The Structure and Annual Variation of Antisymmetric Fluctuations of Tropical Convection and Their Association with Rossby–Gravity Waves

The signature of 4–5-day period Rossby–gravity waves is searched for in the tropical convection field across the Indian-Pacific oceans. The convergence/divergence field of these waves in the lower troposphere is anticipated to produce an antisymmetric fluctuation in tropical convection. Antisymmetric fluctuations of tropical convection are shown to exhibit a pronounced spectral peak at a 4–5-day period only during boreal fall and only within about 30° longitude of the date line. The peak amplitude occurs around 7.5° latitude. These fluctuations propagate westward at 15–20 m s−1 with zonal wavelength of about 7000&–9000 km. The fluctuations of convection are coherent and out of phase with the equatorial meridional wind, which also exhibits a pronounced spectral peak at a 4–5-day period in the lower troposphere near the date line. The antisymmetric zonal wind also is strongly coherent with the antisymmetric convective fluctuations in this region. The horizontal distributions of the 4–5-day power and coherence of the winds and convection are consistent with that produced by a convectively coupled Rossby–gravity wave that is confined near the date line. The localization of the convectively coupled Rossby–gravity wave activity near the date line during boreal fall is postulated to be due to the unique meridional distribution of sea surface temperature at this location. The equatorial minimum flanked by maxima at about 5°–10° latitude is thought to encourage antisymmetric convection, which interacts efficiently with Rossby–gravity waves. The fall maximum in convectively coupled Rossby–gravity wave activity is consistent with these unique sea surface temperatures occurring only during fall.