Observations of Rossby Waves Linked to Convection over the Eastern Tropical Pacific

Rossby wave activity propagating into the eastern tropical Pacific from the midlatitudes during northern winter is examined in some detail. These waves are associated with the intrusion of high potential vorticity air into low latitudes, and they modulate cloudiness, stability, and vertical motion in the vicinity of the ITCZ. In the upper troposphere and lower stratosphere the horizontal phase and group propagation of the wave activity are qualitatively like those of a nondivergent barotropic Rossby wave. As the waves move equatorward, they become more shallow and propagate upward into the stratosphere. The horizontal and vertical propagation is consistent with the tilts of the waves, the large-scale three-dimensional background flow, and with the signatures of momentum and heat fluxes associated with the wave activity. In the lower troposphere, paired cyclonic anomalies on either side of the equator accompany the upper level wave activity to the west of the ITCZ cloudiness signal. These waves amplify following the peak in the ITCZ cloudiness and propagate westward along the equator. This suggests that the upper-level wave activity, and possibly the associated convective heating, can trigger the excitation of the equatorially trapped Rossby modes. The transient wave activity appears to be a crucial component of the momentum balance of the eastern tropical Pacific circulation. There is substantial interannual variability in the wave activity, consistent with observed changes in the large-scale basic state associated with the Southern Oscillation.