Section 4.3: Surface Climate Analysis
65
Figure 4.6: Winter (November-March) pressure difference (mb) between
Ponta Delgada, Azores and Stykkisholmur, Iceland. Year dated by the January. Higher values indicate stronger westerlies over the North Atlantic.
30
25
20
15
10
Azores - leeland
1880 1900 1920 1940 1960 1980
The North AtlanticjEuropean region has probably been studied more than
other parts oft he world, almost certainly because ofthe availabilityoflong (>
150 years) records. The well-known out-of-phase relationship between winter
temperatures over northern Europe (especially Fennoscandia) and Greenland
has long been recognized (van Loon and Rogers, 1978). The link between
surface temperature anomalies in the region and the circulation of the N orth
Atlantic region is evident in indices such as the North Atlantic Oscillation·
(NAO). Figure 4.6 shows seasonal (November to March) values of the NAO
(difference in pressure between Iceland and the Azores). Stronger westerlies
prevailed during the first two decades of the twentieth century and again
during the 1970s and 1980s. The weakest westerlies occurred during the
nineteenth century and the 1960s.
In the North Pacific region a major change in the strength and position
of the Aleutian Low took place du ring the mid-1970s. After about 1977
the strength of the low pressure centre increased and moved slightly south
particularly in winter (Figure 4.7). In many respects the change may have
been a. return to the conditons that occurred during the 1920s, 1930s and
the early 1940s. Some of the change since the 1970s has reversed in the most
recent years. The effects of the change in the mid-1970s in the climate of the
whole North Pacific region have been discussed by numerous workers (e.g.
Douglas et a1. 1982; Nitta and Yamada, 1989 and Trenberth, 1990). The
decreased pressures led to greater advection of warm maritime air on to the
northwest of North America leading to enhanced precipitation and warmer
temperatures. Alaska has experienced the greatest regional warmth over the
65
Figure 4.6: Winter (November-March) pressure difference (mb) between
Ponta Delgada, Azores and Stykkisholmur, Iceland. Year dated by the January. Higher values indicate stronger westerlies over the North Atlantic.
30
25
20
15
10
Azores - leeland
1880 1900 1920 1940 1960 1980
The North AtlanticjEuropean region has probably been studied more than
other parts oft he world, almost certainly because ofthe availabilityoflong (>
150 years) records. The well-known out-of-phase relationship between winter
temperatures over northern Europe (especially Fennoscandia) and Greenland
has long been recognized (van Loon and Rogers, 1978). The link between
surface temperature anomalies in the region and the circulation of the N orth
Atlantic region is evident in indices such as the North Atlantic Oscillation·
(NAO). Figure 4.6 shows seasonal (November to March) values of the NAO
(difference in pressure between Iceland and the Azores). Stronger westerlies
prevailed during the first two decades of the twentieth century and again
during the 1970s and 1980s. The weakest westerlies occurred during the
nineteenth century and the 1960s.
In the North Pacific region a major change in the strength and position
of the Aleutian Low took place du ring the mid-1970s. After about 1977
the strength of the low pressure centre increased and moved slightly south
particularly in winter (Figure 4.7). In many respects the change may have
been a. return to the conditons that occurred during the 1920s, 1930s and
the early 1940s. Some of the change since the 1970s has reversed in the most
recent years. The effects of the change in the mid-1970s in the climate of the
whole North Pacific region have been discussed by numerous workers (e.g.
Douglas et a1. 1982; Nitta and Yamada, 1989 and Trenberth, 1990). The
decreased pressures led to greater advection of warm maritime air on to the
northwest of North America leading to enhanced precipitation and warmer
temperatures. Alaska has experienced the greatest regional warmth over the