A quick glance from an airplane window shows that the ocean looks kind of blue, whereas a satellite view reveals varying shades that reflect the water depth and the amount of material floating near the surface. Waters thick with phytoplankton and dissolved organic matter absorb and scatter light near the surface, shifting the visual properties of the water and changing its appearance.
One variation of light-attenuating material in the water is known as colored detrital matter (CDM), which tints the water yellow. CDM is an agglomeration of dissolved organic molecules and detritus; most of it is material washed from the land into the ocean during storms, but it can also form in situ or be seafloor lifted to the surface by vertical ocean currents. Scientists predict that increasing precipitation in the Northern Hemisphere in the future will deposit more of this yellow material in the coastal aquatic environment.
In a recently published study, Kim et al. explored how outwash affects ocean heating and temperature. The research offers insights into how inputs from coastal environments may alter oceans and how those alterations could influence global climate change.
The researchers ran two Earth system model simulations for 300-year time spans, with CDM data collected by NASA’s Moderate Resolution Imaging Spectroradiometer instrument. The models contained fully coupled land-ocean-atmosphere-ice components. In the “green ocean” control simulation, only phytoplankton attenuated light; in the “yellow ocean” experimental simulation, both phytoplankton and CDM attenuated light.
In the model simulations, the yellow ocean was more opaque than the green ocean, and the conditions in the yellow ocean resulted in lower sea surface temperatures, reduced ocean heat content, and decreased sea surface height after 300 years. Globally, the temperature of the upper 700 meters in the yellow ocean simulation was colder than in the green ocean simulation, which triggered a thermal contraction of ocean waters and a 6-centimeter reduction in sea surface height.
At the surface, the yellow ocean simulation indicated increased solar heating between depths of 0 and 10 meters but less sunlight (and cooler waters) at 20 to 30 meters depth. This result suggests that detritus leads to shallower light absorption, preferentially heating water close to the surface.
The results run counter to recent observations of ocean warming and suggest that an influx of CDM may mitigate the effects of a warming climate. Although the simulations may not match the actual magnitude of changing light conditions in the future ocean, they do highlight how dissolved organic matter can influence ocean heating and circulation. The study illuminates a path to future work exploring the linkages between land and ocean processes, particularly at high latitudes. (Geophysical Research Letters, https://doi.org/10.1029/2018GL077297, 2018)
—Aaron Sidder, Freelance Writer