Analysis of change composites

This is analysis for the composite maps in the previous post

These change composites returned some interesting results.

In the strong cases, the greatest magnitude SLP change is -22hPa over southwest Alaska associated with the low. The maximum change is +12hPa around Juneau associated with the downstream high. This produces a strong isallobaric wind from the east centered around Anchorage. In the weak cases, the greatest magnitude SLP change is +15hPa near 50N/170W associated with the upstream high. The pressure tendency with the low is much weaker and more diffuse than in the strong cases, and the downstream high only has a +5hPa change. The strongest isallobaric wind in this domain is thus over the Aleutians between the upstream high and the low, rather than around Anchorage between the low and the downstream high.

The 500hPa has similar results. The strong class has the greatest change couplet from the trough and downstream ridge, while the weak class has the greatest change couplet from the upstream ridge and trough. The couplet is also much stronger in the strong class: -180m to +150m versus +90m to -80m in the weak class. The differences in height tendency with the downstream ridge is the most significant result. The strong class features a +150m change in the northern GOA, while the weak class has only a +90m change further southeast. As a result, there is also a significant contrast in height change specifically over Anchorage. In the strong class, heights rise 80m, and in the weak class, heights are about constant.

Another point is that the strong class exhibits well defined maxima and minima in height change, with a predominant negative tilt. The weak class has more elongated maxima and minima that are predominantly positively tilted. What this and the above observations suggest is an amplifying wave pattern in the strong class and a more progressive pattern in the weak class.

Much of the evidence here points to the strong cases featuring a much more intense low that remains the dominant feature in the domain. The weak cases exhibit a weaker low that is clearly also undergoing secondary development to the southeast. This weakens the pressure gradient and re-orients it more toward the NE-SW over Anchorage. The dominant feature becomes the upstream high over the northern Pacific.

The tendency in the lifted index shows a significant increase in stability in the strong class versus only a small rise in the weak class. Over Anchorage, the LI increases 5C in the strong class and 1C in the weak class.