In order to study the characteristics of the APW, gridded NCEP/NCAR mean meridional wind data during the month of May were selected for the analysis. Climatology for the 13 year period 1982-1994 available in the CD-ROM supplied along with Kalnay et al (1996) for the 9 levels considered (300, 200, 150, 100, 70, 50, 30, 20, 10 hPa) was used to calculate meridional wind anomaly at each grid point for composites of DRY and WET ISMR years.
In Figs. 1(a-d), meridional wind anomaly for the composite WET ISMR years for 4 levels viz., 300, 200, 100 and 70 hPa and in Fig. 2, meridional wind anomaly for the composite DRY ISMR years for 200 hPa level only are presented. Prominent stationary Rossby wave signal is seen in these figures at all levels. It has a wave number 6 or 7 in zonal structure and is confined between 10° N and 50° N latitudes. It is seen from Fig.1, that the areas of southerly and northerly meridional wind anomaly of the WET composite above 200 hPa levels match with the same at 200 hPa and below. So the wave seen above 200 hPa level is an integral part of the APW reported by JS for 200 hPa and below. Amplitude of this wave decreases with height above 200 hPa and the wave is very weak above 70 hPa (figures not given).
Figure 1(a-d) Meridional wind anomaly (m/s) of 70, 100, 200 & 300 hPa levels for WET ISMR composites
From Fig. 1(a-d), it is clear that the wave is present between 500 and 70 hPa levels with maximum amplitude around 200 hPa. Above and below 200 hPa, its amplitude decreases sharply. It is clear from this figure that the APW couples the upper troposphere and the lower stratosphere. Over the Indian region, northerly anomaly is seen during WET ISMR years and southerly anomaly is seen during DRY ISMR years. It is seen from the 200 hPa WET and DRY composites that the APW shows a phase shift of about 20° longitude between extreme ISMR years as reported by JS. JS showed that the APW at 200 hPa and below possesses major characteristics of stationary barotrophic Rossby Wave. The wave does not have phase shift with height. It is clear from Fig. 1(a-d), that the wave above 200 hPa level also does not show any phase shift with height.
Figure 2 Meridional wind anomaly (m/s) of 200 hPa for DRY ISMR composite
APW and meridional mass
exchange
APW follows the subtropical westerly jet stream over Asian region and then moves southeastwards to the United States of America (JS). Although the wave shows north-south displacement in some regions, generally it is confined between 10°N and 50°N latitudes with large amplitude between 300 and 150 hPa levels. The latitude-height plot of the mean May temperature values of 1989 (normal ISMR year) averaged between 50-100° E longitudes (Indian region) for Northern Hemisphere is presented in Fig.3. The tropical tropopause is situated around 100 hPa and the subtropical tropopause between 200 and 300 hPa levels. Over 30°N and adjoining latitudes, tropical tropopause lies above the subtropical tropopause and a break region exist between them, which we call as tropopause break.
Figure 3 Latitude-Height plot of the temperature averaged between 50-100 E longitudes (Indian region) for May 1989.
Maximum amplitude of APW is in the tropopause break. It is likely that that the large amplitude meridional wind anomaly associated with APW is able to transport subtropical stratospheric air into tropical troposphere and vice versa effectively through the tropopause break region, which in turn can affect the total ozone distribution.
Asia Pacific Wave in
Total Ozone
In order to check the possible
presence of APW induced meridional mass exchange via the tropopause break,
we decided to examine the total ozone anomaly of May. Ozone is an ideal
tracer for this study because relatively long period satellite measured
total ozone data is available on global scale. Also ozone is relatively
abundant in the lower stratosphere. Subtropical stratospheric mass entering
the tropical troposphere may transport ozone rich air mass to relatively
ozone poor tropical region and increase the columnar ozone content over
this region. In contrast, tropical tropospheric mass entering subtropical
stratosphere may transport ozone poor air mass to this relatively ozone
rich stratospheric region and decrease the columnar ozone content over
this region. So it is easy to monitor the meridional mass exchange by analysing
the total ozone anomaly.
Gridded mean May TOMS total ozone data in the latitude range 0.5°N to 50.5°N for the period 1979-92 was available for analysis. Total ozone anomaly from the 14 year climatology is computed for May for each grid point. In Fig. 4(a-b), total ozone anomaly for the composites of WET and DRY monsoon years are presented. Areas of positive and negative total ozone anomalies are seen in the zone between 10°N and 50°N latitudes. It has a 6 or 7 wave number structure in the zonal direction just like APW. Areas of positive (negative) total ozone anomaly correspond to northerly (southerly) meridional wind anomaly of APW. Thus over north India in May of dry (wet) years there is negative (positive) anomaly in total ozone as may be seen from Fig. 4(a-b). In some regions, the areas of total ozone anomaly show small eastward shift while compared to the corresponding locations of the meridional wind anomaly. It is interesting to note that the Indian summer monsoon, which controls the spatial phase of the APW, is also associated with the total ozone distribution around the globe in the interannual time scale.
Figure 4 Total ozone anomaly (DU) for DRY and WET ISMR composites