Introduction
Sea-surface temperature (SST) anomalies over the Niño 3.4 and Niño 1+2
regions over the central and eastern equatorial Pacific Ocean that had remained
negative throughout the second half of 2010, has
since January 2011 become
positive. Atmospheric and oceanic indices over the equatorial Pacific Ocean in the
past 3 months are showing signs that the prevailing La Niña condition has weakened
and neutral ENSO condition is forecasted for the coming months from May to July.
Although the La Niña condition is weakening, however the impacts of it on the
atmospheric circulation in the equatorial western Pacific is still found to be strong
with enhanced convection over the maritime continent of the western Pacific and
Indonesian region, while suppressed convection over the eastern Pacific Ocean
continues. For the first four months of the year, no named tropical storms occurred
over the West North Pacific or South China Sea. Only two tropical depressions
formed in the northern South China Sea in early April and dissipated within a few
days without making landfall. The near equatorial trough associated with the InterTropical Convergence Zone (ITCZ) that was located south of the equator in the early
part of the year has slowly progressed closer to the equator and in April it is slightly
north of the equator. This has started to strengthen the southwesterly flow over the
Indian Ocean while the easterly trade winds over the Southeast Asian region has
begun to weaken, indicating the transition to the southwest monsoon is getting
closer.
1.
Weather Conditions during April 2011
The mean winds at 925-hPa level for April 2011 (Figure 1) shows that light
northeasterly winds of less than 10ms-1 are dominant over the South China Sea, it is
an extension of the easterly trade winds from the western Pacific Ocean. Over the
north Indian Ocean westerly winds were very dominant
and it has pushed the
monsoon trough to about 5ON, that now extends from the northern Indian Ocean
cutting through southern Peninsular Malaysia and into Borneo Island. The winds
over the Malaysian region and Bay of Bengal were generally light and variable in
April.
Figure 1 : 925-hPa Mean Wind Analysis for April 2011 (Wind Speed ms-1)
The mean zonal wind averaged between 10ON and 30ON at 850-hPa for April
2011 is shown in Figure 2. Generally westerly winds were dominant west of 100OE,
throughout April. There were alternating periods when the westerlies penetrated
eastwards into the South China Sea and Western Pacific. During the middle of April
it is very noticeable that the westerlies are becoming more well established as it
continues to strengthen and progress eastwards. By the last week of April westerly
winds were dominant over most of Southeast Asia.
Figure 2 : Time Series of Mean Zonal Averaged between
10ON - 30ON at 850-hPa for April 2011 (Wind Speed ms-1)
2. Outgoing Long Wave Radiation (OLR)
The time series of the OLR anomalies averaged between 7.5OS - 7.5ON from
January to April 2011 is shown in Figure 3. Drier-than-normal condition is indicated
by the positive OLR anomalies and wetter-than-normal conditions is indicated by the
negative OLR anomalies. Over the equatorial region between 90OE and 135OE,
there was generally enhanced convection throughout the entire period. These were
contributed mainly by the eastward propagating Madden Julian Oscillation (MJO)
from the Indian ocean and the westward propagating easterly wave from the western
Pacific Ccean. During this period the MJO had a much longer period of about 60
days, whereas the easterly wave had a period of between 30 to 45 days.
Figure 3 :Time Series of OLR Anomalies (Wm-2) Averaged
Between 7.5OS - 7.5ON from January to April 2011
The monthly distribution of OLR anomaly shown in Figure 4, clearly indicates
that January and March 2011 were the wettest months over the Southeast Asian
region. While in February and April enhanced convection was observed only in
northern Thailand, parts of Sumatra and Jawa, Sulewasi, east Malaysia and
southern Philippines and the rest fo the Southeast Asian region was drier than
normal. The phases of the MJO and the easterly wave are the main reasons for this
anamoly pattern in the convection distribution.
(a)
(b)
(c)
(d)
Figure 4 : Distribution of OLR Anomalies (Wm-2) for (a) January 2011,
(b) February 2011,(c) March 2011 and, (d) April 2011
3. Weather Outlook for June-July-August 2011
The rainfall anomaly forecast for the period of June to August generated using
the seasonal rainfall prediction data from the European Centre for Medium Range
Weather Forecast (ECMWF) is shown in Figure 5. Over most parts of Southeast
Asia, the rainfall anomaly is expected to be within the normal range (-20% to +20%).
Only southern Myanmar, the northern tip of Sumatra Island, Brunei and coastal
areas of eastern Sarawak and western parts of Mindanao Island is expected to
receive slightly below normal amount of rainfall. Above normal amounts of rainfall is
expected over southern Kalimantan, southern and eastern Sulawesi Islands, the
Maluku Islands, southern part of Papua in the Indonesian Archipelago and Luzon
and Cebu Islands of the Philippines.
Figure 5 : June-July-August 2011 Rainfall Anomaly (%) Forecast