Consecutive dry days and extreme rainfall
Trends reveal how Area I sees a decrease in consecutive rainless days during the dry season, while Areas II and III tend to remain unchanged. On the other hand, heavy (P75th) and extreme (P90th) rainfall during the rainy season in Areas I and II reports a significant increase while Area III — the driest region in Indonesia — sees an increasing trend in heavy and extreme rain during the dry season.
This indicates a tendency for regions in southeastern Indonesia, such as East Java, Bali, Lombok, Sumbawa and East Nusa Tenggara, to experience wetter conditions over the last two decades, even though the consecutive dry days that occur in these regions tend to remain constant. These wetter conditions are consistent with the mechanism of shifting a “warm pool” in the Indo-Pacific region from west to east Indonesia, as detailed in recent studies.
This shift in the convective center at BMI involves long-distance interactions or teleconnections between the Pacific Ocean and deep waters in the BMI region. This means rainfall is projected to increase significantly in the southeastern region of Indonesia, due to the impact of global warming. The study has also highlighted the occurrence of seasonal and monthly increases in rainfall in southeastern Indonesia, as a climatological response to climate change. Aside from this, diurnal rainfall patterns undergo changes in phase.
Shift in weather patterns
Studies regarding indications of changes in weather patterns show a diurnal rain cycle that threatens coastal areas, because rain that should fall at sea is found to have shifted to coastal areas. Furthermore, the studies found a number of facts related to the mechanism of storms in BMI, which in several cases contribute to the severity of extreme weather events.
Shift of diurnal cycle of rainfall
Study of the indications of a diurnal cycle in rainfall is an important key to understanding the extreme weather patterns in BMI during the last decade, likely as a result of global warming. Basically, it follows the general pattern of rain on land which is influenced by land-sea winds and gravity waves leading to rain in the afternoon over land and in the morning over the sea.
However, there are variations in the diurnal cycle of rain, in that the heaviest rain over land occurs in the early morning, with significant frequency (~20 percent) for areas in the north of West Java, including Jakarta (Yulihastin et al., 2020). Early morning rain that falls with high or extreme intensity (P99th) has even been proven to be the cause of major foods in Jakarta, in 2007, 2013, 2014 and 2020.
The results of our study show the main characteristics of this phenomenon, namely: (1) strong rain propagation from the sea to land, and vice versa; (2) randomness in terms of the phase of rain in the early morning period (1am to 4am WIB); and (3) early morning rain has a strong connection with extreme rain, which triggers major foods in Jakarta.
The mechanism that causes extreme early morning rain is related to two phenomena that occur at the surface of the atmosphere and sea, in the form of strengthening surface winds and cooling sea surface temperatures in the South China Sea near the Karimata Strait, which then shifts toward the Java Sea. These are called Cross-Equatorial Northerly Surge (CENS) and Cold Tongue (CT).
Storms that trigger extreme weather
In meteorology, a “storm” is defined as a term that describes rain accompanied by strong winds, with various categories based on spatial scales (2-2,000 km) and time (minutes to weeks). From local (short) to broad (long) spatiotemporal scales, the mechanism of storm formation plays a major role in generating extreme weather events.
An extreme weather event is defined as “an escalation of extreme weather that occurs widely to the point of causing catastrophic disasters”. However, studies on the types of storms in the BMI region are still very limited, because physical and dynamic processes at the micro to meso scale are very difficult to detail, considering that there are lack of high resolution tools to monitor the atmosphere over the region in question.
Therefore, scientific documentation by reconstructing each extreme weather event to understand its mechanism is one way to find indications of climate change in extreme weather patterns that have occurred over the last decade in BMI.
The study of storms is then carried out on micro, meso and macro scales, based on case studies of extreme events that cause catastrophic hydrometeorological disasters. The storm categories studied are divided into two main parameters to be used as the basis for identifying storm occurrences: (1) spatial patterns of rain, namely bow-echo and squall-line; (2) spatial cloud patterns, namely, Meso-Convective Complex (MCC); and (3) cyclonic storms.