August Changes

It's been a very chilly few days in the central and eastern interior, with an upper-level trough creating cloud and rain for many.  The high temperature in Fairbanks on Friday was only 52°F, a very low value for this point in August, and yesterday Delta Junction and Northway reached only 46°F and 47°F respectively.  This is the most anomalously cool weather since mid-June.

August is an interesting month for climatologists, because in the far north it is, in some ways, more like an autumn month than a summer month.  This contrasts sharply with its character farther south in the mid-latitude and sub-tropics, where summer drags on or even reaches its zenith of heat and humidity in August (just ask the poor residents of Houston, Texas).

The accelerated timetable of seasonal change in the high latitudes is of course directly tied to the much greater loss of solar heating by this time of year.  At 30°N the sun loses only a small fraction of its power from June to August, and the difference can be imperceptible to many; but the percentage decline is very much larger in the Arctic.

Here's a figure showing the loss of solar radiation from June to August.  Note that this includes the effect of increased cloud cover for many northern areas in late summer, so it's not all attributable to the sinking sun; but the north-south difference is clear.

With solar heating rapidly being lost, the effect on temperatures is inevitable; August is much cooler than July across nearly all Arctic and sub-Arctic land areas.  But the map below immediately reveals another very interesting aspect of August: the northern oceans (except for the ice-covered Arctic Ocean) continue to warm up despite the loss of solar heating.  This is because the heat capacity and vertical transport/mixing of heat are so much greater at the ocean surface than at the land surface: the August sun, weaker as it is, still outweighs the loss of heat from the ocean surface for a time (the air temperature over the northern extratropical oceans typically peaks in the second half of August).

With the distribution of land and ocean being what it is in the Northern Hemisphere, the effect of these seasonal changes is to substantially increase the overall north-south gradient of temperature at about 60°N in August.  And this in turn provides the driving force for a significant increase in westerly winds at that latitude (except farther south in the northwestern Pacific, and a bit farther north in Europe).  Here's a map of the July-August change in westerly winds at 250 mb, which is approximately the level of the jet stream.

The increased energy in the westerly flow causes more disturbed and cloudy weather around the high latitudes, and there's a substantial increase in precipitation compared to July across the North Pacific and North Atlantic - see below.  Interestingly the increase in precipitation doesn't extend to northern land areas in general, and some regions become significantly drier in August; this is at least partly because of the decrease in temperature over land, i.e. cooler air holds less moisture, but over the oceans the warmer water provides more moisture and more rainfall for maritime areas.

Below are zoomed-in versions of the July-August temperature and precipitation change maps for Alaska.  The drop-off in temperature is most pronounced across northern Alaska, and it's minimal along the Gulf Coast, with its heavy maritime influence.

Most of western and southern Alaska are distinctly wetter in August than in July, and it's a large difference for the northeastern Gulf Coast.  Yakutat is a classic example: August produces about 80% more rainfall than July on average.  However, the eastern interior and (according to ERA5 data) parts of the Brooks Range are drier in August.  Northway, for example, sees about 30% less rain in August than in July.

I've used data from the most recent 30-year period for the maps above, so it's interesting to see if the same trends were evident in the previous 30-year window.  The answer is, "essentially yes" - see below.

I also find it interesting to note that August is also considerably cooler than June over much of northern Alaska.  Unlike the comparison to July, this is not universally true across northern land areas, and in fact northern Alaska shows the largest temperature drop from June to August of any high-latitude location.

Finally, the nClimDiv data from NOAA/NCEI gives a good sense of how August temperature and precipitation have changed in the past several decades in relation to June and July.  The following charts show 30-year running mean values on a statewide basis. 

Interestingly, August has warmed up a lot less than either June or July on a statewide basis, and apparently it's only in recent decades that August has become cooler than June.  This is supported by ERA5 data (see below): the August-minus-June analysis for 1965-1994 shows much less pronounced cooling in the northern interior, and southern Alaska apparently used to be more significantly warmer than June.

A more detailed analysis of ground-truth station data and atmospheric circulation patterns would be required to confirm and understand this subtle change in early versus late summer temperatures.