New insights into the reconstructed temperature in Portugal over the last 400 years

Abstract

Abstract. The reliability of an existing reconstructed annual (December–November) temperature series for the Lisbon region (Portugal) from 1600 onwards is assessed in the present study. The consistency of this series with: (1) five local borehole temperature-depth profiles; (2) synthetic temperature-depth profiles generated from both reconstructed temperatures and paleoclimate simulations in Portugal; (3) instrumental data sources over the twentieth century; and (4) temperature indices from documentary sources during the late Maunder Minimum (1675–1715) is assessed. It is found that reconstructed annual mean temperature series in Portugal, after European-wide reconstructions, is not consistent with both borehole profiles and paleoclimate simulations in their long-term variability and trends. Hence, the non-linear trend in the paleoclimate simulations is estimated and added to the reconstructed series (first-stage calibration). The annual reconstructed series is then calibrated in its location and scale parameters, using the instrumental series and a linear regression between them (second-stage calibration). The resulting calibrated series is then in clear accordance with the low-frequency variability of both borehole temperature-depth profiles and paleoclimate simulations. This calibrated series shows clear footprints of the Maunder and Dalton minima, mainly attributed to changes in solar activity and explosive volcanic eruptions, and a strong recent-past warming, attributed to human-driven forcing. Lastly, it is also in overall agreement with independently-derived annual temperature indices for the late Maunder Minimum. Thus, the series resulting of this re-calibration process for Lisbon can be of foremost relevance to improve the current understanding of the driving mechanisms of climate variability in Portugal.