Search Results

You are looking at 1 - 10 of 49 items for

  • Author or Editor: Claude Frankignoul x
  • All content x
Clear All Modify Search
Claude Frankignoul

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.

Full access
Claude Frankignoul

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.

Full access
Claude Frankignoul

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.

Full access
Juliette Mignot and Claude Frankignoul

Abstract

The link between the interannual to interdecadal variability of the Atlantic meridional overturning circulation (AMOC) and the atmospheric forcing is investigated using 200 yr of a control simulation of the Bergen Climate Model, where the mean circulation cell is rather realistic, as is also the location of deep convection in the northern North Atlantic. The AMOC variability has a slightly red frequency spectrum and is primarily forced by the atmosphere. The maximum value of the AMOC is mostly sensitive to the deep convection in the Irminger Sea, which it lags by about 5 yr. The latter is mostly forced by a succession of atmospheric patterns that induce anomalous northerly winds over the area. The impact of the North Atlantic Oscillation on deep convection in the Labrador and Greenland Seas is represented realistically, but its influence on the AMOC is limited to the interannual time scale and is primarily associated with wind forcing. The tropical Pacific shows a strong variability in the model, with too strong an influence on the North Atlantic. However, its influence on the tropical Atlantic is realistic. Based on lagged correlations and the release of fictitious Lagrangian drifters, the tropical Pacific seems to influence the AMOC with a time lag of about 40 yr. The mechanism is as follows: El Niño events induce positive sea surface salinity anomalies in the tropical Atlantic that are advected northward, circulate in the subtropical gyre, and then subduct. In the ocean interior, part of the salinity anomaly is advected along the North Atlantic current, eventually reaching the Irminger and Labrador Seas after about 35 yr where they destabilize the water column and favor deep convection.

Full access
Julie Deshayes and Claude Frankignoul

Abstract

The variability of the circulation in the North Atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003. The interannual-to-decadal variability of the subpolar gyre circulation and the Meridional Overturning Circulation (MOC) is mostly influenced by the North Atlantic Oscillation (NAO). Both circulations intensified from the early 1970s to the mid-1990s and then decreased. The monthly variability of both circulations reflects the fast barotropic adjustment to NAO-related Ekman pumping anomalies, while the interannual-to-decadal variability is due to the baroclinic adjustment to Ekman pumping, buoyancy forcing, and dense water formation, consistent with previous studies.

An original characteristic of the oceanic response to NAO is presented that relates to the spatial patterns of buoyancy and wind forcing over the North Atlantic. Anomalous Ekman pumping associated with a positive NAO phase first induces a decrease of the southern subpolar gyre strength and an intensification of the northern subpolar gyre. The latter is reinforced by buoyancy loss and dense water formation in the Irminger Sea, where the cyclonic circulation increases 1–2 yr after the positive NAO phase. Increased buoyancy loss also occurs in the Labrador Sea, but because of the early decrease of the southern subpolar gyre strength, the intensification of the cyclonic circulation is delayed. Hence the subpolar gyre and the MOC start increasing in the Irminger Sea, while in the Labrador Sea the circulation at depth leads its surface counterpart. In this simulation where the transport of dense water through the North Atlantic sills is underestimated, the MOC variability is well represented by a simple integrator of convection in the Irminger Sea, which fits better than a direct integration of NAO forcing.

Full access
Claude Frankignoul and Nathalie Sennéchael

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.

Full access
Peter Müller and Claude Frankignoul

Abstract

To assess the role of direct stochastic wind forcing in generating oceanic geostrophic eddies we calculate analytically the response of a simple ocean model to a realistic model wind-stress spectrum and compare the results with observations. The model is a continuously stratified, β-plane ocean of infinite horizontal extent and constant depth. All transfer and dissipation processes are parameterized by a linear scale-independent friction law (Rayleigh damping). The model predictions that are least sensitive to this parameterization, the total eddy energy and the subsurface displacement, are in good agreement with observations in mid-ocean regions far removed from strong currents. Properties that depend crucially on the parameterization of nonlinearities and topographic effects are not well reproduced. Observed coherences and seasonal modulations provide direct evidence of wind forcing at high frequencies where motions have little energy. Direct evidence at the more energetic low frequencies will be difficult to detect because the expected coherences are small. Altogether, the present results suggest that direct wind forcing may well be the dominant forcing mechanism for central ocean eddies.

Full access
Claude Frankignoul and Elodie Kestenare

Abstract

The Pan-Atlantic sea surface temperature (SST) anomaly pattern that was found in a previous study to have a significant impact on the North Atlantic Oscillation (NAO) in early winter seemed to reflect the nearly uncorrelated influence of a horseshoe SST anomaly in the North Atlantic and an SST anomaly in the eastern equatorial Atlantic. A lagged rotated maximum covariance analysis of a slightly longer dataset shows that the horseshoe SST anomaly influence is robust, but it deemphasizes the center of action southeast of Newfoundland, Canada. On the other hand, it suggests that the link between equatorial SST and the NAO was artificial and due both to ENSO teleconnections and the orthogonality constraint in the maximum covariance analysis.

Full access
Claude Frankignoul and Antoine Molin

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.

Full access
Arnaud Czaja and Claude Frankignoul

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.

Full access