TKUALA LUMPUR MONSOON ACTIVITY CENT

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ALAYSIAN METEOROLOGICAL DEPARTMENThttp://www.met.gov.my

MINISTRY OF SCIENCE. TECHNOLOGY AND INNOVATIO

Introduction

Atmospheric and oceanic conditions over the tropical Pacific Ocean reflect

ENSO-neutral conditions from June to October 2016 but are more clearly trending

towards La Nina during September to October 2016. Near average Sea Surface

Temperatures (SST) were observed across the central and eastern equatorial Pacific

from June to July 2016, but negative SST anomalies were observed over the east-

central equatorial Pacific during September 2016 and expanded across the eastern

equatorial Pacific Ocean by early October 2016, The expansion of negative SST

anomalies in the central Pacific Ocean were reflected by the increasingly negative Nino-

3.4 index from June to October 2016. Anomalous upper level westerly winds were

observed near the central Pacific Ocean from June to October 2016 accompanied by

suppressed convection over the central Pacific Ocean and enhanced convection over

Indonesia. Weak La Nina conditions were observed during October with negative SST

anomalies in early November stretching across most of the eastern and central

equatorial Pacific Ocean, in conjunction with weakly enhanced lower-level easterly

winds near and west of the International Date Line, together with anomalously westerly

upper-level winds mainly west of the International Date Line. 'Multi-model averages

favour La Nina conditions continuing through winter (December-January-February)

2016-17 with ENSO-neutral favoured beyond February.

The Indian Ocean Dipole (100) index has been negative since May 2016 but it

was observed to have been gradually reverting to neutral since September 2016, and

expected to end in November 2016 according to ensemble mean forecasts. Negative

100 events are associated with anomalously warm waters in the southern South China

Sea, southern Indian Ocean and the Java Sea leading to enhanced convection in

Indonesia and the southern Maritime Continent.

The peak period of the western Pacific tropical cyclone season falls in September

and October, in which many countries in Southeast Asia experience wet weather due to

tropical storm or typhoon landfall or tail-effect. July to October 2016 witnessed twenty-

two tropical cyclones in the western Pacific Ocean reaching tropical storm intensity or

greater. It was close to the average value of 19.6 cases based on the Tokyo Regional

Specialized Meteorological Center (RSMC), over the same period. Half of these tropical

cyclones intensified into typhoons.

The Southwest Monsoon season, which lasts from May to October, is associated

with the rainy season over Southeast Asia north of 10oN, but dry weather conditions in

south of 10 oN.

Analysis of the monthly mean 925-hPa wind circulation indicated intense

southwesterly winds over the Bay of Bengal together with consistent southwesterly

winds over the Maritime Continent from July to October 2016. The intrusion of strong

southwesterly winds from the Bay of Bengal into the South China Sea was observed

from August to September 2016 marking the active phase of the Southwest Monsoon.

Meanwhile, the withdrawal of the Southwest Monsoon from Southeast Asia occurred in

October 2016 whereby direct penetration of easterly winds from the western Pacific

Ocean into Indochina occurred in conjunction with the weakening of the westerly winds

over the Bay of Bengal and the South China Sea in October 2016.

Figure 1: 925-hPa Mean Wind Analysis (ms-1) for (a) July 2016, (b) August 2016, (c) September

2016 and (d) October 2016

The mean zonal wind time series averaged between 10 oN and 30 oN at 850-hPa

from 1st July to 31st October 2016 is shown in Figure 2. Westerly flow is depicted by

yellow to red shading, while easterly flow is indicated by purple shading. Strong westerly

winds from the Bay of Bengal (70 to 100 °E) intruded into the South China Sea (100 to

1200E) in July, August, and September of 2016. However, from mid-September

onwards, easterly winds from the western Pacific Ocean advanced westwards and

prevailed over the South China Sea in October 2016, indicating the withdrawal of the

Southwest Monsoon season.

Figure 2: Time Series of Mean Zonal Wind (ms·1) averaged between 100N and 300N at 850-hPa

from 1st July to 31st October 2016

Figure 3 depicts the mean monthly distributions of anomalous OLR from July to

October 2016, where suppressed convection is indicated by red to orange shading,

while enhanced convection is indicated by the blue shading.

Suppressed convection was generally observed over Indochina, Philippines,

South China Sea, and the tropical western Pacific Ocean in July 2016, followed by a

gradual shift to enhanced convection over these regions in September and October

2016. This change can be attributed to tropical storms and cyclones passing over these

regions during the peak of the western Pacific typhoon season, which falls on

September and October each year on average. An intensifying belt of enhanced

convection was observed from the southern Indian Ocean to Papua New Guinea from

July to October 2016. This may be caused by the atmospheric and oceanic conditions in

the tropical Pacific Ocean showing a trend towards La Nina conditions during this time

of the year. Additionally, the negative 100 event, which persisted since May 2016 may

have contributed to enhanced convection and negative OLR over this region.

Figure 3: Anomalous OLR (Wm-2) for (a) July 2016, (b) August 2016, (c) September 2016 and

(d) October 2016. Anomalies are departures from the 1981-2010 base period monthly means.

Time-Longitude section of anomalous OLR from 151 July to 3151 October 2016 are

shown in Figure 4. In the belt between the Equator and 100N (left panel), negative OLR

anomalies were prevalent over the Malaysian region (100 to 1200E) extending to the

western Pacific Ocean east of 150oE, while positive OLR anomalies persisted to the

west of 150oE. Madden-Julian Oscillation (MJO) was observed by the eastward

propagation of anomalous negative OLR from the Indian Ocean (70 to 100 °E) to the

western Pacific Ocean (east of 120 °E). In the belt between 10 ONand 20 ON, negative

OLR anomalies were dominant in August and October 2016. The enhanced convective

activity during this period, which propagated west into Indochina (95 °E to 110 °E) may

be attributed to tropical cyclones, which formed in the central Pacific Ocean, and made

landfall in the Indochina region.

Figure 4. Time-Longitude section of anomalous OLR (W m-2) averaged between Equator and

10 oN (left panel) and 10 oN to 20 oN (right panel) from 1st July to 31st October 2016. Anomalies

are departures from the 1981 - 2010 base period monthly means. Black arrows in the left panel

show eastward propagating convective systems (MJO) while black arrows in the right panel

indicate westward propagating convective systems (Tropical Cyclones).

The Northeast Monsoon season, which is associated with the rainy season over

most of Southeast Asia particularly over the East Coast of Peninsular Malaysia,

southern Thailand and southern Indonesia, normally occurs from November to March.

Severe floods often occur during this season over the Maritime Continent because of

enhanced cloud formation by the convergence of cold air from Siberia with the tropical

Pacific Easterly wind belt over the South China Sea, which acts as a source of moisture.

Most global climate model outputs together with forecasters' consensus favour

above normal rainfall in the Phillipines, Sulawesi, Papua New Guinea, Sumatra, Java

and northern Australia over the next three months. Normal rainfall is expected over

most states in Malaysia between December 2016 and February 2017 with the exception

of the northwestern states of Peninsular Malaysia and Sarawak. In December 2016 and

January 2017 slightly below normal rainfall is expected in southwestern Sarawak while

in February slightly below normal rainfall is forecasted in interior Sarawak . Meanwhile,

slightly below normal rainfall is expected over the northwestern parts of the Peninsula

during the months of January and February 2017.