scholarly journals Low-frequency variability of the large-scale ocean circulation: A dynamical systems approach

2005 ◽  
Vol 43 (3) ◽  
Author(s):  
Henk A. Dijkstra
Keyword(s):  
Dynamical Systems ◽  
Ocean Circulation ◽  
Large Scale ◽  
Systems Approach ◽  
Low Frequency ◽  
Low Frequency Variability

Decadal physical-biogeochemical changes in the Newfoundland and Labrador ecosystem

2020 ◽  
Author(s):  
Frédéric Cyr ◽  
Olivia Gibb ◽  
David Bélanger ◽  
Guoqi Han ◽  
Gary Maillet ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Newfoundland And Labrador ◽  
Large Scale ◽  
Regional Climate ◽  
Monitoring Program ◽  
Low Frequency ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Physical Conditions ◽  
Low Frequency Variability

<p>Located on a crossroads of some of the main currents associated to the Atlantic meridional overturning circulation (AMOC), Newfoundland and Labrador (NL) shelves are specially affected by changes in large-scale ocean circulation. Such circulation changes impact not only the regional climate, but also the overall water masses composition, with consequences on physical conditions, nutrient availability, oxygen content, pH, etc. Systematic hydrographic observations of this system have been carried out by Canada and other countries since 1948. The observational program was reinforced in 1999 with the creation of the Atlantic Zone Monitoring Program (AZMP), ensuring enhanced seasonal coverage and new biogeochemical observations. In 2014, this monitoring was augmented with the monitoring of ocean acidification parameters. Here we review historical physical-biogeochemical changes on the NL shelves, with an emphasis on low frequency variability and cycles. Results suggest, for example, that the cold intermediate layer (CIL), a cold mid-depth layer that is a key feature of the NL ecosystem, exhibited profound changes during the last 70 years. In the mid 60's, the CIL was anomalously warm compared to the rest of the time series. This warm period was followed by a cold period centered in the early 90's. Historical salinity records also suggest that fresher waters are found during warmer years, and vice-versa. Nitrate/Phosphate ratios suggest recent changes in water masses composition towards less Arctic waters flowing on the shelves. This is concurrent with a reduction in nutrients concentration on the NL shelves since about 2012, together with changes in the strength of the Labrador Current along the shelf.</p>

scholarly journals Low-Frequency Variability, Coherence Resonance, and Phase Selection in a Low-Order Model of the Wind-Driven Ocean Circulation

2011 ◽  
Vol 41 (9) ◽  
pp. 1585-1604 ◽  
Author(s):  
Stefano Pierini
Keyword(s):  
Ocean Circulation ◽  
Kuroshio Extension ◽  
Global Bifurcation ◽  
Low Frequency ◽  
Galerkin Projection ◽  
Coherence Resonance ◽  
Primitive Equation ◽  
Phase Selection ◽  
Low Frequency Variability ◽  
Low Order

Abstract In this paper, a low-order spectral quasigeostrophic (QG) model of the wind-driven ocean circulation is derived and used to analyze the low-order character of the intrinsic low-frequency variability of the midlatitude double-gyre ocean circulation and of the related coherence resonance and phase selection phenomena. The model includes an exponential in the basis functions that allows for westward intensification, retains only four modes in the Galerkin projection, is defined in a rectangular domain, and is forced by deterministic and stochastic winds, thus extending previous low-order QG ocean models. The solution under steady forcing is first obtained, and the results are also analyzed in terms of dynamical systems theory. A homoclinic bifurcation (with the wind amplitude chosen as the control parameter) leads to intrinsic decadal relaxation oscillations (ROs) similar in several respects to those obtained with primitive equation models. The system is then forced with an additional red noise wind, and, in a parameter range preceding the global bifurcation, a coherence resonance scenario very similar to the one found with a primitive equation model of the Kuroshio Extension is obtained: this suggests that such a phenomenon is of low-order character. To study the RO excitation mechanism, a method denoted as phase selection is proposed. The system is forced with additional fictitious periodic winds that produce an emergence of ROs yielding strong phase dependence with the periodic forcing. The subsequent analysis reveals the character of the wind forcing that is most likely to excite a RO. All the results are discussed within the general framework of climate dynamics.

scholarly journals An Assessment of GCM Skill in Simulating Persistence across Multiple Time Scales

2011 ◽  
Vol 24 (14) ◽  
pp. 3609-3623 ◽  
Author(s):  
Fiona Johnson ◽  
Seth Westra ◽  
Ashish Sharma ◽  
Andrew J. Pitman
Keyword(s):  
Interannual Variability ◽  
General Circulation ◽  
Large Scale ◽  
Low Frequency ◽  
General Circulation Models ◽  
Multiple Time Scales ◽  
Multiple Time ◽  
Max Planck ◽  
Reservoir Storage ◽  
Low Frequency Variability

Abstract Climate change impact studies for water resource applications, such as the development of projections of reservoir yields or the assessment of likely frequency and amplitude of drought under a future climate, require that the year-to-year persistence in a range of hydrological variables such as catchment average rainfall be properly represented. This persistence is often attributable to low-frequency variability in the global sea surface temperature (SST) field and other large-scale climate variables through a complex sequence of teleconnections. To evaluate the capacity of general circulation models (GCMs) to accurately represent this low-frequency variability, a set of wavelet-based skill measures has been developed to compare GCM performance in representing interannual variability with the observed global SST data, as well as to assess the extent to which this variability is imparted in precipitation and surface pressure anomaly fields. A validation of the derived skill measures is performed using GCM precipitation as an input in a reservoir storage context, with the accuracy of reservoir storage estimates shown to be improved by using GCM outputs that correctly represent the observed low-frequency variability. Significant differences in the performance of different GCMs is demonstrated, suggesting that judicious selection of models is required if the climate impact assessment is sensitive to low-frequency variability. The two GCMs that were found to exhibit the most appropriate representation of global low-frequency variability for individual variables assessed were the Istituto Nazionale di Geofisica e Vulcanologia (INGV) ECHAM4 and L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL CM4); when considering all three variables, the Max Planck Institute (MPI) ECHAM5 performed well. Importantly, models that represented interannual variability well for SST also performed well for the other two variables, while models that performed poorly for SST also had consistently low skill across the remaining variables.

scholarly journals Comments on “Reconstruction of the Extratropical NH Mean Temperature over the Last Millennium with a Method That Preserves Low-Frequency Variability”

2012 ◽  
Vol 25 (22) ◽  
pp. 7991-7997 ◽  
Author(s):  
Anders Moberg
Keyword(s):  
Large Scale ◽  
Low Frequency ◽  
Temperature Reconstruction ◽  
Noise Variance ◽  
Low Frequencies ◽  
Scale Temperature ◽  
Low Frequency Variability ◽  
High Frequency Variability ◽  
Frequency Temperature ◽  
Correct Estimate

Abstract Christiansen and Ljungqvist have presented an extratropical NH temperature reconstruction using a method (LOC) that they claim “preserves” low-frequency variability, at the expense of exaggerated high-frequency variability. Using theoretical arguments and a pseudoproxy experiment, it is demonstrated here that the LOC method is not guaranteed to preserve variability at any frequency. Rather, LOC reconstructions will have more variance than true large-scale temperature averages at all frequencies. This variance inflation, however, can be negligible at those frequencies where the noise variance in individual proxies is small enough to be effectively cancelled when computing an average over the available proxies. Because the proxy noise variance at low frequencies cannot be directly estimated, and thus has to be regarded as unknown, it is safer to regard a reconstruction with the LOC method as providing an estimate of the upper bound of the large-scale low-frequency temperature variability rather than one with a correct estimate of this variance.

Recruitment to Northwest Atlantic Cod (Gadus morhua) and Haddock (Melanogrammus aeglefinus) Stocks: Influence of Stock Size and Climate

1987 ◽  
Vol 44 (1) ◽  
pp. 26-39 ◽  
Author(s):  
J. Anthony Koslow ◽  
Keith R. Thompson ◽  
William Silvert
Keyword(s):  
Large Scale ◽  
Gadus Morhua ◽  
Reproductive Output ◽  
Atlantic Cod ◽  
Low Frequency ◽  
Melanogrammus Aeglefinus ◽  
Northwest Atlantic ◽  
Biological Variables ◽  
Low Frequency Variability ◽  
Haddock Melanogrammus Aeglefinus

Year-class success of both Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) stocks in the northwest Atlantic exhibits large-scale coherence and low-frequency variability with apparent periodicities of 10–20 yr. Several physical and biological variables in the region exhibit similar large-scale coherence and apparent periodicity. Multiple regression analysis indicates that year-class success in northwest Atlantic cod stocks tends to be associated with large-scale meteorological patterns and offshore winds. Recruitment to most haddock stocks from the Scotian Shelf to Georges Bank is negatively associated with abundance of 0-group mackerel, which may be due to predation over winter and/or to a combination of environmental features including sea-surface temperature, large-scale atmospheric pressure systems, and freshwater outflows. Statistical analyses often did not define a unique set of variables that best predicted fishery recruitment due to widespread intercorrelations among environmental processes and the likelihood that not all relevant processes entered directly into the analyses. There is little evidence that stock reproductive output during the study period was significantly related to year-class success.

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