Structure of the Annual-Mean Equatorial Planetary Waves in the ERA-40 Reanalyses

Abstract

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−1 over the equatorial western Pacific during the boreal winter, implying diabatic heating rates of up to 3°C day−1, 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 Indo-Pacific sector as it is with the conventional Southern Oscillation index based on sea level pressure.