How is rain integrated into the mixed layer salinity structure?

Abstract:

Tropical rainfall has been shown to create small-scale near-surface salinity anomalies that contribute to the upper ocean salinity structure. It is known that atmospheric variability is associated with varying modes of rainfall. However, the implications of different rain modes and stratification conditions on how freshwater gets integrated into the mixed layer have not been investigated. Rain and wind observations from the SPURS-2 field campaign are used to identify the predominant modes of rainfall in the eastern tropical Pacific, which include convective rain events with leading and trailing stratiform rain. The General Ocean Turbulence Model (GOTM), a one-dimensional forced model, is used to determine the salinity response to idealized forcing conditions consistent with rain and wind observations at the SPURS-2 study site. The salinity response to rain events is evaluated by assessing the time and depths at which the salinity structure was modified following rain.

Model results demonstrate that freshwater mixes downward on faster timescales during convective events with leading stratiform rainfall. As a result, these events are associated with a shorter freshening signal at the surface but a longer freshening signal deeper in the mixed layer. In contrast, both isolated convective and convective with trailing stratiform rain events are associated with a salinity response characterized by persistent surface freshening. Existing stratification also significantly influences the salinity response to rainfall: stratification significantly inhibits the downward propagation of freshwater and results in a longer near-surface salinity signal in all rain forcing conditions. These findings indicate that freshwater is integrated into the mixed layer salinity structure on varying timescales dependent on both rain modes and background ocean stratification. Given the frequency at which different modes of rain occur, these results provide insight into the distribution of salinity in the upper ocean and suggest that significant salinity biases may exist between surface and deeper observations due to differing modes of rainfall.