Historical observations indicate a complex relationship between Bangladesh's climate and various climatic indices, such as the Southern Oscillation Index (SOI) and El Niño-Southern Oscillation (ENSO). While the correlation between ENSO strength and rainfall anomalies is not straightforward, certain patterns emerge: high negative SOIs (strong El Niño) tend to bring dry conditions, whereas high positive SOIs (strong La Niña) typically lead to wet conditions in Bangladesh.
Moderate anomalies (moderate SOIs), encompassing both moderate El Niño and La Niña events, show contradictory index-climate relationships. Interestingly, Bangladesh experiences wet conditions during both moderate El Niño and La Niña years. Specifically, strong La Niña events generally forecast a wet climate, with strong events relating to deep flooding and moderate events to milder flooding (Fig 1).
Conversely, strong El Niño events result in dry or drought conditions (Fig 1), while moderate El Niño events often bring wetter weather or moderate flooding (Fig 1). The conventional El Niño, also known as Eastern Pacific El Niño (EPE), denotes a strong El Niño event, whereas a moderate El Niño event may resemble Western Pacific El Niño (WPE) or El Niño Modoki.
According to diagnostic discussions from the US Climate Prediction Center (US-CPC) and the Australian Bureau of Meteorology (AUS-BOM), current conditions indicate a neutral ENSO phase with a 65% chance of La Niña development during July-September, persisting into the Northern Hemisphere winter 2024-25 with an 85% likelihood during November-January.
Another critical factor is the Indian Ocean Dipole (IOD), which presently stands neutral but is expected to trend toward a neutral-to-negative phase in the coming months. A negative IOD typically translates to increased rainfall across the greater Ganges-Brahmaputra-Meghna basin systems. Concurrently, if this aligns with a La Niña event, it can amplify the wet effects across Bangladesh and neighboring countries.
Considering the current statuses of ENSO and IOD, there is a heightened probability of above-normal basin-wide rainfall, implying that Bangladesh may encounter higher-than-normal flooding in 2024. At this juncture, the prospect of lower-than-normal flooding appears improbable.
Md Rashed Chowdhury is Adjunct Professor in the School of Sustainable Engineering and the Built Environment of Arizona State University, USA (Email: [email protected]).
