Upper Ocean Response to Rain Observed From a Vertical Profiler

Abstract

Rainfall induces a vertical salinity gradient directly below the ocean surface, the strength and lifetime of which depend on the size of the rain event, the availability of mixing, and the air-sea heat fluxes. The presence of rain in turn influences the near-surface turbulent mixing and air-sea exchange processes. During a campaign in the midlatitude North Atlantic, the Air-Sea Interaction Profiler (ASIP) was used to investigate changes in the vertical distribution of salinity (S), temperature (T), and turbulent kinetic energy dissipation rate (ϵ) caused by four rain events. During one of the rain events a strong shallow stratification was formed. The buoyancy effect of this freshwater lens changes the dominant wind-driven turbulent mixing. The surface momentum flux was limited to a shallow layer, and below it ϵ is reduced by 2 orders of magnitude. For a different rain event of higher-peak rain rate, the salinity anomaly is smaller and is dispersed deeper into the water column. The difference in ocean response shows that the upper ocean is sensitive to changes in the atmospheric forcing associated with the rain events. The observed salinity anomalies as a function of rain rate and wind speed are compared to relationships from studies with the 1-D turbulence model GOTM and satellite validation. The observations suggest that the vertical salinity anomaly is best described as a function of total rain. A higher-resolution prognostic model for sea surface salinity and temperature is shown to perform well in predicting the observed S and T anomalies.