Stratospheric Nudging And Predictable Surface Impacts (SNAPSI)

Experiments for operational centres to perform that isolate the role of the stratosphere on predictive skill

Recent studies have highlighted that stratosphere-troposphere coupling in the extra-tropics contributes to S2S predictability. However, not all SSWs are followed by impactful weather events; furthermore, there is substantial spread in the ability of models to represent this coupling. Similarly, in the tropics, a focus on the links between the QBO and MJO predictability is very important for surface predictability. The Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project aims to isolate the role of the stratosphere for surface climate and predictability, and also explore the role of stratospheric biases for inter-model spread in the representation of stratosphere-troposphere coupling. Specifically SNAPSI targets three SSW case study events, in which the stratospheric state can be either freely-evolving or nudged towards a climatological or observational state.  The test cases will be NH SSWs during two recent winters with very different tropospheric impact (2018 and 2019) and the 2019 SH warming, which has likely contributed to the extreme wildfires over Australia in 2019/20. The goals of this project are:

a. To assess the contribution of the stratospheric evolution to forecast skill in a controlled fashion.

b. To assess the representation of coupling processes across different operational models.

c. Attribute particular meteorological events to stratospheric conditions

d. To assess the representation of stratospheric wave driving.

The basic experimental protocol consists of a set of forecast ensembles: (1) a standard, free running forecast ensemble, (2) a ‘perfect stratosphere’ forecast in which the stratosphere is relaxed towards the observed evolution, and (3) a ‘control’ forecast in which the stratosphere is relaxed towards climatology. The paper documenting the Experiment Protocol for these experiments was published in 2022. To date, twelve modeling groups at eleven centers have completed integrations following this protocol, and nearly all of the data has been added to the SNAPSI archive at CEDA. This allows for an unprecedented, multi-model comparison of the dynamics underlying the surface responses to sudden stratospheric warmings. Moreover, by including ‘counterfactual’ forecasts in which the stratospheric circulation remains in a climatological state, the experimental protocol allows for formal attribution statements to be made regarding the surface extremes that followed the stratospheric anomalies.