Earth’s oceans are a complex system of interconnected transport highways for heat, nutrients, and the transfer of carbon dioxide between the atmosphere and sea. Meridional overturning circulation is the process by which these key components move from the tropics poleward to the subtropics. Research has found that an increase in this circulation pattern can be beneficial as it results in the ocean storing more heat and therefore draws down global temperature.
New research published in Nature Geoscience focuses on one particular current, the western boundary current in the Pacific, which is a major component of the Equatorial Undercurrent. Also termed the Cromwell Current, this subsurface current at 200 m depth flows eastwards at a rate of 1.5 m/s across the length of the equator in the Pacific Ocean.
Scientists from the National Taiwan University and their collaborators assessed changes in this current over the last century using coral records of the genus Porites obtained from live organisms in the Solomon Sea. This particular coral preserves two distinct nitrogen isotopes (15N and 14N) and the ratio of these is known to decrease as temperature increases.
In addition to this, the researchers found that the western boundary current had strengthened over this time, leading to a consequent strengthening of the Equatorial Undercurrent. They suggest that a slowdown of global warming during the 1940s and 1970s may be attributed to surface waters cooling in the eastern equatorial Pacific.
The overturning circulation occurring in the upper ocean takes place in subtropical-tropical cells, which are predominantly driven by surface winds. Here, subtropical water subducts in the eastern Pacific Ocean and travels west, as well as towards the equator, in the pycnocline layer, which separates surface water from deep water. Upon entering the Equatorial Undercurrent, the water upwells to the surface at the equator and is driven back to the subtropics by surface winds in the Ekman layer. The Solomon Sea, where this research is based, is a western boundary between the equator and subtropics.
2023-07-28 02:48:03
Original from phys.org rnrn