A simple tropical moist model applied to the ‘40‐day’ wave

Abstract

AbstractThe ‘40‐day’ wave is a baroclinic eastward‐propagating disturbance commonly observed in the tropical atmosphere, with a period that actually lies in the band 30 to 60 days. Its structure resembles an equatorially‐trapped Kelvin wave, predominantly with zonal wavenumber 1. In this paper a simple model with marginally stable moist static stability is used to investigate one possible mechanism for its appearance: namely, as the response to variable thermal forcing amplified by the effect of interactive latent heating.The model has just one vertical baroclinic mode, forced by prescribed heating and by interactively‐determined latent heating. Latent heating only occurs when and where there is low‐level convergence and moisture is tending to increase beyond some saturation level: in such regions the static stability takes its moist value, which is considerably less than the dry value acting in regions without precipitation.Analytic solutions are first provided for cases with static stability everywhere constant. Waves propagate more slowly when the moist value is used, so the peak linear response to wave‐like thermal forcing (essentially a resonant response) occurs at longer periods than for dry static stability. For dry conditions the period of a free Kelvin wave with zonal wavenumber one is about 10 days, but for wet conditions the period increases toward the ‘40‐day’ range.When static stability varies the model must be solved numerically. Solutions show that the increase in period predicted analytically is indeed realized, even when precipitation is patchy and dry static stability is acting almost everywhere. Effectively the amplified waves travel at a speed consistent with the maintenance of their latent‐heating energy source. Eastward waves are prominent near the equator. By analysing particular moisture‐dependent combinations of variables, these eastward waves can be dynamically identified as Kelvin waves.Saturation moisture content is closely linked to sea surface temperature. The warm western Pacific and Indian Oceans are regions of high atmospheric moisture content, and hence of very low moist static stability. This ocean temperature effect is studied using the simple model, showing that intraseasonal waves are largest in such regions, as observed.