September Climate Data

With September climate data now available, let's look back at the predominant weather patterns for the month.  The monthly-mean circulation anomaly was quite similar to August, with a high-pressure ridge retaining a strong influence across southern Alaska.  In contrast, low pressure was dominant over the Arctic Ocean, and the Arctic Oscillation was virtually tied with 1975 as the most positive on record in the month of September.  The mid-atmosphere height (pressure) anomaly shows the strong north-south gradient:

The corollary of a strong pressure gradient is, of course, strong wind, and the enhanced westerly flow brought abundant moisture and frequent rainfall to central and northern Alaska.  Again the pattern showed similarities to August, and much drier than normal conditions continued from the Kenai Peninsula to Southeast Alaska.

The monthly precipitation total in Fairbanks was the highest for September since 1925, and as noted in Rick's comments, some rivers saw their highest measured flow on record for so late in the season.

Check out the contrast in sunshine fortunes across the state: western Alaska and the northern interior were extremely cloudy, but southeastern regions were much sunnier than usual.

Despite the broad similarity in the pattern, temperatures were not as overwhelmingly warm in September as in August; the flow direction across central and northern Alaska had more of a southerly component in August.  The figures below highlight a strong north-south gradient that fits with the pressure gradient and the enhanced westerly flow.

Warmth in southern Alaska was amplified by ongoing high sea surface temperatures across the North Pacific to the south of Alaska, and especially to the south of the Aleutians.  Although the core region of unusual warmth is quite a long way from mainland Alaska, southwesterly winds commonly track from the Aleutian region up into southern Alaska, making the oceanic warm anomaly a "source region" for unseasonably warm air.

As for Arctic sea ice, the minimum ice extent on September 12 was the 10th lowest in the satellite-era period of record.  Seasonal meltout was less than might have been anticipated at the start of summer, owing to a relatively cool summer with persistent low pressure across the high Arctic Ocean.  For this reason, the NSIDC describes the melt season as "unusual in the extreme".  My impression, however, is that much of the ice that qualified for "ice extent" (based on the 15% area threshold) on the eastern side of the pole was very tenuous, and a different weather pattern could easily have produced a large drop in extent.  Here's a map of estimated ice concentration just a few days before the date of the seasonal minimum:

Compare the eastern hemisphere coverage to the NSIDC "ice extent" map from the same date: