Carl Wunsch, The Economist and the Gulf Stream

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.

Gulf Stream SSTThe 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).

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