JMAMRIClimate Research Dep.Yuhji Kuroda

Polar-night Jet Oscillation and Annular Mode



The spatial and temporal characteristics of the month-to-month variability of the polar-night jet and its relationship with tropospheric circulation is investigated for both the Northern and Southern Hemisphere winter. The variabilities of the hemispheres have many common characteristics called the Polar-night Jet Oscillation (PJO). These common characteristics include: (1) The anomalous zonal-mean zonal winds shift poleward and downward; (2) The anomalous polar temperatures propagate downward from the stratopause to the upper troposphere; (3) They are made through a wave-mean flow interaction with mainly the planetary wave of zonal wavenumber 1.

Annular modes (AMs) associated with the PJOs appear in both Hemispheres when the zones of maximum polar temperature anomaly descend to the lowermost stratosphere and upper troposphere. The major difference in the PJOs of the two hemispheres is found in their temporal characteristics. In the Southern Hemisphere, the phase of the PJO is closely locked to the annual cycle, while in the Northern Hemisphere it exhibits quasi-periodic variability with its envelope controlled by the annual cycle.

The PJO becomes a nearly zonally symmetric structure from the troposphere to the upper stratosphere when a pattern similar to the AM appears at the troposphere. If the signal of the PJO is removed, the signal of the AM exists only in the troposphere, and the persistency of the signal is largely reduced in the stratosphere. This suggests that the AM is essentially a tropospheric process that tends to strongly interact with the PJO in the active season.

Figure 1. Heterogeneous regression maps of the leading E-SVD between the zonal-mean zonal wind (contour) and the (vertical component of) E-P flux (vector) in (a) the Northern and (b) Southern Hemispheres. The meridional component of the E-P flux was constructed by regression. The numbers above the panels indicate the lag in months, and the initials represent the corresponding months. Shading denotes those regions where the heterogeneous correlations of zonal winds are significant at the 95% level. The contour interval is 2m/s, and the dashed lines indicate negative values. The zero contour line is plotted as a thin solid line. The E-P flux is scaled by the inverse square root of the pressure, and small arrows are omitted. (From Kuroda and Kodera, 2001)

Figure 2. Regression map of the zonal-mean temperature (contour) and the residual velocity (vector) associated with the PJO in (a) the Northern and (b) Southern Hemispheres. The numbers above the panels indicate the lag in months, and the initials represent the corresponding months. Shading denotes regions where the correlations of the temperatures are significant at the 95% level. Contour interval is 1 K. (From Kuroda and Kodera, 2001)

Figure 3. The same as Figure 2, except for the regression map of the polar temperatures (upper panels), displayed as a function of time and height. Contour interval is 1 K. Correlation coefficients between the PJO and the AO or AAO indices are displayed in the bottom panels. (From Kuroda and Kodera, 2001)

Figure 4. (a) 30-day running averaged anomalous polar temperature in the Northern Hemisphere from 1987 to 1989. (b) Interannual standard deviation of monthly mean polar temperature in the Northern Hemisphere. Contour interval is 5 K in (a) and 1 K in (b). Shading indicates negative values in (a), and values larger than 4 K in (b). (From Kuroda and Kodera, 2001)

Figure 5. The same as Figure 4 except for the Southern Hemisphere from 1986 to 1988, and the contour interval is 3 K for (a). (From Kuroda and Kodera, 2001)

Figure 6. Regression maps of the geopotential heights in the NH around the annular stage (lag time -1, 0, 1 months, from left to right) due to the PJO index. Levels of 1, 30, and 500 hPa are shown from top to bottom. Contour intervals for levels of 1, 30, and 500 hPa are 50, 30, and 10 m, respectively, and the dashed lines indicate negative values. The zero contour line is plotted by a thin solid line. Shading denotes significant regions at the 95% level. Numbers below the panels represent the correlation coefficients of the AM index with the PJO index for the respective months. (From Kuroda 2002)

Figure 7. The same as Figure 6, except for the SH winter, and the lag times are -2, 0, and 1 month. (From Kuroda 2002)

Figure 8. Lagged regression of the zonal-mean zonal wind (contour) and the E-P flux (arrow) by the AM indices of the annular stage of the NH (upper panels) and SH (lower panels) winters (D/J for NH and O/N for SH). Contour intervals are 2 m/s, and the dashed lines indicate negative values. The zero contour line is plotted as a thin solid line. Shading denotes significant regions at the 95% level. E-P flux has been scaled by the inverse square root of the pressure. (From Kuroda 2002)

Figure 9. The same as Figure 8, except that the signal due to the PJO has been removed. (from Kuroda 2002)


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