in model parameters and model configuration.

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