Past, present, and future variations of the Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of the global ocean-atmosphere climate system. Due to its role in transporting warm waters northwards, some studies suggest that past shutdowns of the AMOC are associated with abrupt global cooling periods. Although the Coupled Model Intercomparison Project Phase 5 (CMIP5) predict an AMOC shutdown to be unlikely by 2100, a weakening is very likely. On the other hand, studies struggle to reach a consensus regarding whether the recent past or present AMOC has declined, and if so, whether it is due to anthropogenic climate change. This stems from the complexity of the relative contribution of natural and anthropogenic forcings on AMOC variability, and limited observations; the longest continuous trans-basin array being RAPID since 2004. Results from the first chapter of my thesis suggest that 28 to 35 years of data are required to detect a declining AMOC trend (i.e., over a decade more than current observations). This reinforces the demand for improved proxy estimates to understand the AMOC’s long-term behavior. The other two chapters therefore investigate potential reconstructions or indicators of the AMOC’s variability and trend in the past and the future. Using a multi-model approach (with CMIP5 data), the second chapter demonstrates that sea surface height does not have a robust 13-year relationship with the AMOC (even though they are presumably dynamically associated), and therefore tidal gauge data cannot be used to extend the RAPID data into the past. The third chapter then explores salinity transport as a proxy of the AMOC. Firstly, a trend probability analysis is done using the CMIP5 past and future scenarios (from 1850 to 2100) to represent the forced trends and the control simulations representing natural variability. Results show that ensemble mean forced interannual AMOC trends are not signiﬁcantly aﬀected by anthropogenic forcing, however decadal to multi-decadal trends are. Under the ‘business-as-usual’ future scenario (RCP8.5) the probability of a 20-year ‘intense’ decline remains above 50% in the 21st century whereas the ‘stabilisation’ scenario (RCP4.5) recovers its pre-industrial values by 2100. In the second part of this chapter, simultaneous unique features in the AMOC and salinity transport define a 20-year rogue from 1995 to 2015, including the maximum probability and magnitude of an ‘intense’ AMOC decline, and a sustained 20-year decline in subpolar salinity transport caused by internal oceanic (as opposed to atmospheric) feedbacks. This work therefore highlights the potential use of direct observations (after another decade of data), and ensemble mean numerical models to represent changes in past, present, and future natural and forced AMOC variability.