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!).

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