The Intertropical Convergence Zone (ITCZ) is represented by a band of intense convection and associated rainfall which occurs in the equitorial low pressure trough as a result of the convergence of northeast and southeast converging trade winds. The ITCZ migrates seasonally across the tropics, reaching its most northerly position in the boreal summer and its most southerly position in the austral summer. The seasonal migration of the ITCZ is a dominant player in controlling tropical wet/dry seasonality, and so the migration of the ITCZ should produce a characteristic profile of rainfall amount, and also presumably, in the isotopic composition of this rainfall. The purpose of this project was to see whether modern precipitation at tropical sites around the world reflected the seasonal migration of the ITCZ
A series of conceptualized curves was generated indicating relative expected precipitation amount and isotopic composition (because the ITCZ is constrained to the tropics amount effect fractionation is expected) for latitudes at the maximum extents of the ITCZ (~12°N and 12°S) and for sites at the equator. At the northern and southern extents, maximum precipitation (and lowest δ 18 O) was expected in the summer months, and minimum precipitation (and greatest δ 18 O) was expected in winter months. At the equator maximum precipitation (and lowest δ 18 O) was expected in fall and spring months, reflecting the passage of the ITCZ from its positions of maximum extension.
These conceptualized curves were compared to mean monthly averages (precipitation) and weighted mean monthly (δ 18 O) values of sites in tropical Latin America collected in the Global Network of Isotopes in Precipitation (GNIP) database, compiled by the International Atomic Energy Agency. Sites from the GNIP database were divided into five groups by latitude (28°S to 15°S, 15°S to 5°S, 5°S to the equator, the equator to 11°N, and 11°N to 28°N) for comparison to the GNIP database.
Sites between 28°S and 15°S and between 11°N and 28°N exhibited precipitation profiles similar to that predicted for the southernmost and northernmost extents of the ITCZ, respectively. This indicates that the ITCZ affects precipitation patterns well away from its positions of maximum extension. Oxygen isotope curves are lagged behind expected curves by 1-2 months.
Sites between 15°S and 5°S either exhibited characteristics similar to those of the southernmost group of those between 5°S and the equator. The characteristic indicated seems to depend on the distance of the site from the shore, indicating a possible offset in the migration of the ITCZ over land and ocean.
Sites between 5°S and the equator show a maximum in precipitation in March or April but lack the expected peak in the austral spring months.
Sites between the equator and 11°N show a maximum in precipitation in September or October, but lack the expected peak in the boreal spring months.
Both groups of sites on either side of the equator show a lag in isotopic composition behind the month of maximum precipitation.
Sites north of 11°N show a pattern of precipitation similar to that expected for the maximum northern extent of the ITCZ, and show a lag in isotopic values behind expected data.