Risk Wise

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• Sea surface temperature (SST) anomalies

• Soil moisture anomalies

• Atmospheric initial conditions favourable to such a

climate extreme.

A drought can also be exacerbated through a regional

positive feedback in which low soil moisture reduces

local evaporation and in turn precipitation. For

example, the strong drought of the US States of

Oklahoma and Texas in 1998 was established by the

SST anomalies combined with a favourable atmospheric

circulation during April and May. In June-August, the

regional positive feedback associated with lower evap-

oration and precipitation contributed significantly to

the maintenance of the drought. In the autumn, the

drought ended as stronger large-scale weather systems

penetrated the region and overwhelmed the soil-mois-

ture feedback.

The interplay of SST anomalies and atmospheric inter-

nal forcing on local precipitation may be examined using

the long-term measurement of precipitation in Seoul and

the monthly SST data with a resolution of 2.5 degrees

for the period of 1854-1997.

Many modelling studies have shown the significant

effect of soil moisture anomalies in late spring on the

summer rainfall anomalies in North America. On the

other hand, the role of soil moisture seems insignificant

in East Asia.

This conflicting impact of local feedback

and dynamic forcing is attributed to a weakened sensi-

tivity of the monsoonal circulation to the initial soil

moisture. Hence, drought production mechanisms in

Asia are further complicated by large-scale variations in

the Asian monsoon evolution, which affect more than

60 per cent of the world’s population.

South and East Asia are two of the areas with the

highest air pollution and aerosol concentrations due to

rapid population and economic growth. In this region,

air pollution emissions containing aerosol particles

produce widespread layers of brownish haze, so called

‘atmospheric brown clouds’ (ABC). The particles in ABC

(including black carbon, ash, dust and NO

) result in

millions of deaths annually, decreased photosyntheti-

cally active radiation and global dimming (having direct

impacts on ecosystems), stabilized surface-atmosphere

system during dry seasons, altered SST gradients, and

altered regional rainfall patterns – thereby leading to

global cooling and drying.

The aerosol index (AI) is a measure of how much the

wavelength dependence of backscattered ultraviolet

radiation from an atmosphere containing aerosols differs

from that of a pure molecular atmosphere. In South and

East Asia, AI has been continuously increasing with

greater interannual variability, indicating growing

aerosol emissions for the past three decades. Numerous

modelling studies have described the potential effects

of aerosols on monsoon rainfall over the South Indian

subcontinent. Recent observational studies suggest that

although anomalously high aerosols are associated with

deficient precipitation in South Asia in early spring,

• Positive feedback through coupling between water and carbon

cycles in terrestrial ecosystems.

Indebted to the invention of the first rain gauge in 1441 and the pioneer-

ing measurement network around the Korean Peninsula by King Sejong

of the Chosón dynasty, the cyclic nature of drought at a local scale can

be traced through one of the longest precipitation records in Seoul.

Typically, drought is considered relative to some long-term average condi-

tion such as normal. In the case of the annual precipitation from the

traditional Korean rain gauge,

Chugugi

(1777-1907)

and the modern

rain gauges (1908-2007), the recent 30-year normal precipitation is about

1,400 mm. Such a normal can change, however, by more than 30 per

cent depending on the averaging intervals selected from the 230-year

period, reflecting dramatic changes in climate for the last two centuries.

The recurrent dry and wet periods particularly with a prolonged drought

approximately from1880 to 1910 are worth noting. Considering its cyclic

nature, such a severe and extended drought, that has fortunately not

occurred in the past century, may return sooner or later.

The impacts of strong and persistent drought span environmental,

economic and social sectors and can be reduced through proactive

mitigation and preparedness. Establishing drought climatology for a

region provides a better understanding of its characteristics and prob-

ability of recurrence at different levels of severity. Such information

is extremely valuable in the development of mitigation strategies and

preparedness plans. On the other hand, the onset, end and severity

of drought is difficult to determine, requiring better understanding

of the mechanisms that originate, maintain and end an extreme

drought. The potential mechanisms that can produce drought condi-

tions are:

ENSO-like patterns in the tropical pacific

This image was obtained by regressing the unfiltered global Sea Surface

Temperature (SST) field (upon the annually anomalous precipitation time

series of Seoul for the period of 1854-1997). What we observe is an El

Niño Southern Oscillation (ENSO)-like pattern over the tropical Pacific. The

leading pattern of analysis for lowpass-filtered (8-yr) SST indeed reveals

features similar to interdecadal variability of the tropical Pacific (not

shown). These results suggest that there is a link between the long-term

anomalous precipitation of Seoul and ENSO-like variability. It is also

evident that precipitation in Seoul is associated with the Indian Ocean

basin-wide warming. In this analysis, due to the lack of long-term soil

moisture data, it is impossible to examine a physical interaction with soil

moisture anomalies