A recent study by researchers at UNSW Sydney and the UK National Oceanography Centre has revealed how ocean warming has changed the pattern of salinity observed at the surface of the ocean. Their results, published last month in Environmental Research Letters, have significant implications for our ability to measure changes to Earth’s water cycle in a changing climate.
Changes in ocean salinity – the concentration of salt in the ocean – is an important signature of past changes in the water cycle. Over a given area of the ocean, net evaporation increases salt concentration, while net precipitation lowers salt concentration. Observed changes in surface salinity reveal global warming trends: in particular, regions of the globe with high salinity are become more salty, while regions that have low salinity are become less salty.
Figure (a) Mean fresh water flux out of the ocean from reanalysis data. (b) Mean sea surface salinity from observational analysis. From Zika et al. Environmental Research Letters (2018).
This has led climate scientists to think of the ocean itself as a “rain gauge” that can reveal historical changes in the water cycle, which is tightly linked to climate change. Global climate models project overall increases in evaporation and precipitation and rainfall extremes, with wet regions of the globe getting wetter and dry regions getting ever drier. Changes in the global water cycle will critically impact environmental, agricultural, and energy systems relied upon by humanity.
However, the relationship between surface salinity patterns in the ocean and changes in the water cycle is puzzling. While the surface salinity pattern has increased by 5-8% since 1950, the water cycle has changed by a smaller amount, 2-3%, over the same period. So what is driving the discrepancy?
Dr Jan Zika (UNSW Sydney) and colleagues at the National Oceanography Centre in the United Kingdom addressed this puzzle using realistic numerical global ocean models. Their results indicate that surface ocean warming is a key and previously overlooked process driving sea surface salinity changes. Warming increases near-surface stratification, which amplifies surface salinity patterns. As a result, approximately half of the observed surface salinity pattern changes can be accounted for by ocean warming, with the remaining changes due to melting ice and changes in the water cycle.
Dr Zika’s research indicates that changes in Earth’s water cycle can be monitored using sea-surface salinity observations, once ocean warming effects are appropriately accounted for.
Citation: Zika, J.D., Skliris, N., Blaker, A.T., Marsh, R., Nurser, A.J.G. and Josey, S.A. . Improved estimates of water cycle change from ocean salinity: the key role of ocean warming. Environmental Research Letters, 13 (7). https://doi.org/10.1088/1748-9326/aace42