On North Atlantic Intedecadal Variability: A Stochastic View

The North Atlantic climate system is characterized by considerable interdecadal variability. We show examples of interdecadal variability in Figs. 9.1 and 9.2. One of the main modes of the atmosphere over the North Atlantic is the North Atlantic Oscillation (NAO) (e.g. van Loon and Rogers (1978), Hurrell (1995)). The NAO is a dipole in sea level pressure (SLP), with centers of action near Island and the Azores (Fig. 9.Ib), originally described by Walker (1924) and Walker and Bliss (1932). Hurrell (1995) defined an index of the NAO by the difference of the SLPs measured at Lisbon (Portugal) and Stykkisholmur (Iceland). Its time evolution (Fig. 9.1a) exhibits considerable interdecadal variability, with a maximum during the beginning of this century, a minimum during the 1960s, and strongly increasing values thereafter up to present. Moreover, there are observed fairly regular quasidecadal [O(10 years)] variations during the most recent decades. The relatively strong upward trend observed during the last 20 years which contributed strongly to the rise in mean Northern Hemisphere surface temperature (Hurrell (1996)) has been the matter of intense scientific debate, since it is not clear as to whether this trend reflects greenhouse warming or is simply an expression of interdecadal variability.