Floods in the Dry Season
Heavy rain has triggered floods and landslides in a number of regions as Java enters the height of the dry season. The change in rain patterns needs to be understood to reduce the risk of natural disasters.
Heavy rain has triggered floods and landslides in a number of regions as Java enters the height of the dry season. The change in rain patterns needs to be understood to reduce the risk of natural disasters.
Even though the dry season of 2018 arrived in mid-April, heavy rain in late June triggered floods and landslides in several regions from Banyuwangi in Jakarta to Bogor, a marker of change in the weather kitchen suggesting that rain in June is no longer anomalous.
Based on modeling since the 1990s, scientists have been predicting changing rainfall patterns in line with global warming. The rise in temperature triggers faster evaporation, alters the exchange of currents in the air above the oceans and triggers climate change.
The latest events and data confirm this trend. For example, a study by researchers from NASA and the University of Maryland\'s Earth System Science Interdisciplinary Center (ESSIC) published in the journal Nature on May 17 notes changes in the intensity and pattern of rain.
Regions that were previously wet, including high latitudes and tropical regions, are becoming wetter. Meanwhile, dry regions are becoming increasingly dry.
Indonesia does not escape the shift in rainfall patterns. A study by Meteorology, Climatology and Geophysics Agency (BMKG) researcher Supari finds increasing rainfall over Indonesian territory lying north of the equator, specifically in central and northern Sumatra and in eastern Kalimantan.
Conversely, rainfall is decreasing in parts of the country to the south of the equator, such as Java, Bali and Nusa Tenggara. A 2016 study published in the journal Climatology uses climate data from 88 BMKG stations in Indonesia. When viewed on a smaller scale, changes in rain patterns are more complex, with varying effects even on the same islands.
Local conditions
Research by Siswanto and Supari on extreme rainfall pattern changes in Java show look into these variations. Their study, published in the journal Environment and Earth Science (2015), looks at the trends of rain patterns in nine lowland areas and 10 mountains in Java in period of 1981 to 2010.
Its result is that some of the regions experience an increased likelihood of extreme rainfall. Regions with increasing extreme rainfall include Banyuwangi, Malang, Cilacap and Jakarta. Regions experiencing a drop in extreme rainfall include Jember, Solo and Cirebon.
This study points to more common extreme rainfall in highland areas of Banyuwangi. The analysis of data from a 30-year observation period from the Sidomulyo rain monitoring post near Mount Raung indicates increased heavy rain, above 50 millimeters per day.
The high intensity of rainfall is based on meteorological data before the major flooding in Banyuwangi on Saturday (23/6/2018). BMKG data from the Karangploso Climatology Station in Malang, shows that rainfall, monitored through the automated rain gauge (ARG) in ljen Jambu on the slopes of Mount Raung from June 19 to June 23, reached 42.3 mm per day, 19.4 mm per day, 36.2 mm per day and 3 mm per day, respectively,
By the end of June, Songgon\'s rain measurements in the upstream areas of Mount Raung recorded daily rain exceeding 100 mm three times. At the Alas Malang Singojuruh rain monitoring post, daily rainfall above 100 mm occurred four times.
"This data suggests that the intensifying local rain is affected by the orographic effects of the mountains despite drought. Last year, Banyuwangi was flooded in June and July," said Siswanto, a BMKG extreme weather researcher.
From the meteorological aspect, Banyuwangi is experiencing an increased risk of flooding due to hikes in the extreme rain trends in the surrounding mountains. The same can be seen in Jakarta. Siswanto’s study, published in the Royal Meteorological Society (2015), also found that the occurrence of extreme rainfall in Jakarta increased significantly of the period of 1866-2010. The number of days with rainfall above 50 mm per day and above 100 mm per day increased significantly.
The records show that 50 annual events with maximum rainfall of more than 100 mm, which is associated with flooding in Jakarta since the 1900s, did not only occur during the rainy season. Extreme rainfall in Java is hard to guess, it can happen outside of the rainy season. Floods and landslides in Java occurred several times during the dry season from June-September 2016.
According to Supari, each region has different rain mechanisms. For example, rain in the mountains is called an orographic type of rain determined by the slope of the mountain and the main wind direction. For example, during the dry season in Banyuwangi, the eastern part of the mountain gets more rain than in the west, because the eastern slope faces the Australian monsoon.
Moreover, the height of mountains also has an influence. On a not-so-high mountain, rain falls behind the mountain, because the clouds are carried by the wind across its peak. On high mountains, rain falls in front of the mountain, because the wind is unable to push clouds to pass the peak. The terms front or back of the mountain refer to the direction of the wind.
Due to the different mechanisms, orographic rain does not always have the same pattern of change. "East Java\'s western parts, such as Ngawi and Bojonegoro, have experienced a significant decline in rainfall. Central areas, including Surabaya and Pasuruan, have experienced an increase in rainfall," said Supari.
Siswanto said it was difficult to categorize areas with downward and upward trends. "It is not easy to look for patterns of change in extreme rain. Local responses are not uniform on climate change, depending on atmospheric interactions, the biosphere, and environmental hydrosphere," he said.
Mitigation and adaptation
Understanding changes in climate patterns is important to mitigate the potential of disasters. With the increase in the intensity of rain in the mountains, the potential for flash floods and landslides increases. Hydrometeorological disasters are triggered by intense rain and changes in the environmental carrying capacity.
Information on changes in rain patterns is needed in farming. Research by a team from Aalto University in Finland, published in the journal Nature Communications on March 28, 2018, finds a regional-scale climate cycle, such as El Nino, affecting two-thirds of global crop yields.
According to the traditional Javanese seasons, June is the early part of the dry season, while the peak of the rainy season is December – when the springs are becoming big -- (Daldjoeni, 1997). This is based on the observation of past seasonal patterns on the island of Java. This seasons calendar needs to be updated.