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19 Apr 2018

Atmospheric Conditions In  February  - March  2018

PIn March 2018 saw the Asian monsoon begin to weaken. This indicates that Indonesia has entered the transition period. This is illustrated in Figure 1 where the western winds begin to look weaker on the islands of Java, Bali, and Nusa Tenggara due to the weakening influence of the Asian monsoon. The weakening of this monsoon was influenced by the contrast between the northern part of SST (South China Sea) and southern Indonesia (Australia's northern waters) which was smaller in March 2018 (Figure 1). Increased SST appears to occur in Indonesian waters in March 2018 when compared to February 2018, especially in eastern Indonesia. In addition to weakening the Asian monsoon, an increase in SST in almost all Indonesian waters has resulted in a large number of convective clouds, characterized by low OLR values, particularly in southern Sumatra, western and southern Kalimantan, Central Sulawesi and Papua. While the convective activity only slightly in southern Indonesia.

Figure 1. Wind conditions at 850 mb, SST and OLR levels in February and March 2018 based on NCEP / NCAR reanalysis data.

Some atmospheric parameters such as wind surfaces, SST and OLR as shown in Fig. 1, significantly affect the March 2018 rainfall (Fig. 2). The March 2018 rainfall appears to be higher than in February 2018 which occurred almost all over Indonesia, except for southern Indonesia. The increasing distribution of rainfall accumulation mainly occurs in western Java Island, southern part of Sumatera and Arafuru Sea (southern part of Papua), where monthly rainfall is still quite high with accumulated more than 500mm. But on the contrary special in most of Java Island, Bali, Nusa Tenggara rainfall seemed to decrease.

Figure 2. Accumulated rainfall (mm) Indonesia February and March 2017 based on GSMaP data.

Figure 3. OLR conditions based on NCEP / NCAR and CH dasarian reanalysis data (GSMaP) March 2018.

March 2018 rainfall distribution for dasarian 1 is spread along the west coast of Sumatra, Lampung, Java, west and south Kalimantan, central Sulawesi and most of eastern Indonesia and Papua New Guinea. This is due to the number of convective clouds in the dasarian 1. While on dasarian 2, rainfall is centered in eastern Indonesia, the waters southeast of Indonesia, Papua and Papua New Guinea. In dasarian 2, rainfall in the area exceeds rainfall in dasarian 1. In dasarian 3, high rainfall is present in the western waters of Sumatra, the waters between Kalimantan and Sulawesi. However, convective cloud spans appear to be reduced when compared to the previous dasarian and spread over the equatorial region. This is consistent with the MJO observation (source: CPC) explaining that during March the MJO moved from the Indian Ocean to the Western Pacific Parairan. At the beginning of March MJO strong enough and continue to weaken when it is above Indonesia that is until the third week of March. The weakness of MJO while on top of Indonesia causes its influence on weather conditions in Indonesia is also relatively small. Then MJO again increased when it was above the Indian Ocean at the end of March. It is expected that MJO will continue to strengthen in the first week of April and again weaken until the end of April.

The CPC data for ENSO conditions during March 2018 indicates a trend of weak La Niña phenomenon.  This is illustrated by an SST nino 3.4 anomaly of -0.7oC. This value is below the normal ENSO threshold that is between -0,5oC and 0.5oC. The anomalous condition of SST nino 3.4 appears to be less than -0.5 already lasting for more than 3 consecutive months.

Overall, based on atmospheric and oceanic conditions, the IRI predicts that ENSO conditions are expected to be neutral with possibly about 55% in the 2018 M MAM period and the chance of neutral ENSO will increase in the AMJ and MJJ periods. This reinforces the argument that La Nina will disappear into neutral in April 2018.

Dipole Mode conditions in the Indian Ocean in March are also classified as neutral with average index values 0.2oC. Predicted from the Australian Bureau of Meteorology (BOM), the IOD condition remains neutral for the next 2 months with a possible incidence of over 60%. Dipole Mode conditions in the Indian Ocean in early February showed a negative IOD index and increased until the end of the month. but the IOD index for February was not so significant that the February IOD condition was classified as neutral with an average index value of 0.01. Predicted from Bureau of Meteorology (BOM) Australia, IOD condition for next 2 months remain in neutral condition.

Rainfall Prediction for the Next Few Months in Indonesia

Predicted rainfall for April - September 2018 using Conformal Cubic Atmospheric Model (CCAM) data with April 2018 initial condition is shown in Figure 4.

Figure 4. Predicted rainfall and surface wind using CCAM with initial condition of Global Forecasting System (GFS) data of 1 April 2018 and predicted SST from POAMA (m24b_emn) issued on March 28, 2018.

The results of monthly rainfall prediction using CCAM with an initial condition of April 1, 2018 indicates a decrease in rainfall in southern Indonesia in April to July 2018. However, it appears that April 2018 in Java still will rain but with a smaller intensity compared to March 2018 (Figure 5).

Figure 5. Predicted rainfall and surface wind using CCAM with initial conditions from Global Forecasting System (GFS) data of March 1, 2018 and predicted SST from POAMA (m24b_emn) issued on February 26, 2018.

While in May 2018 based on the results of the CCAM, there appears to be a decrease in rainfall that occurs almost in most parts of Indonesia, some of Java will experience the beginning of the dry season. Along with the consistent Australian monsoon that has a dry nature, the peak of the dry season in Java will occur in June 2018, while the equator, will experience an increase in rainfall, such as central Sumatra, western and central Kalimantan and in Papua.

Based on CCAM's prediction results, Australia's monsoon consistency will weaken in September 2018 and enter a transitional season where there is an increase in rainfall in parts of Indonesia.

 (Climate Variability Team 2018 – PSTA LAPAN)

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