Friday, March 18, 2022

Precipitation Changes

As many readers are aware, it's been an extremely wet winter in western and interior Alaska.  Here are percentile maps for this winter's December-February total precipitation compared to all other years in the ERA5 gridded data (top map) and NCEI's climate division data (bottom map):

The 3-month total was the highest for Dec-Feb across large areas in both data sets, and it was the wettest Dec-Feb period on record for the NCEI state average (data since 1925).

This remarkable outcome reinforces the striking change to wetter conditions in recent years in Fairbanks in particular (where December-February this winter was the wettest since 1936-37).  I've written a number of times about the Fairbanks change that occurred in 2014 - see the blog links at the bottom of this post for a review.

The chart below highlights the change effectively; the 12-month running total precipitation has remained at a high level since that extraordinary summer of 2014 (14.5” of precipitation in June-September, the highest on record).  The longer the sustained wet pattern persists, the more it looks like a true “regime change”, i.e. a new climate normal that has suddenly emerged; but of course 8 years is a relatively short period in climate terms, and it remains to be seen how long the new situation will persist.

The chart above also reveals something else that I find very interesting: the green line shows the 12-month precipitation from days with 0.22” or less of precipitation.  From 1930-2013, such days supplied half of the total precipitation in Fairbanks (but more than half in winter, less than half in summer).  Notice that the running totals from these “lighter” precipitation days have seen only a modest increase – certainly not enough to account for the overall precipitation increase, and not that different from what happened, say, in the mid-late 1940s or around 1990-1993.  Notice too that the “lighter” precipitation totals didn’t rise until the wet autumn of 2017, i.e. more than 3 years after the overall total jumped up.

This means, of course, that a change in "heavier" precipitation days has caused the lion's share of the overall precipitation increase.  This is illustrated in the chart below: in earlier decades, the running totals spiked up occasionally in tandem with the heavier amounts, but the joint increase has been sustained since 2014.  From 2014-2021, the frequency of "heavier" days (0.23" or more) increased by about 50% (while the frequency of much-more-common "lighter" days did not change), and for this 8 year period the heavier days supplied over 60% of the total precipitation.

Another interesting facet of the last 8 years is the changing distribution between summer and winter precipitation.  As noted above, it was the summer of 2014 that kicked it all off, and the next two summers were also very wet; but since then, summer rainfall has been closer to (but still above) prior normals.  The chart below shows how the May-September totals have dropped back somewhat since 2014-2016.

Of course, with less excessive summer rains, cold-season precipitation must have picked up to sustain the very high annual totals, and that's easily seen by inverting the chart columns (and note that the October-April numbers are calendar year totals):

Starting in 2017 - in response to the wet autumn of 2017 - the cold-season precipitation jumped up and has remained high for five years, with 2021 setting a new record.

So it turns out that the sustained nature of the high 12-month totals is related to precipitation increases in both summer and winter, with winters picking up the "slack" from the less-extremely-wet summers of the past five years.

It will be interesting to see where we go from here.  If both summer and winter remain much wetter than earlier decades, then the new regime might be reinforced as both ends of the year make big contributions going forward.  But my suspicion is that decadal-scale changes in the North Pacific are a significant part of the explanation for the new "regime", and in due course we're likely to see a shift to something different.  For example, unusual North Pacific warmth associated with the positive North Pacific Mode emerged in mid-2013 and has been a semi-permanent feature ever since.  I could be wrong, but I doubt that particular spatial pattern of sea surface temperatures will be truly permanent.

Here are some earlier posts on precipitation changes, with a focus on the warm season.  There are probably others I've forgotten about!

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