Stratospheric Nudging And Predictable Surface Impacts (SNAPSI): A Protocol for Investigating the Role of the Stratospheric Polar Vortex 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 vortices in sub-seasonal 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, and 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.

The “Polar Vortex” Winter of 2013/2014

“…Future studies should focus on so‐called nudging experiments (Hitchcock et al., 2022), whereby the troposphere and the stratosphere are individually “nudged” toward an observed state or climatology to diagnose the relative roles of variability in each of those layers to the resulting anomalous circulation pattern. Similar experiments could be run with different regions within the troposphere as well (e.g., tropics vs. mid‐latitudes).…”

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

“…The settings for parameterized gravity wave drag are unchanged 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 time-scale of 6h.…”