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The Alaska and Arctic Dry Season

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The Alaska and Arctic Dry Season

When it's Springtime in Alaska…

Rick Thoman
Feb 26
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The Alaska and Arctic Dry Season

alaskaclimate.substack.com

A under appreciated but significant feature of Alaska and Arctic climate is that many regions have pronounced dry and wet seasons, and with days rapidly lengthening we are headed into the dry season. At the largest scale this is not too surprising: sea ice is at its maximum extent in Spring and so there is less ocean water surface exposed to supply “local” moisture through evaporation. Figure 1 shows the 1991-2020 average percent of annual precipitation that falls March through May. If precipitation amounts were equally distributed over the year, then each 3-month season would have about 25 percent of the total. Poleward of 60°N, most areas from central Siberia eastward across North America to western Greenland fall well short of that level, and there are large areas where spring typically bringing less than 15 percent of the annual total. While this “dry spring” is less widespread on the Atlantic side of the Arctic it still appears in interior Scandinavia.

Fig. 1 Percent of annual average precipitation that falls in March through May. All data from ERA5 reanalysis courtesy of ECMWF/Copernicus.

Given these low percentages, it’s not surprising that the driest month of the year occurs in the spring over many land areas. Zooming into Alaska and vicinity, we see in Fig. 2 that either March or April has the lowest average precipitation of any month of the year over a large portion of the region from the Russian Far East eastward into mainland (in some areas there’s hardly any difference), although February is the driest month over parts of southeast eastern Interior Alaska eastward across the Yukon and Northwest Territories. Over the Bering Sea and Gulf of Alaska the driest month (on average) is later, usually either May or June.

Fig 2. The month of the year with the lowest average precipitaiton. Note that in some areas there is no significant diference between the driest and second driest month (in our region typically March and April) but that is not reflected in this graphic. All data from ERA5 reanalysis courtesy of ECMWF/Copernicus.

Hidden in the multi-decade averages is the year-to-year variability. Dry months or seasons in Alaska and the Arctic, like other parts of the world outside the tropics that have well defined dry seasons, e.g. western California Oregon, parts of southern European, central Chile and southwest Australia, tend to have many months/seasons with very low precipitation but with infrequent high precipitation occurrences. When we line up many years this leads to monthly/seasonal variability that does not look anything like a “bell” curve, and is illustrated in Fig 3. This histogram of the March Fairbanks, Alaska precipitation for the 71 years 1952 to 2022 shows that almost half the time March total precipitation has been between 0.1 and 0.3 inches and rarely exceed half an inch. However, there a handful of years that have had more than one inch. This produces a spread of values that is often modeled by a gamma distribution, which is well suited to distributions that are limited by zero (like precipitation) and have a “long right tail”.

Now this is of some practical relevance. Because the typical precipitation totals are not very much above zero, it’s not generally meaningful to talk about drought conditions developing during the spring for much of Alaska and northwest Canada. On the other hand, if snowpack is unusually thin at the end of February, chances are low that spring snowfall will improve that situation very much. Also, by convention at least in the US, standard climate normals are calculated as the average (mean) of the baseline period, but in situations like this the mean is strongly influenced by one or two outliers (this is also an issue with shoulder season snowfall). A non-Arctic example would be July rainfall at San Diego, CA. The NOAA 1991-2020 normal precipitation is 0.08 inches. However, only four times in the 30 years has rainfall been above 0.08 inches. This is because a couple of Julys with significant rain strongly skew the average. The median (half the years more, half less) San Diego rainfall in July is a trace. For parts of central and eastern Interior Alaska in March and April it’s not quite that extreme, but the normal (mean) precipitation is 50 to 75 percent higher than the median.

Fig 3. Histogram of Fairbanks Airport March total precipitation 1952 through 2022.

Technical details: This analysis is based on the ERA5 reanalysis data from ECMWF/Copernicus and used the most current version of the reanalysis available as of this writing (February 2023). ERA5 has a horizontal resolution of about 30km, so while that is the highest resolution of any operational reanalysis it is still quite coarse for work in complex terrain. Fairbanks Airport precipitation data from NOAA/NCEI.

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The Alaska and Arctic Dry Season

alaskaclimate.substack.com
2 Comments
Bill
Feb 26Liked by Rick Thoman

Is it a safe corollary that Alaskan springs are sunny? It would certainly boost shoulder season PV potential.

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