2.7.3 Is There Evidence for Changes in Extreme Weather or
Climate Events?
In this section we assess changes in the intensity and frequency of various
weather phenomena. One aspect of change that is important, but which is beyond
the analysis of present records, relates to changes in the tracks of storms.
Severe storms are often rare, so the analysis of large areas and long lengths
of homo-geneous storm records are required to assess changes. So far this combination
of data is not available.
2.7.3.1 Tropical cyclones
This section updates the information provided in the SAR regarding changes
in tropical cyclones across various ocean basins and those affecting the nearby
continents. As reported in the SAR, a part of the multi-decadal trend of tropical
cyclones occurring in the Australian region (105° to 160°E) is likely
to be artificial, as the forecasters in the region no longer classify some weak
(>990 hPa central pressure) systems as “cyclones” (Nicholls et
al., 1998). By considering only the moderate and intense tropical cyclones (central
pressure 
990 hPa), this artificial
trend is eliminated. The remaining moderate and strong tropical cyclones reveal
a numerical decline since the late 1980s, but the trend is not statistically
significant. Similarly, the trend in intense tropical cyclones (minimum central
pressure below 970 hPa) is not significantly different from zero. Nicholls et
al. (1998) attributed the decrease in moderate cyclones to more frequent occurrences
of El Niño during the 1980s and 1990s. However, a weak trend in the intense
tropical cyclones implies that while ENSO modulates the total frequency of cyclones
in the region, other factors must be more important in regulating their intensity.
For example, new work by Higgins and Shi (2000) and Maloney and Hartmann (2001)
show that 30 to 80 day Madden-Julian oscillations modulate tropical cyclone
activity.
Figure 2.37: Top figure, decadal variations in hurricanes making
landfall in the USA (updated from Karl et al., 1995). Bottom figure,
interannual variability in the number of major hurricanes (Saffir-Simpson
categories 3, 4, and 5) and the long-term average across the North
Atlantic (from Landsea et al., 1999). |
As reported in the SAR, the north-east sub-tropical Pacific has experienced
a significant upward trend in tropical cyclone frequency in the short period
examined, but additional data since that time show no appreciable trend. There
is no appreciable long-term variation of the total number of tropical storm
strength cyclones observed in the north Indian, south-west Indian and south-west
Pacific Oceans east of 160°E. (Neumann, 1993; Lander and Guard, 1998). For
the north-west sub-tropical Pacific basin, Chan and Shi (1996) found that the
frequency of typhoons and the total number of tropical storms and typhoons have
been more variable since about 1980. There was an increase from 1981 to 1994,
which was preceded by a nearly identical magnitude of decrease from about 1960
to 1980. No analysis has been done on the frequency of intense typhoons (having
winds of at least 50 m/s) due to an overestimation of the intensity of such
storms in the 1950s and 1960s (Black, 1993).
There has been an extensive analysis of the North Atlantic basin for the entire
basin back to 1944, and also for the United States landfall tropical storms
and hurricanes back to 1899. The all-basin data, however, have been affected
by a bias in the measurement of strong hurricanes. This bias has been removed
in an approximate way to provide estimates of the true occurrence of intense
(or major) hurricanes since 1944 in the North Atlantic (Landsea, 1993). Earlier
events lack reliable data on the strong inner core of the hurricanes. The United
States record of landfall frequency and intensity of hurricanes is very reliable
because of the availability of central pressure measurements at landfall (Jarrell
et al., 1992). Both of these data sets continue to show considerable inter-decadal
variability, but no significant long-term trends (Figure
2.37, from Landsea et al., 1999). Active years occurred from the late 1940s
to the mid-1960s, quiet years occurred from the 1970s to the early 1990s, and
then there was a shift again to active conditions from 1995 to 1999. Concurrent
with these frequency changes, there have been periods with a strong mean intensity
of the North Atlantic tropical cyclones (mid-1940s to the 1960s and 1995 to
1999) and a weak intensity (1970s to early 1990s). There has been no significant
change in the peak intensity reached by the strongest hurricane each year (Landsea
et al., 1996). As might be anticipated, there is a close correspondence between
the intensity of hurricanes in the North Atlantic and those making landfall
in the United States (Figure 2.37).
Using historical records, Fernandez-Partagas and Diaz (1996) estimated that
overall Atlantic tropical storm and hurricane activity for the years 1851 to
1890 was 12% lower than the corresponding forty year period of 1951 to 1990,
although little can be said regarding the intense hurricanes. They based this
assessment upon a constant ratio of USA landfalling tropical cyclones to all-basin
activity, which is likely to be valid for multi-decadal time-scales. However,
this also assumes that Fernandez-Partagas and Diaz were able to uncover all
USA landfalling tropical cyclones back to 1851, which may be more questionable.
2.7.3.2 Extra-tropical cyclones
Extra-tropical cyclones are baroclinic low pressure systems that occur throughout
the mid-latitudes of both hemispheres. Their potential for causing property
damage, particularly as winter storms, is well documented, where the main interest
is in wind and wind-generated waves. In place of direct wind measurements, which
suffer from lack of consistency of instrumentation, methodology and exposure,
values based on SLP gradients have been derived which are more reliable for
discerning long-term changes. Over the oceans, the additional measurements of
wave heights and tide gauge measurements provide additional ways of indirectly
evaluating changes in extra-tropical storm strength and frequency (see Chapter
11, Section 11.3.3). Global analyses of changes in
extra-tropical storm frequency and intensity have not been attempted, but there
have been several large-scale studies. Jones et al. (1999c) developed a gale
index of geostrophic flow and vorticity over the UK for the period 1881 to 1997.
This revealed an increase in the number of severe gale days over the UK since
the 1960s, but no long-term increase when considering the century period. Serreze
et al. (1997) found increases in cold season cyclones in the Arctic region for
the period 1966 to 1993. Angel and Isard (1998) found significant increases
in strong cyclones (<993 mb) in the Great Lakes region from 1900 to 1990
during the cold season. Graham and Diaz (2001) find evidence for increases in
strong cyclones over the Pacific Ocean between 25 and 40°N since 1948 and
link the increase to increasing sea surface temperatures in the western Tropical
Pacific. Alexandersson et al. (1998, 2000) similarly studied extreme geostrophic
wind events in the north-western European area based on homogenised observations
during the period 1881 to 1998. These studies revealed an increase in the number
of extreme wind events around and to the north of the North Sea. The WASA group
(1998) similarly investigated the storm related sea level variations at gauge
stations in the south-eastern part of the North Sea. They found no long-term
trend during the last 100 years, but a clear rise since a minimum of storminess
in the 1960s, which is consistent with the rise in extreme geostrophic wind
found by Jones et al. (1999c). This increase is also consistent with changes
in the NAO (Figure 2.30). Some analyses have focused
on hemispheric changes in cyclone activity. Lambert (1996) analysed gridded
SLP over both the North Atlantic and North Pacific Oceans for the period 1891
to 1991. He found a significant increase in intense extra-tropical storms, especially
over the last two decades of his analysis, but the data were not completely
homogenised. Simmonds and Keay (2000) used data from 1958 to 1997 in the Southern
Hemisphere and found an increase in cyclone activity through 1972 before decreasing
through 1997 with strong decreases during the 1990s.
Hourly values of water levels provide a unique record of tropical and extra-tropical
storms where stations exist. Zhang et al. (1997a) have analysed century-long
records along the East Coast of the United States. They calculated several different
measures of storm severity, but did not find any long-term trends. On the other
hand, they did find that the effect of sea level rise over the last century
has exacerbated the beach erosion and flooding from modern storms that would
have been less damaging a century ago.
Another proxy for cyclone intensity is wave height (see Chapter
11, Section 11.3.3). Several studies report increased
wave height over the past three decades in the North Atlantic (approximately
2.5 cm/yr) and in coastal areas, though no longer-term trends were evident (Carter
and Draper, 1988; Bacon and Carter, 1991; Bouws et al., 1996; Kushnir et al.,
1997; WASA Group, 1998).
It appears that recent work points towards increases over time in extra-tropical
cyclone activity during the latter half of the 20th century in the Northern
Hemisphere, and decreased activity in the Southern Hemisphere. However, the
mechanisms involved are not clear, and it is not certain whether the trends
are multi-decadal fluctuations, or rather part of a longer-term trend. Furthermore
decreased cyclone activity in higher latitudes of the Southern Hemisphere is
not obviously consistent with an increase in the positive phase of the Antarctic
Oscillation in the last fifteen years or so (Section 2.6.6).
A more fundamental question is whether we would expect more or fewer extra-tropical
cyclones with increased warming. As pointed out by Simmonds and Keay (2000),
the specific humidity increases as temperatures increase, and this increased
moisture should enhance extra-tropical cyclones, but Zhang and Wang (1997) suggest
that cyclones transport energy more efficiently in a more moist atmosphere,
therefore requiring fewer extra-tropical cyclones (see Chapters
7 and 10 for more discussion).
