Climate Sense

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Changes in the climate of

East Asia tropical cyclones

Johnny C. L. Chan, Guy Carpenter Asia-Pacific Climate Impact Centre, City University of Hong Kong

ropical cyclones (TCs) are one of the most devastating

weather phenomena. Over the ocean, they pose extreme

hazards to all vessels, from small fishing boats to large

container ships and oil tankers. As they make landfall, they can

bring about heavy rain and severe winds, causing significant

damage or even fatalities in the coastal regions. A better under-

standing of the climate variability of TC activity in a region can

help produce good seasonal predictions of TC activity, which

will enable relevant authorities to be better prepared for poten-

tial disasters. Such an understanding will also help project

changes in TC activity under global warming.

Some recent results in understanding the changes in the climate

of East Asia TCs based on the analyses of various datasets are

presented here. All these results show that the frequency of TC

landfall in various parts of East Asia and that of TC occurrence

in the entire western North Pacific Ocean have large variations

on timescales, ranging from annual to decadal. Because of these

large variations, it is difficult to detect any long-term trend that

may exist in such occurrences. However, this does not necessar-

ily mean that global warming will not cause a trend in the future.

How such changes may be projected for the future under global

warming scenarios is also discussed.

Landfall in the Philippines during the last 100 years

The Philippines is frequently hit by TCs every year. It is there-

fore of great importance to understand whether the frequency of

landfall has any detectable trend. Recently, Kubota and Chan

created a dataset of the annual number of TCs making landfall in

the Philippines by combining the historical observation records

of the Monthly Bulletins of Philippine Weather Bureau and the

US Joint Warning Typhoon Center best-track data for the period

1902 to 2005. During these 104 years (except for the Second

World War period), the annual number of landfall TCs does not

show any long-term trend. Rather, the annual landfall number

has an apparent oscillation of up to 32 years before 1939 and

between 10 and 22 years after 1945. Such variations are found to

be related to different phases of the El Niño/Southern Oscillation

(ENSO) and the Pacific Decadal Oscillation (PDO). During the

low PDO phase, the annual number significantly decreases in El

Niño but increases during La Niña years. During the high PDO

phase, however, the difference between ENSO phases becomes

unclear. These results suggest that natural variability related to

ENSO and PDO phases appears to prevail in the interdecadal

variability of the annual number of TCs making landfall in the

Philippines.

Landfall in East Asia during the last 60 years

On average, about 12-15 TCs every year make landfall

in various countries in East Asia, which can produce

significant casualties and substantial property loss.

Understanding the variability in the annual number

of landfalling TCs in various parts of East Asia is

therefore crucial in disaster preparedness and mitiga-

tion. Chan and Xu

recently made a detailed study

of such variability during the period 1945–2004 by

dividing the coastal areas into three regions: South

(Philippines, Vietnam and south China), Middle (east

China, including the island of Taiwan) and North

(Korean Peninsula and Japan). Consistent with the

findings already described, none of the time series

of the annual number of landfalling TCs for each of

these three regions shows a significant linear trend.

Instead, each time series shows large interannual (2–8

years) and multi-decadal (16–32 years) variations. In

some periods, the annual number of landfalling TCs

varies in unison among all regions of Asia. In others,

one region would have an above normal number of

landfalling events, while the other regions have below

normal numbers. Changes in the frequency of landfall

in various regions on multi-decadal timescales have

been found to be related to variations in the atmos-

pheric flow patterns on similar timescales, which led

to changes in the steering flow and hence the TCs

made landfall in different locations.

Tropical cyclone activity in the western North

Pacific Ocean during the last 50 years

Among all ocean basins, the western North Pacific

has the largest share of TCs,

with an average of

up to 27 tropical storms and typhoons per year.

However, this number varies quite significantly

from year to year, ranging from a low of 17 in 1998

to 39 in 1964. It is also obvious from the graphed

data that this number does not have any rising or

decreasing trend during the last 50 years. Similarly,

no trend can be detected in the annual number of

the strongest of these TCs, those with intensities

in the Category 4 or 5 in the Saffir-Simpson scale.

It must be pointed out that before 1965, no mete-

orological satellite was available and therefore some

TCs might have been missed. Furthermore, in the

early days of meteorological satellite coverage, the

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