Predictability of Long-lived of Rossby Wave Packets during Southern Hemisphere Summer
Authors: Perez Iago , Barreiro Marcelo 
Published: June 2022

&lt;p&gt;&lt;span&gt;Rossby wave packets (or RWPs) are synoptic scale perturbations that appear in the high atmosphere of mid-latitude&lt;/span&gt;&lt;span&gt;s &lt;/span&gt;&lt;span&gt;that are&lt;/span&gt;&lt;span&gt; linked to the occurrence of extreme weather events. &lt;/span&gt;&lt;span&gt;Normally these packets last between 3-6 days, but u&lt;/span&gt;&lt;span&gt;nder certain conditions, th&lt;/span&gt;&lt;span&gt;ey can gain enough &lt;/span&gt;&lt;span&gt;stability &lt;/span&gt;&lt;span&gt;to last&lt;/span&gt; &lt;span&gt;from &lt;/span&gt;&lt;span&gt;several days &lt;/span&gt;&lt;span&gt;to &lt;/span&gt;&lt;span&gt;2-3 weeks &lt;/span&gt;&lt;span&gt;in the atmosphere&lt;/span&gt;&lt;span&gt; before disappearing, RWPs that last more than 8 days in the atmosphere are referred as long-lived RWPs or LLRWPs. Thus, a skillful prediction of the apparition and propagation of LLRWPs can improve extreme weather prediction in the sub-seasonal scale. In this study we &lt;/span&gt;&lt;span&gt;aim to &lt;/span&gt;&lt;span&gt;measure the NCEP CFSv2 S2S model &lt;/span&gt;&lt;span&gt;skill at&lt;/span&gt; &lt;span&gt;forecasting the &lt;/span&gt;&lt;span&gt;development&lt;/span&gt;&lt;span&gt; and propagation of&lt;/span&gt;&lt;span&gt; LLRWPs, and how climatic modes such as El Ni&amp;#241;o-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) affect the skill on LLRWPs forecasting. &lt;/span&gt;&lt;span&gt;First, &lt;/span&gt;&lt;span&gt;we calculated the envelope of austral summer meridional winds at 300 hPa from ERA 5 reanalysis, and applied a tracking algorithm&lt;/span&gt; &lt;span&gt;to detect the formation and propagation of LLRWPs&lt;/span&gt;&lt;span&gt;. &lt;/span&gt; &lt;span&gt;Secondly, &lt;/span&gt;&lt;span&gt;we repeated the same methodology but &lt;/span&gt;&lt;span&gt;using &lt;/span&gt;&lt;span&gt;NCEP reforecast data&lt;/span&gt;&lt;span&gt;,&lt;/span&gt;&lt;span&gt; starting the &lt;/span&gt;&lt;span&gt;analysis the &lt;/span&gt;&lt;span&gt;dates a LLRWPs &lt;/span&gt;&lt;span&gt;was detected&lt;/span&gt;&lt;span&gt; in the &lt;/span&gt;&lt;span&gt;reanalysis &lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;o &lt;/span&gt;&lt;span&gt;that &lt;/span&gt;&lt;span&gt;we can track the trajectory of the RWPs forecasted by the model, &lt;/span&gt;&lt;span&gt;these packets will be referred as&lt;/span&gt;&lt;span&gt; Forecasted Rossby Wave packets (FRWPs). &lt;/span&gt;&lt;span&gt;Next,&lt;/span&gt; &lt;span&gt;we studied the propagation characteristics of the &lt;/span&gt;&lt;span&gt;FRWPs&lt;/span&gt;&lt;span&gt; against the original &lt;/span&gt;&lt;span&gt;LLRWPs&lt;/span&gt;&lt;span&gt; tracked in the &lt;/span&gt;&lt;span&gt;reanalysis, &lt;/span&gt;&lt;span&gt;and &lt;/span&gt;&lt;span&gt;classified the FRWPs &lt;/span&gt;&lt;span&gt;according to&lt;/span&gt;&lt;span&gt; the dominant &lt;/span&gt;&lt;span&gt;phases&lt;/span&gt;&lt;span&gt; of ENSO and SAM to &lt;/span&gt;&lt;span&gt;assess &lt;/span&gt;&lt;span&gt;whether&lt;/span&gt;&lt;span&gt; the &lt;/span&gt;&lt;span&gt;predictability of the LLRWPs is&lt;/span&gt;&lt;span&gt; affected by large scale climate modes. Results showed that the model is able to detect the formation and propagation of FRWPs in 88% of the simulations, but only 40 % of the &lt;/span&gt;&lt;span&gt;FRWPs&lt;/span&gt;&lt;span&gt; su&lt;/span&gt;&lt;span&gt;rpass&lt;/span&gt;&lt;span&gt; the 8 days threshold. &lt;/span&gt;&lt;span&gt;I&lt;/span&gt;&lt;span&gt;n addition, forecasted packets show an eastward bias displacement and propagate &lt;/span&gt;&lt;span&gt;slower &lt;/span&gt;&lt;span&gt;compared to the original &lt;/span&gt;&lt;span&gt;LLRWPs&lt;/span&gt;&lt;span&gt;. During ENSO events, years with El Ni&amp;#241;o, Neutral and La Ni&amp;#241;a events, the model was able to forecast &lt;/span&gt;&lt;span&gt;F&lt;/span&gt;&lt;span&gt;RWPs in 85, 96 and 76% of simulations respectively. Nonetheless, the model struggle&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; to predict long episodes of RWPs during Neutral ENSO events. Nonetheles, during negative (positive) phases of SAM, the model FRWPs detection is around 85% (65 %) , and near 88% in neutral SAM phases.&lt;/span&gt;&lt;/p&gt;