Assessing the impacts of tropical rainfall on near-surface salinity and turbulence using observations from the SPURS-2 experiment and a 1-D forced model

In tropical areas of the world ocean, rain often falls intermittently in a patchy structure. This creates small-scale near-surface salinity anomalies which are mixed into the larger scale salinity structure through turbulent mixing, an important link in the global water cycle. During the recent 2016 and 2017 SPURS-2 cruises to the eastern tropical Pacific, measurements of turbulent kinetic energy (TKE) dissipation rate were collected from microstructure sensors at 37 cm depth from the ship-towed Surface Salinity Profiler (SSP). Atmospheric conditions varied significantly on the cruises and observations were made over a wide range of conditions.

These data have been used to show that dissipation rates and near-surface mixing were dependent on atmospheric conditions, particularly wind speed. Near-surface rain-formed stratified layers also significantly influenced turbulence levels, as dissipation rates were suppressed at 37 cm at low wind speeds when fresh lenses were generally confined above 37 cm, but enhanced within fresh lenses at higher wind speeds. The General Ocean Turbulence Model (GOTM), a one-dimensional forced model, was used to further explore the relationships between rain and near-surface salinity. GOTM simulations were compared to observations to determine the best model parameterizations for simulating near-surface salinity and dissipation rate, including the TKE equation, turbulence length scale, and wave breaking TKE injection. Additional simulations were run to investigate the effect different types of rain events, including low intensity long duration stratiform rain compared to intense short duration convective rain, have on near-surface salinity. These results provide insight into the mechanisms through which the low salinity region in the eastern tropical Pacific is created.