Meridional circulation

Meridional circulation in two-dimensional stratospheric models has been specified based on observations or general circulation model calculations recendy efforts have been undertaken to calculate circulations from first principles, within the stratospheric models themselves. An important limitation of using models in which circulations are specified is that these caimot be used to study the feedbacks of changing atmospheric composition and temperature on transport, factors which may be important as atmospheric composition is increasingly perturbed. 

We have now to go one step further and to build stellar evolution models where the transport of angular momentum will be followed self-consistently under the action of meridional circulation, shear turbulence, and internal gravity waves. In this path some important aspects still need to be clarified Can we better describe the excitation mechanisms and evaluate in a more reliable way the quantitative properties of the wave spectra What is the direct contribution of 1GW to the transport of chemicals, especially in the dynamical shear layer produced just below the convective envelope by the wave-mean flow interaction What is the influence of the Coriolis force on IGW How do 1GW interact with a magnetic field Work is in progress in this direction. 

Great care has to be given to the physics of rotation and to the treatment of its interaction with mass loss. For differentially rotating stars, the structure equations need to be written differently [9] than for solid body rotation. For the transport of the chemical elements and angular momentum, we consider the effects of shear mixing, meridional circulation, horizontal turbulence and in the advanced stages the dynamical shear is also included. Caution has to be given that advection and diffusion are not the same physical effect. 

MAIN SEQUENCE ABUNDANCES, MASS LOSS AND MERIDIONAL CIRCULATION... 

ABSTRACT Constraints that abundance anomalies observed on main sequence stars put on turbulence, meridional circulation and mass loss are reviewed. The emphasis is on recent observations of Li abundances. 

Fig. 2 Meridional circulation stream lines (full lines) as a function of the angle from the polar axis. The position of convection zones is indicated by dotted lines. That identified by A is for a 12000 R main sequence star while that indicated by B is for a 6400 K star.

For the HgMn stars, Wolff and Preston (1978) obtain an upper limit of 100 km s l for the V sin i at which they are observed. The meridional circulation is slow enough to allow the disappearance of the He convection zone by He settling for rotational velocities up to 75 km s 1 (Charbonneau and Michaud 1987a). Once the He convection zone has... 

Fig. 3 On part a) is shown the Li abundance as a function of time in a T ft = 7000 K star. The curves are identified by the equatorial rotational velocity in km s 1. The effect of meridional circulation on the Li abundance starts to be felt for rotational velocities of 50 km s 1. This can be understood from part b) of the figure where the total vertical transport velocity is shown as a function of the angle from the rotation axis. For rotational velocities of up to some 40 km s 1, the transport velocity is everywhere positive.

One can similarly use the meridional circulation fields to test its effect on the diffusion of Li in the F stars of clusters (Charbonneau and Michaud (1987). It turns out however that the upper limit of the equatorial rotation velocity is much smaller. This can be traced to the increase in the depth of the convection zone. The diffusion velocity decreases considerably due to the p 1 dependence of the diffusion coefficient while the meridional circulation velocity is nearly constant as one goes deeper in the star. While the critical velocity in the middle of the gap is about 15 km s 1, there are stars in the middle of the gap of the Hyades with a V sin i of 50 km s 1 (Boesgaard 1987). These stars have very low Li abundance and if the low Li abundance in the gap is to be explained by diffusion it is clear that the calculations of Tassoul and Tassoul (1982) do not apply to F stars. 

Fig. 4 The Li underabundance caused by the matter brought, to the convection zone, by meridional circulation from the region where Li burns, is shown as a function of Teff for the Hyades (8 10s yr) and UMa (4 10s yr). It is compared to observations for these two clusters (Boesgaard, Budge and Burck 1988).

In Table 1 are shown a number of stars that have large Li abundances and rotational velocities. Are indicated both the measured rotational velocities and the limiting rotational velocity beyond which Li should be strongly depleted by the mechanism just described. These contradict the model just described. They require that the penetration of the convection zone by meridional circulation be at most partial. 

Another explanation of the lithium gap in the Hyades could be found in terms of turbulent diffusion and nuclear destruction. Turbulence is definitely needed to explain the lithium abundance decrease in G stars. If this turbulence is due to the shear flow instability induced by meridional circulation (Baglin, Morel, Schatzman 1985, Zahn 1983), turbulence should also occur in F stars, which rotate more rapidly than G stars. Fig. 2 shows a comparison between the turbulent diffusion coefficient needed for lithium nuclear destruction and the one induced by turbulence. Li should indeed be destroyed in F stars This effect gives an alternative scenario to account for the Li gap in the Hyades. The fact that Li is normal in the hottest observed F stars could be due to their slow rotation. 

Enhancement of helium and nitrogen may require mixing due to meridional circulation because the convection in the current model is too shallow. The observed enhancement of s-process elements, Sr, Ba (Williams 1988) might be related to this mixing. 

We have addressed several aspects of STE of ozone and the impact on tropospheric ozone levels. Using ozone observations in the upper troposphere and lower stratosphere from MOZAIC, we have examined the rdation between ozone and PV in the lower stratosphere. A distinct seasonality in the ratio between ozone and PV is evident, with a maximum in spring and minimum in fall associated with the seasonality of downward transport in the meridional circulation and of the ozone concentrations in the lower stratosphere. The ozone-PV ratio is applied in our tropospheric chemistry-climate model to improve the boundary conditions for ozone above the tropopause, to improve the representativity of simulated ozone distributions near synoptic disturbances and realistically simulate cross-tropopause ozone transports. It is expected that the results will further improve when the model is applied in a finer horizontal and vertical resolution. 

Rosenlof, K.H. (1995) The seasonal cycle of the residual mean meridional circulation in the stratosphere, J. Geophys. Res. 100,5173-5191. 

Climate extremes will intensify (heat waves, heavy showers, etc.) the intensity of tropical cyclones (typhoons) will increase due to further SST rise at low latitudes meridional circulation in the North Atlantic will decrease, on average, by 25% extra-tropical cyclones will shift toward the poles. 

FIGURE 5.15 Cumulative, standardized (mean = 0, STD = 1) series of the winter (IFM) frequency of weather types with dominating meridional circulation (MC, dots) producing north winds over West Europe (1891-1968) from data of Girs (1971) and that of the WIBIX (open circles) note the negative (positive) trend in the MC (WIBIX) during the maritime climate mode (1903-1939) identified in Fig. 5.14. 

Investigations of characteristic anomalies in the sea-level pressure field over the North Atlantic and Europe preceding MBls were carried out by Schinke and Matthaus (2003). Up to 3 years preceding a season with MBls, the various anomalies identified seem to have no influence on the occurrence of MB Is. In contrast to the investigations carried out by Bomgen et al. (1990), close connections have not been found between the variations of the meridional circulation over the North Atlantic and the occurrence or absence of MBls. Favorable conditions for the occurrence of MBls seem to be almost exclusively generated in the transition area and the Baltic Sea area itself. 

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