9 On North Atlantic Intedecadal
Variability: A Stochastic View
MOJIB LATIF, AxEL nMMERMANN, ANSELM GROTZNER, CHRISTIAN ECKERT,
REINHARD Voss
Max-Planck-Institute jUr Meteorologie, Hamburg, Germany
9.1 Introduction
The North Atlantic climate system is characterized by considerable interdecadal
variability. We show examples ofinterdecadal variability in Figs. 9.1 and 9.2. One
ofthe main modes ofthe 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. 9.1b), originally described by Walker (1924) and Walker and Bliss
(1932). Hurrell (1995) defined an index ofthe NAO by the difference ofthe SLPs
measured at Lisbon (Portugal) and Stykkisholmur (lceland). Its time evolution
(Fig. 9.1a) exhibits considerable interdecadal variability, with a maximum during
the beginning ofthis century, a minimum during the 1960s, and strongly increasing
values thereafter up to present. Moreover, there are observed fairly regular quasidecadal [0(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.
Similar low-frequency variations on interdecadal time scales can be observed in
the North Atlantic Ocean (Fig. 9.2). The potential temperature in the Labrador Sea
in the depth range 800-2000m (curve labeled c), which is a good index of convection in this region, exhibits a time evolution, which is highly anti-correlated with
the NAO index (Dickson et al. (1996». Colder (warmer) temperatures imply stronger (weaker) convection, so that the cooling trend observed in the Labrador Sea
since 1970 can be identified with enhanced convection. Interdecadal changes are
also observed in the convective activity in the Greenland Sea (Fig. 9.2d) and in the
salinity ofthe Sargasso Sea (Fig. 9.2e). The behaviours in the Labrador and Greenland Seas during the recent decades can be readily understood by atmospheric forcing. As the NAO strengthened and the Icelandic low intensified during the last few
decades, more polar air masses are blown over the Labrador Sea, resulting in
colder SSTs and stronger convection. Likewise temperatures in the Greenland Sea
rised and convection weakened. This is visualized by Fig. 9.3 which shows the
trends in SST and SLP observed over the last few decades. The trend in sea level
pressure (Fig. 9.3b) is very similar to the NAO pattern (Fig. 9.1b), and consistent
Variability: A Stochastic View
MOJIB LATIF, AxEL nMMERMANN, ANSELM GROTZNER, CHRISTIAN ECKERT,
REINHARD Voss
Max-Planck-Institute jUr Meteorologie, Hamburg, Germany
9.1 Introduction
The North Atlantic climate system is characterized by considerable interdecadal
variability. We show examples ofinterdecadal variability in Figs. 9.1 and 9.2. One
ofthe main modes ofthe 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. 9.1b), originally described by Walker (1924) and Walker and Bliss
(1932). Hurrell (1995) defined an index ofthe NAO by the difference ofthe SLPs
measured at Lisbon (Portugal) and Stykkisholmur (lceland). Its time evolution
(Fig. 9.1a) exhibits considerable interdecadal variability, with a maximum during
the beginning ofthis century, a minimum during the 1960s, and strongly increasing
values thereafter up to present. Moreover, there are observed fairly regular quasidecadal [0(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.
Similar low-frequency variations on interdecadal time scales can be observed in
the North Atlantic Ocean (Fig. 9.2). The potential temperature in the Labrador Sea
in the depth range 800-2000m (curve labeled c), which is a good index of convection in this region, exhibits a time evolution, which is highly anti-correlated with
the NAO index (Dickson et al. (1996». Colder (warmer) temperatures imply stronger (weaker) convection, so that the cooling trend observed in the Labrador Sea
since 1970 can be identified with enhanced convection. Interdecadal changes are
also observed in the convective activity in the Greenland Sea (Fig. 9.2d) and in the
salinity ofthe Sargasso Sea (Fig. 9.2e). The behaviours in the Labrador and Greenland Seas during the recent decades can be readily understood by atmospheric forcing. As the NAO strengthened and the Icelandic low intensified during the last few
decades, more polar air masses are blown over the Labrador Sea, resulting in
colder SSTs and stronger convection. Likewise temperatures in the Greenland Sea
rised and convection weakened. This is visualized by Fig. 9.3 which shows the
trends in SST and SLP observed over the last few decades. The trend in sea level
pressure (Fig. 9.3b) is very similar to the NAO pattern (Fig. 9.1b), and consistent