JJAG: Comparison between JJA Alpine temperature anomaly, the anomaly
derived from the Burgundy GHD (Grape Harvest Date) and the ETA (European
Temperature Anomaly, Mariani & Zavatti, 2017). The low pass filters of
the series (green, red and blue respectively) have a 50, 20 and 30 years
The strong divergence from the beginning of LIA (1350 CE) to 1730 CE has been
assigned to local climates by the authors (for one year, 1639 which appears
milder in Burgundy than in the Alps) but the differences span for several
decades and in Burgundy "kills" the coldest LIA period, so some more
structured and stable situation must had been present or we need to assume
that grape harvest date cannot be a proxy for temperature in cold periods.
After 1730 (last phase of the LIA and the recovery from a cold climate) the
smoothed curves are quite similar. It must be noted that the anomaly from
GHD has a different base than JJA (April to August mean temperature of Dijon
between 1960 and 1989 instead of 1901-2000); ETA has a 1961-1990 base
period, so some difference could derive from the bases.
Spectra show some variety: GHD and ETA seem similar at long periods (e.g 300 yr) while JJA and ETA show the same for shorter periods (20-80 yr) also if some coincidence could appear casual.