Low-level high-frequency modes in the Tropical Atlantic and their relation to precipitation in the equatorial South America

Abstract

¶Evolving aspects of the dominant high-frequency modes of the 925-hPa v-component and precipitable water in the Tropical South American and Atlantic sector are studied for the austral summer and winter seasons. Their relations to the precipitation anomalies over the equatorial South America are also investigated. The highest percentages of the total variance contained in the high-frequency time-scale for the 925-hPa v-component have a clear seasonal dependency such that they are confined in the Tropical South Atlantic (TSA) during the austral summer and in the Tropical North Atlantic (TNA) during the austral winter. The high-frequency variability for the TSA during the austral summer stems from the combined effects of equatorward incursions of midlatitude synoptic and transient wave systems, equatorial trades and westward traveling disturbances in equatorial latitudes. All these systems together have a pronounced effect in modulating the summer precipitation over northeastern South America, though the isolated effect of the easterly waves is weak. In contrast, the high-frequency variability for the TNA during the austral winter is mainly due to wave-type westward traveling tropical disturbances. These disturbances have an important role in modulating the daily precipitation of northern part of the South America during the austral winter.