Abstract
AbstractThe winter of 2019–2020 was dominated by an extremely strong stratospheric polar vortex and positive tropospheric Arctic Oscillation (AO). Here, we analyze forecasts from six different prediction systems contributing to the C3S seasonal forecast database. Most performed very strongly, with consistently high skill for January–March 2020 from forecasts launched through October–December 2019. Although the magnitude of the anomalies was underestimated, the performance of most prediction systems was extremely high for a positive AO winter relative to the common hindcast climate. Ensemble members which better predicted the extremely strong stratospheric vortex better predicted the extreme tropospheric state. We find a significant relationship between forecasts of the anomalous midlatitude tropospheric wave pattern in early winter, which destructively interfered with the climatological stationary waves and the strength of the stratospheric vortex later in the winter. Our results demonstrate a strong interdependence between the accuracy of stratospheric vortex and AO forecasts.
Highlights
The Northern Hemisphere (NH) winter of 2019–2020, January–March (JFM) 2020, was characterized by a strong and persistent positive phase of the Arctic Oscillation (AO) (Hardiman et al, 2020; Lawrence et al, 2020)—the leading mode of extratropical tropospheric wintertime variability, analogous to the surface Northern Annular Mode (NAM) (e.g., Black & McDaniel, 2004; Thompson & Wallace, 1998, 2001) and closely related to the North Atlantic Oscillation (NAO) (Feldstein & Franzke, 2006)
We find a significant relationship between forecasts of the anomalous midlatitude tropospheric wave pattern in early winter, which destructively interfered with the climatological stationary waves and the strength of the stratospheric vortex later in the winter
We analyze data from six prediction systems that contribute to the Copernicus Climate Change Service (C3S) seasonal forecast database—namely, from the United Kingdom Met Office (UKMO), the European Centre for Medium‐range Weather Forecasts (ECMWF), Météo‐France, Deutsche Wetterdeinst (DWD), the National Centers for Environmental Prediction (NCEP), and the Euro‐Mediterranean Center on Climate Change (Centro euro‐Mediterraneo sui Cambiamenti Climatici, CMCC)
Summary
The Northern Hemisphere (NH) winter of 2019–2020, January–March (JFM) 2020, was characterized by a strong and persistent positive phase of the Arctic Oscillation (AO) (Hardiman et al, 2020; Lawrence et al, 2020)—the leading mode of extratropical tropospheric wintertime variability, analogous to the surface Northern Annular Mode (NAM) (e.g., Black & McDaniel, 2004; Thompson & Wallace, 1998, 2001) and closely related to the North Atlantic Oscillation (NAO) (Feldstein & Franzke, 2006). Additional influences on the seasonal‐mean NAO/AO and SPV include tropical sea surface temperatures (SSTs) and precipitation, including the El Niño‐Southern Oscillation (ENSO) and Indian Ocean SSTs (Baker et al, 2019; Domeisen et al, 2019; Fletcher & Cassou, 2015; Hall et al, 2017; Trascasa‐Castro et al, 2019), Atlantic SSTs (Hall et al, 2017; Rodwell & Folland, 2002; Wang et al, 2004) and North Pacific SSTs (Hurwitz et al, 2012) These can interact directly through forcing tropospheric Rossby wave trains, or indirectly by influencing the strength of the SPV (e.g., Ineson & Scaife, 2009) through modulation of vertically propagating wave activity. We investigate possible drivers of the extreme winter pattern
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