Twelve people are now known to have died, with another eighteen injured and 27 missing after two landslides hit the village of Cihanjuang in West Java Province, Indonesia, on Saturday 9 January 2020. The first incident happened at about 4.00 pm local time, with a second landslide at about 7.30 pm burying several rescue workers trying to help people buried by the initial incident. The landslides are thought to have been triggered by heavy rain in the area associated with the northeast monsoon. Landslides are a common problem after severe weather events, as excess
pore water pressure can overcome cohesion in soil and sediments,
allowing them to flow like liquids. Approximately 90% of all landslides
are caused by heavy rainfall.
The aftermath of a pair of landslides that hit the village of Cihanjuang in West Java Province, Indonesia, on Saturday 9 January 2020. Antata Photo/Novrian/Reuters. Landslides are a common problem in Java, particularly during the Northeast Monsoon, which lasts from November to February, with peak rainfall in January and February, and can result in an annual
rainfall of around of 4000 mm in parts of Central Java. This problem has been made worse as expanding populations
has led to people farming higher on hillslopes, in an area where soils
tend to be volcanic in action and poorly consolidated (i.e. lack much
cohesion), making them more prone to landslides.
Monsoons
are tropical sea breezes triggered by heating of the land during the
warmer part of the year (summer). Both the land and sea are warmed by
the Sun, but the land has a lower ability to absorb heat, radiating it
back so that the air above landmasses becomes significantly warmer than
that over the sea, causing the air above the land to rise and drawing in
water from over the sea; since this has also been warmed it carries a
high evaporated water content, and brings with it heavy rainfall. In the
tropical dry season the situation is reversed, as the air over the land
cools more rapidly with the seasons, leading to warmer air over the
sea, and thus breezes moving from the shore to the sea (where air is
rising more rapidly) and a drying of the climate.
Java has two distinct Monsoon Seasons, with a Northeast Monsoon driven
by winds from the South China Sea that lasts from November to February
and a Southwest Monsoon driven by winds from the southern Indian Ocean from March to October. Such a double Monsoon Season is common
close
to the equator, where the Sun is highest overhead around the equinoxes
and lowest on the horizons around the solstices, making the solstices
the coolest part of the year and the equinoxes the hottest.
The winds that drive the Northeast and Southwest Monsoons in Southeast Asia. Mynewshub. Indonesia, and other countries of the western Pacific margin, are
experiencing a particularly wet year, due to a prevailing La Niña weather system over the Pacific. The La Niña weather system is the opposite of the El Niño weather
system, in which unusually cold surface temperatures spread across the
equatorial Pacific from the upwelling zone on the South American coast.
This traps warm water from the western Pacific, preventing it from
spreading east and warming the central Pacific. This leads to lower
evaporation over the (cooler) east Pacific, leading to low rainfall on
the west coast of South America, and higher evaporation over the
(warmer) west Pacific, leading to higher rainfall over East and
Southeast Asia and northern Australia.
The effects of a La Niña weather system in December-February. NOAA.
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