Search Results
You are looking at 1 - 8 of 8 items for
- Author or Editor: L. Terray x
- Refine by Access: Content accessible to me x
Abstract
Effects of Atlantic sea surface temperature (SST) anomalies on the North Atlantic low-frequency atmospheric variability are examined by analyzing two ensembles of integrations of the ARPEGE general circulation model (GCM) forced with differently configured observed SSTs and sea ice extents (SIE) over the 1948–98 period. An attempt is made to separate the forced atmospheric response from internal atmospheric variability by using a signal-to-noise maximizing empirical orthogonal function (EOF) analysis. This method yields an estimate of the most detectable common forced response given the knowledge of internal variability provided by the ensemble. Applying the algorithm to North Atlantic atmospheric model data leads to an atmospheric response associated with a tripole pattern in North Atlantic SSTs. The spatial structure of the forced response, which is most consistent in winter, shows a dipole pattern in mean sea level pressure projecting onto the North Atlantic Oscillation. Examination of other atmospheric variables shows a very coherent signal with a quasi-barotropic signature. Additional atmospheric integrations with idealized SST anomaly patterns demonstrate the primary role of the tropical North Atlantic SST anomalies in generating the forced response. The physical mechanism involves related changes in tropical convection, Hadley circulation, and the modulation of the stationary and transient planetary-scale waves by the low-frequency variability in subtropical winds induced by the persistent tropical circulation anomalies.
Abstract
Effects of Atlantic sea surface temperature (SST) anomalies on the North Atlantic low-frequency atmospheric variability are examined by analyzing two ensembles of integrations of the ARPEGE general circulation model (GCM) forced with differently configured observed SSTs and sea ice extents (SIE) over the 1948–98 period. An attempt is made to separate the forced atmospheric response from internal atmospheric variability by using a signal-to-noise maximizing empirical orthogonal function (EOF) analysis. This method yields an estimate of the most detectable common forced response given the knowledge of internal variability provided by the ensemble. Applying the algorithm to North Atlantic atmospheric model data leads to an atmospheric response associated with a tripole pattern in North Atlantic SSTs. The spatial structure of the forced response, which is most consistent in winter, shows a dipole pattern in mean sea level pressure projecting onto the North Atlantic Oscillation. Examination of other atmospheric variables shows a very coherent signal with a quasi-barotropic signature. Additional atmospheric integrations with idealized SST anomaly patterns demonstrate the primary role of the tropical North Atlantic SST anomalies in generating the forced response. The physical mechanism involves related changes in tropical convection, Hadley circulation, and the modulation of the stationary and transient planetary-scale waves by the low-frequency variability in subtropical winds induced by the persistent tropical circulation anomalies.
Abstract
The relationship between large-scale atmospheric circulation and November–March precipitation over France during the twentieth century is investigated. A long daily MSLP dataset is used to derive daily weather types that are discriminant for precipitation. A linear regression model is then used to relate the November–March-accumulated precipitation amount and the occurrence frequency of the weather types. This simple model shows that an important part of the interannual variability of precipitation is directly linked to large-scale circulation changes. Trends in observed precipitation and precipitation series reconstructed by regression are computed and compared. Spatially coherent trends in November–March precipitation during the second half of the twentieth century are observed, with an increase in the north and a decrease in the south. The spatial pattern of the trends in reconstructed precipitation is very similar to that observed, even if an underestimation of the positive trends in the north is seen, indicating that other mechanisms play a role. A detection study then leads to a better understanding of the respective roles of anthropogenic forcing (greenhouse gases and sulfate aerosol) and sea surface temperature in the evolution of the weather-type occurrence. Finally, it is shown that intratype dynamical variability has also played a role in precipitation changes in northern France, whereas no impact of temperature changes is seen.
Abstract
The relationship between large-scale atmospheric circulation and November–March precipitation over France during the twentieth century is investigated. A long daily MSLP dataset is used to derive daily weather types that are discriminant for precipitation. A linear regression model is then used to relate the November–March-accumulated precipitation amount and the occurrence frequency of the weather types. This simple model shows that an important part of the interannual variability of precipitation is directly linked to large-scale circulation changes. Trends in observed precipitation and precipitation series reconstructed by regression are computed and compared. Spatially coherent trends in November–March precipitation during the second half of the twentieth century are observed, with an increase in the north and a decrease in the south. The spatial pattern of the trends in reconstructed precipitation is very similar to that observed, even if an underestimation of the positive trends in the north is seen, indicating that other mechanisms play a role. A detection study then leads to a better understanding of the respective roles of anthropogenic forcing (greenhouse gases and sulfate aerosol) and sea surface temperature in the evolution of the weather-type occurrence. Finally, it is shown that intratype dynamical variability has also played a role in precipitation changes in northern France, whereas no impact of temperature changes is seen.
Abstract
Turbulent velocity spectra measured beneath wind waves show a large enhancement about the central wave frequency. A “5/3" frequency dependence can be seen both above and below the central peak, but with an apparent increase in spectral density at high Frequencies.
We show that these features can be understood via a generation of Taylor's hypothesis to the case in which frozen, isotropic, homogeneous turbulence is bodily convected past a fixed probe by a combination of drift and wave orbital motions. In a monochromatic wave field turbulent energy is aliased into harmonics of the wave frequency fp . We show qualitatively how drift currents or a random wave field broaden these lines into a continuous spectrum, and present the results of direct calculations which demonstrate clearly the transition from “line-like” to a smooth “5/3" spectrum. We calculate the leading asymptotic behavior in the limit of large and small frequencies for an arbitrary wave-height spectrum. For wave orbital velocities larger than the mean drift (in the direction of wave propagation) we findwhen U denotes an rms velocity. This result provides a possible explanation for the observed increase in spectral densities for frequencies above the peak.
Abstract
Turbulent velocity spectra measured beneath wind waves show a large enhancement about the central wave frequency. A “5/3" frequency dependence can be seen both above and below the central peak, but with an apparent increase in spectral density at high Frequencies.
We show that these features can be understood via a generation of Taylor's hypothesis to the case in which frozen, isotropic, homogeneous turbulence is bodily convected past a fixed probe by a combination of drift and wave orbital motions. In a monochromatic wave field turbulent energy is aliased into harmonics of the wave frequency fp . We show qualitatively how drift currents or a random wave field broaden these lines into a continuous spectrum, and present the results of direct calculations which demonstrate clearly the transition from “line-like” to a smooth “5/3" spectrum. We calculate the leading asymptotic behavior in the limit of large and small frequencies for an arbitrary wave-height spectrum. For wave orbital velocities larger than the mean drift (in the direction of wave propagation) we findwhen U denotes an rms velocity. This result provides a possible explanation for the observed increase in spectral densities for frequencies above the peak.
Abstract
The authors present the distribution of a coupled ocean–atmosphere global circulation model. The atmospheric (ARPEGE) and the oceanic (OPA) components run separately at different sites; the coupling is achieved through the exchanges of fluxes via the coupler (OASIS) and the three independent programs communicate together through the 2-Mbit RENATER network. The coupling and distributing procedure is based on the PVM software and is validated by 1-yr simulations. Performances and difficulties raised by the distributed environment are also presented.
An additional study deals with the sensitivity to the precision in exchanged data in coupled mode. This question is addressed by introducing small artificial perturbations into the forcings of each component. The transient growth of these perturbations is first followed over 10 days on specific grid points. A global spatio-temporal analysis is then performed over the planet for 1-yr simulations.
During the first 10 days of the experiments, the “error” dynamics is amplified by the atmosphere with a doubling time of the order of 5 days, while the upper ocean simply relaxes toward equilibrium. For long time ranges of simulation, errors tend to saturate and oscillate around a plateau, following the seasonal cycle. Spatio-temporal studies prove that the most sensitive areas to the precision in exchanged forcings are related to the regions where the variability is the most pronounced. These analyses are integrated into the general studies of predictability in coupled ocean–atmosphere models.
Abstract
The authors present the distribution of a coupled ocean–atmosphere global circulation model. The atmospheric (ARPEGE) and the oceanic (OPA) components run separately at different sites; the coupling is achieved through the exchanges of fluxes via the coupler (OASIS) and the three independent programs communicate together through the 2-Mbit RENATER network. The coupling and distributing procedure is based on the PVM software and is validated by 1-yr simulations. Performances and difficulties raised by the distributed environment are also presented.
An additional study deals with the sensitivity to the precision in exchanged data in coupled mode. This question is addressed by introducing small artificial perturbations into the forcings of each component. The transient growth of these perturbations is first followed over 10 days on specific grid points. A global spatio-temporal analysis is then performed over the planet for 1-yr simulations.
During the first 10 days of the experiments, the “error” dynamics is amplified by the atmosphere with a doubling time of the order of 5 days, while the upper ocean simply relaxes toward equilibrium. For long time ranges of simulation, errors tend to saturate and oscillate around a plateau, following the seasonal cycle. Spatio-temporal studies prove that the most sensitive areas to the precision in exchanged forcings are related to the regions where the variability is the most pronounced. These analyses are integrated into the general studies of predictability in coupled ocean–atmosphere models.
Abstract
We present the results of an analysis of field data collected by Donelan who used a miniature drag sphere to measure velocities beneath wind waves on Lake Ontario. Linear statistical techniques are used to separate the velocity into wave and turbulent parts. While we mostly aim at demonstrating the effects of surface wind waves on the statistical characteristics of the turbulent field in the upper mixed layer, we also interpret several features of the data on the hags of recent theoretical results.
One of the most intriguing features of the turbulent velocity spectra so obtained is a large peak near the dominant wave frequency. We review various possible explanation for this behavior although we prefer a model in which the turbulence is assumed frozen on the timescale of the Waves. This model requires no new dynamics and gives explicit formulae relating the dissipation rate to the magnitude of the spectral densities for high and low frequencies. On this basis we have determined a dissipation length from the data. The dependence of this quantity on depth is inconsistent with pure shear produced turbulence. Moreover the observed turbulent velocities shows a strong dependence on wave energy,. which cannot be explained solely within the framework of similarity theory for the inner (constant flux) layer.
Abstract
We present the results of an analysis of field data collected by Donelan who used a miniature drag sphere to measure velocities beneath wind waves on Lake Ontario. Linear statistical techniques are used to separate the velocity into wave and turbulent parts. While we mostly aim at demonstrating the effects of surface wind waves on the statistical characteristics of the turbulent field in the upper mixed layer, we also interpret several features of the data on the hags of recent theoretical results.
One of the most intriguing features of the turbulent velocity spectra so obtained is a large peak near the dominant wave frequency. We review various possible explanation for this behavior although we prefer a model in which the turbulence is assumed frozen on the timescale of the Waves. This model requires no new dynamics and gives explicit formulae relating the dissipation rate to the magnitude of the spectral densities for high and low frequencies. On this basis we have determined a dissipation length from the data. The dependence of this quantity on depth is inconsistent with pure shear produced turbulence. Moreover the observed turbulent velocities shows a strong dependence on wave energy,. which cannot be explained solely within the framework of similarity theory for the inner (constant flux) layer.
Abstract
A systematic modular approach to investigate the respective roles of the ocean and atmosphere in setting El Niño characteristics in coupled general circulation models is presented. Several state-of-the-art coupled models sharing either the same atmosphere or the same ocean are compared. Major results include 1) the dominant role of the atmosphere model in setting El Niño characteristics (periodicity and base amplitude) and errors (regularity) and 2) the considerable improvement of simulated El Niño power spectra—toward lower frequency—when the atmosphere resolution is significantly increased. Likely reasons for such behavior are briefly discussed. It is argued that this new modular strategy represents a generic approach to identifying the source of both coupled mechanisms and model error and will provide a methodology for guiding model improvement.
Abstract
A systematic modular approach to investigate the respective roles of the ocean and atmosphere in setting El Niño characteristics in coupled general circulation models is presented. Several state-of-the-art coupled models sharing either the same atmosphere or the same ocean are compared. Major results include 1) the dominant role of the atmosphere model in setting El Niño characteristics (periodicity and base amplitude) and errors (regularity) and 2) the considerable improvement of simulated El Niño power spectra—toward lower frequency—when the atmosphere resolution is significantly increased. Likely reasons for such behavior are briefly discussed. It is argued that this new modular strategy represents a generic approach to identifying the source of both coupled mechanisms and model error and will provide a methodology for guiding model improvement.
Abstract
Ensemble experiments are performed with five coupled atmosphere–ocean models to investigate the potential for initial-value climate forecasts on interannual to decadal time scales. Experiments are started from similar model-generated initial states, and common diagnostics of predictability are used. We find that variations in the ocean meridional overturning circulation (MOC) are potentially predictable on interannual to decadal time scales, a more consistent picture of the surface temperature impact of decadal variations in the MOC is now apparent, and variations of surface air temperatures in the North Atlantic Ocean are also potentially predictable on interannual to decadal time scales, albeit with potential skill levels that are less than those seen for MOC variations. This intercomparison represents a step forward in assessing the robustness of model estimates of potential skill and is a prerequisite for the development of any operational forecasting system.
Abstract
Ensemble experiments are performed with five coupled atmosphere–ocean models to investigate the potential for initial-value climate forecasts on interannual to decadal time scales. Experiments are started from similar model-generated initial states, and common diagnostics of predictability are used. We find that variations in the ocean meridional overturning circulation (MOC) are potentially predictable on interannual to decadal time scales, a more consistent picture of the surface temperature impact of decadal variations in the MOC is now apparent, and variations of surface air temperatures in the North Atlantic Ocean are also potentially predictable on interannual to decadal time scales, albeit with potential skill levels that are less than those seen for MOC variations. This intercomparison represents a step forward in assessing the robustness of model estimates of potential skill and is a prerequisite for the development of any operational forecasting system.
Abstract
The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean–atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global means coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model.
The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others by an ITCZ that migrates across the equator with the seasons; neither behavior is realistic. There is considerable spread in the simulated seasonal cycles of equatorial SST in the eastern Pacific. Some simulations do capture the annual harmonic quite realistically, although the seasonal cold tongue tends to appear prematurely. Others overestimate the amplitude of the semiannual harmonic. Nonetheless, the results constitute a marked improvement over the simulations of only a few years ago when serious climate drift was still widespread and simulated zonal gradients of SST along the equator were often very weak.
Abstract
The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean–atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global means coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model.
The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others by an ITCZ that migrates across the equator with the seasons; neither behavior is realistic. There is considerable spread in the simulated seasonal cycles of equatorial SST in the eastern Pacific. Some simulations do capture the annual harmonic quite realistically, although the seasonal cold tongue tends to appear prematurely. Others overestimate the amplitude of the semiannual harmonic. Nonetheless, the results constitute a marked improvement over the simulations of only a few years ago when serious climate drift was still widespread and simulated zonal gradients of SST along the equator were often very weak.
