Search Results

You are looking at 1 - 10 of 13 items for

  • Author or Editor: Francis Codron x
  • Refine by Access: All Content x
Clear All Modify Search
Francis Codron

Abstract

In a zonally symmetric climatology with a single eddy-driven jet, such as prevails in the Southern Hemisphere summer, the midlatitude variability is dominated by fluctuations of the jet around its mean position, as described by the Southern Hemisphere annular mode (SAM). To study whether this result holds for a zonally asymmetric climatology, the observed variability of the Southern Hemisphere winter is analyzed. The mean state in this case is characterized by relatively weak stationary waves; yet there exist significant zonal variations in the mean strength and meridional structure of the subtropical jet stream.

As in summer, the winter SAM signature is annular in shape and the corresponding wind anomalies are dipolar; but it is associated with two different behaviors of the eddy-driven jet in different longitudinal ranges. Over the Indian Ocean, the SAM is associated primarily with a latitudinal shift of the jet around its mean position. Over the Pacific sector, it is instead characterized by a seesaw in the wind speed between two distinct latitudes, corresponding to the positions of the midlatitude and subtropical jets. Composites of eddy forcing and baroclinicity over both sectors appear consistent with the two different behaviors. As in the zonal-mean case, high-frequency eddies both force and maintain the low-frequency wind anomalies associated with the SAM. The positive feedback by eddies is, however, not local: changes in the eddy forcing are influenced most strongly by zonal wind anomalies located upstream.

Full access
Francis Codron

Abstract

The annular modes emerge as the leading mode of extratropical month-to-month climate variability in both hemispheres. Here the influence of the background state on the structure and dynamics of the Southern Hemisphere annular mode (SAM) during the austral summer when the climatology is characterized by a single, well-defined, eddy-driven jet is studied. Subsets of the climatology are constructed for early and late summer, and for contrasting polarities of the ENSO cycle. The analysis is based both on observations and on perpetual-state GCM experiments. The main differences between the subsets involve variations of the latitude of the mean jet.

It is found that in all the cases, the SAM is characterized by latitudinal shifts of the jet about its mean position, reinforced by a positive momentum flux feedback from baroclinic waves. This result is consistent with previous studies of the dynamics of the zonally averaged circulation but is found here to hold over all longitudes and for different positions of the mean jet. The low frequency eddies exert a weaker negative feedback upon the mean flow, with a less zonally symmetric structure.

The strong differences in the amplitude of the SAM among the various climatologies seem to be determined by a combination of 1) the variance of the “random” forcing by transient eddies and 2) the strength of the positive feedback component of this forcing. The latter mechanism increases the variance at low frequencies only and lengthens the decorrelation time of zonal-mean zonal wind anomalies. It tends to become stronger when the mean jet moves equatorward.

Full access
Virginie Guemas and Francis Codron

Abstract

This article examines the sensitivity of the Laboratoire de Météorologie Dynamique Model with Zoom Capability (LMDZ), a gridpoint atmospheric GCM, to changes in the resolution in latitude and longitude, focusing on the midlatitudes. In a series of dynamical core experiments, increasing the resolution in latitude leads to a poleward shift of the jet, which also becomes less baroclinic, while the maximum eddy variance decreases. The distribution of the jet positions in time also becomes wider. On the contrary, when the resolution increases in longitude, the position and structure of the jet remain almost identical, except for a small equatorward shift tendency. An increase in eddy heat flux is compensated by a strengthening of the Ferrel cell. The source of these distinct behaviors is then explored in constrained experiments in which the zonal-mean zonal wind is constrained toward the same reference state while the resolution varies. While the low-level wave sources always increase with resolution in that case, there is also enhanced poleward propagation when increasing the resolution in longitude, preventing the jet shift. The diverse impacts on the midlatitude dynamics hold when using the full GCM in a realistic setting, either forced by observed SSTs or coupled to an ocean model.

Full access
Ara Arakelian and Francis Codron

Abstract

Fluctuations of the Southern Hemisphere eddy-driven jet are studied in a suite of experiments with the Laboratoire de Météorologie Dynamique, version 4 (LMDZ4) atmospheric GCM with varying horizontal resolution, in coupled mode and with imposed SSTs. The focus is on the relationship between changes in the mean state brought by increasing resolution, and the intraseasonal variability and response to increasing CO2 concentration.

In summer, the mean jet latitude moves poleward when the resolution increases in latitude, converging toward the observed one. Most measures of the jet dynamics, such as skewness of the distribution or persistence time scale of jet movements, exhibit a simple dependence on the mean jet latitude and also converge to the observed values. In winter, the improvement of the mean-state biases with resolution is more limited.

In both seasons, the relationship between the dominant mode of variability—the southern annular mode (SAM)—and the mean state remains the same as in observations, except in the most biased winter simulation. The jet fluctuations—latitude shifts or splitting—just occur around a different mean position. Both the model biases and the response to increasing CO2 project strongly onto the SAM structure. No systematic relation between the amplitude of the response and characteristics of the control simulation was found, possibly due to changing dynamics or impacts of the physical parameterizations with different resolutions.

Full access
Francis Codron, Augustin Vintzileos, and Robert Sadourny

Abstract

To take into account the strong nonlinearities of vertical fluxes due to small-scale heterogeneities of surface properties, more and more coupled general circulation models compute part of their atmospheric physical parameterizations, either the surface fluxes or the whole package, on the finer grid of their ocean or land model. A modification of a traditional interpolation scheme is presented to calculate the values of atmospheric variables over surface model grid points. In addition to the desirable properties of flux conservation and preservation of a constant field, the new scheme allows discontinuities in the interpolated fields at the surface model’s boundaries and orographic jumps, while remaining continuous elsewhere. It can also be tuned separately for each variable.

The modified scheme is then evaluated using the circulation model of the Laboratoire de Météorologie Dynamique coupled to the Laboratoire d’Océanographie Dynamique et de Climatologie tropical Pacific Ocean model using the delocalized physics method. The results show a large improvement of heat and humidity fluxes near the focus region of the South American coast in the southeastern equatorial Pacific, and a subsequent westward propagation of significant cold SST anomalies.

Full access
Francis Codron, Augustin Vintzileos, and Robert Sadourny

Abstract

This study examines the response of the climate simulated by the Institut Pierre Simon Laplace tropical Pacific coupled general circulation model to two changes in parameterization: an improved coupling scheme at the coast, and the introduction of a saturation mixing ratio limiter in the water vapor advection scheme, which improves the rainfall distribution over and around orography. The main effect of these modifications is the suppression of spurious upwelling off the South American coast in Northern Hemisphere summer. Coupled feedbacks then extend this warming over the whole basin in an El Niño–like structure, with a maximum at the equator and in the eastern part of the basin. The mean precipitation pattern widens and moves equatorward as the trade winds weaken.

This warmer mean state leads to a doubling of the standard deviation of interannual SST anomalies, and to a longer ENSO period. The structure of the ENSO cycle also shifts from westward propagation in the original simulation to a standing oscillation. The simulation of El Niño thus improves when compared to recent observed events. The study of ENSO spatial structure and lagged correlations shows that these changes of El Niño characteristics are caused by both the increase of amplitude and the modification of the spatial structure of the wind stress response to SST anomalies.

These results show that both the mean state and variability of the tropical ocean can be very sensitive to biases or forcings, even geographically localized. They may also give some insight into the mechanisms responsible for the changes in ENSO characteristics due to decadal variability or climate change.

Full access
Loïc Robert, Gwendal Rivière, and Francis Codron

Abstract

The variability of midlatitude jets is investigated in a long-term integration of a dry three-level quasigeostrophic model on the sphere. As for most observed jets, the leading EOF of the zonal-mean wind corresponds to latitudinal shifts of the jet, and the second EOF to pulses of the jet speed. The first principal component (PC1) is also more persistent than the second one (PC2); this longer persistence arises from different eddy feedbacks both in the short term (i.e., within a few days following the peak of the PCs) and in the long term. The short-term eddy feedbacks come from two distinct mechanisms. First, a planetary waveguide effect acts as a negative feedback on both PCs. The positive phases of PC1 and PC2, which correspond to poleward-shifted and accelerated jets, respectively, are first driven then canceled by planetary waves reflecting on the equatorward flank of the jet. A similar process occurs for the negative phases when planetary waves reflect on the poleward flank of the jet. Second, synoptic waves also exert a short-term negative feedback on PC2: when the jet accelerates, the enhanced meridional wind shear increases the barotropic sink of eddy energy and depletes it very rapidly, therefore preventing synoptic eddies from maintaining the accelerated jet. Finally, at lags longer than their typical time scale, synoptic eddies drive a positive feedback on PC1 only. This feedback can be explained by a baroclinic mechanism in which the jet shift modifies the baroclinicity, causing, first, eddy heat flux anomalies and, then, momentum convergence anomalies. This feedback is absent for PC2, despite some changes in the baroclinicity.

Full access
Loïc Robert, Gwendal Rivière, and Francis Codron

Abstract

The sensitivity of the variability of an eddy-driven jet to the upper- and lower-level baroclinicity of the mean state is analyzed using a three-level quasigeostrophic model on the sphere. The model is forced by a relaxation in temperature to a steady, zonally symmetric profile with varying latitude and intensity of the maximum baroclinicity. The leading EOF of the zonally and vertically averaged zonal wind is characterized by a meridional shift of the eddy-driven jet. While changes in the upper-level baroclinicity have no significant impact on the persistence of this leading EOF, an increase in lower-level baroclinicity leads to a reduced persistence. For small lower-level baroclinicity, the leading EOF follows a classical zonal index regime, for which the meridional excursions of the zonal wind anomalies are maintained by a strong positive eddy feedback. For strong lower-level baroclinicity, the jet enters a poleward-propagation regime, for which the eddy forcing continuously acts to push the jet poleward and prevents its maintenance at a fixed latitude. The enhanced poleward propagation when the lower-level baroclinicity increases is interpreted as resulting from the broader and weaker potential vorticity gradient that enables the waves to propagate equatorward and facilitates the poleward migration of the critical latitude. Finally, the decrease in the persistence of the leading EOF as the lower-level baroclinicity increases is shown not to result from the impact of changes in the mean climatological jet latitude.

Full access
Blandine L’Hévéder, Francis Codron, and Michael Ghil

Abstract

This paper explores the impact of anomalous northward oceanic heat transport on global climate in a slab ocean setting. To that end, the GCM LMDZ5A of the Laboratoire de Météorologie Dynamique is coupled to a slab ocean, with realistic zonal asymmetries and seasonal cycle. Two simulations with different anomalous surface heating are imposed: 1) uniform heating over the North Atlantic basin and 2) concentrated heating in the Gulf Stream region, with a compensating uniform cooling in the Southern Ocean in both cases. The magnitudes of the heating and of the implied northward interhemispheric heat transport are within the range of current natural variability. Both simulations show global effects that are particularly strong in the tropics, with a northward shift of the intertropical convergence zone (ITCZ) toward the heating anomalies. This shift is accompanied by a northward shift of the storm tracks in both hemispheres. From the comparison between the two simulations with different anomalous surface heating in the North Atlantic, it emerges that the global climate response is nearly insensitive to the spatial distribution of the heating. The cloud response acts as a large positive feedback on the oceanic forcing, mainly because of the low-cloud-induced shortwave anomalies in the extratropics. While previous literature has speculated that the extratropical Q flux may impact the tropics by the way of the transient eddy fluxes, it is explicitly demonstrated here. In the midlatitudes, the authors find a systematic northward shift of the jets, as well as of the associated Ferrel cells, storm tracks, and precipitation bands.

Full access
Gwendal Rivière, Loïc Robert, and Francis Codron

Abstract

A three-level quasigeostrophic model on the sphere is used to identify the physical nature of the negative planetary wave feedback on midlatitude jet variability. A first approach consists of studying the nonlinear evolution of normal-mode disturbances in a baroclinic westerly zonal jet. For a low-zonal-wavenumber disturbance, successive acceleration and deceleration of the jet occur as a result of reflection of the wave on either side of the jet. The planetary wave deposits momentum in opposite ways during its poleward or equatorward propagation. In contrast, a high-zonal-wavenumber disturbance is not reflected but absorbed within the subtropical critical layer. It thus only induces poleward momentum fluxes, which accelerate the jet and shift it slightly poleward. A long-term simulation forced by a relaxation toward a zonally symmetric temperature profile is then analyzed. Planetary waves are shown to be baroclinically excited. When they propagate equatorward, they induce an acceleration of the jet together with a slight poleward shift. About two-thirds of the planetary waves are absorbed by the subtropical critical layer, which allows the accelerated poleward-shifted jet to persist for a while. For the remaining third, the potential vorticity equatorward of the jet is so well homogenized that a reflection occurs. It is followed by an abrupt jet deceleration during the subsequent poleward propagation. The reflection of planetary waves on the poleward side of the jet is more systematic because of the quasi-permanent presence of a turning latitude there. This negative planetary wave feedback is shown to act more on pulses of the jet than on its latitudinal shifts.

Full access