Decrease In Thermocline Depth Linked To Climate Changes

By James Dennett

Branwen Williams and Andrea G Grottoli, who are both students of the School of Earth Sciences at the Ohio State University, recently completed and published the findings of a scientific study that used the physical characteristics of ocean coral in the eastern Pacific to determine how the boundary between the warm surface water and the colder, deeper water has shifted over time. According to the researchers, the movement of this boundary, known as the thermocline, can be linked to the increase in the ocean’s average temperature due to global warming.

Much like trees, when coral plants grow, they continually add new growth rings around their existing “trunk”, and if a section of the trunk is cut open, scientists can observe the chemical makeup of each ring, as well as their size and spacing, to determine what the environmental conditions were during the period in which the ring was growing. The reason that the researchers used coral samples is because directly measuring the position of the thermocline over a large period of time and over a large area is both time-intensive and incredibly difficult. Additionally, some of the coral samples yielded information dating back almost 100 years, which is a large enough time span to allow some sort of pattern to emerge.

In order to use the chemical composition to identify the location of the thermocline, Williams and Grottoli focused on the ratio between two common nitrogen isotopes, nitrogen-15 and nitrogen-14. The amount of each isotope present depends chiefly on the amount of nutrients present in the water, with greater amounts of nitrogen-14 present in areas with a higher nutrient concentrations, and higher amounts of nitrogen-15 present in areas with a low nutrient level. Armed with the knowledge that ocean water above the thermocline consistently has a lower nutrient level than water below the thermocline, the researchers hypothesized that they could deduce the average thermocline movement over the last century using only the nitrogen isotope ratio present in the coral samples.

When conducting their research, Williams and Grottoli sampled coral trunks at depths of 5 metres, 85 metres, and 105 metres, so that their observations were not concentrated in a small area of water, which would have reduced the usefulness of their results, because it would not have given them a very accurate representation of “the big picture”.

After completing their field research and tabulating the data they collected, Williams and Grottoli found that a sudden decrease in the depth of the thermocline occurred sometime between 1976 and 1977, which they state corresponds with a shift in the Pacific Decadal Oscillation, or PDO, which is a pattern of climate variation that shifts phases approximately every 20 to 30 years. In 1977, the PDO shifted into what is called a “warm” phase, which results in an overall cooling of the western Pacific Ocean – the area in which Williams and Grottoli conducted their experiment. This corresponds with their results showing a decrease in the depth of the thermocline, which suggests that the thermocline depth is directly influenced by the temperature of the surrounding environment. This means that if global warming continues – assuming that it is happening at all – a large shift in thermocline depth may be observed sometime in the near to mid future.

The implications of an abrupt thermocline shift have not been totally deduced yet, due to a lack of historical data, but the researchers suggest that the upwards thermocline shift may increase the size of the nutricline, the area of the ocean with sufficient nutrients to sustain plant life, which would lead to an overall increase in both plant growth as well as the area of the ocean which can support plant life. However, the researchers do not mention whether or not this increase in plant life will have more positive or negative implications on the surrounding environment.

Both Williams and Grottoli agree that at this point in time, they are only able to speak with certainty about the thermocline level in the small area of ocean they were able to study, but they say that they hope to repeat the experiment several more times, moving gradually east across the Pacific Ocean, so that they can gain a more complete understanding of global thermocline movement, and what it could mean for the environment.

References

Williams B. and Grottoli, A. 2010. Recent shoaling of the nutricline and thermocline in the western tropical Pacific. American Geophysical Union