Malte Jansen
University of Chicago
"Understanding the Deep Ocean’s Response to Global Temperature Change on Time Scales from Decades to Millennia"
Abstract: Changes in the deep ocean circulation are thought have played a major role in past climatic changes and may again play an important role in the future. Climate model simulations suggest a weakening and shoaling of the Atlantic Meridional Overturning Circulation (AMOC) in response to global warming, and some recent observational studies lend support to this result. Proxy data, however, also suggest a shallower and possibly weaker AMOC during the cold glacial climate, which may appear at odds with the expectation of a weaker and shallower AMOC in a warmer climate. To make sense of these seemingly opposing results we examine the expected response of the deep ocean overturning circulation to global temperature change on time-scales from decades to millenia, using a hierarchy of models of varying complexity. Our results suggest that the ocean’s equilibrium response to global cooling is associated with a shoaling and weakening of the AMOC (consistent with proxy evidence for the last glacial maximum), while warming results in a deepening and strengthening of the AMOC. However, the transient response is reversed, with surface warming initially causing a shoaling and weakening of the AMOC - consistent with results from coupled climate simulations. The initial shoaling, which is attributed to a rapid warming of the North Atlantic, occurs on decadal time-scales and lasts for centuries. The eventual recovery and strengthening of the AMOC instead is shown to be associated with diffusive adjustment of the abyssal ocean and continues for many millennia. The results highlight the fundamental difference between the transient and equilibrium response of the ocean circulation to changes in the climate, which needs to be considered when attempting to use past climates as analogs for the coming decades or centuries.
Biography: Malte Jansen, is an Assistant Professor in the Department of the Geophysical Sciences and the University of Chicago. His research aims to improve the understanding of the dynamics of the oceans, the atmosphere, and the coupled climate system. He is particularly interested in the processes that govern the fluxes of heat and other constituents in the ocean. Understanding the mechanisms of these transport processes is key to understanding the changes in the climate system during Earth’s past and future. He tackles these questions using a combination of theoretical fluid dynamics, numerical simulations and analysis of observational data.
