Climatology, ENSO-related interannual variability, and MJO-related intraseasonal variation of gravity waves in the southern hemisphere subtropical stratosphere revealed by high-resolution AIRS observations

Kaoru Sato* and Chikara Tsuchiya, M. Joan Alexander, Lars Hoffmann
Department of Earth and Planetary Science, The University of Tokyo

A new temperature retrieval from Atmospheric Infrared Sounder with a fine horizontal resolution of 13.5 km was used to examine gravity wave (GW) characteristics in the austral summer at an altitude of 39 km in the subtropical stratosphere over eight years from 2003-2011. Using an S-transform method, GW components were extracted, and GW variances, horizontal wavenumbers and their orientations were determined at each grid point and time. Both climatology and interannual variability of the GW variance were large in the subtropical South Pacific. About 70 % of the interannual variation in the GW variance was regressed to ENSO index. The regression coefficient exhibits a geographical distribution similar to that of the precipitation. In contrast, the regression to the QBO was not significant in the South Pacific. These results indicate that the interannual variability of GW variance in the South Pacific is controlled largely by the convective activity modulated by the ENSO. An interesting feature is that the GW variance is maximized slightly southward of the precipitation maximum. Several possible mechanisms causing the latitudinal difference were discussed. Next, the intraseasonal variability of GWs was examined. Composite maps were made of GW temperature variances, large-scale zonal winds around the tropopause, and precipitation based on the real-time multivariate MJO index. Regional distributions of these quantities are synchronized with the MJO: The GW variances are larger for stronger precipitation, and for more strongly westward wind around the tropopause at a given precipitation. These results suggest that the GWs observed by AIRS in the stratosphere originate from convection. Moreover, it is shown that the zonal wind around the tropopause likely controls the GW propagation into the stratosphere by a critical level filtering mechanism. This means that the MJO can modulate the middle atmospheric circulation by regulating the GWs in two ways, namely, generation and propagation.


*email: kaoru@eps.s.u-tokyo.ac.jp
*Preference: Poster