An Unexpected Outcome Followed an Apparent Forecast of Opportunity and Prolonged Drought in Southwest Asia (Invited Presentation)

Forecast above-average Southwest Asia precipitation during October-December 2023 was expected to allay an ongoing three-year drought that resulted in water shortages and acute food insecurity among several other hazards. Confident above-average precipitation predictions in Southwest Asia were based on several initialized forecast systems from the prior September, making October-December 2023 an apparent seasonal forecast of opportunity, and bringing hope to a region that has been plagued by frequent drought and heat since the beginning of the 21st Century. However, below-average Southwest Asia precipitation was observed in October-December 2023, which prolonged the ongoing multi-year drought, delayed planting of crops for the 2023-2024 agricultural season because of low soil moisture, and low led to low snowpack accumulation, which provides water during the spring and summer seasons in Afghanistan.

Confident above-average precipitation predictions in Southwest Asia during October-December 2023 were due to attendant El Niño and positive Indian Ocean Dipole (PIOD) events. Uninitialized model simulations indicate that the simultaneous behavior of these two climate modes is related to the tropical forcing of the midlatitude circulation over Asia resembling a Gill-Matsuno response over India and China, which is associated with precipitation-enhancing low pressure over western Asia that increases moisture flux into Southwest Asia. The co-action of these two climate modes is related to greater chances of above-average Southwest Asia precipitation than if El Niño were operating alone.

The same uninitialized model simulations indicate a 10% chance of below-average October-December precipitation in Southwest Asia during simultaneous El Nino and PIOD events. Diagnosis of all such events indicates large differences in the atmospheric circulation between them over Eurasia, which, combined with similarities in their tropical atmospheric forcing that resembles composite conditions related to simultaneous El Nino and PIOD events, suggests no systematic cause of these unlikely below-average precipitation occurrences. The lack of systematic cause of below-average October-December Southwest Asia precipitation related to El Nino and PIOD indicates that the predictability of such an outcome was low ahead of the 2023 season.