Saturday, May 30, 2015

Patterns that Produce Extreme Temperatures

In view of recent heat extremes in the eastern interior, I was curious to look at the upper-level flow patterns that are typically associated with these events, and to see how these heat-generating patterns vary through the year.  To that end, I took the largest departures from normal of the daily high temperature in Fairbanks since 1948 and created a composite map of the 500 mb height anomaly (departure from normal) for these dates.  The table below shows these maps for each month of the year, with the composites for warm events in the left column, and for cold events in the right column.  Click on the maps to enlarge.

A few features are worth pointing out:

- The height anomalies associated with temperature extremes are generally much larger in winter; this is because the height variance is much larger in the cold season.

- The winter maps consistently identify southeast Alaska and western Canada as the focal point for an upper-level ridge that can bring extreme warmth to Fairbanks in the cold season.  This flow orientation advects warm air from the south and creates downsloping (chinook) flow off the Alaska range.  The pressure gradient also creates wind that disturbs the low-level inversion.

- In April through August, the axis of a heat-generating ridge is much closer to Fairbanks and in July it's located right over the Alaskan interior.  This indicates that unusual heat in summer is caused less by southerly chinook winds, and more by subsidence in place under high pressure aloft.

- The heat-generating pattern for September is unusual, showing a high pressure anomaly over far northwestern Canada.  More investigation would be required to explain this.

- The wintertime cold pattern is not simply the inverse of the warm pattern: the trough axis is located closer to Fairbanks than the ridge in the warm composites, and there is a tendency for strong ridging over the Bering Sea in the cold events.  These features reflect the optimal configuration to bring a cold airmass to Fairbanks along with surface high pressure and calm winds.

- The warm season cold composites emphasize a trough over the interior or North Slope of Alaska, which would be associated with both a cold airmass and an abundance of cloud and precipitation in Fairbanks.



























4 comments:

  1. Wow Richard, what a climate treat! Great info and thanks for ref on the subsidence under highs aloft during summer heat spells.

    Gary

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    1. Thanks Gary, I'm glad I am not the only one who finds this interesting. The NCEP global reanalysis data is a wonderful tool, making analyses like this relatively straightforward.

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  2. I too appreciate these big picture analyses. The way I see it , the blob of air moving north in summer and south in winter makes sense when you consider that the jet stream does that with the seasons. It just so happens that the interactions with the land allows for ridges and troughs to strengthen without interruption. One way to show this is to plot the average position of the jet stream with relation to the air masses.

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