Jeff Masters, a meteorologist at wunderground.com has blogged about the Atlantic multidecadal mode, with a figure showing the AMM index even [LINK]. Unfortunately he doesn't give his references, although I'm sure he isn't making anything up.
The weird thing is that he says, "One of the leading theories is that changes in the ocean's salt content causes a speed up or slow down of the Gulf Stream, due to the fact that density differences between lighter fresh water and heavier salty water drive weaker and stronger ocean currents, respectively. This circulation (called the "thermohaline circulation") is thought to cause the warm phase of the AMO and warmer Atlantic SSTs when it speeds up, and cooler SSTs and a cool AMO phase when it slows down." This doesn't actually make sense to me, and I think Masters just got a little confused about some terms. The gulf stream is a result of western intensification... also known as a western boundary current. It is almost entirely wind driven; basically the trade winds blow the water south and west over the tropical Atlantic and the Coriolis force, conservation of momentum, and the existence of the continent (stopping the water from going further west) all combine to make a very strong, narrow current, which we call the gulf stream. There are similar currents on all the major western boundaries. The thermohaline circulation, as its name tries to imply, is driven by density differences of sea-water that arise from differences in temperature and saltiness. Thus, the two currents are fundamentally different phenomena (wind (dynamics) versus density (thermodynamics) ). My guess is that Masters just shouldn't have said "gulf stream" at all, as the rest of the sentence seems to implicate the THC. Even so, it is a useful post, and a good blog.
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