IMAGES - The hydrological cycle and ocean temperatures: A paleo-perspective

Abstract

runoff . Such records would provide information on past changes in the hydrological cycle, in which surface ocean temperature and circulation changes are strongly tied with fl uctuations in precipitation over the continents. PAGES has identifi ed landocean climatic linkages as a major objective for the coming years. As water vapor exerts a strong eff ect on Earth’s climate, improved future scenarios of climate change need to take into account changes in the hydrological cycle more than before. The principle processes of past linkages between ocean circulation changes, ocean-atmosphere water exchange, and shifts in the hydrological regimes over the continents are well understood. However, not much is known about the magnitudes of change, such as variations in the evaporation-precipitation balance between the ocean and adjacent continents or between diff erent ocean basins. Quantitative estimates of this kind could be very helpful in testing the performance of Earth system models with a hydrological cycle and therefore, essential for more accurate climate change predictions. The study of past changes in surface ocean temperatures and salinity is one of IMAGES major challenges. Examples from the equatorial Atlantic using paired Mg/ Ca and δ 18 O measurements on planktonic foraminiferal shells, show that the western and eastern basins have historically experienced very diff erent sea surface temperature and salinity variations, particularly during the Younger Dryas Period and the Holocene (Fig. 1). This can be explained by diff erences in the response to remote and local forcing. The western equatorial Atlantic is strongly infl uenced by the thermohaline circulation via the Brazil Current, whereas the eastern Atlantic is intimately coupled to the West African monsoon moisture transport and continental runoff and thus depends on changes in tropical ocean surface temperatures as well as on shifts of the Intertropical Convergence Zone (ITCZ).