The curves shown (PDSI units) represent the

SUM OF INDICES OF THE PREVIOUS 21 YEARS

that is, a duodecadal “lookback.”

For the relevant regions (SW Canada, Montana, Utah) the onset of glacier retreat coincides with the sum dropping below zero (that is, the average for the previous 20 years is hitting zero as the PDSI plummets after 1840).

Similarly, for the acceleration of the retreat in the 1920s, the lookback index again hits zero in these three regions.

The history of the southern regions (Arizona, Southern California) is much shifted in phase relative to the northern regions, suggesting some sort of anitcorrelation plays a role in the histories of northern and southern western regions.

PSDI Fourier periodograms SW Canada, for 1730-1850 (blue) and 1859-1979 (purple)

For SW Canada, the climate shift in the 1850s consists of increased amplitudes of more or less the same cycles, but with supression of periods near 8 and 9 and new emphasis near 7y.

Similar periods to those seen in SW Canada series. Again, solar var. entirely suppressed. A move from 4.3 and 12.7 periods to 3, 2 and 33 y periods (both shorter and longer) is apparent.

In Utah, variability stayed roughly the same across the 1850s shift, the 33-y period is abandoned (the reverse of the Montana trend) and periods around 18 are strengthened. A solar signal (10-11) is present.

data E. Cook

In Arizona, the tendency is toward lesser variability, and long-term oscillations, centered near 80 y, moving out from periods near 22.3 and 11.5 y.

post-1859

Periodograms of drought series of Arizona before and after 1855. Overall pattern: Move from duodecadal (22.3) and monodecadal (11.5) into subcentennial cycle (near 79) and subdecadal NAO (near 7.5 y, arrow).

Cycles in common in the Southwest, since 1730. Note log scale. Three families of cycles, related by factors of 2 and 3. 23-y period is prominent because it is linked to all groups.

Summarizing: western drought cycles move from 22-y and 5.9-y periods into 67-y, 33-y and 12.4-y periods. The change is distinct, but unexplained. There is no direct evidence for a stronger solar influence.

Common periods in western drought (PDSI series) 1730-1850 and 1859-1979 (by product in F dimension: SW Canada, Mo, Ut, Az, S Calif.