1. Flood season 2013
1.1 Rainfall situation
During the five months of the flood season 2013, the critical rainfall situation in the Lower Mekong Basin occurred during the end of June, the month of July, the beginning of August, and the middle of September, as a result of Southwest Monsoon activity, low pressure troughs, storm and typhoon’s appearances in the South China Sea. In terms of total seasonal rainfall, the general picture was higher than the average years (see Figure 1-1). The total of rainfall in the flood season 2013 at stations of the LMB was higher than the long-term average (LTA) and higher than in the previous flood season 2012.
The spatial and temporal variation of rainfall was high, indicating that the intensity of heavy rain situations along with the Lower Mekong Basin from upstream to downstream occurred as a function of time (Annex A: 1. Graphs and Tables for monthly observed rainfall distribution during flood season):
• The wet season started at the end of June; the heavy rain mostly occurred in the middle reaches (from Paksane to Pakse) of the LMB.
• During July - August, the intensive and continued rain covered the entire LMB and appeared more frequently during this period.
• As is usual, from September to October intensive rainfall occurred in lower reach.
However this year it covered the entire LMB as a result of many Tropical Storms and Typhoons related activities.
In 2013, three main weather patterns caused heavy rains, which are presented below:
• Southwest monsoon: influenced the Mekong River Basin from the end of June to October; the critical activity mostly occurred in July. Typically, heavy rain event from 21st - 31st July in the upper and middle reaches of LMB, which caused rapid rise in water level. From mid-September, there was moderate to weak SW monsoon, which prevailed over Indochina Peninsula as a common phenomenon.
• Tropical Low Pressure (TLP) and Inter Tropical Convergence Zone (ITCZ): these periodically appeared from early June to the mid of October with on average 3 to 7 days duration. In the flood season 2013, the frequent appearances of TLP and ITCZ during almost the entire flood season were one of the main phenomena which caused continuous heavy rain and rising water along the Mekong River. In August, TLP and ITCZ were observed and had significant influence on the upper and middle reaches of the LMB while the influence on the lower reach took place mostly in September and October. Figure 1-3 shows an illustration of the appearances and influences of TLP and ITCZ to the LMB in August and September.
• Tropical depressions (TD), tropical storms (TS) or typhoons (TY): The flood season 2013 presented more than 15 tropical depressions, storms and typhoons, which came to East Sea (see Figure 1-4), but only some of them affected the Mekong River Basin. Of these, the four storms BEBINCA, MANGKHUT, JEBI, WUTIP, UTOR and NARI were the most noticeable.
BEBINCA was formed as a Tropical Storm (TS) on the 21st June, moved from East Sea in north-westerly direction over the Bay of Tonkin, then entered into North-eastern provinces of Viet Nam on 23rd June. BEBINCA’s Storm Track is shown in Figure 1-4. Weather maps for BEBINCA’s situation are shown in Figure 1-5 and 6, respectively
Tropical storm JEBI was upgraded by Tropical Depression (TD) which was present over the East Sea areas at the end of July, and then the TD gained strength and became TS and moved in north-westerly direction and landed at Hai Phong Province of Viet Nam. On 04th August the TD was over the North of Viet Nam and was downgraded to a Tropical Depression. It brought moderate to heavy rainfall in these areas. JEBI’s Storm Track and its weather maps with the situation of the TS are shown in Figure 1-8 and Figure 1-9, respectively.
Tropical storm MANGKHUT was upgraded by Tropical Depression (TD) which moved from the East Sea areas on 5th August quickly in north-westerly direction and landed at Thanh Hoa Province of Viet Nam. It brought moderate to heavy rain in these areas. MANGKHUT Storm Track is shown in Figure 1-7 and its weather maps with the situation of the TD are shown in Figure 1-8 and Figure 1-9, respectively.
UTOR was formed as a Typhoon (TY) on 13th August over the East Sea, and was moving in north-westerly direction with a speed of about 24 km/h. On 15th August the TY was downgraded to Tropical Depression (TD) after it made landfall over Guangxi, South of China. UTOR’s Storm Track is shown in Figure 1-7 and its weather maps with the situation of the TD are shown in Figure 1-8 and Figure 1-9, respectively
WUTIP was formed as a Typhoon (TY) on the 28th September, started from upper of South of the East Sea and then was downgraded to Tropical Depression (TD) on 01st October, then over the central part of Lao PDR and moved Westwards to the Northeast of Thailand before dissipating as low pressure area.
NARI was formed as TY on 12th October over the Central part of the Philippines and finally made landfall over Da Nang Province, Viet Nam before it was downgraded to TD over Lao PDR on 16th October 2013. Figure 1-10 presents the track of TY NARI and NESAT’s weather maps before and after landing are shown in Figure 1-11 and Figure 1-12, respectively.