An Unexpected Outcome Followed an Apparent Seasonal Forecast of Opportunity and Prolonged Drought in Southwest Asia

Abstract

Despite forecasts to the contrary, Southwest Asia precipitation was unexpectedly below normal in October–December 2023, which extended an ongoing three-year drought that was responsible for water shortages and acute food insecurity. Expectations for above-normal precipitation in this season were based on predictions made the prior September from initialized forecast systems, which indicated a greater than 60% chance of such an occurrence. Confident above-normal precipitation predictions, making October–December 2023 an apparent forecast of opportunity, were due to attendant El Niño and positive Indian Ocean Dipole (PIOD) events. An ensemble of model simulations during 1991–2020 indicates that the simultaneous behaviour of these two phenomena is related to the tropical forcing of the mid-latitude circulation over Asia resembling a Gill–Matsuno response over India and China, which is associated with precipitation-enhancing low pressure over Southwest Asia. The co-action of these two modes is related to greater chances of above-normal Southwest Asia precipitation than if El Niño were acting alone. Southwest Asia precipitation in October–December 2023 was 13 mm below average (15 percentile) and was principally caused by two periods of protracted dryness that each lasted up to 3 weeks. During 26 November to 14 December, high pressure moved slowly eastward across western Asia at the same time as a strong MJO event moved across the Indian Ocean in its Phases 4 and 5, which are related to below-average Southwest Asia precipitation. Cumulative regional precipitation while the MJO was in Phases 4 and 5 during this period was −6 mm, accounting for 46% of the seasonal precipitation deficit in the region. During 26 October to 19 November, high pressure persisted with very little eastward movement over Southwest Asia while the MJO was weak, which suggests that the precipitation deficit during this time was caused by internal atmospheric variability in the extratropics.