Analyses of historic air temperature records using the MONACLE filter indicate periods of reduced signal lasting between 30 to 70 years. Such periods appear “flatter” in the autocorrelation field and so could be associated with higher inter-annual variability in the temperature field (periods of weaker climate persistence).

Two examples are shown below, for 19th century England and early 20th century New England. However, since little is known about historic oceanic conditions, it remains to be proven if these examples were connected to major oceanic changes.

1800s
A long relatively "flat" signal occurred in the autumn autocorrelation series in the early 1800s [1810-1880, between the vertical lines] for central England and coincided with a change or lack of cycles in regional fish catches.

Swedish Bohusian Fisheries Norwegian (spring) herring Solid black line is Climate Index	North Sea - Cod/Gadold Russel - larvae Russel - Gadold

This 70-year period exhibits a pattern change in all the MONACLE analyses across Europe with maritime-inland differences being more evident than for prior or subsequent periods.

A WET time for Europe?
An unusual event can be seen in 900 year long climate records for Western Europe.

A persistent 80-year period, (between the vertical lines), in the differences in Lamb summer wetness index between England and Germany, coincides with the same period as the “flatter” UK autocorrelation/feedback.

1900s
A similar “flat” or low inter-decadal variability period occurs in the 1900s for New England. This also coincides with a fall off in the regional mackerel fisheries [gap between blue and pink plots in top panel].

Black lines give autumn feedback signal, top to bottom (marine) New England, Lakes Erie, Ontario & Huron. Coloured lines give commercial fish catch (green, total lake catch; vertical lines mark the 1906-1942 period). Relative to prior and post periods, the interval between 1906-1942 shows less variability over marine and Great Lakes regions.

FREEZE-UP of Great Lakes
For New England a 30 year period with reduced fishing activity might tie in to regional climate change.

The feedback signal during this period (black) shows a marked similarity to changing freeze-up dates for Lake Superior (red).

SUMMARY
The MONACLE analysis was originally devised to generate indices representative of bulk ocean conditions. From the above two historic examples, the indexing method may have keyed into additional components of the global hydrological cycle; possibly long term moisture related processes over 19th century Europe and the hydrological system of the Great Lakes for 20th century New England. These additions provide further insight into potential physical mechanisms for feedback within the climate system. This pilot study is still exploring additional oceanic and moisture-related datasets.