Prediction of near-surface conditions following the 2023/24 sudden stratospheric warming by the S2S project models

Abstract

The stratospheric disturbances and their impact on predictability of near surface extreme events are one of crucial issues in the subseasonal to seasonal (S2S) prediction project. This study examines the 2023/24 winter when frequent stratospheric disturbances occurred, including minor and major sudden stratospheric warmings (SSWs). The stratospheric circulation was disturbed multiple times, with rapid circumpolar westerly wind deceleration and even zonal wind reversal. Corresponding wave pulses were observed in the troposphere and lower stratosphere, with large eddy heat flux pulses appearing before every stratospheric perturbation. The stratospheric perturbation was examined from the spatiotemporal evolution of the annular mode index, revealing two instances of evident downward propagation. Nevertheless, the near surface did not respond in a typical negative NAM pattern associated with the stratospheric signal. The study further analyzed the predictability of the near surface and its relation with the stratospheric disturbance using common initializations in January and February 2024 from S2S models. The results indicate that the near surface predictability was not enhanced in the 2023/24 winter albeit with frequent stratospheric disturbances, and the contribution of the stratospheric disturbance to the surface predictability was limited. Although the multimodel ensemble means forecast warm spots over broad regions of lands and dry spots in part of China and US, the stratospheric circulation error nearly did not explain the near surface forecasting error among S2S models most of the time. The subseasonal predictability of the near surface conditions over the course of the 2023/24 winter seldom originated from the stratospheric disturbances, and other predictability sources such as the warm tropical Pacific Ocean and increased Arctic sea ice should be considered.