SYNOPTIC CLIMATOLOGY FOR OCTOBER 2015
The monthly mean circulation in the lower troposphere for October 2015 (Figure 1) shows that
the southern near equatorial trough is located close to its mean position of between 5°S latitude
and the equator from the Southern Indian Ocean until the Western Pacific Ocean at around
170°E. Meanwhile, the northern trough divided into two parts; lies along 10°N across Arabian
Sea to the Sri Lanka region before extending northwards to the Bangladesh region, and the
another part is stretching from Western Pacific Ocean at 10°N crossing the Philippines into the
South China Sea at 5°N. The northern subtropical ridge is located at 23°N in western part of
India to 32°N in eastern China, after which it extends to about 28°N in the western Pacific
Ocean. It is within its mean climatological position of 25°N – 35° N. The Peninsular Malaysia
and Sarawak experienced southeasterly winds for the October 2015, while Sabah received
southwesterly winds of 5 knots in this month.
The 200hPa monthly average atmospheric circulation of October 2015 is showed in Figure 2.
The subtropical ridge of the northern hemisphere is located between 15°N and 22°N across the
Indian subcontinent until the Western Pacific Ocean. It lies close to its climatological mean
position for October. Meanwhile, the southern hemisphere ridge is located over 10°S – 13°S
from the Indian Ocean to the Western Pacific Ocean at 13°S. It is located close to its average
climatological position for the month of October and remained quasi-stationary compared to the
last month. Upper level easterly to northeasterly winds of 10 – 20 knots prevails over our region,
during this period.
The decadal mean circulation patterns for October 2015 at the 850hPa level are shown in
Figures 3A, 3B and 3C, respectively. The northern subtropical ridge was well established in the
first two decades over a region of 20°N – 30°N across mainland China until the Western Pacific
Ocean. In the third decade, this subtropical ridge was split by two parts in the Western Pacific
Ocean; located between 10°N – 20°N and another one was between 20°N – 30°N. The northern
monsoon trough was well established during the first and the second decade. In the first decade,
Typhoon Mujigae emerged into the South China Sea, where weak vertical wind shear and
warm sea surface temperature favoured the development of the system. Due to radial outflow,
Mujigae began with a phase of rapid deepening, by convection wrapping around and JMA and
JTWC classify Mujigae as a typhoon Category 4. In the second decade, two typhoons were
formed over the Pacific Ocean, named Koppu and Champi (located just east of Koppu). The
interaction between these both typhoons and the warm air, caused the northern subtropical ridge
on top of them (over East China Sea) became strengthen. This subtropical ridge was effectively
trapping typhoon Koppu over the Philippines region for a number of days. The formation of
these three typhoon during the first two decade has significantly affected the wind pattern over
Malaysian region, where the winds blew southeasterly to southerly in the first decade and
southeasterly to southwesterly in the second decade. In the third decade, the wind that prevailed
over our region was easterly winds, which indicate the beginning of the monsoon season
(Northeast Monsoon) over Malaysian region. This easterly winds transport the moisture over the
Pacific basin into our region, which resulting more wet days in the third decade.
The mean circulation patterns of the three decades of October 2015 at 200hPa are depicted by
Figures 4A, 4B and 4C. The northern ridge has been remained quasi-stationary for the first and
second decades and lay within 15°N – 30°N. For the third decade, it lay within 10°N – 25°N and
shifted slightly southward compared to the first and second decades. In the southern hemisphere,
the ridge was broad and it lay within a range of 5°S to 15°S for the first and second decades. In
the third decade, the southern hemisphere ridge split into two parts. First part located over Indian
Ocean between 10°S and 15°S and then stretched over Indonesian region at 2°S to 7°S. The
second part located over Western Pacific Ocean and lay between 5°S and 15°S. Easterly to
northeasterly winds kept blowing to prevail over our region with the speed of 25 - 30 knots
during the first decade to around 10 - 20 knots during the second decade. Meanwhile, in the third
decade winds became decreasing to speed of around 5 - 10 knots over Malaysian region. The
wind direction became northwesterly and northeasterly, because of the outflows of Typhoon
Koppu and Typhoon Champi over the Philippines region.