Abstract

Anthropogenic climate change threatens to disrupt the global water cycle and consequently the human and ecological systems which rely on freshwater. Salinity observations offer a route to quantifying historical water cycle change in the face of scarce direct observations of rainfall, runoff and evaporation. Due to the dynamic nature of ocean circulation and mixing, however, robustly relating local (i.e. in a Eulerian reference frame) surface salinity changes to local fresh water fluxes remains challenging. In this presentation, we harness water mass-based methods to define insightful metrics for water cycle change based on ocean salinity observations.

First, we explore whether fresh regions of the ocean are getting fresher and salty regions are getting saltier by analysing the distribution of sea water as a function of salinity. As the contrast between fresh and salty regions intensifies, the distribution becomes wider. This widening in the distribution can be expressed as a metric which can be attributed to water cycle change.

Second, we investigate the climatological regions these changes in fresh water transport occur between, namely, the hot tropics, warm sub-tropical regions or cool sub-polar regions. To do this we consider the ‘T-S’ curve of classical oceanography. From 1970 to 2014, the T-S curve has become ‘curvier’, indicating a dramatic shift in fresh water from the warm to cold oceans – a trend not captured by climate reconstructions.

Finally, we present a new method based on optimal transport theory which allows us to attribute air-sea water flux changes to geographically distinct regions. The method optimises the contributions of mixing, circulation and air-sea heat and fresh water fluxes necessary to explain observed changes in the ocean’s water mass distribution. Based on this approach, we provide new estimates of the geographical distribution of water flux changes over the ocean surface.