Persistence

Continuing with July's theme of wet weather, western and southern parts of Alaska (except Southeast) have remained much wetter than normal so far this month.  Here's a "percent of normal" analysis based on NWS precipitation estimates for the latest available 14-day period.  Flood watches and warnings are out for several regions in the west. 

The wet pattern illustrates the idea that weather patterns often tend to persist for weeks or even months, but as we saw earlier in the summer, a dramatic reversal can occur too:

Thinking about this, I started wondering if weather patterns have become more or less "persistent" over the decades in Alaska.  A simple measure of persistence can be constructed by counting the frequency with which the precipitation (or temperature, wind etc) departure from normal reverses sign from month to month.

For example, if the pattern flips from warm to cold and back again every month of the year, then this persistence index is zero: the monthly anomalies have opposite signs in consecutive months.  But if all 12 months are warmer than normal, then the index is 100%: no sign reversals occurred between consecutive months.

Using statewide data from NOAA/NCEI, and using a trailing 30-year mean as "normal", the result looks like this:

Higher numbers correspond to more month-to-month persistence.  The first thing that strikes me here is how non-persistent statewide temperatures are; I would have expected higher persistence for temperature, given the large influence of nearby (and very persistent) ocean temperature anomalies on Alaska climate.  But having said that, the atmospheric circulation pattern governs a very large fraction of the temperature variability from month to month, so perhaps it shouldn't be surprising.

It's also interesting to see that persistence was relatively low for both temperature and precipitation in around 2005-2015 (the chart uses a 10-year running average), but temperature persistence has increased substantially in recent years.

The recent prevalence of month-to-month persistence is partly related to the sharp uptrend in statewide temperatures: if it's "warm all the time", then the persistence index will be high.  However, it's worth noting that the sharp increase in temperatures in the 1980s did not produce an equally dramatic rise in temperature persistence.  Here's a chart of actual decadal temperatures and precipitation; both have increased over the last 75 years.

To counter the effects of trend on the persistence index, it seems worthwhile to detrend the data.  After doing this with a simple linear trend (calculated for each month of the year separately), the persistence index looks like this:

We still see that temperature anomaly persistence has increased a lot in recent years, but the detrended perspective suggests that the 1970s and early 1980s were similarly persistent.

The precipitation result is quite interesting, with a slow and steady rise of persistence into the 1990s, a sharp change to more "flip-flopping" patterns after 2005, and then a recovery to a more normal situation recently.

How about summer and winter separately?  The chart below shows results for June through August (with the June values compared to May, so that June can be included).  Here we see a big spike in temperature persistence in the 1990s, but this is likely because summers started becoming much warmer then.  Historically cool summers became a rare breed starting in about 1987.

As for winter, the recent uptick in temperature persistence (see below) must be related to the rapid warming trend, although again it's curious that the warming in the 1980s didn't involve a similar increase in persistence - evidently there were still plenty of colder winter months interspersed with extremely warm months in that period.

Finally, the decrease in winter precipitation persistence this century looks like a "real" change compared to the more persistent regime of the 80s and 90s.  In the last 20 years or so, a wet winter month has been more likely than not to be followed by a dry month, and vice versa, i.e. the statewide monthly precipitation is anti-persistent in winter.  I find this interesting and not a little surprising.