Spectrum Analysis of Large-Scale Wave Disturbances in the Tropical Lower Troposphere

Abstract

The study consists of two parts: a detailed investigation of wave disturbances in the tropical lower troposphere during a single 6 month period, and a brief survey of wave activity at a single station during four successive 6-month periods. Cross spectrum analysis of wind, temperature, relative humidity and surface pressure data reveals the existence of at least three types of disturbances: Easterly waves, with periods in the range of 4–5 days and horizontal wavelengths on the order of 3000 km. Contrary to the results of earlier studies, the axes of the waves show no inclination with height. There is some indication of a cold core structure, but the temperature fluctuations in the waves are very small. Some stations show a tendency for high relative humidities to occur in the troughs of the waves, in agreement with earlier studies. Low-frequency oscillations with periods >10 days and horizontal wavelengths on the order of 10,000 km. These are most strongly evident in the zonal wind component, for which they account for most of the variance during the period of the study. Planetary-scale pressure fluctuations with periods near 4 days and amplitudes on the order of 1 mb at the equator. The relative importance of the easterly waves and the low-frequency oscillations as measured by their contributions to the total variance is found to vary considerably from one year to another and from one season to another.