Variability in background stratospheric aerosols over the tropics and its association with atmospheric dynamics

摘要

Spatiotemporal variability of background stratospheric aerosols over the tropics (30°S-30°N) and its association with atmospheric dynamics are studied using the zonal-averaged Stratospheric Aerosol and Gas Experiment II (SAGE-II) derived aerosol extinction at 525 nm along with lidar data at 532 nm from Gadanki (13.5°N, 79.2°E) during the period 1998-2005. In general, a pronounced increase in aerosol loading is observed in the spatial distribution of aerosols over the equatorial region compared to the off-equatorial regions. The particulate optical depth (τ_P) in the lower stratospheric region showed an increasing trend particularly after 2002 in both these regions. This increase could partly be attributed to a corresponding increase in the number of feeble/minor volcanic eruptions. Over and above this steady increase, the τ_P also showed significant periodic variations. In addition to the prominent short-period variations, the stratospheric Tp showed a significant biennial variation (with a period of –30 months) in association with stratospheric quasi-biennial oscillation in zonal wind (QBOu). During the westerly phase of stratospheric QBO_U, the mean particulate optical depth decreases rapidly with increase in latitude on either side of equator in both the hemispheres, while it remains fairly steady around the equator up to 15° in latitude on either side with a small bite-out around the equator during the easterly phase, particularly during the very quiet period of 1998-2002. During the latter half (after 2002), when the stratosphere was mildly disturbed, the latitudinal variation of τ_P was fairly similar in both phases of QBOu with a higher value of τ_P near the equator during the westerly phase. The QBO signal in aerosol extinction around 25 km is found to be in opposite phase with that observed in the upper regime (28-32 km) and lower regime (18-22 km), depicting the influence of the secondary meridional circulation produced owing to vertical shear of QBOu in the equatorial region.