Some Features of Global-Scale 4–5 Day Waves

Abstract

Routine perusal of surface maps covering tropical and subtropical regions frequently reveals the presence of large-scale patterns in the 24 h pressure tendency. Previous analyses of surface pressure, 1000 mb heights and 500 mb heights have determined that these oscillations, have a preferred frequency and are caused by a global-scale wave with a wavelength equal to the circumference of the earth and a period of about 5 days. In order to investigate this wave motion in more detail, the troughs of 60 waves were followed from eastern Africa to the western Caribbean, and both surface and upper air data were composited relative to the troughs. The surface pressure analysis shows that the wave is nearly in phase from 30°N to 30°S and that the minimum amplitude occurs near the equator. Upper air observations in the region from 10?30°N indicate that the wave features do not tilt significantly in the vertical from the surface to 70 mb. Although the amplitude of the waves in the surface pressure field is less than 1 mb throughout the tropics; and the horizontal scale is zonal wavenumber 1 or 2, the data show that the wave affects the occurrence of both precipitation and thunder.