Stratospheric Nudging And Predictable Surface Impacts (SNAPSI): a protocol for investigating the role of stratospheric polar vortex disturbances in subseasonal to seasonal forecasts

Abstract

Abstract. Major disruptions of the winter season, high-latitude stratospheric polar vortices can result in stratospheric anomalies that persist for months. These sudden stratospheric warming events are recognized as an important potential source of forecast skill for surface climate on subseasonal to seasonal timescales. Realizing this skill in operational subseasonal forecast models remains a challenge, as models must capture both the evolution of the stratospheric polar vortices in addition to their coupling to the troposphere. The processes involved in this coupling remain a topic of open research. We present here the Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project. SNAPSI is a new model intercomparison protocol designed to study the role of the Arctic and Antarctic stratospheric polar vortex disturbances for surface predictability in subseasonal to seasonal forecast models. Based on a set of controlled, subseasonal ensemble forecasts of three recent events, the protocol aims to address four main scientific goals. First, to quantify the impact of improved stratospheric forecasts on near-surface forecast skill. Second, to attribute specific extreme events to stratospheric variability. Third, to assess the mechanisms by which the stratosphere influences the troposphere in the forecast models. Fourth, to investigate the wave processes that lead to the stratospheric anomalies themselves. Although not a primary focus, the experiments are furthermore expected to shed light on coupling between the tropical stratosphere and troposphere. The output requested will allow for a more detailed, process-based community analysis than has been possible with existing databases of subseasonal forecasts.

Changes in Stratospheric Dynamics Simulated by the EC‐Earth Model From CMIP5 to CMIP6

“…Our findings highlight that more comprehensive and extended model outputs are useful for understanding model biases and their origin, including evaluating the realism of stratospheric processes and to quantify benefits from model developments. However, given the complex interactions between waves and mean flow in the stratosphere, nudging experiments (Hitchcock et al., 2022; Morgenstern et al., 2017) would be useful to identify the origins of biases.…”

Increased vertical resolution in the stratosphere reveals role of gravity waves after sudden stratospheric warmings

“…The settings for parameterized gravity wave drag are un-changed between the different vertical-resolution configurations. To verify whether changes in gravity wave drag are caused directly by changes in the vertical resolution as opposed to changes in the stratospheric zonal-mean basic state, we performed nudged simulations following the SNAPSI protocol (Hitchcock et al, 2022). Specifically, zonal-mean temperature, vorticity, and divergence above 90 hPa are relaxed towards the "observed" time series of ERA5 on a timescale of 6 h. Consequently, there are no significant differences in the zonal-mean zonal wind and polar-cap temperature anomaly between the different model configurations in the nudged simulations.…”

Identifying Perturbations That Tipped the Stratosphere Into a Sudden Warming During January 2013

“…Historical analysis suggest at least one third of SSWs immediately follow anomalous tropospheric wave activity, with the longer‐term accumulation of upward wave activity also important (Birner & Albers, 2017; de la Cámara et al., 2019; Polvani & Waugh, 2004). Understanding the roles of these different processes within subseasonal forecasts is an active area of research (Hitchcock et al., 2022).…”

“…Understanding the roles of these different processes within subseasonal forecasts is an active area of research (Hitchcock et al, 2022).…”