Abstract

I will start by asking how we can detect changes in the global water cycle from changes in ocean salinity. The water cycle transports over 2.5×10^9 litres of fresh water from high salinity regions to low salinity regions of the surface ocean every second. This fresh water is returned within the ocean by mixing. I frame this balance in terms of a distribution function of water mass in salinity coordinates. The distribution is made wider by the water cycle – creating greater contrast between high and low salinity – and collapsed by mixing – bringing these waters back towards a single salinity. The latter is characterised by an e-folding timescale of approximately 50 years. The salinity distribution has been found to widen over the observational period suggesting an amplification of the water cycle of approximately 3%.

I will then investigate the mechanisms by which heat is transported from the earth’s surface to the ocean interior offsetting global warming. To this end I will project ocean circulation into pressure-temperature coordinates – analogous to the Carnot Diagram of classical thermodynamics. Traditional models describe the deep circulation as a thermally direct ‘heat engine’ balanced by vertical mixing. It is suggested here that the deep ocean may have both a thermally direct and a thermally indirect ‘heat pump’ component – the latter driven by either wind and/or salinity effects. The two components of the circulation, the pump and the engine, can balance one and other in steady state. As the surface of the ocean warms under transient climate change, the pump continues to warm the deep ocean while the engine can be temporarily extinguished. Whether the ocean is purely a weak heat engine or has both a vigorous engine and pump is a strong predictor of how fast the ocean will absorb heat with global warming.

Finally, I will review some recent work on distilling the ‘thermohaline circulation’ of the ocean and the analogous ‘hydrothermal circulation’ of the atmosphere using thermodynamic coordinates and discuss opportunities for future work.