The Role of Topography in the Low-Frequency Variability of the Large-Scale Midlatitude Circulation

1995 ◽  
Vol 52 (14) ◽  
pp. 2497-2508 ◽  
Author(s):  
Anthony R. Hansen ◽  
Alfonso Sutera
Keyword(s):  
Large Scale ◽  
Low Frequency ◽  
Low Frequency Variability

Depth-integrated equation for hydro-acoustic waves with bottom damping

2015 ◽  
Vol 766 ◽  
Author(s):  
Ali Abdolali ◽  
James T. Kirby ◽  
Giorgio Bellotti
Keyword(s):  
Acoustic Waves ◽  
Large Scale ◽  
Model Equation ◽  
Low Frequency ◽  
3D Models ◽  
Sediment Layer ◽  
Sea Bed ◽  
Weakly Compressible ◽  
Acoustic Wave Generation

AbstractWe present a depth-integrated equation for the mechanics of generation, propagation and dissipation of low-frequency hydro-acoustic waves due to sudden bottom displacement in a weakly compressible ocean overlying a weakly compressible viscous sediment layer. The model is validated against a full 3D computational model. Physical properties of these waves are studied and compared with those for waves over a rigid sea bed, revealing changes in the frequency spectrum and modal peaks. The resulting model equation can be used for numerical prediction in large-scale domains, overcoming the computational difficulties of 3D models while taking into account the role of bottom dissipation on hydro-acoustic wave generation and propagation.

scholarly journals Contribution of low-frequency climatic–oceanic oscillations to streamflow variability in small, coastal rivers of the Sierra Nevada de Santa Marta (Colombia)

2019 ◽  
Vol 23 (5) ◽  
pp. 2379-2400 ◽  
Author(s):  
Juan Camilo Restrepo ◽  
Aldemar Higgins ◽  
Jaime Escobar ◽  
Silvio Ospino ◽  
Natalia Hoyos
Keyword(s):  
Energy Spectrum ◽  
Sierra Nevada ◽  
Large Scale ◽  
Low Frequency ◽  
Maximum Power ◽  
Simultaneous Occurrence ◽  
Atmospheric Processes ◽  
Low Frequency Oscillations ◽  
Streamflow Variability

Abstract. This study evaluated the influence of low-frequency oscillations, that are linked to large-scale oceanographic–atmospheric processes, on streamflow variability in small tropical coastal mountain rivers of the Sierra Nevada de Santa Marta, Colombia. We used data from six rivers that had > 32 years of complete, continuous monthly streamflow records. This investigation employed spectral analyses to (1) explore temporal characteristics of streamflow variability, (2) estimate the net contribution to the energy spectrum of low-frequency oscillations to streamflow anomalies, and (3) analyze the linkages between streamflow anomalies and large-scale, low-frequency oceanographic–atmospheric processes. Wavelet analyses indicate that the 8–12-year component exhibited a quasi-stationary state, with a peak of maximum power between 1985 and 2005. These oscillations were nearly in phase in all rivers. Maximum power peaks occurred for the Palomino and Rancheria rivers in 1985 and 1995, respectively. The wavelet spectrum highlights a change in river variability patterns between 1995 and 2015, characterized by a shift towards the low-frequency oscillations' domain (8–12 years). The net contribution of these oscillations to the energy spectrum was as high as 51 %, a value much larger than previously thought for rivers in northwestern South America. The simultaneous occurrence of hydrologic oscillations, as well as the increase in the amplitude of the 8–12-year band, defined periods of extremely anomalous wet seasons during 1989–1990, 1998–2002 and 2010–2011, reflecting the role of low-frequency oscillations in modulating streamflow variability in these rivers. Cross-wavelet transform and wavelet coherence revealed high common powers and significant coherences in low-frequency bands (>96 months) between streamflow anomalies and Atlantic Meridional Oscillation (AMO), Pacific Decadal Oscillation (PDO) and the Tropical North Atlantic Index (TNA). These results show the role of large-scale, low-frequency oceanographic–climate processes in modulating the long-term hydrological variability of these rivers.

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.

scholarly journals Low-Frequency Variability and Remote Forcing of Gap Winds over the East Pacific Warm Pool

2008 ◽  
Vol 21 (19) ◽  
pp. 4901-4918 ◽  
Author(s):  
Kristopher B. Karnauskas ◽  
Antonio J. Busalacchi ◽  
Raghu Murtugudde
Keyword(s):  
Wind Stress ◽  
Large Scale ◽  
Southern Oscillation ◽  
Low Frequency ◽  
Warm Pool ◽  
Secondary Importance ◽  
Remote Forcing ◽  
Low Frequency Variability ◽  
Western Caribbean ◽  
Gap Winds

Abstract The low-frequency variability of gap winds at the Isthmuses of Tehuantepec and Papagayo is investigated using a 17-yr wind stress dataset merging the remotely sensed observations of Special Sensor Microwave Imager (SSM/I) and Quick Scatterometer (QuikSCAT) satellite sensors. A decadal signal is identified in the Tehuantepec gap winds, which is shown to be related to the Atlantic tripole pattern (ATP). Using linear regression and spectral analysis, it is demonstrated that the low-frequency variability of the Tehuantepec gap winds is remotely forced by the ATP, and the Papagayo gap winds are primarily governed by El Niño–Southern Oscillation (ENSO) with the ATP being of secondary importance. The Tehuantepec (Papagayo) time series of wind stress anomalies can be better reconstructed when the local cross-isthmus pressure difference and large-scale climate information such as the ATP (ENSO) are included, suggesting that there is important information in the large-scale flow that is not transmitted directly through the background sea level pressure gradient. The geostrophic modulation of the easterly trades in the western Caribbean also serve as a remote driver of the Papagayo gap winds, which is itself not fully independent from ENSO. Finally, it is suggested that precipitation variability in the Inter-Americas region is closely related to the same remote forcing as that of the Tehuantepec gap winds, being the ATP and associated large-scale atmospheric circulation.

scholarly journals Low-Frequency Modulation of Turbulent Diapycnal Mixing by Anticyclonic Eddies Inferred from the HOT Time Series

2013 ◽  
Vol 43 (4) ◽  
pp. 824-835 ◽  
Author(s):  
Zhao Jing ◽  
Lixin Wu
Keyword(s):  
Time Series ◽  
Dissipation Rate ◽  
Low Frequency ◽  
Diapycnal Mixing ◽  
Dissipation Rates ◽  
Low Frequency Variability ◽  
Increasing Trend ◽  
The Mean ◽  
Anticyclonic Eddies

Abstract Profiles of potential density obtained from CTD measurements during the Hawaii Ocean Time series (HOT) program in the vicinity of the island of Oahu, Hawaii, are used to evaluate low-frequency variability of turbulent kinetic dissipation rates based on a finescale parameterization method. A distinct seasonal cycle, as well as an increasing trend of dissipation rates, is found in the upper 300–600 m. The trend is mainly due to the much weaker diapycnal mixing in the first four years of the record, that is, 1988–92. In the upper 300–600 m, enhanced diapycnal mixing is found under anticyclonic eddies with the mean dissipation rate about 53% larger than that under eddy-free conditions. The modulation of dissipation rates by anticyclonic eddies becomes more evident with increasing eddy strength. The role of cyclonic eddies in modulating diapycnal mixing is almost negligible compared with that of anticyclonic eddies. The mean dissipation rate under cyclonic eddies is comparable to that under eddy-free conditions with a difference of less than 10%. Seasonality of the dissipation rates is partly modulated by the seasonal variation of anticyclonic eddies.

Role of the Low-Frequency Deformation Field on the Explosive Growth of Extratropical Cyclones at the Jet Exit. Part I: Barotropic Critical Region

2006 ◽  
Vol 63 (8) ◽  
pp. 1965-1981 ◽  
Author(s):  
G. Rivière ◽  
A. Joly
Keyword(s):  
Critical Region ◽  
Large Scale ◽  
Sudden Change ◽  
Storm Track ◽  
Low Frequency ◽  
Reanalysis Data ◽  
Deformation Field ◽  
Extratropical Cyclones ◽  
Exit Region

Abstract By using new theoretical results on perturbation growth in spatially and temporally complex quasigeostrophic flows, this paper investigates the role of the large-scale deformation field on extratropical cyclones and especially on their explosive growth in the jet-exit region. Theoretical ideas are tested by decomposing the atmospheric flow into a high- and a low-frequency part and by analyzing four-dimensional variational data assimilation (4DVAR) reanalysis data of the Fronts and Atlantic Storm-Track Experiment (FASTEX) during February 1997 as well as reanalysis data for the end of December 1999. Regions where the low-frequency deformation magnitude is greater than the absolute value of the low-frequency vorticity are shown to correspond to regions where synoptic disturbances at the same level tend to be located. These regions in the upper troposphere are intrinsically related to the horizontal inhomogeneities of the low-frequency large-scale upper-tropospheric jet but cannot be detected by looking separately at the deformation or vorticity. Transitions from one such large-scale region to the next furthermore can be accompanied by a sudden change of the dilatation axes orientation: this combination defines a barotropic critical region (BtCR). Reasons why a BtCR is a specific place where barotropic development is likely to occur are exposed. Two very differently located BtCR regions in two apparently similar zonal-like weather regimes are shown to be the preferred regions where synoptic eddies tend to cross the jet from the south to the north. BtCRs are also special regions where constructive association between barotropic and baroclinic processes is favored, indeed constrained to cooperate. This is illustrated through the detailed analysis of the last growth stage of Intensive Observation Period 17 (IOP17) of FASTEX. It happens precisely around a BtCR area located in the jet-exit region. Two processes explain this IOP17 development; one involves the barotropic generation rate resulting from the low crossing the BtCR and the other one is baroclinic interaction, which is strongly maintained far away from the baroclinicity maximum because of the new favorable baroclinic configuration resulting from the first process.

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