Seasonal Predictability over Europe Arising from El Niño and Stratospheric Variability in the MPI-ESM Seasonal Prediction System

Abstract

Predictability on seasonal time scales over the North Atlantic-Europe region is assessed using a seasonal prediction system based on an initialized version of the Max Planck Institute Earth System Model (MPI-ESM). For this region, two of the dominant predictors on seasonal time scales are El Niño-Southern Oscillation (ENSO) and sudden stratospheric warming (SSW) events. Multiple studies have shown a potential for improved North Atlantic predictability for either predictor. Their respective influences are however difficult to disentangle, since the stratosphere is itself impacted by ENSO. Both El Niño and SSW events correspond to a negative signature of the North Atlantic Oscillation (NAO), which has a major influence on European weather.This study explores the impact on Europe by separating the stratospheric pathway of the El Niño teleconnection. In the seasonal prediction system, the evolution of El Niño events is well captured for lead times of up to 6 months, and stratospheric variability is reproduced with a realistic frequency of SSW events. The model reproduces the El Niño teleconnection through the stratosphere, involving a deepened Aleutian low connected to a warm anomaly in the northern winter stratosphere. The stratospheric anomaly signal then propagates downward into the troposphere through the winter season. Predictability of 500-hPa geopotential height over Europe at lead times of up to 4 months is shown to be increased only for El Niño events that exhibit SSW events, and it is shown that the characteristic negative NAO signal is only obtained for winters also containing major SSW events for both the model and the reanalysis data.

Satellite observations of middle atmosphere gravity wave absolute momentum flux and of its vertical gradient during recent stratospheric warmings

“…The forecast skill of weather prediction is related to the occurrence of SSWs (e.g., Sigmond et al, 2013;Domeisen et al, 2015), and this can be utilized for an improvement of weather forecasts.…”

The Stratospheric Pathway of La Niña

“…Several indices have been used in the literature, considering different oceanic regions, such as Nino-3 (N3) (5°N-5°S, 150°-90°W) (Hoerlinget al 1997;Manzini et al 2006) or Nino-4 (N4) (5°N-5°S, 160°E-150°W) (Kug and Ham 2011;Zhang et al 2015). However, most recent studies used the Nino-3.4 (N3.4) (5°N-5°S, 170°-120°W) index from the National Centers for Environmental Prediction/Climate Prediction Center (NCEP/CPC) (e.g., Free and Seidel 2009;Butler and Polvani 2011;Garfinkel et al 2012;Butler et al 2014;Barriopedro and Calvo 2014;Domeisen et al 2015). For this reason, we choose the N3.4 index from the NCEP/CPC.…”

“…Finally, different thresholds have been applied to select the events. Some studies se lected La Nina events below -0.5°C (Domeisen et al 2015) or -0.7 standard deviations (SD) (Pozo-Vazquez et al 2005), while other studies applied higher thresholds of -1 SD (Mitchell et al 2011) or -1 K (Free and Seidel 2009). As discussed in the introduction, the use of unequal thresholds could lead to a diverse range of responses.…”

“…Related zonal mean zonal wind anomalies in the stratosphere propagate downward into the troposphere, having an impact on the North Atlantic and European (NAE) region (Cagnazzo and Manzini 2009;Ineson and Scaife 2009;Bell et al 2009). It has been indeed shown that El Nino has the potential to improve seasonal predictability over Europe (Domeisen et al 2015).…”

“…It should be noted that the short record (and thus small signal-to-noise ratio) could be perhaps the reason for the absence of an evi dent robust stratospheric La Nina response in observa tions and reanalysis data. For this reason, a relatively low threshold is sometimes used to increase the num ber DOI: 10.1175/JCLI-D-16-0230.1 of identified La Nina events (Butler and Polvani 2011;Barriopedro and Calvo 2014;Domeisen et al 2015). The different thresholds used to select cold ENSO events in the literature hamper a direct comparison among these studies.…”