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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 ā€œforced noise levelā€ 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.

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Claude Frankignoul

Abstract

A simple SST anomaly model is used to demonstrate that midlatitude statistical atmospheric models bias the airā€“sea fluxes toward positive airā€“sea feedback and distort the coupling between ocean and atmosphere.

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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 MĆ¼ller. The possible influence of critical layer absorption is briefly discussed.

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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.

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Claude Frankignoul
and
Antoine Molin

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.

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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 1ā€“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.

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Claude Frankignoul
and
Nathalie SennƩchael

Abstract

A lagged maximum covariance analysis (MCA) of monthly anomaly data from the NCEPā€“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 Pacificā€“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 Kāˆ’1 in the MCA and 20 m Kāˆ’1 in the regressions. In early winter, the maximum atmospheric perturbation at 250 mb ranges between 70 m Kāˆ’1 in the MCA and about 35 m Kāˆ’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.

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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.

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Claude Frankignoul
and
Elodie Kestenare

Abstract

The dominant airā€“sea feedbacks that are at play in the tropical Atlantic are revisited, using the 1958ā€“2002 NCEP reanalysis. To separate between different modes of variability and distinguish between cause and effect, a lagged rotated maximum covariance analysis (MCA) of monthly sea surface temperature (SST), wind, and surface heat flux anomalies is performed. The dominant mode is the ENSO-like zonal equatorial SST mode, which has its maximum amplitude in boreal summer and is a strongly coupled oceanā€“atmosphere mode sustained by a positive feedback between wind and SST. The turbulent heat flux feedback is negative, except west of 25Ā°W where it is positive, but countered by a negative radiative feedback associated with the meridional displacement of the ITCZ. As the maximum covariance patterns change little between lead and lag conditions, the in-phase covariability between SST and the atmosphere can be used to infer the atmospheric response to the SST anomaly. The second climate mode involves an SST anomaly in the tropical North Atlantic, which is primarily generated by the surface heat flux and, in boreal winter, wind changes off the coast of Africa. After it has been generated, the SST anomaly is sustained in the deep Tropics by the positive windā€“evaporationā€“SST feedback linked to the wind response to the SST. However, north of about 10Ā°N where the SST anomaly is largest, the wind response is weak and the heat flux feedback is negative, thus damping the SST anomaly. As the in-phase maximum covariance patterns primarily reflect the atmospheric forcing of the SST, simultaneous correlations cannot be used to describe the atmospheric response to the SST anomaly, except in the deep Tropics. Using instead the maximum covariance patterns when SST leads the atmosphere reconciles the results of recent atmospheric general circulation model experiments with the observations.

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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 NiƱo 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.

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