Seasonal migration of tropical rainfall is examined in the South American sector and compared with that over Africa, using satellite and reanalysis data. While the African rain band moves continuously back and forth across the equator following the seasonal march of the sun, the South American one displays a peculiar asymmetry between its northward and southward migration. The rain band moves gradually northward from October to July from the Amazon toward the Caribbean Sea, while its return to the Amazon is an abrupt event, with convection developing rapidly in October around 10°S without going through the equator. Over the equatorial Amazon during July-October, equivalent potential temperature (θ_e) is kept low by the easterly advection of low temperature and humidity air from the equatorial Atlantic, coinciding with the seasonal development of ocean upwelling and a cold tongue in sea surface temperature. The low θ_e values prevent convection from developing in the equatorial Amazon while warm SST supports convection on the north coast of South America during August-October. Meanwhile solar radiation continues to heat up the land surface to the south, eventually triggering the onset of deep convection there in October.
Atmospheric general circulation model experiments were conducted to examine the effect of the Atlantic cold tongue on tropical rain bands. Without the seasonal development of the Atlantic cold tongue, surface θ_e remains high in September, and rainfall in the equatorial Amazon switches to a pronounced semi-annual cycle with a peak in each equinox. These results illustrate the role of the Atlantic cold tongue in the peculiar meridional migration of the observed South American rain band.