EOF of 500 hPa Heights: Comparing July/August to mid-August/September

Below will be a look at the variability due to seasonality in the EOF components. This look is based on the fact that through my event classification scheme the preponderance of events occur later in the year, late August through September, then seen in previous works, July through August.

Figure 1: Principal Component 1 for both seasons

Figure 2: Principal Component 2 for both seasons
Figure 3: Variance of the first 10 components for both seasons

With PC #1 a well defined trough (lower heights region) is present over central Alberta that stretches southward into southern California. But, with a shift in the time frame from July/August to mid-August/September we see this feature become much more well defined. This makes physical sense (we are moving closer to a time of year where more mid-latitude 'action' is found further south) and it also falls in line with our findings of trough interactions being important components or drivers of surge events in the late Summer.

Comparing the first to the second principal component shows that the July/August season is picking up more of a trough signature than that of the later season. This trough feature is associated with roughly 20% of the variance for mid-Aug/September and roughly 14% for July/August. It is reassuring to see that this trough feature is the leading component for the mid-Aug/September season in that it lends more foundation to our findings of the importance of these features.

The one issue I am having qualms with is the absence of the monsoon ridge over the Four Corners region. I believe we could be seeing some indication of this feature in PC #1 for July/August, but it appears very weak in nature. The problem could be that we are simply looking slightly too high in the atmosphere to pick out a strong signal with this feature, or perhaps my domain is slightly too large to pick out what might be considered a subtle feature in context of the overall synoptic and planetary patterns.

-jamie

500 hPa Height Pattern Composite

Here is a composite of the 500 hPa height pattern associated with strong events. Though we were questioning whether the 500 hPa height anomalies were showing weakening of the monsoonal ridge or an actual trough, it appears quite clear from the actual height patterns that the anomaly feature is indeed associated with a strong trough moving in from the Pacific.

Ridging, though relatively weak in nature, does appear to be in place slightly east of the Four Corners (i.e., Four Corners' High) on Day -4 and -3 to the event. At this time we are already seeing signs of a deepening trough off Northern California. As we progress closer to the event day the trough deepens dramatically, nearly explosively, and progrades into regions where it could potentially affect the characteristics and development of the NAM system. The actual height patterns and the anomaly fields display a strong spatial relationship to one another, and thus, I feel confident say that the anomalies we are seeing are associated with a trough feature.

There are still quite a few questions regarding the origin of this trough feature though. More on that to come soon.

Same Bat-channel...Same Bat-Time.

-jamie