Timescales of the Ocean Salinity Response to Water Cycle Intensification
[12-Feb-2018] Levang, S. and Schmitt, R.W.
Presented at the 2018 Ocean Sciences Meeting
Global patterns of salinity are an imprint of the atmospheric water cycle, which tends to intensify as the climate warms, making fresh regions fresher and salty regions saltier. However, transport and stirring of salt in the ocean complicate the details of this response. Here we use the Simple Ocean Data Assimilation (SODA) v3.4.1 to investigate timescales of exchange between evaporative and precipitative regimes of the world oceans. Salinity does not have significant feedbacks on atmospheric freshwater fluxes, meaning salt behaves primarily as a passive tracer with a long 'memory' for freshwater forcing which occurs in the surface mixed layer. Therefore, a Lagrangian analysis of water parcels provides an upper bound on salinity mixing timescales without considering additional small-scale diffusive processes. By releasing a large ensemble of ~380,000 synthetic floats within the SODA velocity fields, a statistical distribution of exchange between basins is produced.
On the timescale of climate change, which occurs over decades to centuries, the exchange of water between the subtropics and tropics and across the equator is relatively fast. The exchange between subtropical and subpolar gyres, and between global ocean basins, is much weaker on this timescale. Therefore, the influence of new freshwater forcing should be felt quickly across the ocean surface at low latitudes, but slowly mixed between subtropical and subpolar gyres. These results agree with simulations from CMIP5, which show weak exchange of salinity changes across the gyre boundaries. This behavior has implications for density in the high-latitude deep water formation sites, in that local freshening is not significantly counteracted by subtropical salinification, and for use of the ocean as a 'rain gauge'.