An observational study of the tropical tropospheric circulation
This observational study focuses on four different aspects of the general circulation of the tropical troposphere: the seasonality of the mean meridional circulation (MMC), the zonal momentum balance of the tropical troposphere, the structure of the equatorial stationary waves and the variability of the equatorial stationary waves.The annual march of the climatological MMC is shown to be dominated by two components of comparable mean-squared amplitude: (1) a seasonally invariant pair of "Hadley cells" with rising motion centered just to the north of the equator and subsidence in the subtropics, and (2) a seasonally reversing, sinusoidally varying "solsticial" cell with ascent in the outer Tropics of the summer hemisphere and subsidence in the outer Tropics of the winter hemisphere.The analysis of the seasonal cycle of the zonal-mean zonal momentum balance in the tropics shows that the climatological stationary waves in the tropical upper troposphere produce an equatorward eddy flux of westerly momentum in the equatorial belt. The resulting westerly acceleration is balanced by the advection of easterly momentum associated with the cross-equatorial MMC. The eddy momentum fluxes and the cross-equatorial flow are strongest during the monsoon seasons.The three-dimensional analysis of the annual mean equatorial stationary waves in the ECMWF model reveals a complex vertical structure of various fields associated with these waves. There seems to be something 'special' about the warm pool that enables the planetary-scale ascent to extend higher than elsewhere in the tropics. It is conceivable that the slow, hydrostatically-balanced ascent in the planetary-scale stationary waves in this region rather than the updrafts in the convective clouds is responsible for maintaining the extremely cold temperatures in the tropopause transition zone in this region.The main modes of tropical variability (ENSO, the MJO and the annual cycle) are shown to share a number of common characteristics: they can all be described as oscillations of the planetary-scale waves between two preferred states. One state can be described as a relaxed pattern, in which the geopotential height is practically flat over the whole tropical belt and the zonal and meridional winds in the equatorial belt are relatively weak. Such a state is characteristic of situations with reduced precipitation over the warm pool and a weakening of the equatorial cold tongue in the Eastern Pacific. The perturbed state can be generally described as having opposite characteristics to the relaxed state.
- Atmospheric sciences