Examining mechanisms of variability within the Pacific storm 1 track : upstream seeding and jet core strength

5 This paper examines how variations in two mechanisms, upstream seeding and jet core 6 strength, relate to storminess within the cold season (Oct Apr) Pacific storm track. It is 7 found that about 17% of observed storminess co-varies with the strength of the upstream 8 source, and the relationship is robust throughout the cold season and for both the Pacific 9 and Atlantic basins. Further analyses of the intraseasonal variability in the strength and 10 structure of the wintertime (DJF) Pacific jet stream draw upon both Eulerian-variance and 11 feature-tracking statistics. We diagnose why winter months with a strong-core jet stream 12 have weaker storminess than those with a weak-core jet stream. Contrary to expectations, 13 we show that the basic spatial patterns actually conform with a simple linear picture: regions 14 with a weaker jet have weaker storminess. The overall decrease in storminess is most strongly 15 linked to the weaker amplitude of individual storms in strong-core months. Previously 16 proposed mechanisms are evaluated in the context of these new results. Lastly, this analysis 17 provides further evidence that the midwinter suppression in storminess over the North Pacific 18 Ocean is primarily due to a notable lack of storminess upstream of the Pacific storm track 19 in the heart of winter. 20