Investigating the emergence of salinity and temperature changes in the ocean interior in response to human-induced warming and water-cycle amplification

Ocean temperature and salinity are key signatures of climate variability and anthropogenic forcings. On multi-decadal time scales, the upper ocean mean salinity patterns have been found to intensify, a change inconsistent with internal variability of the climate system alone but coherent with a human-induced amplification of the hydrological cycle and with concurrent ocean warming. Here, we will first review density-compensated salinity and temperature changes in the global ocean interior as signatures of simultaneous changes in heat and freshwater fluxes at the ocean surface, causing anomalies subducted along isopycnal layers. We will address when they can be expected to emerge from climate variability by conducting Time of Emergence (ToE) diagnostics at the local and regional scale using an ensemble of CMIP5 models. Then, we will explore the role and time scales of different mechanisms involved in causing salinity and temperature anomalies in the ocean interior, such as the individual response to an amplified water cycle, surface heat uptake or changing winds. To investigate such processes and their relative time scales, we propose a new modelling study in the context of the IPSL-CM6A-LR large ensemble, where a number of sensitivity experiments to individual surface flux perturbations are conducted on a fixed-flux ocean replicating the period 1850-2100, to disentangle these different processes. Passive temperature and salinity tracers are implemented in every simulation to quantify the evolution of the added heat and freshwater. We quantify locally and regionally the emergence of the anthropogenic temperature and salinity change from background climate variability for each of the physical processes individually and discuss the attribution of the total response in the large ensemble to these different mechanisms.