Geoscience Reference
In-Depth Information
on the degree of cyclonic activity or of blocking in the Newfoundland-Greenland
area during the Northern Hemisphere winter and spring, with blocking leading to
severe drought in north-east Brazil. Later work showed a close link between SST in
the South Atlantic and drought in north-east Brazil, with drought unlikely when the
SSTs are warmer than 24.5
°
C and flooding unlikely when the SSTs are colder than
23.5
C (Markham and McLain,
1977
). Chang et al. (
1997
) found a decadal oscillation
between SST in the tropical Atlantic on either side of the equator, and concluded that
this dipole mode reflected ocean-atmosphere interactions.
°
23.9 The Pacific Oscillation and drought
We have seen that historic floods and droughts are associated with the zonal Walker
Circulation (see
Section 23.2
), which propagates from east to west across the equat-
orial Pacific and reflects the magnitude and frequency of ENSO events. The Pacific
Oscillation is another contributor to climatic instability in this region (Salinger et al.,
2001
; Mantua and Hare,
2002
; Pierce,
2002
). Salinger et al. (
2001
) recognised three
phases of what they termed the Interdecadal Pacific Oscillation (IPO) during the
twentieth century: a positive phase (1922-1944), a negative phase (1946-1977) and
another positive phase (1978-1998). During the most recent positive phase, mean sea
level atmospheric pressure (SLP) in the area west of 170
°
W increased compared to
the previous negative phase and decreased east of 170
W. The result was a 30 per cent
increase in annual precipitation north-east of the South Pacific Convergence Zone.
The IPO modulates the influence of ENSO in the Pacific region and can accentuate
it in some areas while weakening it in others. ENSO events only last about a year
and generally occur about every five to eight years, in contrast to IPO phases, which
last for several decades. Using corals and tree rings, Mantua and Hare (
2002
) found
evidence of periodicities associated with the Pacific Decadal Oscillation (PDO) dating
back to 1600, and they identified two main periodicities, one of fifteen to twenty years
and the other of fifty to seventy years.
°
23.10 Assessing drought severity
There have been various attempts to assess the severity of drought, of which the best-
known and most widely used is the Palmer Drought Severity Index (PDSI) (Palmer,
1965
). This index is a measure of drought severity and soil moisture deficit, and it
was designed to allow a rapid comparison between different regions in the United
States affected by drought, based on the rapid mapping of drought-affected areas.
Cook et al. (
1999
) provided a detailed explanation of the methods used to reconstruct
the PDSI for the continental United States between 1700 and 1978, based on 425 tree-
ring chronologies from across the United States, and concluded that the 1930s 'Dust
Bowl' drought was the most severe in this region since 1700. Cook et al. (
2004
;
2007
)
