Understanding Ocean Circulation: Science vs. Climate Skeptics

“the conveyor-belt model no longer serves the community well— not because it is a gross oversimplification but because it ignores crucial structure and mechanics of the ocean’s intricate global overturning” [Lozier, 2010: 1511]

"I repeat my earlier assertion: if the conveyor belt model is wrong then none of the IPCC's model results can be taken seriously” (Global warming Skeptics). 

Conceptual approaches to global ocean circulation are the subject of today’s post. Before asking paleoclimatological questions concerning how ocean circulation may affect climates past and present, it is vital we understand ocean circulation. As indicated above, this can frame our imagination of the ocean’s role in the climate system.

Wallace, S. Broecker (1991: 79) hypothesised a 1-dimensional ‘ocean conveyor belt’ model to describe the role of the oceans in influencing climate (Figure 1).In a nutshell, the oceans transport water from the North Atlantic via the ‘lower limb’ of the conveyor to the rest of the Global Ocean (Broecker, 1991: 79). It is here that water is upwelled and transported to the oceans, and then to the ‘upper limb’ (This can take several thousands of years), repeating the process (Broecker, 1991: 79). 

Figure 1: The Ocean Conveyor Belt

Source: Broecker (1991).

This analogy has often been used to describe the ‘thermohaline circulation’ (THC) (the term can be accredited to Stommel, 1961), which refers to a circulation based on salinity and temperature. The THC works on the premise that if the temperature of surface water increases, the density of the surface water will decrease and it will have increased buoyancy compared with deep water, and addition of freshwater to locations where sea sinks to depth are often considered the starting point of the circulation (North Atlantic deepwater) (Wunsch, 2002). This circulation has been framed in the context of the climate change debate, with concerns over future inputs of freshwater (subject of a later blog post!). The THC must be considered as a separate entity from the conveyor belt as it refers to transport of heat and salt, not a circulation whose mechanics are not yet known (Wunsch, 2002). The ocean conveyor belt has come under increasing scrutiny and the term ‘meridional overturning circulation’ has been postulated as an alternative (Lozier et al., 2010, Wunsch, 2010).

The ocean conveyor belt or (MOC) in the context of ocean circulation, is inaccurate for several reasons:

1)      The conveyor belt is an analogy- while it is a useful to describe how heat is conveyed from the tropics to high latitudes, the model is not the subject of reality (Wunsch, 2002, Wunsch, 2010).If our predictions become shaped by this analogy as in THE DAY AFTER TOMMOROW when the paleoclimatologist shows a similar diagram to the politicians, we then should be worried! (Sorry Dennis Quaid…)

2)      The model contradicts ocean physics - it is focused on one region, the North Atlantic, where water sinks and fills the deep ocean. However, water elsewhere in the ocean moves in a 3 D fashion, thus the metaphor and geophysical fluid dynamics become polarised. (Wunsch, 2002).

3)      Recent work emphasises the greater role of tidal motions and wind-field (Tyrell, 2011) - convective motion cannot generate the turbulence to carry the circulation as viewed in the model, thus discrediting the notions of continuous upper and lower limb (Wunsch, 2002).  Tidal motions are different today due to lower sea levels than in glacial times, shifts qualitatively can influence ocean mixing (Wunsch, 2002). Wind fields can determine areas of sinking and account for the 3 D properties that are observed (Wunsch, 2002). Whilst our understandings of both have improved vastly, the goal of quantification is still afar.      

4) Also the Overturning and mechanisms associated with this are uncertain (issues of which are subject of later blogs).

The fact is clear, air temperatures on Earth are projected to rise between 1.1- 6.4 °C for the 21^st century (Tyrell, 2011). Even if we achieve climate stabilization, we are committed to approximately 0.5 °C regardless, with the slow response of the oceans a key factor in that, partly due to basic chemistry (Tyrell, 2011).Water has a higher specific heat capacity (4 J cm^-3 K^-1 than air (0.001 J cm^-3 K^-1 ) at sea level, meaning the heat capacity of the ocean is approximately 1000 times the atmosphere in total (Tyrell, 2011). Therefore, it is of paramount importance (as will be shown in later posts) that our imagination of ocean circulation is as accurate as possible if we are to make successfully predict the effects on climate, past and future. 

So climate skeptics, rather than view ocean circulation as a conveyor belt, take heed of Wunsch (2002: 1179):

‘The ocean is thus best viewed as a mechanically driven fluid engine, capable of importing, exporting and transporting vast quantities of heat and freshwater, this transport is a nearly passive consequence of mechanical machinery’.

It’s just that we do not understand the engine yet…..