Variability of the Marine ITCZ over the Eastern Pacific during the Past 30,000 Years

Abstract

The Intertropical Convergence Zone (ITCZ) is manifested as a circum-global atmospheric belt of intense, moist convection and rainfall, marking the confluence of the northern and southern trades and the rising branch of the Had-ley cell. It regulates the hydrologic cycle over the tropical continents and interacts tightly with the tropical oceans, notably with the seasonal appearance of the equatorial cold tongues of the Atlantic and Pacific. While it undergoes a regular seasonal migration, today the ITCZ maintains a nearly permanent Northern Hemisphere bias. Here we address the question of variability in the mean latitude of the marine ITCZ over the eastern Pacific on time scales of 100–10,000 years, with emphasis on the past 30,000 years. Our strategy relies on reconstructing the intensity of the prominent oceanographic front of the cold tongue-ITCZ complex, using oxygen isotope and magnesium thermometry techniques. We show that a weaker cold tongue-ITCZ front prevailed during the last glacial maximum (LGM), which indicates a more southerly ITCZ at that time. We further show that the Holocene history of sea surface temperature (SST) near the Galapagos Islands is consistent with progressive southward migration of the ITCZ during the last ~7,000 years, in accord with records from South America and the tropical Atlantic. In the more recent past, evidence from eastern Pacific corals supports a northward ITCZ shift since the end of the Little Ice Age (LIA), in agreement with the hydrologie record of the nearby Cariaco Basin. Collectively, the evidence points to coherent behavior of the Pacific, Atlantic, and South American ITCZs over a broad range of time scales. All regions have responded to Northern Hemisphere cooling by southward (equatorward) ITCZ displacements. In the Pacific, such displacements are likely to have been unfavorable to divergent upwelling at the equator, resulting in weaker zonal and meridional SST gradients and more uniform equatorial SSTs, analogous to modern El Niño conditions.