Anomalously Wet Dry Seasons in Indonesia and Moisture Convergence Anomalies during The Cool Phase of Different ENSO Types
Gammamerdianti and Tri Wahyu Hadi

Institut Teknologi Bandung

Abstract

Dry season in Indonesia typically occurs from June to September, overlapping with the Indian Summer Monsoon (ISM) period, during which India sub-continent and its surroundings receive large amount of moisture transported from oceans in the Indo-Pacific region. However, there have been years with anomalously wet dry season, more commonly known as ^wet dry-season^ (WDS) among the media, in Indonesia. On the other hand, the Meteorology, Climatology, and Geophysical Agency of Indonesia (BMKG) officially defines seasonal rainfall variations by three categories i.e., above normal (AN), normal (N), below normal (BN). However, there is no clear definition of WDS referring to AN criteria of the dry-season rainfall. This study attempts to make clear definition of WDS by applying the criteria of AN, N, and BN the seasonal rainfall variations on gridded rainfall data. Moisture convergence anomalies associated with identified years of wet-dry season are also analyzed.

The results show that, during the period of 1993 to 2022, WDS is identified in eight years i.e., 1995, 1996, 1998, 2010, 2013, 2016, 2021 and 2022. General cool-phase ENSO (La Nina) year (such that defined using NINO 3.4 index) identification only explains 40% of the WDS events. When definitions of Eastern- and Central-Pacific (including mixed type) are used, 80% of the events can be associated with La Nina. These WDS events are characterized by distinct dipole moisture convergence patterns, especially over Indonesia and South Asia. Moisture convergence indices developed over those regions show anomalous evolution that might indicate a precursor to WDS events. More studies are needed to understand cases of WDS that cannot be directly associated with La Nina.

Keywords: Rainfall in dry season, anomalous wet-dry season identification, ENSO, moisture transport convergence

Topic: Atmospheric Sciences