Carl Wunsch usually has very interesting things to say about the climate system, and although his arguments don’t necessarily win everyone completely over, they often generate an improvement in the level of scientific discussion. In this week’s Economist, he has a letter printed concerning the mis-definition of the ‘Gulf Stream’ concept in the magazine’s climate change survey a couple of weeks ago. This is essentially a reprint of his letter to Nature that was published in 2004, which stated correctly that the Gulf Stream is basically a wind driven phenomenon and will not stop or reverse while the wind still blows and the Earth still turns.
The offending Economist statement was ‘The Gulf Stream is driven both by the rotation of the Earth and by a deep water current called the thermohaline circulation’ in an article discussing the likelihood of a ‘shutdown of the Gulf Stream’. Senso stricto, Wunsch is absolutely correct; the Gulf Stream in oceanographic terms refers to the very strongly intensified current on the western boundary of the Atlantic running from Florida to the Carolinas and which heads off into the mid-Atlantic at Cape Hatteras (see figure). These kinds of currents appear on the western boundaries of basins everywhere in the mid-latitudes and arise from the basic pattern of the winds (easterlies in the tropics, westerlies in the mid latitudes) and the rotation of the Earth (they do also require some kind of rotational gradient like you get on a spherical Earth – they wouldn’t exist on a cylindrical rotating planet, for instance – look up the ‘beta effect‘ if you are interested).
However, the Economist is using the term in a much more colliquial (and common) sense that conflates this current with the Meridional Overturning Circulation (MOC, often conflated with the Thermohaline Circulation) which involves convection in the waters around Greenland and the deep currents that cool the deep ocean. This use of the term is often synomymous with northward ocean heat transport (the North Atlantic Current) that contributes to Europe’s warmth and which have often been fingered as a particularly sensitive aspect of the climate. While in one sense the water flow associated with the MOC does contribute to the Gulf Stream, it is definitely the junior partner, and so any changes in the MOC are not going to threaten the Gulf Stream in any existential way. However, a shutdown in the MOC does not make as good a headline as a shutdown in the Gulf Stream, and so this misuse persists in the media and public alike (though not in The Day After Tomorrow – they used ‘North Atlantic Current’ throughout!).
If the definition of Gulf Stream was really all that this was about, I doubt Wunsch would have picked up his pen, however, what Wunsch really objects to is the casual use of the word ‘driven’. This is a much more subtle point and one which even the scientific community hasn’t fully assimilated yet. There is a standard theorem of oceanography called Sandström’s theorem which basically states that it is really hard to do any work on a system, like the ocean, if you are cooling and heating at the same level (i.e. the surface). By contrast, the atmosphere is heated from below (since the atmopshere is mostly transparent to solar radiation), and cooled from above and so can behave as a relatively efficient heat engine. Since we observe however that the ocean does have a large-scale deep circulation, it can’t therefore be ‘driven’ (in an energetic sense) from the cooling of waters in the high latitudes. The alternate name, ‘the Thermohaline Circulation’, does tend to perpetuate the idea that it is the temperature and salinity fluxes that ‘drive’ the circulation, and so is slowly falling into disfavour (though it is still widely used and defended).
But if this deep circulation doesn’t derive its energy from density contrasts, where does it get the energy from? Most of the energy in the oceans is derived from two sources, the winds and the tides. Both of these forces generate small scale turbulence and internal waves which cause mixing of ocean waters. It is this mixing which energetically fuels the deep ocean circulation.
Since the winds will continue to blow and the Earth continue to turn, does this mean that there can’t be any changes to the MOC? Emphatically no. The circulation may well derive it’s energy from the winds and tides, but it is heavily steered by density contrasts and the stratification of the ocean (witness the difference between the North Pacific and the North Atlantic). Changes in that modulation can have profound effects on the currents, and in particular, additions of fresh water from massive lake drainages (i.e. the 8.2 kyr event) or ice sheet collapses (the Heinrich events) most likely caused severe slowdowns or shutdowns of the MOC in the past. Wunsch is a little sceptical of this research (he calls fresh water the ‘deus ex machina’ of climate change), but in this he is probably mistaken – for instance, there is enough information from the 8.2 kyr event to reasonably attribute it to the drainage of Lake Agassiz into Hudson Bay.
Thus while density changes don’t ‘drive’ the circulation (in an energetic sense) they can ‘drive’ (in a modulating sense) changes in that circulation. If this seems complicated, think of the example of greenhouse gases – they don’t drive the climate in an energetic sense (the sun does), but they can drive changes in the climate (by modulating radiation flow in the atmosphere).
In summary, these definitional issues may seem like pendantry at times, but there is often a more subtle point of understanding at issue. It would be nice if everyone used words in the exact same way, but until that happy utopia dawns, letters like Wunsch’s are probably needed every so often to keep us all on the straight and narrow.
Update: Stefan rightly notes below that that the term ‘Thermohaline Circulation’ has a long history of applying similarly ‘driven’ flows and is not strictly synonymous with the MOC. His paper on the subject is worth reading.