The Role of the Intertropical Convergence Zone on the Aerosol Transport Pathways From Africa Towards the Western Tropical Atlantic Ocean

Abstract

This study investigates the pathways of aerosol transport from Africa towards the Western Tropical Atlantic Ocean (WTAO), and the consequent deposition of terrigenous particulates in the ocean's surface, especially relating the deposition of these elements with the position of the Intertropical Convergence Zone (ITCZ). The work uses meteorological, oceanographic and geochemical data collected in situ during the PIRATA (Prediction and Research Moored Array in the Tropical Atlantic) BR-XVIII (hereinafter P18) campaign performed in October–November 2018 in along the 38° W meridian between 1° S and 15° N. Atmospheric radiosondes and Expendable Bathythermographs (XBTs), launched from aboard the Brazilian Navy, Research Vessel (R/V) Vital de Oliveira, allowed us to delineate the exact latitudinal band of the ITCZ and the associated ocean divergence occurring in the study area and period. Surface water samples were taken along the ship's track at the same locations where radiosondes and XBTs were launched. After processing in the laboratory, the water samples allowed us to estimate the concentrations of lithogenic, suspended particulate aluminium and iron in surface water. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back-trajectory model was used to reconstruct the transport paths of aerosols (including aluminium and iron) originating from the African continent towards the WTAO, as well as the relation of these paths with the ITCZ position during the study period. Our results underline the influence of the ITCZ position on the aerosol paths and on the aluminium and iron deposition in the WTAO. The ITCZ position during the study period was, particularly, important for modulating the deposition of aluminium and iron in surface waters at about 10° N, the northern edge of the ITCZ at the time. This modulation was evident in the spatial distribution of the studied minerals at sea surface along the 38° W meridian, suggesting a barrier effect of the ITCZ on aerosol transport south of 10° N. Using satellite-derived data of several sources, this study also identified the seasonal variations in aerosol inputs from Africa towards the open sea, with mineral dust predominantly originating from the Sahara and biomass burning aerosols originating from central and sub-Saharan Africa. The unique integration of observational atmospheric, oceanographic and geochemical data used here allowed a comprehensive understanding of the complex interactions between the atmosphere and the ocean for modulating the deposition of aluminium and iron in the WTAO. These minerals are key to more complex processes such as the ocean's fertilisation and consequent productivity. Nevertheless, the possibility of other sources of minerals contributing to the concentrations found in the study area cannot be ruled out. These other sources can be the Amazon River discharge and the proximity of the Brazilian coast in the southern part of the 38° W meridian. In this sense, future complementary studies are indicated to be performed in our study region.