Storm Track Variability from the Perspective of Individual Storms

Abstract

A combination of feature-tracking and variance techniques are employed to investigate synoptic-scale controls on mid-latitude storm tracks. Most notably, the way in which an upstream wave source (a ``seed" disturbance) propagates downstream and acts to control a downstream storm track is investigated in detail for the observational record. It is found that, in general, about 17\% of observed storminess within the two Northern Hemisphere storm tracks co-varies with the strength of the upstream wave source, and the relationship is robust throughout the cold season (October through April). Three specific cases are studied for the possible impacts of upstream waves: the relative minimum in storminess during winter within the Pacific storm track (the ``midwinter suppression"), the inverse relationship that exists between the strength of the Pacific storm track and the intensity of the Pacific jet stream in winter (the ``inverse relationship"), and the effects of Alpine lee cyclones (a canonical example of a ``seed" disturbance) on climatological timescales. These three are each summarized briefly below. It is shown that a reduction in the frequency and amplitude of disturbances entering the Pacific storm track from mid-latitude Asia is substantially reduced during winter relative to fall and spring and that the magnitude of this reduction is more than sufficient to account for the midwinter suppression. That the midwinter suppression is caused by upstream effects is unexpected based on expectations from previous work, and this is the first published evidence that an upstream wave source can have important consequences for a storm track downstream. It is shown that upstream seeding does not explain why winter months with a stronger-than-average Pacific storm track tend to have weaker-than-average jet streams (the ``inverse relationship"). Most notably, results show that the inverse relationship occurs because Pacific storm track disturbances are weaker and shorter-lived when the jet stream is the strongest; connections with the lifecycles of individual storms are implicated. Finally, Alpine lee cyclones are studied on climatological timescales to evaluate the synoptic events preceding surface cyclogenesis as well as the effects felt downstream due to these events. It is shown that Alpine lee cyclones are largely responsible for the location of the Mediterranean storm track, however it does not appear that the cyclones are surface-driven events on average. Instead, lee cyclones are most often cases of orographically-modified cyclogenesis, and represent cyclogenesis events that would have otherwise occurred in the region but that are geographically localized by the presence of the mountains.