Deconstructing the Ocean Conveyor Belt
Monday, June 21, 2010 12:27:35 PM
Let me start this post on ocean circulation with a brief description of three concepts.
1. Thermohaline circulation. This term refers to the part of the large-scale ocean circulation that is driven by density. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content. Temperature and salinity together determine the density of sea water. Cold water is heavier than warm water, and salt rich water is heavier than salt poor water. At the poles cold dense water sinks to the bottom and flows towards the tropics where it return to the surface through upwelling.
2. Global conveyor belt. This term stresses the fact that the ocean circulation flows through all oceans and transfer heat from the tropics to the polar regions. Here I prefer the image of the circulation below, where the currents around Antarctica are better visible.
3. Meridional overturning circulation. (Meridional means "along a meridian" or "in the north-south direction"). This term stresses that the the ocean currents are overturned by whatever forces. It is the pattern followed by currents which is driven by wind, density, or other factors.
For decades, oceanographers have embraced the idea that Earth’s ocean overturning currents operate like a giant conveyor belt, continuously transporting deep, cold polar waters toward the equator and warm equatorial surface waters back toward the poles along narrow boundary currents. Thus more or less assuming that the three concepts I described above are more or less one and the same thing. But are they? And is the conveyor belt model not only an extreme oversimplification (no doubt about that), but what is worse also totally wrong?
A paper in the 18 June 2010 issue of Science reviews the growing body of evidence that suggests it’s time to rethink the conveyor belt model.
The conveyor belt model ignores crucial elements such as eddies and winds, to a great extent because of lack of exact measurements. May I here refer to my post on Atlantic Meridional Overturning Circulation, where I wrote about the setting up of an array of instruments measuring the variations in the bottom pressure, temperature and salinity along 26.5°N. Results from this array are providing an unprecedented view of the temporal variability of the overturning and the challenges accompanying such variability. interestingly, the overturning transport showed more range over the course of one year than had previously been expected for decades of change. In the context of an ocean conveyor belt, such a result would have been surprising, but with the emerging understanding of the contributions from the ocean’s wind and eddy field to the transport of mass and heat in the ocean, such a result is decidedly unsurprising.
Despite such recent advances in the understanding of how the ocean’s overturning operates, the Science article also reviews what remains unknown about the ocean’s overturning. A major unknown question is: How do changes in the production of deep water masses at high latitudes affect overturning changes? As the surface waters warm and/or freshen due to climate variability and climate change, how might the overturning change? All such questions are important as there is clear and convincing evidence that the ocean waters do overturn and that this overturning impacts the Earth’s climate.
What I find an interesting question is whether the overturning is primarily caused by “push” or “pull” - pushed by wind or pulled by density. A question similar to the question in plate tectonics - are the plate movements “pushed” by upwelling magma at the spreading ridges, or “pulled” by cold, dense slabs at subduction zones. Today it is generally acknowledged that plate tectonics are driven by gravity. The gravity-controlled sinking of a cold, dense, oceanic slab into a subduction zone (called "slab pull") is considered the main driving mechanism. The role of upwelling at spreading ridges is however still debated.
Apart from raising questions about the validity of the conveyor belt conceptual model the author of the paper also reviews the emerging view of the overturning circulation within a historical framework that chronicles significant scientific developments in the field, from the first reported measurement of ocean overturning in 1751 through the present.