JMA„MRI„Climate Research Dep.„Yuhji Kuroda„

Role of medium]scale waves on the Southern Annular Mode



We examined the role of short]period waves on the Southern Annular Mode (SAM) through analysis of 6]hourly reanalysis data of the European Centre for Medium]Range Weather Forecasts (ECMWF ERA]Interim). We focused on medium]scale waves, those with periods shorter than about 2 days. Although medium]scale waves generally have much smaller amplitudes than synoptic waves, they have a much higher correlation with the SAM index. As a result, about one third of wave]driven meridional circulation and zonal wind acceleration associated with the SAM is generated by medium]scale waves. Their contribution is large in spite of their climatologically smaller amplitude. Moreover, two thirds of SAM]related meridional circulation and zonal wind acceleration comes from waves of periods shorter than about 3 days. Our analysis demonstrates a positive feedback between westerly winds around 60‹S and amplification of synoptic and medium]scale waves, which sustains hemispheric SAM variability. For the feedback process between zonal wind and waves, baroclinic conversion is a key process to both synoptic waves and medium]scale waves. Our analysis also shows that wave forcings calculated from daily data explain only half of the total forcings and that the contribution from short]period waves is very significant around 50‹S.

Figure 1, (a) Annual mean climate of zonal mean zonal wind, (b) amplitude of synoptic waves, and (c) amplitude of medium]scale waves. Abscissa shows latitude from equator to South Pole, and ordinate shows height from surface to 100 hPa level. Contour interval is 5 m/s for zonal wind and 10 m for wave amplitudes. Shading indicates month]to]month standard deviation of deseasonalized variations: light, medium, and thick shading indicates 1, 2, and 3 m/s, respectively, for zonal wind and 1, 2, and 5 m, respectively, for wave amplitudes.
Figure 2, Regression patterns of (a) the geopotential height at 850 hPa level and height]latitude sections of (b) the zonal mean zonal wind and (c) the Eulerian mass stream function associated with the SAM index. Shading indicates statistical significance at a 99.9% level (correlation higher than 0.23). Contour intervals are 10 m (Figure 3a), 0.5 m/s (Figure 3b), and 5 ~ 108 kg/s (Figure 3c). Zero contour line is plotted as a thin solid line, and dashed lines indicate negative values.
Figure 3, (a-c) Calculated Eulerian meridional circulation, (d-f) acceleration of zonal mean zonal wind, and (g-i) the surface pressure tendency from total eddy forcings (Figures 3a, 3d, and 3g), frictional forcings (Figures 3b, 3e, and 3h), and the sum of all forcings (Figures 3c, 3f, and 3i). Arrows indicate velocities on the meridional plane. The contour interval is 5 ~ 108 kg/s for the mass stream function in meridional circulation, 0.1 m/s/d for zonal wind acceleration, and the unit of ordinates is 0.01 hPa/d for the surface pressure tendency. Zero contour line is plotted as a thin solid line, and dashed lines indicate negative values. Small vectors are not shown.
Figure 4, Same as Figure 2 except for (left) correlation or (right) regression of wave amplitude with the SAM index: (a) Correlation and (b) regression of synoptic waves and (c) correlation and (d) regression of medium]scale waves. Contour interval is 0.1 for correlations and 1 mfor regressions. For the correlation plot, only contours of absolute value greater than 0.5 are shown.
Figure 5, Same as Figure 3 except from (a, c, e) synoptic and (b, d, f) medium]scale waves.
Figure 6, Same as Figure 2 except for regression of (a, c) total wave]energy transfer or (b, d) baroclinic energy transfer (first term of equation (A4)) from zonal mean fields to eddies. Contour intervals are 1 ~ 10-5 W/m3.

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