02 Climate oscillation in the North Atlantic

In addition to this shift in the intensity distribution towards the higher categories, changes may also be observed in the total frequency in some regions. The number of cyclones occurring throughout the world every year on average is 80 (margin of deviation: 20) without any distinctive trend.

A general increase in frequency is observed in the North Atlantic since 1970, that means from a comparatively cool period to the current "warm phase" in terms of sea surface temperatures (Fig. 3). Accordingly, the hurricane season of 2005 has set an absolute record in terms of the number of named tropical storms (27, old record 21) and hurricanes (15, old record 12).

If further research findings of recent years are taken into account (Goldenberg (2001), Science; Trenberth (2005), Science), the result for the North Atlantic is such that cyclone activity is determined there both by a natural climate oscillation and by a superimposed linear warming process — most probably not explainable without anthropogenic global warming.

There are alternating phases lasting for several decades with exceptionally warm or exceptionally cool sea surface temperatures, the margin of deviation being around 0.5°C. The natural climatic fluctuation is driven by the ocean's large-scale currents (thermohaline circulation, Knight et al. (2005) GRL, Willoughby/Masters (2005)). Warm phases produce a distinct increase in hurricane frequency and also more intense storms, whereas cold phases have the opposite effect. So in the current warm phase, for example, 4.1 strong hurricanes have already occurred per year on average while in the previous cold phase this figure only was 1.5 (this means an increase by 173%). Of course, a definitive value for the average annual level of activity for the whole of the current warm phase can only be given when this phase has ended. The figures correspond to the observation possible up to 2005.