CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) (FR)
Living Planet Fellowship research project carried out by Audrey Hasson.
Mode waters (MWs) transport a large volume of heat, carbon and other properties across basins at seasonal to longer time-scales and thus play a major role in the modulation of the Earth climate. In the context of anthropogenic global warming, unlocking the understanding of the MWs transport and characteristics is critical. MWs in the South Pacific Ocean are of particular interest because of their likely interaction with the El Nino Southern Oscillation (ENSO).
Variations in the MWs, their relation with the observed long-term changes and possible implication for ENSO remain unknown. This proposal offers to investigate the MWs characteristics in surface salinity (SSS), temperature (SST) and sea level (SL), which are all Essential Climate Variables (ECV) emphasized by three European Climate Change Initiatives (CCIs). Their link with interannual to longer time scale variability of the Pacific Ocean also need further examination.
MWs are subducted from the subtropical and sub-Antarctic Pacific mixed layers and subsequently flow equatorward at the subsurface or intermediate depth. They export the characteristics acquired at the surface into the subtropical gyre and the equatorial region. Surface observations can in consequence give us insight of the future characteristics found at depth at lower latitudes. According to IPCC (2013), it is likely that both the subduction of SSS anomalies and the movement of density surfaces due to warming have contributed to the observed changes in subsurface salinity. We will investigate properties of the formation areas and associated variations that will drive the volume and characteristics of the MWs.
As MWs shoal, they modify the equatorial mixed layer characteristics, and could affect ENSO events. Studies indeed have shown that western equatorial Pacific SST and SSS modulate ENSO through vertical stratification. We will therefore to characterize the mean MWs pathways, properties and associated variations.
In conclusion, the South Pacific Ocean is at the forefront of interannual variability to long-term modifications associated with climate change. It is therefore essential to study the observed SSS changes as they impact ENSO and SL variations. Satellite observations associated with in situ and modelling would ultimately enable us to unlock our understanding of the role of MWs SSS signature on interannual to longer timescale variability of the South Pacific Ocean.
The work done supported by the LPF has enabled to characterise the surface signatures of mesoscale eddies in the Southern Ocean and shows the unprecedented use of satellite salinity data to describe the interaction of eddies with subsurface MW. Much still need to be understood about eddies in the Southern Ocean but this study offers great perspectives on using the most recent satellite borne SSS datasets. Better understanding the interaction of MW with the ocean surface is vital to better characterise their role in the Southern Ocean overturning circulation and thus the current global changes in our climate.