Radiative-dynamical processes modulating the vertical structure of the stratospheric polar vortex

1. 1. Introduction The stratospheric polar vortex in Winter is driven by radiative cooling in high latitudes, leading to an increased equatorward temperature gradient between the mid and high latitudes and stronger thermal wind. Disturbances of the polar vortex result from planetary waves which under certain conditions propagate into the stratosphere, break there and deposit momentum. The vortex variability may also be determined by changes in the radiative forcing. While dynamical consequences of the spring ozone minimum and the negative heating anomaly associated therewith have been discussed in the literature (e.g. [1]), the effects of midwinter ozone variability are less understood. In this work, we investigate possible dynamical effects of radiative forcing associated with the intra-seasonal and interannual ozone anomalies during the polar night. 4. Results Fig. 6. Autocorrelation of intra-seasonal anomalies of PC1 and of the time series of the area-weighted average of total ozone north of 75ºN and the index U(2-10). The negative auto-correlation at a lag of approximately 2 months partly explains the reversal of the sign of the correlations in Fig. 5 between lags 0 and ±2 months. Fig. 5. Lagged correlation between PC1 and time series of the area-weighted average of total ozone north of 75ºN for intra-seasonal and total variabilities. Positive lags means that PC1 is leading, and for negative lags O3 is leading. The total ozone and PC1 are strongly anti-correlated at zero lag. The positive correlations of intra-seasonal variabilities may be due to both persistence and the polar night jet oscillation [2]. 2. The basic idea The following figures show lagged correlations among the above mentioned time series. During the polar night from 1 November to 11 February the polar cap north of ~75ºN is in the shade. During this period the radiative heating/cooling at high latitudes is mainly due to LW (long wave) transfer. Ozone LW emission contributes to cooling the middle/upper stratosphere. Therefore, negative O3 anomalies must force positive temperature anomalies over the polar cap, and poleward temperature gradient anomalies (see Fig. 1). This temperature gradient anomaly must imply a thermal wind shear anomaly. Hence the intra-seasonal O3 anomalies (Fig. 3) may force vortex anomalies, which feed back on the O3 through changes in transport. Fig. 2. Latitude-height section of NDJ-mean temperature differences, 1993-2002 minus 1979-1988, derived from the ECMWF reanalysis. Dark (light) shaded areas denote regions where temperature differences are significantly different from natural variability at the 99% (95%) level. The positive temperature anomalies in the high-latitude middle stratosphere are consistent with Fig. 1. Fig. 1. Decadal temperature changes for January calculated with a radiative transfer model forced by the observed ozone losses from 1980 to 2000 (contour interval 0.5K/decade, positive values are shaded). The ozone losses were obtained linearly extrapolating the 1980-1996 trend to 2000. From [3, Fig. 9]. Fig. 7. As in Fig. 5, but for correlation between PC1 and wind shear index U(2-10). The effect of polar night jet oscillation is evident in the curve for intra-seasonal variability. However, the absolute values of correlations are higher when PC1 leads by about a month. This may be due to time integration radiative O3 cooling rate anomalies. Fig. 8. TOVS composites based on the polar vortex strength in the NCEP/NCAR reanalysis. Solid and dashed curves are for months in which the vortex strength is half a standard deviation above or below the mean strength in the analyzed period, respectively. The thick curves represent the composite means. These results strongly suggest that the anti-correlation in Fig. 5 is not an artifact of the reanalysis. Fig. 3. Monthly mean anomalies of the area-weighted average total ozone north of 75ºN from ECMWF reanalysis for November to February. The open circles indicate months in which no observations were assimilated. The intra-seasonal and interannual fluctuations of total ozone have an amplitude at least as large as the trend i.e. the ozone anomalies to which the temperature anomalies in Fig. 1 correspond. Fig. 9. As in Figure 5 but with the wind shear index U(2-10) instead of PC1. The correlations for intra-seasonal variabilities still reveal the effect of the polar night jet oscillation. However, now the correlations are stronger when the O3 anomalies lead by 1-2 months. This is in accordance with a time integration of radiative cooling rates of O3. 3. Data and method • The data used in this work consist of the zonal wind, temperature and total ozone from ERA40 reanalysis, and TOVS total ozone. • The following time series were constructed: • PC1 of the zonal mean zonal wind in the high latitudes (45ºN to 85ºN) stratosphere (all ERA40 levels from 150 to 1hPa). This time series represents the vortex strength (Fig. 4). • Zonally averaged total O3, and area-weighted average of total O3 north of 75ºN. • The vortex shear in the middle/upper stratosphere, U(2-10)=<U>(2hPa)-<U>(10hPa), where <U> represents area-weighted averages of the zonal wind in the latitudinal band 57.5ºN-75ºN. • These time series were used both unfiltered and filtered by a 31-day moving average. 5. Conclusion Our results suggest that, besides dynamical forcing, the variability of the vertical structure of the polar night vortex responds also to the LW radiative forcing of ozone anomalies associated with the vortex variability itself. A positive feedback loop may exist in which negative anomalies of the wind shear, associated with a stronger vortex in the lower stratosphere and planetary wave reflection, reduce poleward transport of ozone, thereby causing a reduction of LW cooling over the polar cap, hence poleward temperature gradient anomalies and a further intensification of the negative wind shear. References [1] Hu, Y. Y., and K.-K. Tung, 2003: Possible ozone-induced long-term changes in planetary wave activity in late winter. J. Climate, 16(18), 3027-3038. [2] Kodera, K., Y. Kuroda, and S. Pawson, 2000: Stratospheric sudden warmings and slowly propagating zonal-mean zonal wind anomalies. J. Geophys. Res., 105(D10): 12351-12359 MAY 27 2000 [3] Langematz, U., M. Kunze, K. Kruger, et al., 2003: Thermal and dynamical changes of the stratosphere since 1979 and their link to ozone and CO2 changes. J. Geophys. Res., 108(D1), Art. No. 4027. Fig. 4. Principal component analysis of the ECMWF zonal mean zonal wind in the stratospheric layer (150-1hPa). The wind data were weighted by the area and the mass. The fist row shows the first two EOF, and the last column the correlation between the leading PC and the zonal mean zonal wind in the NH at all ECMWF reanalysis levels. The second row shows the empirical PDFs for the first two PCs, and the eigenvalue spectrum. Radiative-dynamical processes modulating the vertical structure of the stratospheric polar vortex José M. P. Silvestre, José M. Castanheira, and Juan Ferreira Department of Physics, University of Aveiro, 3810-193 Aveiro, Portugal jsilvestre@fis.ua.pt, jcast@fis.ua.pt, juan@fis.ua.pt