Water cycle amplification inferred from broadening of the ocean’s salinity distribution

Abstract:

The water cycle leaves an imprint on ocean salinity through evaporation and precipitation. It has been proposed that observed changes in salinity could be used to infer changes in the water cycle. Here salinity is characterized by the distribution of water masses in salinity coordinates. Mixing acts to collapse the distribution, homogenizing salinity, while evaporation and precipitation stretch the distribution, maintaining the contrast between water masses. A simple model is developed to describe the relationship between the breadth of the distribution, the water cycle and mixing, the later being characterized by an e-folding (mixing) timescale. From observations, mean E-P over the ocean maintains the mean salinity distribution with a mixing timescale of order 50 years. Using observed changes in salinity we thus estimate that the hydrological cycle has increased by approximately 3% over the 60-year record of observations - although the spread in observations is large. In state of the art climate models we compute both the mixing timescale setting the steady (or nearly steady) balance in the pre-industrial climate and the time scale setting the response of ocean salinity to water cycle change. The models tend to be more diffusive than observations, potentially explaining why they display weak salinity amplification in response to warming.