A 35 year record of Catalina rainfall and what it reveals

Thanks to our friends at Our Garden just off Columbus Blvd. here in Catalina, about 1 mi northwest of my location, we have a rainfall record that goes back to the later 1970s.   This is fantastic because there are no official reporting stations nearby that reflect our rainfall climate, one this close to the Catalina Mountains.   The closest to us in rainfall is Oracle Ranger Station, but that’s at 4400 feet elevation.    We’re about 3000 to 3200 feet here in Catalina.  The long term climate stations, ones under the aegis of the government, are also at lower elevations, and more importantly are farther away from the Catalinas than we are.  Those lower, farther away stations have annual or “water year” (October 1st through September 30th) totals of only around 12 inches.   So, they don’t reflect our wetter Catalina climate.

Our Catalina average rainfall since the 1977-78 water year?  17.04 inches!  Median, 16.59 inches.  Oracle, at 4400 feet elevation for comparison, 21 inches; Mt Sara Lemmon, 30 inches. Too, you could have guessed that we receive much greater rainfall than stations farther away from the Catalinas by our more “lush”, and, being from Washington State,  I use that word advisedly, vegetation hereabouts.  Here is a chart with the water year rainfall values from Our Garden plotted on it:    (Discussion continues below)

Most of the two of you who read this blog will find this 1) quite interesting, and 2) upsetting since there is a clear downward trend in water year rainfall since the folks at Our Garden started maintaining records. Perhaps, you will wonder,  “will it become too dry to sustain life here? Maybe we should sell now and move to a wetter locale like Mobile, AL, before the bottom drops out of the real estate market in the dessicated conditions ahead.”  (Oops, the bottom has already has dropped out of the real estate market.)

But, following the words of Douglas Adams’ in Hitchhiker’s Guide to the Galaxy, “Don’t Panic” .

Let’s look at this a little more closely, shall we?  First, let’s see where this godawful decline in rainfall is located in the records.   Summer?  Winter?  Or both?

Here is our summer rainfall in Catalina (June through September) that will be VERY illustrative of where the decline is taking place because….its not here!

(Discussion continues below)

Yay! As you will plainly see, our summers are the same as ever over the past 35 years or so!  FANTASTIC!   The same number of great storms, lightning, molasses-like dense rainshafts and colorful summer skies are likely still ahead for us between June and September.  No indication here of any “climate change” over the very period in which the earth’s temperature began a noticeable rise following the coolness of the late 1940s through mid-1970s.  As we all know, that rise in temperature since the mid-1970s has been attributed to CO2.  Until recently, global temperatures and the rise in the atmospheric CO2 fraction have been in lockstep.  (Not so over the past 10 or so years for reasons that are “not well understood.”)

But our summer graph is foreboding.  Panic now!

No change in summer rainfall means that ALL of the decline in rainfall over the past 35 years is going to be contained in the “cool” season rainfall records for October through May.  Those cool season rains are due to disturbances in the jet stream; “troughs” that pass over us.  They must be decreasing in frequency over this period.   Furthermore, such a “trough” in the westerlies must have its maximum wind (i.e.,  at the 500 mb level) sag to the south of us to get rain1.

Here is the awful graph which you have now anticipated from the summer rain graph; take a deep breath:

(Discussion continues below)

As you will see, this graph is so shocking I considered calling a news conference about climate change after I plotted it, even though I am not really a climatologist, but rather, a nephologist2.   So, it wouldn’t be quite right to do that.

However, my claim in such a bogus news conference would be that it will likely stop raining in the wintertime in southern AZ within 40 years, if the downturn continues.   I will assign this decline to global warming, since almost all negative-bad news trends are these days.  And, I might have widespread credibility, perhaps a headline.  “Arizona to go dry by 2040!”  The media are, as we know, primed to accept these kinds of claims today it seems without really investigating.

But it will be poor science, an outrage, really.  It should be taken to the Catalina refuse transfer station.

What’s the truth here about our downward trend?  Its certainly a REAL decline over this time span, and,  “what does a longer record show?”

The fact is, Jenny and Wayne at Our Garden happened to start their rainfall record during one of the wettest regimes of the 20th century, and even longer!  Remember how the Great Salt Lake was filling up in the early 1980s?  Remember the huge rains in Cal and AZ in the late 1970s and early 1980s?  Well, of course, IF you DON’T have a good 100 years of experience here, you can’t recall the similar big wet spell in the SW after the turn of the century (1904-05 to about 1920) and the drying that took place afterwards, too.  Those that have been here since 1945 or so are thinking, “If you think its dry now (last ten years), you shoulda been here in the 1950 and 1960s!”

So you can begin to appreciate that our shocking trend since 1977-78 is not one that will continue forever, is not associated with global warming, but is fortuitous because of the starting and end points of our available record.   Our climate in AZ is always oscillating from dry to wet regimes and back again, even in the slightly warmer years likely ahead in the coming decades due to “global warming.”

An example of this oscillation can be seen in the 30 year STATEWIDE averages for rainfall in Arizona.   Check these out from our friends at the Western Regional Climate Center (here). They are shocking and illuminating.

As you will see, the mostly droughty 1941-1970 statewide AVERAGE for Arizona is a ghastly TWO inches LESS (11.69 inches) than that for the frequently rainy era of 1971-2000 (13.59 inches).  Incredible.   That is a HUGE difference when you average over the whole state!   That change to wetter conditions would likely be attributed to the Pacific Decadal Oscillation change in 1977-78 that rolled on for the next 20 years or so.

So, without a grand picture, even 30 years of data can be misleading, produce spurious trends.

Gee, kind of lecture-ish here, boring.  Oh, well.

So, what’s really ahead for Arizona?

Of course, nobody knows for sure, and the climate models with global warming are dicey on long term regional precip changes, such as in southern AZ.

However, I’m one of those (remember, though, not really a climo) who feels that a prediction of just the opposite conditions in the winters ahead might be a “buzzer beater”, a correct one, a winning one; that we are likely to experience a phase shift into wetter winters in the decade or two ahead rather than dryer ones as a projection of the trend line would suggest.

Enough!

The End

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1 If you still have a copy of Willis and Rangno (1971–Final Report to the BurRec, Colorado River Basin Pilot Project, Durango) you of course will already suspect this because that is also the case in Colorado as we reported.

2 Nephologist:  one who studies and has reported stuff in peer-reviewed journals about clouds, but not climate).