Seasonal variability for the 40-50 day oscillation in wind and rainfall in the tropics

Time spectral analysis is performed on long records of wind and precipitation from stations in the tropical Indian Ocean-Pacific Ocean area. The spectra are done separately for winter and summer half-years. Statistically significant spectral peaks in the 40-50-day period range show strong seasonal variability. The 40-50-day peaks in the 200-mb zonal wind spectra are stronger and more prevalent during the Northern Hemisphere winter half-year. Spectral peaks in the 850-mb wind show a preference for summer in the Northern Hemisphere. Precipitation spectra show significant 40-50-day peaks at selected locations in the Indonesian region and along the South Pacific convergence zone in the central Pacific during Southern Hemisphere summer. These oscillations in precipitation are coherent with nearby zonal wind oscillations. No significant oscillations in precipitation were found for stations significantly north of the Equator during either half-year. In particular, no significant peaks in precipitation spectra were found for composites of stations on the Indian Peninsula during summer, where it has been proposed that the 40-50-day oscillation modulates monsoon precipitation. It is concluded that the 40-50-day oscillation is sustained by interactions between the large-scale flow and convective-scale processes and that these interactions take place in restricted areas where intensely convective regions are near the Equator. The wind oscillation occupies a larger area, particularly at upper tropospheric levels, principally by horizontal wave propagation away from the excitation regions. Because the oscillation does not appear to be forced over India, it is conjectured that the seasonal variation in the intensity of the oscillation is attributable, in part, to the fact that the tropical convection is drawn away from the Equator by the Indian summer monsoon. When the convection is drawn off the Equator, the efficiency of the interaction with equatorially trapped modes declines; hence, the amplitude of the oscillation is less during Northern Hemisphere summer.