The Climate‐system Historical Forecast Project: do stratosphere‐resolving models make better seasonal climate predictions in boreal winter?

Abstract

Using an international, multi-model suite of historical forecasts from the World Climate Research Programme (WCRP) Climate-system Historical Forecast Project (CHFP), we compare the seasonal prediction skill in boreal wintertime between models that resolve the stratosphere and its dynamics ("high-top") and models that do not ("low-top"). We evaluate hindcasts that are initialized in November, and examine the model biases in the stratosphere and how they relate to boreal wintertime (Dec-Mar) seasonal forecast skill.We are unable to detect more skill in the high-top ensemble-mean than the low-top ensemble-mean in forecasting the wintertime North Atlantic Oscillation, but model performance varies widely. Increasing the ensemble size clearly increases the skill for a given model. We then examine two major processes involving stratosphere-troposphere interactions (the El Niño-Southern Oscillation/ENSO and the Quasi-biennial Oscillation/QBO) and how they relate to predictive skill on intra-seasonal to seasonal timescales, particularly over the North Atlantic and Eurasia regions. High-top models tend to have a more realistic stratospheric response to El Niño and the QBO compared to low-top models. Enhanced conditional wintertime skill over high-latitudes and the North Atlantic region during winters with El Niño conditions suggests a possible role for a stratospheric pathway.

The Stratospheric Pathway of La Niña

“…As Butler and Polvani (2011) stated, in the reanalysis record the SSW fre quency of occurrence is enhanced, with respect to neu tral winters, during both El Nino and La Nina winters. At the same time, the ENSO response is also influenced by the QBO (Garfinkel andHartmann 2007,2008;Calvo et al 2009;Richter et al 2015).…”

“…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.…”

“…For this reason, we choose the N3.4 index from the NCEP/CPC. In addition, these indices have been computed using sea surface temperature (SST) anomalies (e.g., Butler and Polvani 2011;Garfinkel et al 2012) or their standardized values (e.g., Hoerling et al 1997;Mitchell et al 2011). Finally, different thresholds have been applied to select the events.…”

“…Table 1 lists the La Nina winters identified by both thresholds. For comparison, we included the winters identified by Free and Seidel (2009) and Butler and Polvani (2011), who used thresholds of -1 and -0.5°C, respectively. The comparison shows that for the same period, the use of SST anomalies or their SD does not change the La Nina winter selection (see Table 1).…”

“…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.…”