Validation of Aquarius sea surface salinity with Argo: Analysis of error due to depth of measurement and vertical salinity stratification

We validate Aquarius sea surface salinities against Argo 1-7 m salinities for the period 27 August 2011 through 1 October 2013, a period of similar to 25 months. The validation consists of comparison of 20,149 collocated Argo/Aquarius data pairs. The global mean of the difference between Aquarius and Argo salinities is +0.018 PSU, with latitudinal variations of approximately +/- 0.2 PSU. The standard deviation of this difference ranges from about 0.3 PSU in the tropics to 0.7 PSU at high latitudes. We discuss errors due to geographic and temporal displacement and depth of measurement and show that these are insignificant for global validation of Aquarius. In particular, we use NASA's Tropical Rainfall Measuring Mission 3B42 product to analyze the contribution of vertical salinity stratification in the ocean to validation error. This salinity stratification is important for understanding the hydrological cycle of the oceans and has been cited as a potential source of error in validation of satellite-based sea surface salinity because of the similar to 1-7 m minimum depth of most Argo profiles. We show evidence of heavy precipitation events causing stratification greater than 0.1 PSU and lasting similar to 2-8 h, but note that these events occur infrequently and contribute less than 0.03 PSU bias in the tropics and 0.025 PSU globally. It is demonstrated that the existing global Argo array provides sufficient data for large-scale validation of Aquarius sea surface salinity. We also discuss the potential to exploit large salinity gradients in the upper mixed layer as a signature of rain in the tropical ocean.