National Seminar on
"Socio-Economic Impacts of
Extreme Weather and Climate Changes."
June 21-22, 2007, Kuala Lumpur
Local to global scale atmospheric circulation features
associated with the floods in Malaysia in Dec.2006 and Jan.2007
- A Preliminary Analysis -
Tetsuzo YASUNARI
Hydrospheric-Atmospheric Research Center, Nagoya University
Collaborators:
Hiroshi TAKAHASHI, Hiroki ICHIKAWA,
Kunihiko KODERA, Masatake HORI,
Hironari KANAMORI and Hatsuki FUJINAMI
1. Introduction
Abnormally heavy rainfall occurred in southern part of the Malaysian
Peninsula for several days in late December 2006 and in the middle of January 2007,
associated with cold surges of the northeast winter monsoon, and caused severe floods
in the southern states of Malaysia (Malaysian Met. Department, 2006, 2007).
Some important and urgent meteorological and climatological questions are
raised here, listed as follows:
1) Why these two heavy rainfall events hit for the whole of the Peninsula, rather than
to hit the northeast coastal region as usual cold surge cases.
2) Why these two events were so abnormally severe, though several cold surge events
appeared in this winter monsoon season?
3) How these rainfall events might be connected with the anomalous climatic condition
of the winter of 2006/2007?
4) May these extreme events tend to occur more frequently in the global warming in
the future?
This paper addresses some quick diagnostic analysis of these events and
possible implication as a climatic extreme in the winter monsoon climate.
2. Regional monsoon circulation of the events
According to the cold surge index by Chang et al. (2005), the northeast winter
monsoon over the South China Sea started in the beginning of December 2006, and
strong cold surge events occurred 6 times during December through January. However,
strong surge events associated with heavy rainfall (HR) in southern Malaysia occurred
only two times in 17-20 December and 11-14 January. The difference of regional
circulation near the surface and lowter troposphere shows a distinct difference between
HR cases and other cases. The HR cases show stronger northeasterlies over the southern
South China Sea and its max. wind axis hit close to the southern tip of the Peninsula,
whereas the other cases northeasterlies hit northward with less strong wind speeds. A
notable feature for the HR cases are meso-scale cyclonic circulation near the equator,
which may be closely related to the meso-scale convective systems over the southern
Peninsula, bringing heavy rainfall there. Another interesting feature of the northeasterly
wind is its large-scale starting from east Asia near Korean Peninsula. Strong
northeasterly winds produces strong cyclonic shear zones along its southern flank of the
low level jet axis, which may also be responsible for meso-scale cloud systems (of
about 100-200km scales) over the South China Sea.
3. Tropical circulation and convective systems associated with the events
Some studies have noted that the so-called Madden Julian Oscillation (MJO)
disturbances passed in late December, but the 1st HR event occurred before the passage
of this MJO disturbance. However, relatively strong low level easterly along the equator
associated with the MJO disturbance over the Indian Ocean may have been favorable
for the HR events in Malaysia. In the beginning of January, a large-scale cyclonic vortex
(sometimes called the Borneo Vortex) appeared just after the passage of the MJO
disturbance due to the strong equatorial westerlies and cyclonic shear to the north
associated with a Rossby-wave response of the MJO disturbance. This caused a
moderate rainfall event in Malaysia, though it is not so strong as the HR events.
We may also need to note relatively high SST condition over and around the
Malaysian Peninsula due to El Nino condition up the January. Interestingly, this warm
SST condition in the western Pacific and the Indian Ocean was likely to be changed to
the La Nina condition in timing with the passage of the MJO disturbance.
4. Mid to high latitude circulations in the northern hemisphere
We have noticed that the HR events with the strong northeasterly cold surges
are associated with amplified and southward extended cold trough over the East Asia
coupled with a blocking high in the middle to upper troposphere (and even in the lower
stratosphere) over the north Atlantic. The similar circulation pattern was also noticeable
in the HR case in December 2001, associated with the abnormal hit of Typhoon Vamei
to Malaysia (Chang et al., 2005). In addition, the hemispheric atmospheric circulation
in the lower stratosphere shows strong upward motion near the equator (downward
motion in high latitudes) probably associated with the formation of blocking high in the
north Atlantic. This condition in the stratosphere may have been coupled with the
large-scale convection near the equator, though causal relation is, at this moment, not
yet clear. We should point out here, however, that the HR events are not local
phenomena, but definitely coupled with higher latitude circulation, and even with the
stratospheric circulation in the northern hemisphere.
5. Long-term change of the extreme rainfall events in Malaysia in the past decades
Preliminary analysis and review has also been made on the long-term change
or trend of rainfall and its extremes in winter monsoon season in Malaysia. Endo et
al.(2007), by using 40-year data (1961-2000) have examined linear trends of
precipitation over southeast and east Asia, suggesting that no clear trends have been
detected in Malaysian Peninsula, both in rainfall amount and heavy rainfall frequency.
However, our regional analysis for the recent 20 years suggest that the noreast monsoon
rainfall in the east coast seems to have shifted some along the coast of the main rainfall
zone, though further analysis is necessary.
6. some implication under the global warming climate
It should be noted here that the global warming in the recent years show
general weakening of the cold surge in the East Asia, and possibly of the northeast cold
surges to the southeast Asia. However, as shown by Endo et al. (2007) the winter
monsoon rainfall in Vietnum and south China show increasing trend, though it is not
clear in Malaysian Peninsula. It may be plausible in Malaysia that rainfall event like the
HR in 2006 and 2007 appears in a more complex manner due probably to interaction of
land-ocean and orography with slight change of strong jet axis of the cold surge from
the South China Sea. In fact, the HR events have suggested that to predict these events
we need new cold surge index to better indicate formation and devilment of meso-scale
cloud systems near and over Malaysian Peninsula.
In addition, a predicted composite winter monsoon circulation for the 2100s
associated with the IPCC Scenario (Hori and Ueda, 2006) has suggested that the global
warming in the future is likely to produce anomalous circulation with the intensified
Aleutian Low coupled with intensified north Pacific High coupled with warmer SST
condition in the tropical Pacific, which is, to some extent, a similar pattern to the
2006/2007 winter. We should keep in mind that possible warm winters in East Asia
in the future may not necessarily imply mild and weak northeast monsoon in Southeast
Asia.
References:
Chang, C.P., et al., 2005 : synoptic disturbances over the equatorial South China Sea
and the western maritime continent during the boreal winter. Mon.Wea.Rev.,
133, 489-503.
Endo, N. et al., 2007: Inter-annual and long-term variations in precipitation
characteristics in Southeast Asia. To be submitted.
Hori, M. E., and H. Ueda, 2006: Impact of global warming on the East Asian winter
monsoon as revealed by nine coupled atmosphere-ocean GCMs. Geophys. Res.
Let., 33, L03713, doi:10.1029/2005GL024961, 2006.
Malaysian Meteorological Department, 2006: Report on heavy rainfall that caused
floods in Johor, Melaka, Negeri Sembilan and Pahang during the period 17th
-20th December 2006.
Malaysian Meteorological Department, 2007: Report on the second heavy rainfall
episode that caused floods in Johor and southern Patang during the period 11th
-14th January 2007.