The oceans take up roughly 90 per cent of the heat absorbed by the Earth. Hence it is no surprise that significant warming trends have been observed in the mid to intermediate depths of the ocean over the past few decades consistent with a warming climate. However, detection of significant warming trends of the deep and abyssal waters of the ocean has been more challenging because of the limited number of long-term temperature observations.
Meinen et al  take advantage of a decade long, hourly temperature time series fortuitously associated with a moored array in the south-west Atlantic that was primarily designed to measure the ocean transport as part of the South Atlantic Meridional Overturning Circulation (SAMOC). What was surprising was the range of variability captured by the continuous time series ranging from daily to interannual variations, as well as the long-term warming trends detected at all sites.
The shorter period variations less than a month are likely related to tides and topographic interactions of currents and waves. The deep and abyssal waters are originally formed at the surface in the polar regions where the densest waters sink to eventually flood the bottom depths of the global oceans. The annual to decadal variations including the long-term trends carry with them the fingerprint of changes in the air-sea forcing and global warming experienced in the source regions.
Along with the continuation of time series such as the SAMOC array, the development and future implementation of a global profiling array of deep Argo floats that measure temperature and salinity at depths down to 4000 to 6000 meters will enable better monitoring of long-term changes in the deepest layers of the ocean.
Citation: Meinen, C. S., Perez, R. C., Dong, S., Piola, A. R., & Campos, E. . Observed ocean bottom temperature variability at four sites in the Northwestern Argentine basin: Evidence of decadal deep/abyssal warming amidst hourly to interannual variability during 2009–2019. Geophysical Research Letters, 47, e2020GL089093. https://doi.org/10.1029/2020GL089093
—Janet Sprintall, Editor, Geophysical Research Letters
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