Characterising water mass and circulation change using heat and carbon co-variability during the anthropogenic era

Abstract

The ocean is a major sink of both CO₂ and heat, having absorbed approximately a third of cumulative carbon emissions to data and 93% of the additional heat contained in the climate system. Whilst not straightforward, it is possible to identify this additional carbon in the ocean, allowing the quantification of the global and regional ocean carbon sinks. However, for a number of reasons, it is far more difficult to decompose heat changes in an analogous fashion in order to identify the ‘excess’ heat. In this work, two new and related techniques are developed for the identification of excess heat. The first removes the ‘anthropogenic’ carbon signal from total carbon changes, leaving changes in carbon which are not driven by increased atmospheric CO₂. By relating the remaining changes in temperature and carbon, the redistributed temperature is identified, and the excess isolated by residual. This technique is applicable to additional tracers, for example salinity. This technique is demonstrated in the NEMO GCM, finding significant excess salinity changes generally precede excess temperature changes, but that excess temperature changes dominate later in the model run. Previous work has also shown that changes in anthropogenic carbon and excess heat are linked by a transient response coupling, and are therefore linearly related. By combining the previous technique and this transient response coupling, a second technique, which does not require an explicit carbon decomposition is developed. This technique is then applied in the Subtropical North Atlantic, and to the full GLODAP dataset, to produce global fields of excess heat and salinity accumulation. As expected, excess heat content increases smoothly with time, with the majority of excess heat accumulation in the upper thousand metres. Additionally, the rate of excess heat storage is higher in the Atlantic than in other ocean basins, in agreement with previous studies. Patterns of excess salinity storage are less spatially uniform and exert a strong influence on excess density changes, suggesting that changes to the water cycle may impact ocean circulation to a similar or greater degree than additional heat content.

More information

Identifiers

Catalogue record

Export record