Seasonal Variations of the 40-50 Day Oscillation in the Tropics

Daily rawinsonde data from 19 near-equatorial stations are examined to learn more about annual variations of the 40–50 day oscillations. Lengths of the available time series range from 5 to 28 years. A technique is devised to isolate spectral and cross-spectral quantities as a function of season. It is determined that a variance of the zonal wind in a relatively broad band centered on 47-day periods generally exceeds that in adjacent lower and higher frequency bands by the largest amount during December January and February (DJF) and at stations in the Indian and western Pacific Oceans during all seasons. The coherence between lower-and upper-tropospheric zonal winds tends to be largest in the summer hemisphere for stations in the Indian and western Pacific Oceans. Upper tropospheric zonal and meridional winds are coherent and out of (in) phase at several stations there during DJF [June, July and August (JJA) These results. coupled with composited wind and outgoing longwave radiation data, lead us to conclude that in the Indian and western Pacific Oceans the eastwardd-waving regions of enhanced convection associated with the 40-50 day oscillation force a Kelvin-like wave to the east and anticyclonic, Rossby-like waves to the west. The anticyclonic eddies are found in the summer hemisphere during solstice seasons and cause local surges in upper-level southeasterlies (northeasterlies) during DJF (JJA).