The annual march of the climatological mean meridional circulations (MMCs) in the NCEP-NCAR reanalyses is dominated by two components of roughly comparable mean-squared amplitude: 1) a seasonally invariant pair of ''Hadley cells'' with rising motion centered near and 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 meridional structure and seasonal evolution of the solsticial cell are suggestive of a close association with the monsoons. These results are consistent with previous analyses of the mean meridional circulation based on radiosonde data.

The seasonal cycle of the zonal-mean zonal momentum balance in the Tropics is investigated using NCEP reanalysis data. It is found that the climatological stationary waves in the tropical upper troposphere, which are dominated by the equatorial Rossby wave response to tropical heating, produce an equatorward eddy flux of westerly momentum in the equatorial belt. The resulting westerly acceleration in the tropical upper troposphere is balanced by the advection of easterly momentum associated with the cross-equatorial mean meridional circulation. The eddy momentum fluxes and the cross-equatorial flow both tend to be strongest during the monsoon seasons, when the maximum diabatic heating is off the equator, and weakest during April–May, the season of strongest equatorial symmetry of the heating. The upper-level Rossby wave pattern exhibits a surprising degree of equatorial symmetry and follows a similar seasonal progression. Solutions of the nonlinear shallow water wave equation also show a predominantly equatorially symmetric response to a heat source centered off the equator.

The three-dimensional structure of the annual-mean equatorial planetary waves in the 40-yr ECMWF Re-Analysis (ERA-40) is documented. The features in the free atmosphere are predominantly equatorially symmetric, driven by east-west heating gradients. The geopotential height and wind perturbations are strongest at or just below the 150-hPa level. Below the level of maximum amplitude, the circulations in the waves are thermally direct with latent heat release in deep convective clouds and radiative cooling in the intervening cloud-free regions. Within the overlying capping layer, the wave-related circulations are thermally indirect, with rising of the coldest air and sinking of air that is less cold. At the cold point, just above the 100-hPa (17 km) level, the ERA-40 annual-mean vertical velocity in the equatorial belt ranges up to 3 mm s Ϫ1 over the equatorial western Pacific during the boreal winter, implying diabatic heating rates of up to 3°C day Ϫ1 , an order of magnitude larger than typical clear-sky values. Strong heating is consistent with evidence of widespread thin and subvisible cirrus cloud layers over this region. It is hypothesized that the air mass as a whole is rising (as opposed to just the air in the updrafts of convective clouds) and that this plume of ascending air spreads out horizontally at or just above the cold point, ventilating and lifting the entire lower stratosphere.El Niño years are characterized by anomalously weak equatorial planetary waves in the Indo-Pacific sector and slightly enhanced waves over the Atlantic sector and cold years of the El Niño-Southern Oscillation (ENSO) cycle by the opposite conditions. Equatorial Pacific sea surface temperature is as well correlated with the strength of the equatorial planetary waves in the upper troposphere over the IndoPacific sector as it is with the conventional Southern Oscillation index based on sea level pressure.

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Statistical analyses of long-term instrumental and proxy data emphasize a distinction between two quasi-decadal modes of climate variability. One mode is linked to atmosphere-ocean interactions ('the internal mode') and the other one is associated with the solar sunspots cycle ('the solar mode'). The distinct signatures of these two modes are also detected in a high-resolution sediment core located in the Cariaco basin. In the oceanic surface temperature the internal mode explains about three times more variance than the solar mode. In contrast, the solar mode dominates over the internal mode in the sea-level pressure and upper atmospheric fields. The heterogeneous methods and data sets used in this study underline the distinction between these decadal modes and enable estimation of their relative importance. The distinction between these modes is important for the understanding of climate variability, the recent global warming trend and the interpretation of high-resolution proxy data.