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- Author or Editor: Claude Frankignoul x
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Abstract
The isotropy of the internal wave field is investigated from short-time spectra of horizontal current measured in the deep sea. A significant anisotropy is detected at high frequencies during most periods of large mean current. The possible cause of this phenomenon is discussed. A theoretical investigation of Doppler distortion of the measurements shows that the Doppler effect cannot produce the observed anisotropy, which is more likely due to the interaction between internal waves and mean shear currents. Data collected during the MODE-0 experiment suggests this anisotropy over short periods of time is produced by the modulation of the internal wave field by the spatial gradients of low-frequency currents, in good agreement with the theory of Müller. The possible influence of critical layer absorption is briefly discussed.
Abstract
The isotropy of the internal wave field is investigated from short-time spectra of horizontal current measured in the deep sea. A significant anisotropy is detected at high frequencies during most periods of large mean current. The possible cause of this phenomenon is discussed. A theoretical investigation of Doppler distortion of the measurements shows that the Doppler effect cannot produce the observed anisotropy, which is more likely due to the interaction between internal waves and mean shear currents. Data collected during the MODE-0 experiment suggests this anisotropy over short periods of time is produced by the modulation of the internal wave field by the spatial gradients of low-frequency currents, in good agreement with the theory of Müller. The possible influence of critical layer absorption is briefly discussed.
Abstract
The stochastic character of short time scale atmospheric variables induce unpredictable fluctuations in the climatic system. These fluctuations are treated here as a “forced noise level” to be taken into account in climate modeling. A simple statistical model based on recent work by Hasselmann (1976) is used to estimate the resulting error in the calculated means of climate variables over finite intervals. This error may be large for short record lengths, as illustrated for the sea surface temperature. Some consequences relevant to joint ocean-atmosphere models and climate change experiments are discussed.
Abstract
The stochastic character of short time scale atmospheric variables induce unpredictable fluctuations in the climatic system. These fluctuations are treated here as a “forced noise level” to be taken into account in climate modeling. A simple statistical model based on recent work by Hasselmann (1976) is used to estimate the resulting error in the calculated means of climate variables over finite intervals. This error may be large for short record lengths, as illustrated for the sea surface temperature. Some consequences relevant to joint ocean-atmosphere models and climate change experiments are discussed.
Abstract
A simple SST anomaly model is used to demonstrate that midlatitude statistical atmospheric models bias the air–sea fluxes toward positive air–sea feedback and distort the coupling between ocean and atmosphere.
Abstract
A simple SST anomaly model is used to demonstrate that midlatitude statistical atmospheric models bias the air–sea fluxes toward positive air–sea feedback and distort the coupling between ocean and atmosphere.
Abstract
The climate variability in the North Atlantic sector is investigated in a 325-yr integration of the ECHAM1/ LSG coupled ocean–atmosphere general circulation model. At the interannual timescale, the coupled model behaves realistically and sea surface temperature (SST) anomalies arise as a response of the oceanic surface layer to the stochastic forcing by the atmosphere, with the heat exchanges both generating and damping the SST anomalies. In the ocean interior, the temperature spectra are red up to a period of about 20 years, and substantial decadal fluctuations are found in the upper kilometer or so of the water column. Using extended empirical orthogonal function analysis, two distinct quasi-oscillatory modes of ocean–atmosphere variability are identified, with dominant periods of about 20 and 10 years, respectively. The oceanic changes in both modes reflect the direct forcing by the atmosphere through anomalous air–sea fluxes and Ekman pumping, which after some delay affects the intensity of the subtropical and subpolar gyres. The SST is also strongly modulated by the gyre currents. In the thermocline, the temperature and salinity fluctuations are in phase, as if caused by thermocline displacements, and they have no apparent connection with the thermohaline circulation. The 20-yr mode is the most energetic one; it is easily seen in the thermocline and can be found in SST data, but it is not detected in the atmosphere alone. As there is no evidence of positive ocean–atmosphere feedback, the 20-yr mode primarily reflects the passive response of the ocean to atmospheric fluctuations, which may be in part associated with climate anomalies appearing a few years earlier in the North Pacific. The 10-yr mode is more surface trapped in the ocean. Although the mode is most easily seen in the temperature variations of the upper few hundred meters of the ocean, it is also detected in the atmosphere alone and thus appears to be a coupled ocean–atmosphere mode. In both modes, the surface heat flux acts neutrally on the associated SST anomalies once they have been generated, so that their persistence appears to be due in part to an overall adjustment of the air–sea heat exchanges to the SST patterns.
Abstract
The climate variability in the North Atlantic sector is investigated in a 325-yr integration of the ECHAM1/ LSG coupled ocean–atmosphere general circulation model. At the interannual timescale, the coupled model behaves realistically and sea surface temperature (SST) anomalies arise as a response of the oceanic surface layer to the stochastic forcing by the atmosphere, with the heat exchanges both generating and damping the SST anomalies. In the ocean interior, the temperature spectra are red up to a period of about 20 years, and substantial decadal fluctuations are found in the upper kilometer or so of the water column. Using extended empirical orthogonal function analysis, two distinct quasi-oscillatory modes of ocean–atmosphere variability are identified, with dominant periods of about 20 and 10 years, respectively. The oceanic changes in both modes reflect the direct forcing by the atmosphere through anomalous air–sea fluxes and Ekman pumping, which after some delay affects the intensity of the subtropical and subpolar gyres. The SST is also strongly modulated by the gyre currents. In the thermocline, the temperature and salinity fluctuations are in phase, as if caused by thermocline displacements, and they have no apparent connection with the thermohaline circulation. The 20-yr mode is the most energetic one; it is easily seen in the thermocline and can be found in SST data, but it is not detected in the atmosphere alone. As there is no evidence of positive ocean–atmosphere feedback, the 20-yr mode primarily reflects the passive response of the ocean to atmospheric fluctuations, which may be in part associated with climate anomalies appearing a few years earlier in the North Pacific. The 10-yr mode is more surface trapped in the ocean. Although the mode is most easily seen in the temperature variations of the upper few hundred meters of the ocean, it is also detected in the atmosphere alone and thus appears to be a coupled ocean–atmosphere mode. In both modes, the surface heat flux acts neutrally on the associated SST anomalies once they have been generated, so that their persistence appears to be due in part to an overall adjustment of the air–sea heat exchanges to the SST patterns.
Abstract
A lagged maximum covariance analysis (MCA) of monthly anomaly data from the NCEP–NCAR reanalysis shows significant relations between the large-scale atmospheric circulation in two seasons and prior North Pacific sea surface temperature (SST) anomalies, independent from the teleconnections associated with the ENSO phenomenon. Regression analysis based on the SST anomaly centers of action confirms these findings. In late summer, a hemispheric atmospheric signal that is primarily equivalent barotropic, except over the western subtropical Pacific, is significantly correlated with an SST anomaly mode up to at least 5 months earlier. Although the relation is most significant in the upper troposphere, significant temperature anomalies are found in the lower troposphere over North America, the North Atlantic, Europe, and Asia. The SST anomaly is largest in the Kuroshio Extension region and along the subtropical frontal zone, resembling the main mode of North Pacific SST anomaly variability in late winter and spring, and it is itself driven by the atmosphere. The predictability of the atmospheric signal, as estimated from cross-validated correlation, is highest when SST leads by 4 months because the SST anomaly pattern is more dominant in the spring than in the summer. In late fall and early winter, a signal resembling the Pacific–North American (PNA) pattern is found to be correlated with a quadripolar SST anomaly during summer, up to 4 months earlier, with comparable statistical significance throughout the troposphere. The SST anomaly changes shape and propagates eastward, and by early winter it resembles the SST anomaly that is generated by the PNA pattern. It is argued that this results via heat flux forcing and meridional Ekman advection from an active coupling between the SST and the PNA pattern that takes place throughout the fall. Correspondingly, the predictability of the PNA-like signal is highest when SST leads by 2 months. In late summer, the maximum atmospheric perturbation at 250 mb reaches 35 m K−1 in the MCA and 20 m K−1 in the regressions. In early winter, the maximum atmospheric perturbation at 250 mb ranges between 70 m K−1 in the MCA and about 35 m K−1 in the regressions. This suggests that North Pacific SST anomalies have a substantial impact on the Northern Hemisphere climate. The back interaction of the atmospheric response onto the ocean is also discussed.
Abstract
A lagged maximum covariance analysis (MCA) of monthly anomaly data from the NCEP–NCAR reanalysis shows significant relations between the large-scale atmospheric circulation in two seasons and prior North Pacific sea surface temperature (SST) anomalies, independent from the teleconnections associated with the ENSO phenomenon. Regression analysis based on the SST anomaly centers of action confirms these findings. In late summer, a hemispheric atmospheric signal that is primarily equivalent barotropic, except over the western subtropical Pacific, is significantly correlated with an SST anomaly mode up to at least 5 months earlier. Although the relation is most significant in the upper troposphere, significant temperature anomalies are found in the lower troposphere over North America, the North Atlantic, Europe, and Asia. The SST anomaly is largest in the Kuroshio Extension region and along the subtropical frontal zone, resembling the main mode of North Pacific SST anomaly variability in late winter and spring, and it is itself driven by the atmosphere. The predictability of the atmospheric signal, as estimated from cross-validated correlation, is highest when SST leads by 4 months because the SST anomaly pattern is more dominant in the spring than in the summer. In late fall and early winter, a signal resembling the Pacific–North American (PNA) pattern is found to be correlated with a quadripolar SST anomaly during summer, up to 4 months earlier, with comparable statistical significance throughout the troposphere. The SST anomaly changes shape and propagates eastward, and by early winter it resembles the SST anomaly that is generated by the PNA pattern. It is argued that this results via heat flux forcing and meridional Ekman advection from an active coupling between the SST and the PNA pattern that takes place throughout the fall. Correspondingly, the predictability of the PNA-like signal is highest when SST leads by 2 months. In late summer, the maximum atmospheric perturbation at 250 mb reaches 35 m K−1 in the MCA and 20 m K−1 in the regressions. In early winter, the maximum atmospheric perturbation at 250 mb ranges between 70 m K−1 in the MCA and about 35 m K−1 in the regressions. This suggests that North Pacific SST anomalies have a substantial impact on the Northern Hemisphere climate. The back interaction of the atmospheric response onto the ocean is also discussed.
Abstract
The large-scale patterns of covariability between monthly sea surface temperature (SST) and 500-mb height anomalies (Z 500) in the Atlantic sector are investigated as a function of time lag in the NCEP–NCAR reanalysis (1958–97). In agreement with previous studies, the dominant signal is the atmospheric forcing of SST anomalies, but statistically significant covariances are also found when SST leads Z 500 by several months. In winter, a Pan-Atlantic SST pattern precedes the North Atlantic oscillation (NAO) by up to 6 months. Such long lead time covariance is interpreted in the framework of the stochastic climate model, reflecting the forcing of the NAO by persistent Atlantic SST anomalies.
A separate analysis of midlatitudes (20°–70°N) and tropical (20°S–20°N) SST anomalies reveals that the bulk of the NAO signal comes from the midlatitudes. A dipolar anomaly, with warm SST southeast of Newfoundland and cold SST to the northeast and southeast, precedes a positive phase of the NAO, and it should provide a prediction of up to 15% of its monthly variance several months in advance. Since the “forcing” SST pattern projects significantly onto the tripole pattern generated by the NAO, these results indicate a positive feedback between the SST tripole and the NAO, with a strength of up to ≃25 m K−1 at 500 mb or 2–3 mb K−1 at sea level. Additionally, a warming of the tropical Atlantic (20°S–20°N), roughly symmetric about the equator, induces a negative NAO phase in early winter. This tropical forcing of the NAO is nearly uncorrelated with and weaker than that resulting from the midlatitudes, and is associated with shorter lead times and reduced predictive skill.
Abstract
The large-scale patterns of covariability between monthly sea surface temperature (SST) and 500-mb height anomalies (Z 500) in the Atlantic sector are investigated as a function of time lag in the NCEP–NCAR reanalysis (1958–97). In agreement with previous studies, the dominant signal is the atmospheric forcing of SST anomalies, but statistically significant covariances are also found when SST leads Z 500 by several months. In winter, a Pan-Atlantic SST pattern precedes the North Atlantic oscillation (NAO) by up to 6 months. Such long lead time covariance is interpreted in the framework of the stochastic climate model, reflecting the forcing of the NAO by persistent Atlantic SST anomalies.
A separate analysis of midlatitudes (20°–70°N) and tropical (20°S–20°N) SST anomalies reveals that the bulk of the NAO signal comes from the midlatitudes. A dipolar anomaly, with warm SST southeast of Newfoundland and cold SST to the northeast and southeast, precedes a positive phase of the NAO, and it should provide a prediction of up to 15% of its monthly variance several months in advance. Since the “forcing” SST pattern projects significantly onto the tripole pattern generated by the NAO, these results indicate a positive feedback between the SST tripole and the NAO, with a strength of up to ≃25 m K−1 at 500 mb or 2–3 mb K−1 at sea level. Additionally, a warming of the tropical Atlantic (20°S–20°N), roughly symmetric about the equator, induces a negative NAO phase in early winter. This tropical forcing of the NAO is nearly uncorrelated with and weaker than that resulting from the midlatitudes, and is associated with shorter lead times and reduced predictive skill.
Abstract
To study the transient atmospheric response to midlatitude SST anomalies, a three-layer quasigeostrophic (QG) model coupled to a slab oceanic mixed layer in the North Atlantic is used. As diagnosed from a coupled run in perpetual winter conditions, the first two modes of SST variability are linked to the model North Atlantic Oscillation (NAO) and eastern Atlantic pattern (EAP), respectively, the dominant atmospheric modes in the Atlantic sector. The two SST anomaly patterns are then prescribed as fixed anomalous boundary conditions for the model atmosphere, and its transient responses are established from a large ensemble of simulations.
In both cases, the tendency of the air–sea heat fluxes to damp the SST anomalies results in an anomalous diabatic heating of the atmosphere that, in turn, forces a baroclinic response, as predicted by linear theory. This initial response rapidly modifies the transient eddy activity and thus the convergence of eddy momentum and heat fluxes. The latter transforms the baroclinic response into a growing barotropic one that resembles the atmospheric mode that had created the SST anomaly in the coupled run and is thus associated with a positive feedback. The total adjustment time is as long as 3–4 months for the NAO-like response and 1–2 months for the EAP-like one. The positive feedback, in both cases, is dependent on the polarity of the SST anomaly, but is stronger in the NAO case, thereby contributing to its predominance at low frequency in the coupled system. However, the feedback is too weak to lead to an instability of the atmospheric modes and primarily results in an increase of their amplitude and persistence and a weakening of the heat flux damping of the SST anomaly.
Abstract
To study the transient atmospheric response to midlatitude SST anomalies, a three-layer quasigeostrophic (QG) model coupled to a slab oceanic mixed layer in the North Atlantic is used. As diagnosed from a coupled run in perpetual winter conditions, the first two modes of SST variability are linked to the model North Atlantic Oscillation (NAO) and eastern Atlantic pattern (EAP), respectively, the dominant atmospheric modes in the Atlantic sector. The two SST anomaly patterns are then prescribed as fixed anomalous boundary conditions for the model atmosphere, and its transient responses are established from a large ensemble of simulations.
In both cases, the tendency of the air–sea heat fluxes to damp the SST anomalies results in an anomalous diabatic heating of the atmosphere that, in turn, forces a baroclinic response, as predicted by linear theory. This initial response rapidly modifies the transient eddy activity and thus the convergence of eddy momentum and heat fluxes. The latter transforms the baroclinic response into a growing barotropic one that resembles the atmospheric mode that had created the SST anomaly in the coupled run and is thus associated with a positive feedback. The total adjustment time is as long as 3–4 months for the NAO-like response and 1–2 months for the EAP-like one. The positive feedback, in both cases, is dependent on the polarity of the SST anomaly, but is stronger in the NAO case, thereby contributing to its predominance at low frequency in the coupled system. However, the feedback is too weak to lead to an instability of the atmospheric modes and primarily results in an increase of their amplitude and persistence and a weakening of the heat flux damping of the SST anomaly.
Abstract
The GISS general circulation model (GCM) is used to investigate the influence of a positive sea surface temperature (SST) anomaly in the subtropical North Pacific on the Northern Hemisphere wintertime circulation. As the set of model data is small, the signal-to-noise ratio is low, and statistical significance is difficult to establish. Only local effects are detected with the univariate t-test, but a multivariate analysis based on hypothesis testing shows that the SST anomaly induces a small signal in the middle and upper troposphere. The same technique is applied to investigate whether the GCM response can be predicted in part from a specification of the SST anomaly, using a simple linear quasi-geostrophic equivalent barotropic model. The model is forced by a vorticity sink proportional to the SST anomaly, as the latter induces condensation heating and upper level divergence. When the barotropic model is linearized about the zonally symmetric part of the mean GCM state in the control runs, no satisfactory prediction of the GCM response is obtained. However, when the zonal variations of the GCM basic control state are taken into account, the linear model prediction is statistically significant, even if only a small fraction of the GCM signal is accounted for. The agreement with the GCM data is best when the equivalent barotropic level is in the upper troposphere, and it depends little on the amount of dissipation. On the other hand, it is very sensitive to the details of the mean flow waviness.
Abstract
The GISS general circulation model (GCM) is used to investigate the influence of a positive sea surface temperature (SST) anomaly in the subtropical North Pacific on the Northern Hemisphere wintertime circulation. As the set of model data is small, the signal-to-noise ratio is low, and statistical significance is difficult to establish. Only local effects are detected with the univariate t-test, but a multivariate analysis based on hypothesis testing shows that the SST anomaly induces a small signal in the middle and upper troposphere. The same technique is applied to investigate whether the GCM response can be predicted in part from a specification of the SST anomaly, using a simple linear quasi-geostrophic equivalent barotropic model. The model is forced by a vorticity sink proportional to the SST anomaly, as the latter induces condensation heating and upper level divergence. When the barotropic model is linearized about the zonally symmetric part of the mean GCM state in the control runs, no satisfactory prediction of the GCM response is obtained. However, when the zonal variations of the GCM basic control state are taken into account, the linear model prediction is statistically significant, even if only a small fraction of the GCM signal is accounted for. The agreement with the GCM data is best when the equivalent barotropic level is in the upper troposphere, and it depends little on the amount of dissipation. On the other hand, it is very sensitive to the details of the mean flow waviness.
Abstract
The influence of a North Pacific sea surface temperature (SST) anomaly on the wintertime atmospheric circulation is investigated with the GISS general circulation model (GCM) 2. Although no signal could be detected by the standard univariate t-test, a multivariate statistical analysis based on the assumption that the atmospheric response is primarily at large scales shows that the SST anomaly has an influence on the model Northern Hemisphere climate. The signal, primarily barotropic, is strongest at zonal wavenumbers 3 to 5. It is above the noise level in the middle and upper troposphere, but not near the ground. For realistic SST magnitudes, the change in geopotential height could reach several tens of meters, suggesting that midlatitude SST anomalies may have a weak climatic impact. However, the signal is model-dependent since it differs from the response of the (less realistic) GISS model 1 to the same SST anomaly. The signal is also inconsistent with 500 mb height anomalies observed during two periods with similar SST anomalies in the North Pacific.
A two-layer quasi-geostrophic linear model with a zonally symmetric basic state is then used to investigate whether the GCM response can he interpreted in terms of forced stationary waves. When the mean zonal flow and the anomaly heating field are prescribed from the GCM data, it is found that the linear model prediction is consistent with the GCM signal, although only a small fraction of the anomaly variance can be explained. A simple linear wave model is thus useful to analyze the GCM experiments, but it cannot be used for predictive purposes, unless the relation between SST and diabatic heating anomaly can be better established in the mid-latitudes.
Abstract
The influence of a North Pacific sea surface temperature (SST) anomaly on the wintertime atmospheric circulation is investigated with the GISS general circulation model (GCM) 2. Although no signal could be detected by the standard univariate t-test, a multivariate statistical analysis based on the assumption that the atmospheric response is primarily at large scales shows that the SST anomaly has an influence on the model Northern Hemisphere climate. The signal, primarily barotropic, is strongest at zonal wavenumbers 3 to 5. It is above the noise level in the middle and upper troposphere, but not near the ground. For realistic SST magnitudes, the change in geopotential height could reach several tens of meters, suggesting that midlatitude SST anomalies may have a weak climatic impact. However, the signal is model-dependent since it differs from the response of the (less realistic) GISS model 1 to the same SST anomaly. The signal is also inconsistent with 500 mb height anomalies observed during two periods with similar SST anomalies in the North Pacific.
A two-layer quasi-geostrophic linear model with a zonally symmetric basic state is then used to investigate whether the GCM response can he interpreted in terms of forced stationary waves. When the mean zonal flow and the anomaly heating field are prescribed from the GCM data, it is found that the linear model prediction is consistent with the GCM signal, although only a small fraction of the anomaly variance can be explained. A simple linear wave model is thus useful to analyze the GCM experiments, but it cannot be used for predictive purposes, unless the relation between SST and diabatic heating anomaly can be better established in the mid-latitudes.
Abstract
The transient atmospheric response to interactive SST anomalies in the midlatitudes is investigated using a three-layer QG model coupled in perpetual winter conditions to a slab oceanic mixed layer in the North Atlantic. The SST anomalies are diagnosed from a coupled run and prescribed as initial conditions, but are free to evolve. The initial evolution of the atmospheric response is similar to that obtained with a prescribed SST anomaly, starting as a quasi-linear baroclinic and then quickly evolving into a growing equivalent barotropic one. Because of the heat flux damping, the SST anomaly amplitude slowly decreases, albeit with little change in pattern. Correspondingly, the atmospheric response only increases until it reaches a maximum amplitude after about 1–3.5 months, depending on the SST anomaly considered. The response is similar to that at equilibrium in the fixed SST case, but it is 1.5–2 times smaller, and then slowly decays away.
Abstract
The transient atmospheric response to interactive SST anomalies in the midlatitudes is investigated using a three-layer QG model coupled in perpetual winter conditions to a slab oceanic mixed layer in the North Atlantic. The SST anomalies are diagnosed from a coupled run and prescribed as initial conditions, but are free to evolve. The initial evolution of the atmospheric response is similar to that obtained with a prescribed SST anomaly, starting as a quasi-linear baroclinic and then quickly evolving into a growing equivalent barotropic one. Because of the heat flux damping, the SST anomaly amplitude slowly decreases, albeit with little change in pattern. Correspondingly, the atmospheric response only increases until it reaches a maximum amplitude after about 1–3.5 months, depending on the SST anomaly considered. The response is similar to that at equilibrium in the fixed SST case, but it is 1.5–2 times smaller, and then slowly decays away.
