Ocean temperature and salinity responses to 50 year changes in surface conditions

Changes in the global ocean's temperature and salinity in the last decades are evidence of the Earth's warming climate. These interior ocean changes are driven by changes in ocean surface fluxes of heat, freshwater and momentum. The warming atmosphere induces increased Sea Surface Temperature (SST) and amplifies the water cycle (evaporation-minus-precipitation) and in turn the Sea Surface Salinity (SSS) pattern. There is strong evidence of significant changes in the temperature and salinity fields in the ocean interior, but little is known of the relative contribution to these trends from each of the surface forcings. Changing surface winds also impact on ocean circulation and penetration of surface properties into the ocean interior. However, coincident wind changes that have occurred alongside ocean temperature and salinity changes are not well known and available reanalyses that provide our only coherent insight to wind changes are sometimes contradictory. Using a global ocean model, changes from each independent surface forcing is decomposed in idealized experiments. The results show that to reproduce the observed pattern of salinity changes in each major ocean basin using a density space coordinate for the analysis, both the SSS pattern amplification and SST increase need to be taken into consideration. Changes in SSS are transmitted to the subsurface salinity field mainly through subduction while the warming ocean results in migration of isopycnals relative to the mean salinity field creating apparent salinity changes in density space. The SSS pattern amplification results in a subsurface warming in the ventilated gyres and subpolar regions of similar amplitude to that from heat subducted as a result of the increased SST. Warming in the subpolar regions is mainly driven by the reduced convective heat loss as a result of the fresher and less dense surface water which strengthens vertical stratification. To investigate wind changes three different reanalyses datasets are used to compare how imposing the surface forcing due to wind changes affect the subsurface properties. Equatorial cooling between 100 and 300 m across the Pacific and Indian Oceans is consistent across datasets, which is in agreement with a reported strengthening of the trade winds. There is a ~0.5 to ~1oC and ~0.1 PSS-78 changes under the subtropical gyres induced by changes in the surface wind reanalyses but the results are not inconsistent with different reanalyses winds and are inconsistent with the available observations. This suggests that there are some inconsistencies in the available wind reanalyses which affects the temperature and salinity under the subtropical gyres, where the ocean is most sensitive to small changes in wind stress curl. Each of the three surface forcings acts linearly together in these idealized experiments to produce most of the observed key features of multidecadal subsurface temperature and salinity changes as estimated from a suite of independent temperature and salinity analyses.